Survey Assessment of Semi-pelagi Gadoids: The Example of Walleye Pollok, Theragra halogramma, in the Eastern Bering Sea WILLIAM A KARP and GARY E WALTERS Introdution Diret assessment provides essential information for the management of many marine fish stoks Frequently, demersal stoks are assessed by means of bottom trawl surveys, and pelagi stoks are assessed using aousti tehniques together with some form of diret sampling suh as midwater trawling Eah approah has its own strengths and limitations but these types of routine surveys provide ritial information for many stoks When a stok is semi-pelagi (or semi-demersal) in habit, however, it is diffiult to aomplish overall assessment with a single tehnique, and it may be neessary to assess the pelagi and demersal omponents independently Beause the biases assoiated with eah tehnique differ, diffiulties may be enountered when attempting to ombine the data to produe a omprehensive estimate To address this problem, survey objetives should be evaluated If the assessment proess requires a surveybased estimate of overall abundane, problems assoiated with ombining the two sets of data require areful onsideration But if it is satisfatory to provide indies of the abundane of ertain portions of the stok, suh as speifi age groups, it may be possible to onsider the pelagi and demersal assessments as independent soures of information, and problems assoiated with ombining data sets would then be of less onern The walleye pollok, Theragra halogramma, resoure of the ontinental shelf and slope of the Eastern Bering Sea (EBS) supports major fisheries ativities The speies is semipelagi and The authors are with the Alaska Fisheries Siene Center, National Marine Fisheries Servie, NOAA, 76 Sand Point Way NE, Seattle, WA 98115 is generally found in pelagi and demersal regions over bottom depths of 25 4 m, although it does our in the pelagi zones of deeper waters (Sample and Bakkala, 19) Greatest abundanes are found along the outer ontinental shelf over water depths of 1 3 m (Wespestad and Megrey, 199) Sientists from the NMFS Alaska Fisheries Siene Center (AFSC) ondut the assessment of this stok The demersal omponent of the stok is assessed annually during a multi-speies bottom trawl survey of the EBS shelf Smallsale surveys began in the early 197's, and the present survey overage was first established in 1975 and has been done annually sine 1979 Also beginning in 1979, an expanded triennial bottom trawl survey has been onduted; this has overed a greater area of the shelf and the waters of the upper ontinental slope During the triennial surveys, the pelagi omponent of the pol ABSTRACT-Assessmentofwalleye pollok, Theragra halogramma, in the eastern Bering Sea is ompliated beause the speies is semi-pelagi in habit Annual bottom trawl surveys provide estimates of demersal abundane on the eastern Bering Sea shelf Every third year (starting in 1979), an extended area of the shelf and slope is surveyed and an eho integrationmidwater trawl survey provides estimates ofpollok abundane in midwater Overall age-speifi population and biomass estimates are obtained by summing the demersal and midwater results, assuming that the bottom trawl samples only pollok inhabiting the lower 3 m ofthe water olumn Total population estimates have ranged from 134 x J(p fish in 1979 to 27 x 1 9 fish in 1988 The very high abundane observed in 1979 reflets the appearane of the unusually large 1978 year lass Changes in age-speifi abundane estimates have doumented the passage of strong (1978, 1982, and 1984) and weak year lasses through the fishery In general, older fish are more demersally oriented and younger fish are more abundant in midwater, but this trend was not always evident in the patterns ofabundane of1- and 2-year-oldfish As the average age of the population has inreased, so has the relative proportion of pollok estimated by the demersal surveys Consequently, it is unlikely that either tehnique an be used independently to monitor hanges in abundane and age omposition Midwater assessment depends on pelagi trawl samplesfor size and age omposition estimates, so both surveys are subjet to biases resulting from gear performane and interations between fish and gear In this review, we disuss survey methodology and evaluate assumptions regarding athability and availability as they relate to demersal, midwater, and overall assessment 8 Marine Fisheries Review
lok stok has also been assessed by means of an eho integration-midwater trawl (EIMWT) survey In this paper we evaluate the methodology and results of the pollok assessments onduted during the triennial surveys as an example of semi-pelagi gadoid assessment In onsidering the soures of bias, we offer suggestions for researh and hanges in methodology whih may lead to improvements Methods Detailed information on survey methodology was presented by Bakkala and Wakabayashi (1985), Bakkala et al (1985), Walters et al (1988), and Bakkala et al (1992) Here we provide an overview of bottom trawl and EIMWT tehniques Bottom Trawling Assessment of demersal pollok on the EBS shelf and slope is onduted within the broader objetives of a multispeies bottom trawl survey designed to assess the ondition of stoks of several speies Surveys are performed annually during the months of June, July, and August when migratory ativities are believed to be minimal Every third year a more omprehensive survey, overing a larger area of the shelf and slope, is arried out Details of the triennial survey design are presented by Bakkala (1988) and an illustration of the area sampled is presented in Figure I An Eastern otter trawl (type 83-112) with 9 kg steel V-doors has been used for sampling sine 1982 (Table 1) Previously, a similar but smaller net, the 4 mesh Eastern trawl with 57 kg doors, was used Both trawls were onstruted of the same mesh sizes (Table 1), however, the 83-112 was rigged with 3 m hain extensions on the footrope ends to improve bottom tending harateristis Prior to 1988, trawl width (wingspread) measurements were made on only a few tows made by any vessel with a given trawl The averages of the measured widths were then used for all tows within an annual survey by that vessel and trawl Beginning in 1988, the aquisition of suffiient mensuration equipment allowed measurements for almost every tow Examination of these data revealed that trawl operating width is primarily a funtion of the amount of trawl warp extended (Rose and Walters, 199) This analysis indiated mean widths-per-tow of 12-2 m over bottom depths of 2-2 m Warp extended over these depths ranged from 9 to 55m Time on the bottom and the distane fished for eah haul were determined from the time and loation where the winh brakes were set to the time and loation of the beginning of haul bak Based on depth readings from the trawl mensuration equipment, trawl settling time was onsidered insignifiant at depths enountered on the EBS shelf The loations of the