Length and species-dependent diurnal variation of catch rates in the Norwegian Barents Sea bottom-trawl surveys

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1 ICES Journal of Marine Science, 56: Article No. jmsc , available online at on Length and species-dependent diurnal variation of catch rates in the Norwegian Barents Sea bottom-trawl surveys K. Korsbrekke and O. Nakken Korsbrekke, K. and Nakken, O Length and species-dependent diurnal variation of catch rates in the Norwegian Barents Sea bottom-trawl surveys. ICES Journal of Marine Science, 56: Diurnal variation in capture efficiency may add to the variability in swept area estimates (or indices) of abundance from bottom-trawl surveys. In the present study the relationship between the day/night ratio of swept area estimates and fish length was examined for five species observed in the Barents Sea bottom-trawl survey in winter in the years Generally, most species showed increased catch rates during daylight at all sizes as compared with darkness. For cod the day/night ratio peaked at a length interval cm with a substantial reduction for larger fish. For haddock the ratio was highest at the minimum size group, cm, and decreased with increasing size. Some possible behavioural explanations for these findings are discussed. A change in groundgear during the time period analysed had a pronounced effect on the day/night ratios for all species. In addition there seem to be a tendency for the day/night ratios of catch rates for both cod and haddock to increase with stock size. This is a matter to be aware of when survey results are interpreted and used in stock assessments International Council for the Exploration of the Sea Key words: diurnal variation, catch rates, bottom trawl. Received 27 March 1998; accepted 30 November K. Korsbrekke and O. Nakken: Institute of Marine Research, P.O. Box 1870 Nordnes, N-5024 Bergen, Norway. Correspondence to K. Korsbrekke: tel: ; fax: ; knutk@imr.no Introduction The reliability of indices of abundance from bottomtrawl surveys depends on a range of factors. Usually one can use the coefficient of variation as an indicator of the precision in the estimates. In the Norwegian bottomtrawl survey in the Barents Sea in winter the coefficient of variation of the abundance indices for cod by 5 cm length groups seems to have a tendency to peak between 25 and 40 cm, with some changes between years (Korsbrekke et al., 1995). This is not so for haddock where the coefficient of variation seems to increase from the abundant lower length groups up to low abundant larger sizes. As this bottom-trawl survey yields one of the most important tuning series in the annual stock assessment work, all knowledge on uncertainties related to the results is useful. Catch rates in bottom trawl are known to vary with the time of day (Shepherd and Forrester, 1987; Walsh, 1988; Engås et al., 1988; Atkinson, 1989; Ehrich and Gröger, 1989). The catch composition by size groups is also known to vary (Engås and Soldal, 1992; Michalsen et al., 1996; Aglen et al., 1997a). In the present work we have used 12 years of survey data to describe how the day/night ratios in bottom-trawl catch rates for species (cod, haddock, beaked redfish, golden redfish, and long rough dab) vary with fish size and type of groundgear. For cod and haddock we have also studied the dependency in the day/night ratio on the density or abundance. The implications of the findings for the conduct of the surveys and the reliability of swept area estimates/indices are considered. Materials and methods The Barents Sea bottom-trawl survey A combined acoustic and bottom-trawl survey for demersal fish in the Barents Sea is conducted annually, January to March, by The Institute of Marine Research, Bergen (IMR). The main aim of the survey is to map the spatial distribution and obtain indices of abundance of the most important commercially exploited demersal fish species in the Barents Sea. The target species are cod /99/ $30.00/ International Council for the Exploration of the Sea

2 Diurnal variation of catch rates 285 Figure 1. Strata (numbers) and subareas (letters) used in the bottom-trawl survey. Trawl stations in winter 1996 are shown. (Gadus morhua), haddock (Melanogrammus aeglefinus), golden redfish (Sebastes marinus), beaked redfish (Sebastes mentella), and Greenland halibut (Reinhardtius hippoglossoides). A description of the survey can be found in (Jakobsen et al., 1997). Data and results are used in the stock assessments in ICES and in several projects at IMR, especially multispecies research and development of new methods (Korsbrekke et al., 1995). Data from the bottom-trawl suvey in the years are presented in this work. They comprise catch rates at length for each species from 190 to 300 bottomtrawl stations each year carried out in 3 4 weeks by 2 3 vessels (Jakobsen et al., 1997) in positions determined prior to the survey. The survey area was increased in 1993, but only data from the central region (A, B, C, and D) which had a good coverage in all years are used (see Figure 1). The effort and equipment used in these years have changed (Jakobsen et al., 1997). Both the change from bobbins to rockhopper groundgear in 1989 and the reduction of mesh size in the codend from 40 to 22 mm in 1994 introduced an increase in abundance indices of small fish. In this work, however, it is assumed that the change in mesh size had little effect on the day/night ratio between catch rates of the smallest size groups. Estimation and analysis A description of the estimation of abundance indices by length and age can be found in (Jakobsen et al., 1997). in the present study, length-based indices of abundance are used. Usually abundance indices are computed for each 5 cm interval in length. However, in order to favour direct comparisons between length groups we have chosen to keep the relative range of length groups constant, i.e. (L max L min )/L mean =constant, and thus generated indices for length groups with equal range on a logarithmic scale (cm): 12 15; 16 22; 23 31; 32 44; 45 63; Independent night and day estimates of abundance were produced by defining night hauls as stations taken when the sun was 5 or more below the horizon. Thus stations taken in twilight conditions were included in the day estimates. This increased the number of day stations, which otherwise would have been low due to the short days in winter at these latitudes, and reduced the coefficient of variation in the day estimates. Some of the strata (especially the northern ones) were, in some years, sampled poorly during daytime. Poor sampling was mainly in areas of low abundance for all species, and the possible effect on the total day/night ratio of abundance indices was negligible. Night and day indices were calculated for all years for the following species: cod, haddock, golden redfish, beaked redfish, and long rough dab (Hippoglossoides platessoides). Ratios between night and day indices were produced for comparison. The yearly ratios for each lengthgroup represent one observation in the analysis. Gear and stock size effects were studied using a covariance model established as follows: The