vessel at the start and end of eah haul were obtained from LORAN C instruments Biologial information was obtained from the athes so that biomass and population abundane ould be estimated by speies, size, and age; athes greater than I metri ton (t) were subsampled using proedures designed to ensure randomness (Hughes, 1976; BakkaIa et ai, 1985) Analytial proedures were desribed by Wakabayashi et al (1985) An area swept tehnique whih inorporates the wingspread and distane fished measurements desribed above, was used to develop biomass, population, and size omposition estimates (Alverson and Pereyra, 1969; Doubleday and Rivard, 1981) Eah ath was Table 1 - Charateristis Years trawl used Trawls used during the triennial eastern Bering Sea groundfish surveys Horizontal opening while fishing (m) Vertial opening while fishing (m) Headrope Length (m) Footrope Length (m) Mesh sizes (mm) Wing Body Intermedible Codend Codend iiner Door (m) Length Height Dandyline Length (m), 1988 onfiguration/1991 onfiguration Bottom trawls 4-mesh Eastern trawl 1979 122 17 216 287 12 12 32 21 15 83-112 trawl 1982-91 12-2 2-23 253 341 12 12 32 27 18 standardized into ath per unit of area The standardized athes within a stratum were then used to estimate stratum biomass, and the stratum estimates were summed over the entire area Lengthfrequeny data and age-length keys, developed from fish sampled during the survey and aged from otoliths, were applied to the data to provide stratum and overall survey estimates of size omposition and age omposition Until 199, the standardized athes of eah speies by eah vessel were ompared using a Bayesian approah (Geisser and Eddy, 1979) Ifsignifiant differenes between vessels were found for a partiular speies, the athes of the least effiient vessel were adjusted to be equivalent to athes from the most effiient vessel for that speies by the ratio of the mean athes per unit effort (CPUE) Beause this method was based on the estimation of a ratio, it was sensitive to oasional large CPUE values Beginning in 199, a new method developed by Kappenman I was used to ompare the distribution of CPUE values based on a power transformation and develop a saling fator for adjustment This method has been applied to the time series of data bak through 1982 I Kappenman, R F 1992 Estimation of the fishing power orretion fator US Dep Commer, NOAA, Natl Mar Fish Serv, Alaska Fish Si Cent Pro Rep 92-1,1 p Norsenet trawl 1979 35 27-3 Unknown Unknown 1 1 8-8 46 38 3 2 Northern Gold 12 trawl 1988-91 4-5/Unk' 3-4/18-25' 98/945' 849/5' Rope 163-76 96-32 27 18 Midwater trawls Diamond 1 trawl 1982-85 165-183 16-19 54 54 46 813-38 21-27 15-18 Gourok rope wing trawl 1982 274-329 27-33 121 121 Rope 1626-813 46-121 32 6 6 455 549 165 823 549 914 56(1), 1994 9
I _::::::C) ::::> _ i 65 DON m,j " tlj CON ::!1 '" 1 In l B '" 61 ' *, L6 : s "'- -s' " \ I '2 7 "'-,::,-;:" "unlrnol<-p", ALASKA f" CON Jr " 57 DON I I- 6 OON I i I I iii i 54 OON 177 ODE 18 DOW 177 DOW 17-4 DOW 171 DOW 168 DOW 165 DOW 162 OOW 1551 DOW 156 DOW / / \ "'" 'l'iyal< }!( DON : J 1f:v\ 69 OON 4, 1 J, R/V MILLER FREEMAN I "- _ :"- o R/V ALASKA F/V OCEAN HOPE TOM I MARU NO 51 11 3 \ O)--Cf ;;,,"'-J! Islands '-"," ) 1(t'5' f) A" '" ::l Figure I - Trawl stratifiation and survey design used in 1988 over the eastern Bering Sea shelf and slope L55 aon
Aousti and Midwater Trawl Assessment The aousti method for pelagi stok assessment is based on the priniple of eho integration An eho sounding system transmits disrete pulses of sound into the water and waits for a period of time to reeive ehoes from targets in the insonified volume of water The reeived ehoes, in the form of voltages, are then fed into an eho integrator whih squares and sums the voltage samples The output of the eho integrator is proportional to the density of the fish insonified (Dragesund and Olsen, 1965; Forbes and Nakken, 1972; Burzynski, 1982) Conversion of relative to absolute biomass estimates is dependent upon onsistent system performane as monitored by alibration proedures and information regarding the aousti properties of the fish in the form of mean aousti target strength (TS) TS is dependent on speies, size, behavior, and, in some ases, depth Sine small hanges in mean TS an give rise to large errors in biomass estimation, diret in situ measurement of TS is generally reommended (Ehrenberg, 1983; Foote, 1991) Tehniques for the US EIMWT surveys were desribed by Karp and Traynor (19) and Traynor and Nelson (1985) Additional details were reported by Bakkala et al (1985) and Walters et al (1988) Transet lines were surveyed by means of a sientifi quality 38 Khz aousti system onsisting of a transmitter, a towed transduer, a reeiver, and a omputer based digital eho integrator The aousti system was installed in a portable van that ould be loated on the dek of the survey vessel When onditions were suitable, in situ target strength studies were onduted in order to ollet target strength distribution information for a range of fish sizes and behavioral pattems Dualbeam and split-beam tehniques (Traynor and Ehrenberg, 1979) have been employed for this purpose, although most data has been olleted with a split-beam system in reent years Sine we do not yet have suffiient information to enable us to apply field measurements ofts during analysis of survey data, altemative approahes have been used as an interim measure Before 1988, a TS value of-3 dblkg was applied in the onversion of integrator values to estimates of biomass; starting in 1988, however, the empirial target strength/length relationship developed by Foote and Traynor (1988) (TS = 2 log (Fork Length (m)) -66) has been employed and has provided results whih are generally onsistent with in situ measurements The aousti system was alibrated before and after eah survey The underwater aousti alibration system available at the University of Washington's Applied Physis Laboratory was employed to ondut standard transmit and reeive response and equivalent beam angle measurements Also, beginning in 1988, the standard target tehnique, as desribed by Foote et al (1987) was adopted and is now arried out in situ to monitor system performane at intervals during eah survey Midwater trawling was an integral part of eah survey A large midwater trawl was used to ollet biologial samples when signifiant eho sign was enountered during the aousti transets Midwater trawl types and speifiations have hanged several times during the period that these surveys have been onduted (Table 1) As indiated in the table, different trawls were used for sampling juvenile and adult pollok sign The vertial opening of eah net was monitored with a netsonde Towing speed in all surveys was approximately 15 m/se Fishing was arried out on an opportunisti basis in order to ollet adequate samples from the different types of eho sign enountered throughout the survey area Sine ontamination with other speies ourred infrequently, the primary objetive of this sampling was to provide suffiient data for partitioning the aousti estimates of biomass by size and age, and developing size and age speifi population estimates The information olleted from these trawls was not used to provide quantitative information on abundane Cathes were proessed in a manner similar to that desribed for the demersal trawl surveys Age omposition was determined by means of age-length keys obtained by analyzing otoliths taken from fish sampled randomly from most athes All EIMWT triennial surveys have taken plae in the summer, during the same general time period as the bottom trawl surveys In 1979 only the outer portion of the shelf and the upper slope were surveyed (Traynor and Nelson, 1985) In 1982 the entire shelf and upper slope over bottom depths from about 4 to 5 m was surveyed with a zigzag transet design (Bakkala et ai, 1985) Subsequent surveys have overed most of the shelf waters deeper than 5 m and the slope Starting in 1985, equidistantly-spaed parallel transet survey designs (eg, Fig 2) have been employed (Walters et ai, 1988; Bakkala et ai, 1992) Results Beause the general objetive of this ontribution is to disuss the methodology for overall pollok assessment in the EBS, this setion will onentrate on the types of information produed during the triennial surveys Results of the 1979, 1982, 1985, and 1988 triennial shelf/slope surveys were reported by Bakkala and Wakabayashi (1985), Bakkala et al (1985), Walters et al (1988), and Bakkala et al (1992), and the results of more detailed analysis of data from these surveys were presented by Karp and Traynor (19), Sample and Bakkala (19), and Traynor et al (199a) Results of the 1991 survey were not available when this report was prepared Rather than present detailed figures of pollok distribution for eah survey, the 1988 results are provided as an example (Fig 3-5) Similar figures for the preeding triennial surveys were provided by Karp and Traynor (19) The 1988 survey results indiated patterns of distribution similar to those observed during previous triennial surveys Taken independently, neither the bottom trawl nor the EIMWT survey results provide omplete information on the horizontal distribution of pollok (Fig 3,4) Most demersal pollok ourred in waters deeper than 1 m, and 56(1),1994 Jl
""' tv 65N SIBERIA 54N I 53 OON ALASKA 162N 61 OON 6N 59 DON 58N 57N I l56n 55 DON s 54N '" 174 ODE 177 OOE 18 OOW 177 OOW 174 OOW 171 OOW 168 OOW 165 OOW 162 OOW 159 OOW 156 OOW :::l '" Figure 2 - The eho integration and midwater trawl survey design used in 1988 on the eastern Bering Sea shelf and slope '" '" ;;;
' ' Figure 3 - Distribution and abundane of age I and older walleye pollok near bottom as determined during the 1988 triennial eastern Bering Sea shelf and slope bottom trawl survey
-l> 65 aon SIBERIA 64 DON o, ALASKA I 63 aon 162N 61 aon 6 DON POLLOCK IN ),(IDWATER I\laB Cl'I1ILlrED DENSITY (KGjHA) ' 59 DON 58 aon 57 DON NO CATCH > - 5 >5-1 I l5b OON >1-3 55 DON >3 ::I ;, 174 ODE 177 OOE 18W 177 OOW 174 DOW 171W 168W 165 DOW 162 DOW 159W 156 OOW '" Figure 4 - Distribution and abundane of age I and older walleye pollok in midwater as determined during the 1988 triennial eastern Bering Sea shelf and slope EIMWT survey ::I '" :: '" ;; ;< 54 aon
' 65N SIBERIA I ;J - ALASKA 64 DDN 63 OON "'" 162N 61 OON 6 DON 59 OON 58N 1988 EsrulA7ED DENSrIY (KG/HA) NO CATCH > - 5 >5-1 >1-3 I 57 DON l56n 55 DON >3 I I I iii iii iii iii I Iii I ", 't'""""'i' I i I I I I I I I 54 OON 174 OOE 177 OOE 18 OOW 177 OOW 174 OOW 171 OOW 168 DOW 165 OOW 162 OOW 159W 156W Figure 5 - Distribution and abundane of age 1 and older walleye pollok near bottom and in midwater as determined by ombining bottom trawl and EIMWT estimates for the 1988 triennial eastern Bering Sea shelf and slope survey v,
few were found in trawls onduted in water shallower than 5 m Pelagi pollok were more abundant in waters deeper than 1 m Loalized areas of high abundane generally ourred in the viinity of Unimak Pass and south of the Pribilof Islands, and overall pollok abundane was usually higher to the north and west of the Pribilof Islands than elsewhere (see Figure I for loation of depth ontours and geographi sites) EIMWT surveys alone would not have doumented the presene of pollok in shallower waters of the ontinental shelf, but, by ombining the two types of survey data, it was possible to produe a more omprehensive map of distribution whih indiated the trend of inreasing overall abundane with depth (Fig 5) Comparison of results from the four ombined demersal trawl and EIMWT surveys onduted over a IO-year period indiates substantial differenes in abundane and vertial distribution between years (Fig 6) Differenes in the age-speifi proportions of pollok Sn 8 14 6 4 [SJMidwater DDemersal 12 2 / // CI) 1 18 1979 16 " :D 8 14 12 6 1 " " 8 4 6 4 " 2 CI) 2 :D :; 2 " 2 1 8 6 4 2 F1R "- "- [SJMidwater 12 34567 8 911112 123456711112 " 1985 "- :> " " [SJMidwater DDemersal "- " "- I\ DDemersal 1982 I\ "- n IR= 1 --------------, 8 6 [SJMidwater DDemersal 1988 14 -r----------------,--,a I A o 1 2 3 4 5 6 7 8 9 11112 Age (years) o 1 2 3 4 5 6 7 8 9 11112 Age (years) Figure 7 -Age-speifi walleye pollok population estimates for midwater and demersal assessments onduted in 1979, 1982, 1985, and 1988 1979 1982 1985 1988 14 -r-----------b 12 1 8 z o 6 ::> Q 4 2 b:sl Midwaler C]Demersal 1979 1982 1985 1988 SURVEY YEAR Figure 6 - Eastern Bering Sea shelf and slope walleye pollok biomass (A) and population (B) estimates for 1979, 1982, 1985, and 1988, illustrating proportions ofestimates in midwater and near bottom found in midwater and on bottom are also apparent (Fig 7, 8) For example, the proportion of 1- and 2-year-old fish in midwater was muh greater than on bottom in 1979, whereas the proportion of age 1 fish on bottom exeeded that in midwater in subsequent survey years; age 2 and 3 fish were more abundant in midwater than on bottom during eah survey, but age 4 fish were more abundant demersally in 1979 and in midwater in 1982, 1985, and 1988 The proportion of fish older than 5 years assessed by bottom trawl always exeeded the EIMWT derived proportion; this supports the pereption that demersal orientation is more ommon for older fish (Fig 9) The extremely strong 1978 year lass undoubtedly influened the unusual vertial distribution of juvenile fish that was observed in 1979 Our ability to trak the progress of this year lass over a IO-year period has been greatly enhaned by the use of both assessment methods It is apparent that the variability of age-speifi distribution in the midwater and demersal zones, overlaid on the general trend of inreased demersal orientation with age, ould not have been adequately doumented by one assessment method alone In addition, this ombined assessment approah has enabled us to better doument the progression of the above average year lasses of 1982 and 1984 Many of the trends and patterns observed in the time series of data an be reasonably attributed to differenes in year lass strength, overall abundane, and reruitment to the shelf and slope stoks Nevertheless, it is likely that some of our pereptions 16 Marine Fisheries Review
have been influened by biases that are inherent in our survey tehniques For example, the total abundane of the 1981 year lass appeared to inrease between 1982 and 1985 This suggests that this year lass was not fuily available to either (or both) surveys at age 1 5 Limitations Biases of onern to sientists onduting these surveys an be lassified into three prinipal ategories: those assoiated with defiienies in biologial knowledge; those assoiated with fishing gear, fish behavior, and the athing proess; and those assoiated with the tehnique of eho integration 4 3 Q E 1Il 1Il 2 E a m E2SJMidwater DDemersal 1979 123456711112 Biologial Considerations The ategory of biologial knowledge inludes migratory ativities and fators relating to stok identifiation Detailed knowledge ofpatterns ofstok distribution in spae and time is an essential prerequisite for assessment survey design Sine these surveys have been onduted over relatively short time periods, when EBS pollok are believed to be in a nonmigratory feeding mode, horizontal migrations are not onsidered to be a serious soure of bias However, assumptions regarding the geographial distribution of the stok are based on limited information 5 4 3 2 E2SJ Midwater DDemersal 1982 123456711112 and it is generally reognized that the stok extends into unsurveyed areas of the western Bering Sea Stok identifiation is of partiular onr,ern with regard to the origin of fish aught in the entral Bering Sea and adjaent to Bogoslof Island (lat 54oN, long 168OW) The midwater and demersal surveys have not always been synhronous This is a potential soure of bias if signifiant migration does our Vertial migration is also of some onern The demersal trawl surveys are onduted only during daylight hours to avoid possible inonsistenies EIMWT surveys have been arried out on a 24 hour per day basis Even though diel hanges in vertial distribution are apparent, it has been assumed that movement between the pelagi and demersal zone is not signifiant and that possible biases are minimal due to the short darkness period during the summer Interannual hanges in vertial distribution have been observed and the resultant hanges in availability to eah survey tehnique are a ause of serious onern -::- 1 l CD 14 -,-------------, E::::S:]Ages 6+ A 12 DAges 1-5 5 4 3 Q E 1Il 1Il E Q CD 2 o F9r:::1 " 1'\ " " " E2SJMidwater DDemersal 1985 Fin 123456711112 Age (years) 5 --,----------------, 4 E2SJMidwater DDemersal 1988 123456711112 Age (years) Figure 8 - Age-speifi walleye pollak biomass estimates for midwater and demersal assessments onduted in 1979, 1982, 1985, and 1988 14 1 e 8 z 6 l a 4 1979 1982 1985 1988 E:SJAges 6+ 12 DAges 1-5 2 1979 1982 1985 1988 Survey Year Figure 9 - Eastern Bering Sea shelf and slope walleye pollok biomass (A) and population (B) estimates for 1979, 1982, 1985, 1988, illustrating proportions of estimates of age 1-5 and age 6 and older fish B 56(1),1994 17
Fishing Gear and Fish Behavior Conerns regarding vessel avoidane, gear performane, and interations between gear and fish during bottom trawl surveys have reeived a great deal of attention (Clark, 1979; Engas and GodliS, 1986; Koeller, 1991; Olsen, 199; ana and GodliS, 199; Wardle, 1986) The development of instruments for monitoring some aspets of gear performane has been of great importane in this ontext Suh equipment is now used on a routine basis during EBS bottom trawl surveys (Rose and Walters, 199) The estimates of wingspread used prior to 1988 introdued bias into the pollok abundane assessment proess beause they did not aount for the effet of varying sope These early estimates were based on averages of small numbers of measurements taken in various loations, and for eah survey the mean wingspread for the net aboard eah vessel was used to estimate area swept Work by GodliS and Engas (19) and Rose and Walters (199), among others, has demonstrated that trawl width inreases with sope Most pollok are found in deeper water, where sope lengths and, therefore, wingspreads are greater than average Thus for the greater proportion of pollok athes, net wingspread would have been underestimated and biomass overestimated Sine juvenile pollok are generally found in shallower waters than adults, it is likely that this soure of bias inorporates an age-speifi omponent In the area-swept alulations, it is assumed that the effetive width of the net is equivalent to the wingspread Work by Engas and GodliS (19a) indiates that the effetive width of a demersal trawl used for sampling gadoids is related to fish size In general, they demonstrated that, as sweep length (distane between trawl doors and net) was inreased, ath rates of Atlanti od, Gadus morhua, and haddok, Melanogrammus aeglefinus, inreased with inreasing fish length They onluded that the herding effet is greater for larger fish Loss of small fish under the net, as observed by Engas and GodliS (19b) is thought to be minimal in the EBS surveys beause the net does not have roller gear and is rigged to dig into the bottom slightly in order to better sample rabs The seond omponent of the areaswept estimate is distane travelled along the bottom Net ontat with bottom does not our until after the winh brakes are set and the gear settles The rate at whih the gear settles is influened by winh and vessel speed, water urrents, depth, and net onstrution At the end of the tow the net does not lift off as soon as the winhes are turned on However, the atual forward motion of the net aross the bottom during haul bak is diffiult to determine Even if the exat distane were known, the degree to whih it is appropriate to orret for time off bottom is not lear beause of the onfounding effet of speies and size related patterns of behavior Beginning in 1993, new tehniques will be used to address this problem By using a time-depth reorder fitted to the net and Global Positioning System (GPS) navigation, exat time and position of settling and lift off an be determined Thus haul duration an be measured and anomalous gear behavior identified However, tehniques are not yet available for observing how well the gear stays in ontat with the bottom (bottom tending) There is also some onern that the area swept is based on distane travelled rather than the quantity of water passing through the net Gear flowmeters are available to investigate this issue but they have not yet been used during AFSC surveys The proedure for fishing power orretion (FPC), or standardization of observations to the most effiient vessel-gear ombination has also hanged in reent years The new tehnique is onsidered to be more statistially appropriate than the method employed previously However, the FPC proedure onsiders only speies-speifi effets and does not take size-speifi fators into onsideration Also, the deision to adjust area swept estimates to the most effiient vessel-gear ombination is based on the impliit assumption that athability annot exeed unity Younger pollok tend to remain in the water olumn and older fish are generally more demersally oriented Several researhers (eg, Olsen, 199; Nunnallee 2 ) have demonstrated that gadoids may dive in response to pereived disturbane from vessels and/or trawls This behavior ould lead to hanges in relative availability to eah assessment method Size speifi differenes in patterns of avoidane are also likely With the exeption of 1979, the demersal survey biomass estimate ofage 1fish has always been higher than that obtained by EIMWT Midwater estimates of age 2 and 3 pollok are generally higher than in the demersal survey, but the reverse is true for most ages greater than 3 in the majority of years If the demersal trawl gear that was introdued in 1982 sampled age 1 fish with greater effiieny than before, this ould partially explain the unusually high proportion of age 1 fish observed by the EIMWT method in 1979 It should be noted, however, that the 1978 year lass was extremely large, and its midwater abundane in 1979 was very high The hange in gear type has undoubtedly ompromised the integrity of the time series of data in a number of ways Unfortunately, however, limited resoures and inlement weather have preluded any meaningful omparison of the two trawl types Size and age omposition of the midwater omponent of the stok is estimated by applying the information obtained from midwater trawling to the eho integration results Thus, the degree to whih the trawl samples represent the atual omposition of the eho sign is a fundamental onern The proess of judging ehograms and assigning biologial harateristis based on trawls is somewhat subjetive Hylen et al 3 believe the proess ofassigning bio 2 Nunnallee, E P 1991 An investigation of the avoidane reations of Paifi whiting (Merluius produtus) demersal and midwater trawl gear Pap pres to Fish Capture Committee, Int Coun Explor Sea, em 1991, paper/ B:5, Session U 3 Hylen, A, O Nakken, and K Sunnana 1986 The use of aousti and bottom trawl surveys in the assessment of north-east Arti od and haddok In M Alton (Compiler), A workshop on omparative biology, assessment, and management of gadoids from the North Paifi and Atlanti oeans, part II, p 473-498 Rep on file at Northwest and Alaska Fish Cent, Natl Mar Fish Serv, NOAA, 76 Sand Point Way NE, Seattle, WA 98115 18 Marine Fisheries Review
logial harateristis to be a prinipal soure of error in the determination of size and speies speifi aousti abundane estimates (Aglen, 19) Errors that result from avoidane, seletivity, or herding will be refleted in the sizeand age-speifi abundane estimates Of partiular onern are size-speifi phenomena suh as seletivity, or differenes in avoidane reations The observations of Engas and Godjlj (19b) regarding the greater degree to whih larger gadoids are herded into the path of a bottom trawl are probably also appliable in the pelagi zone; one might also expet greater esapement of small fish through the rope wings of the midwater trawl It is not known if the diving avoidane behavior observed by Nunnallee 2 ours to a greater degree in larger fish, but their stronger swimming abilities and greater stamina suggest this possibility Eho Integration Calibration is an essential omponent of a quantitative aousti assessment program The system was alibrated frequently, and eho sounder performane was monitored during the surveys Due to the limited dynami range of the eho sounder and the need to set an integration voltage threshold high enough to eliminate extraneous noise, we undoubtedly obsured aousti returns from low density distributions of pollok during these surveys The degree to whih this resulted in underestimation of abundane is unknown, but our general observations of the pattern of pollok aggregation and distribution suggest that this problem was not severe, espeially sine the survey area is relatively shallow Before 1988, a onstant average TS value was used in biomass estimation alulations Seletion of too low a mean TS value would have led to an overestimate of fish abundane; this would have been more likely when smaller fish predominated, suh as in 1979 This is beause, in general, the TS per unit weight for small fish is greater than for large fish The use of observed fish length ompositions and the empirial TS-length relationship of Foote and Traynor (1988) to ompute mean TS in 1988 makes some progress towards addressing the problem but relies heavily on the assumption that the midwater trawl athes aurately represent the size omposition of the fish sampled aoustially (further impliations of this assumption are disussed below) In the EBS, shools ofjuvenile fish are generally spatially separate from shools of adults, so that the effets of seletivity may be less severe in this ontext Neither TS estimation tehnique has been able to take into aount behavioral and other fators that influene fish target strength (Traynor et ai, 199b) An aurate measure of the aousti pulse width is essential if biomass estimates are to be obtained by eho integration The aousti pulse width provides one dimension of the omputation used to determine the amount of energy transmitted into the water If unorreted, an erroneous assumption that the pulse width is too wide will lead an underestimate of abundane The reverse is also true A omputer program was developed to sample the retified voltage signal from individual fish targets and alulate the effetive integration pulse width (Traynor and Nelson, 1985) Sound energy dereases with distane from the transduer due to spreading and attenuation To ompensate for this, eho sounder reeivers inorporate a time varied gain (TVG) amplifier and provide for an estimate of attenuation due to absorption by seawater The AFSC aousti system TVG funtion is measured using a omputer program that samples alibration osillator signals at I m intervals Deviations from the theoretially orret TVG were determined and orreted as desribed by Traynor and Nelson (1985) If unorreted, deviations from the theoretially orret TVG ould lead to biased estimates of abundane Overorretion (too muh gain) would ause overestimation and underorretion would ause underestimation The aousti system and operating proedures were designed to minimize the effets of self noise Self noise is the sum of noise omponents ontributed by the vessel and the eletroni equipment Sine low-noise, sientifi quality instruments were used, the prinipal potential soure of self noise was the survey vessel During these surveys, vessel noise was minimized by seletion of appropriate engine speed and propeller pith, and, on oasion, suspending survey operations during inlement weather Reverberatory noise ours when the transmitted sound pulse interepts bubbles or biologial sound satterers (ie, organisms other than the target speies) In general, during the EBS surveys, most ambient or reverberatory soures of noise were eliminated or redued to an insignifiant level by setting an appropriate signal threshold and eliminating data from nontarget speies during data proessing It should be noted, however, that the proess of setting an appropriate system threshold is one of ompromise; setting a threshold high enough to eliminate most extraneous noise will result in the elimination of returns from fish, espeially at low levels of density The transduer was generally towed deep enough to avoid near-surfae noise from air bubbles but no deeper than the shallowest expeted ourrene of pollok (Traynor et ai, 199b) Separation of fish ehoes from bottom ehoes is subjet to physial and operational limitations System parameters limit the absolute ability of an eho sounder to separate near-bottom targets from the bottom itself and fish in ontat with the bottom annot be assessed by eho integration Provided that eho integration is arried out in small disrete depth intervals, it is usually possible to eliminate unwanted bottom ehoes from the data and inlude aousti returns from fish within a short distane (2-3 m) of the bottom This proedure was followed during AFSC data olletion and analysis The methodology disussed above refers to the AFSC aousti system that was in use through the 1988 triennial survey Beginning in 1991, a new set of instruments, with improved stability, dynami range, and signal-to-noise ratio has been employed (Knudsen, 199) The basi methodology has not hanged, but tehnial improvements have re 56(1),1994 19
dued the potential impat of some of the aforementioned onerns Overall Estimates All the aforementioned biases are of onern when ombining the results of the pelagi and demersal assessments Eah method is subjet to a series of biases and it is likely that ombining the two sets ofestimates will exaerbate the effets The basi assumption impliit in this proedure is that the effetive sampling height of the bottom trawl is 3 m, that all demersal fish are fully available only to the bottom trawl, and all pelagi fish are fully available only to the aousti assessment It is also assumed that all sizes of fish in eah zone have a athability of unity to the respetive assessment method In the preeding paragraphs we have presented evidene to suggest that these assumptions may not be ompletely valid For example, diving avoidane behavior may inrease the availability of pelagi fish to the demersal trawl and redue their availability to the pelagi trawl This may result in inauraies in both demersal and pelagi size and age omposition estimates God and Wespestad (199) postulate that this is indeed the ase, and that it led to substantial overestimation of older fish in 1988, as indiated by a divergene of fishery based and survey based estimates in that year Their argument is plausible beause it onsiders the tendeny of the bottom trawl to sample larger fish with greater effiieny in the horizontal plane due to herding and in the vertial plane due to diving avoidane behavior The diving behavior would probably also lead to an underestimate of the proportion of larger fish in the pelagi region Preliminary results of the 1991 triennial survey add further weight to this argument (Williamson)4 With the new 4 Williamson, NJ NMFS Alaska Fisheries Siene Center, 76 Sand Point Way NE, Seattle, WA 98115 Personal ommun Further information on reent stok assessments is provided in the 1993 stok assessment and fishery evaluation report for the groundfish resoures of the Bering Sea/Aleutian Islands, and it is available from the North Paifi Fishery Management Counil, PO Box 13136, Anhorage, AK 9951 aousti assessment system, pelagi pollok biomass estimates overed the water olumn down to 1 m off bottom The preliminary pelagi biomass estimate was 21 million t and the preliminary bottom trawl estimate (overing the lower 3 m) was 5 million t Extrapolation of the eho integration results to the bottom inreases the overall EIMWT estimate by a relatively small amount This suggests that the effetive width of the bottom trawl is onsiderably greater than the distane between wingtips and the effetive height is greater than 3 m As mentioned earlier, bottom trawl surveys are onduted during daylight, while aousti and midwater trawl sampling are arried out during daylight and darkness Muh work has been done on the effet of diel hanges on fish behavior and assessment results throughout the world (eg, Engas and Soldal, 1992; Wardle, 1986; Woodhead, 1964) but this phenomenon has not been investigated extensively with regard to Bering Sea pollok assessment The impat of these phenomena on survey results an be substantial, and onsiderable researh is alled for It also seems reasonable to evaluate the manner in whih survey data is used to tune the fishery based analysis; perhaps the demersal and pelagi estimates ould be onsidered as independent estimates of the ondition of ertain sets of age groups Sample and Bakkala (19) demonstrated a highly signifiant regression when omparing bottom trawl and ohort analysis estimates of age 4-9 pollok abundane for the period 1979-86 Sine the population ageing proess is gradual, one might expet the size dependent phenomena disussed above to have influened the annual bottom trawl (and triennial EIMWT) results gradually over a period of years If this had been the ase, it might have been apparent in the work of Sample and Bakkala (19); it would be interesting to extend their analysis with reently obtained data Toward Improved Assessments As we reognize the various problems assoiated with bottom trawl and EIMWT surveys we must address them in a systemati manner In the light of reent work arried out at AFSC and elsewhere, appropriate researh is being planned Relationships between net width and trawl warp determined by Rose and Walters (199) have been applied to the histori 83-112 trawl data for whih diret width measurements were not made This enabled us to make orretions in the area swept estimates and investigate the degree of bias assoiated with assumptions regarding the horizontal opening of the net Biases leading to overestimates in the earlier analyses (espeially for trawls made in deeper water) were apparent in all but one year Overall, the magnitude of the bias was approximately 3%, a value less than we originally expeted For speies with predominantly inshore, shallow-water distributions, underestimation biases were indiated The CPUE values have now been used to reevaluate the FPC estimates neessary to omplete a new set of biomass estimates for the time series After applying these two sets of orretions, it will be neessary to review the multi-year data set It must be reognized that these orretions will not aount for all soures of error For example, if wingspread exeeds design limitations, hanges in gear performane an be expeted It has been argued that the FPC approah is no more than an interim solution and will be eliminated when we better understand fish behavior in relation to fishing gear and an take signifiant phenomena into aount in our assessment proess (Munr 5 ) Researh will be direted towards in situ monitoring ofgear performane and establishing tehniques for maintaining performane within design limitations The work of Engas and Ona (1991) on the use of restritors for door spread shows promise in this regard The innovative approah to determining settling and haulbak time and distane fished desribed above will help redue errors in area swept alulations and enable us to better identify poor 5 Munro, P NMFS Alaska Fisheries Siene Center, 76 Sand Point Way NE, Seattle, WA 98115 Personal ommun 2 Marine Fisheries Review
hauls We are also onerned with the problems of subsampling large athes and interested in the reent work whih investigated the advantages and disadvantages of shorter tow durations (God et ai, 199) Other problems in fishing gear tehnology are more diffiult to researh and will require a major effort It is essential that researh on gear design and fish behavior in relation to fishing gear be assigned high priority if we are to understand and address the biases in our survey estimates The data presented above suggest that larger pollok are more demersally oriented but we are not sure of the extent to whih this pereption is tainted by interations between fish and sampling gear We are now able to ollet ontinuous TS measurements through the water olumn during the surveys Provided that suitable target densities an be enountered, and the single target seletion algorithm is equally effetive through the range of depths sampled, we will be able to use TS measurement data to examine possible trends in size with proximity to the bottom It should also be possible to design experiments to investigate how suh trends are influened by the passage of a trawl Be ause of the influene of fish behavior on TS these types of observations will have to be evaluated with aution The work ofengas and God (19a) suggests that we must investigate the effet of sweep length on the size (and speies) omposition in our athes This leads to a onsideration of priorities Beause the trawl survey is designed to assess a multispeies ommunity, it will probably never be possible to optimize the design with respet to pollok Sine most of the speies sampled by the bottom trawl are truly demersal, perhaps we need to develop an alternate tehnique for assessment of this semi-pelagi speies Regardless, it is essential that we investigate the size and speies-speifi sampling effiienies of our sampling trawls We believe that it may be possible to develop the EIMWT tehnique to produe satisfatory total water olumn estimates of pollok abundane Reent work on problems of near-bottom de tetion (Ona, 1988 (ited in God and Wespestad, 199) and Mitson, 1983) suggests that it may be possible to ollet useful integration data very lose to the bottom and make appropriate orretions for regions of the aousti beam that are obsured in this near bottom region While the physial limitations have not hanged, our understanding of the limitations has improved, and newly developed tools for olleting and proessing eho integration data will make it easier for us to ondut researh in this area (Knudsen, 199; Foote et ai, 1991) It is important to bear in mind that this approah will still require us to make assumptions about fish distribution and behavior lose to the bottom, and these assumptions may be diffiult to test We will still be faed with some questions regarding trawl sampling biases when it omes to developing age speifi abundane estimates This will beome even more ritial if we are to expand our EIMWT method into the demersal zone An alternative approah would involve using aousti measurements to investigate and doument availability of pollok to demersal gear Muh researh remains to be done if we are to develop effetive tehniques for ombining biologial information from midwater and bottom trawls while aounting for the biases inherent in the use of eah type of gear The methods desribed above are designed to provide estimates of absolute abundane However, we have presented overwhelming evidene of sub stantial bias in our survey results This supports the argument that survey results should be onsidered as indies of relative abundane that have signifiane only as elements in a time series Even under this onstraint, we must work under assumptions regarding the onsisteny of bias whih may be diffiult to support Realistially, however, we must onsider the manner in whih survey results are applied Stok assessment models that are used to determine allowable biologial ath (ABC) levels for ommerial fisheries require a soure of absolute abundane information This an be obtained from analysis of historie fisheries data or from survey results Analysis of fisheries data requires reliable time series of ath and effort information and realisti estimates of natural mortality and terminal fishing mortality rates For some fisheries, data are insuffiient, and in other ases assumptions regarding mortality rate estimation may be diffiult to support This leaves us on the horns of a dilemma Are we more willing to aept the assumptions impliit in developing absolute abundane estimates from survey data than those impliit in the analysis of fishery data? The problem an be resolved by taking a pragmati approah, reognizing the limitations under whih we are fored to work and making best use of the data that is available Inevitably we will sometimes have no hoie but to use survey results as measures of absolute abundane This serves only to emphasize the need for all sientists involved in stok assessment to understand the limitations in their data and investigate approahes to improving the quality of information used in the stok assessment proess Aknowledgments We wish to aknowledge the advie of Arill Engas, David Somerton, Jim Traynor, Mark Wilkins, and Russell Nelson in preparing this doument This manusript was originally presented at the 1991 statutory meeting of the International Counil for Exploration of the Sea in La Rohelle, Frane Literature Cited Aglen, A 19 Reliability of aousti fish abundane estimates Dotoral Dissert, Univ of Bergen, Norway, L5 p Alverson, D L, and W T Pereyra 1969 Demersal fish explorations in the northeastern Paifi Oean - An evaluation of exploratory fishing methods and analytial approahes to stok size and yield foreasts J Fish Res Board Can 26: 1985-2 I Bakkala, R G 1988 Struture and historial hanges in the groundfish omplex of the east em Bering Sea Ph D Dissertation, Hokkaido Univ, Hakodate, Japan 187 p, W A Karp, G E Walters, T Sasaki, M T Wilson, T M Sample, A M Shimada, D Adams, and C E Armistead 1992 Distribution, abundane and biologial harateristis of groundfish in the eastern Bering Sea based on the results of US-Japan bottom trawl and hydroaousti surveys during June September, 1988 US Dep Commer, NOAA Teh Memo NMFS FINWC-2J3, 362 p 56(1), 1994 21
_-;:-;-,---_ ' J J Traynor, K Teshima, A M Shimada, and H Yamaguhi 1985 Results ofooperative US-Japan groundfish investigations in the eastern Bering Sea during June November 1982 US Dep Commer, NOAA Teh Memo NMFS FINWC- 87, 448 p _-=:----,_' and K Wakabayashi (Editors) 1985 Results ofooperative US-Japan groundfish investigations in the Bering Sea during May August 1979 Int North Pa Fish Comm, Bull 44, 252 p Burzynski, J J 1982 Introdution to the use of sonar systems for estimating fish biomass FAO Fish teh pap FIRM/191 Rev I, p Clark, S H 1979 Appliation of bottom-trawl survey data to fish stok assessment Fisheries 4:9-15 Doubleday, W G, and D Rivard (Editors) 1981 Bottom trawl surveys Can Spe Publ Fish Aquat Si 58, 273 p Dragesund,, and S Olsen 1965 On the possibility of estimating year-lass strength by measuring eho abundane of O-group fish FiskDir Skr Havunders 13(8):48-75 Ehrenberg, J E 1983 A review of in situ target strength estimation tehniques In O Nakken, and S C Venema (Editors), Symposium on fisheries aoustis, p 85-9 FAO Fish Rep FIRMIR3 Engas, A, and O R God!'! 1986 Influene of trawl geometry and performane and fish vertial distribution on fish sampling with bottom trawl J Northw Atl Fish Si 7:35-42 _-;-:-;:;;-_' and 19a The effet of different sweep lengths on the length omposition of trawl athes J Cons int Explor Mer 45: 263-268,-_' and 19b Esape offish under the fishing line of a Norwegian sampling trawl and its influene on survey results J Cons into Explor Mer 45:269-276, and E Ona 1991 A method to redue survey bottom trawl variability Int Coun Explor Sea, CM 19911B:39, 6 p _---:-_' and A V Soldal 1992 Diurnal variation in bottom trawl athes of od and haddok and their influene on abundane indies Int Coun Explor Sea, J Mar Si 49:-95 Foote, K G 1991 Summary of methods for determining fish target strength at ultrasoni frequenies Int Coun Explor Sea, J Mar Si 48:211-217 _=----,,' H P Knudsen, R J Korneliussen, P E Nordb!'!, and K Roang 1991 Postproessing system for eho sounding data J Aoust So Am 9(1):37-47 _-::-,, G Vestnes, D N Ma- Lennan, and E J Simmonds 1987 Calibration of aousti instruments for fish density estimation: a pratial guide Coop Res Rep Cons int Explor Mer 144,69 p _---:-_,, and J J Traynor 1988 Comparison of walleye pollok target strength estimates determined from in situ measurements and alulations based on swimbladder form J Aoust So Am 88(1):9-17 Forbes, S T, and O Nakken (Editors) 1972 Manual of methods for fisheries resoure survey and appraisal Part 2 The use of aousti instruments for fish detetion and abundane estimation FAO Manuals Fish Si 5, 138 p Geisser, S, and W F Eddy 1979 A preditive approah to model seletion J Am Stat Asso 74(365):153-16 God!,!, O R, and A Engas 19 Swept area variation with depth and its influene on abundane estimates from trawl surveys J Northw Atl Fish Si 9:133-139 _=_' M Pennington, and J H V!,!lstad 199 Effet of tow duration on length omposition of trawl athes Fish Res 9: 165-179 God!'!, O R, and V G Wespestad 199 Monitoring hanges in abundane of gadoids with varying availability to the applied survey tehnique In O R God!'!, Fators affeting auray and preision in abundane estimates of gadoids from sientifi surveys, p 139-169 Dotoral Dissert, Univ of Bergen, Norway Hughes, S E 1976 System for sampling large trawl athes of researh vessels J Fish Res Board Can 33:833-839 Karp, W A, and J J Traynor 19 Assessment of the abundane of eastern Bering Sea walleye pollok stoks In Pro Int Symp BioI Manage Walleye Pollok, p 433-456 Alaska Sea Grant Rep AK-SG--1 Knudsen, H P 199 The Bergen Eho Integrator: an introdution J Cons int Explor Mer 47:167-174 Koeller, P A 1991 Approahes to improving groundfish survey abundane estimates by ontrolling the variability of survey gear geometry and performane J Northw Atl Fish Si 11:51-58 Mitson, R B 1983 Fisheries sonar Fish News Books Ltd, Farnham, Surrey, Engl, 287 p Olsen, K 199 Fish behavior and aousti sampling Rapp P-v Reun Cons int Explor Mer 1: 147-158 Ona, E 1988 Manual for bruk av bunnkanal, bakstep, d!'!dsonekorreksjon, luftabsorbsjonskorreksjon under flerbestandstoktet 1988 Inst Mar Res, Bergen, 14 p, and O R God 199 Fish reation to trawling noise: the signifiane for trawl sampling Rapp P-v Reun Cons int Explor Mer 1:159-166 Rose,, and G E Walters 199 Trawl width variation during bottom trawl surveys: Causes and onsequenes In L L Low, (Editor), Proeedings of the Symposium on Appliation of Stok Assessment Tehniques to Gadids, p 57 67 Int North Pa Fish Comm, Bull 5 Sample, T M, and R G Bakkala 19 Assessment of walleye pollok of the eastern Bering Sea based on bottom trawl surveys In Pro Int Symp BioI Manage Walleye Pollok, p 457-469 Alaska Sea Grant Rep AK-SG -1 Traynor, J J, and J E Ehrenberg 1979 Evaluation of the dual beam aousti fish target strength measurement method J Fish Res Board Can 36: 165-17 _-:-:-=--,' W A Karp, M Furusawa, T Sasaki, K Teshima, T M Sample, N J Williamson, and T Yoshimura 199a Methodology and biologial results from surveys of walleye pollok (Theragra halogramma) in the eastern Bering Sea and Aleutian Basin in 1988 In L L Low (Editor), Proeedings of the symposium on appliation of stok assessment tehniques to gadids, p 69-99 Int North Pa Fish Comm, Bull 5 _--:---:-:,-' and M O Nelson 1985 Methods of the US hydroaoustis (eho integratormidwater trawl) survey In R G Bakkala and K Wakabayashi (Editors), Results of ooperati',e US-Japan groundfish investigations in the Bering Sea during May-August 1979, p 33-4 Int North Pa Fish Comm, Bull 44 _---=--' N J Williamson, and W A Karp 199b A onsideration of the auray and preision offish-abundane estimates derived from eho-integration surveys Rapp P-v Reun Cons int Explor Mer 1:11-111 Wakabayashi, K, R G Bakkala, and M S Alton 1985 Methods of the US-Japan demersal trawl surveys In R G Bakkala and K Wakabayashi (Editors), Results of ooperative US-Japan groundfish investigations in the Bering Sea during May-August 1979, p 7-26 Int North Pa Fish Comm, Bull 44 Walters, G E, K Teshima, J J Traynor, R G Bakkala, K L Halliday, W A Karp, K Mito, N J Williamson, and D M Smith 1988 Distribution, abundane, and biologial harateristis of groundfish in the eastern Bering Sea based on results of the US- Japan triennial bottom trawl and hydroaousti surveys during May-September 1985 US Dep Commer, NOAA Teh Memo NMFS FI NWC-154, 41 p Wardle, C S 1986 Fish behavior and fishing gear In T J Pither (Editor), The behavior of teleost fishes, p 463-495 Johns Hopkins Univ Press, Baltimore Wespestad, V G, and B A Megrey 199 Assessment of walleye pollok stoks in the eastern North Paifi Oean: An integrated analysis using researh survey and ommerial fisheries data RappP-v Reun Cons int Explor Mer 1:33-49 Woodhead, P M J 1964 Diurnal hanges in trawl athes of fishes Rapp P-v Reun Cons int Explor Mer 155:35-44 22 Marine Fisheries Review