3 286 K. Korsbrekke and O. Nakken Rockhopper 0 Bobbins 0 Cod Haddock Beaked redfish Golden redfish Long rough dab Figure 2. Ratios between day and night indices. The lines connect mean values for each gear type. Vertical bars represents 2 standard errors of the mean. assumption that day and night estimates are proportional can be expressed for each length group l as: N day l=k l N night l (1) where N day l and N night l are the independent lengthbased abundance indices calculated from day stations and night stations and k l is the ratio between day and night indices. Taking logarithms: In order to investigate if and to what extent the change in gear (bobbins to rockhopper) as well as varying abundance or stocksize (N l ) has effected the ratio between day and night indices we write Equation (2) in a form that permits an analysis of covariance: Where we have replaced Log(k l ) with α l and added terms for the gear effect (β g ), the stock size effect (λ) and an error term (ε).a difference between the two gear types would show up as β g significantly different from 0.

4 Diurnal variation of catch rates Beaked redfish Golden redfish 2.0 Cod Long rough dab Haddock Figure 3. Mean ratios day/night by size for all species (only rockhopper gear). 1 Estimates of λ being significantly different from 0 would indicate that the ratios between day and night estimates are not only length dependent, but also stock size dependent. N l was calculated for each of the length groups using VPA estimates (ICES, 1999) as estimators of stock size. VPA numbers at age were distributed among the length groups by assuming a normal distribution of lengths within each age class. The observed mean lengths by age and years were used together with a constant variance given from the mean of the CV estimated over all ages and years. Results The ratios for all five species are presented in Figure 2. The results for the period (bobbins gear) and (rockhopper gear) are shown separately. For comparison purposes the mean day/night ratio for each species in (only rockhopper gear) is shown in Figure 3. The ratio between day and night indices was significantly different from unity for all species at some size group and generally higher than unity at all lengths for the surveys using rockhopper groundgear. The largest differences between day and night indices (ratio >3) were found for small haddock and for beaked redfish cm in length while the day/night ratio for long rough dab was just slightly above 1 at all lengths for rockhopper gear. The change from bobbins groundgear to rockhopper generated an increase in the day/ night ratio between catch rates for all species and lengths. Cod and haddock showed strong size dependence in the day/night ratios but the patterns were different in the two species. For cod the day/night ratio increased from the smallest lengthgroup, peaked at length cm (mainly 3-year-olds and some 2-year-olds) and then decreased with increasing length. For haddock the largest ratio was observed for the smallest length group decreasing with length to approximately one for the larger fish. The results from the analysis of covariance are given in Tables 1 and 2. For both cod and haddock the year range was used. Due to very low and imprecise indices of haddock larger than 60 cm, the upper size group (64 89 cm) was excluded from the analysis. In both species the models showed that size is the dominating explanatory variable. The second most important effect was the type of groundgear. The stock size explained only a small part of the remaining variation. When the stock size effect was included the goodness of fit increased from R 2 =0.44 to R 2 =0.49 for cod and from R 2 =0.66 to R 2 =0.71 for haddock. The results from the analysis of covariance are illustrated in Figure 4. The bars connect predicted values for

5 288 K. Korsbrekke and O. Nakken Table 1. Analysis of covariance results for cod. Model fitting d.f. SS MS F p Model Error Total R CV Type III tests d.f. SS MS F p α β g λ Parameter estimate T p Standard error α α α α α α β bobbins λ Table 2. Analysis of covariance results for haddock. Model fitting d.f. SS MS F p Model Error Total R CV Type III tests d.f. SS MS F p α β g λ Parameter estimate T p Standard error α α α α α β bobbins λ minimum and maximum observed stock size for each of the periods (gear type); the height of the bars being proportional to the range of the logarithms of stock size for each of the two periods. The relatively modest fraction of variation explained by the stock size is also illustrated in Figure 5 where the residuals from a model with only length and gear effect are plotted against stock size.

6 Diurnal variation of catch rates Cod 2.0 Bobbins Rockhopper Haddock Rockhopper Bobbins Figure 4. Predicted ratios day/night for each gear type against fish size. Discussion Our study of the observed variations in day/night ratio between catch rates has been using data from a series of annual trawl surveys. Other studies (Engås and Soldal, 1992; Michalsen et al., 1996; Aglen et al., 1997a) differs from this approach by using data from repeated trawl hauls at the same station or a limited number of such stations. As our study only reveals overall effects more detailed studies using a number of repeatedly trawled stations can focus on possible local effects such as the effect of varying fishing depth, temperature, fish density, and tidal current. The results show clearly that the day/night ratio in catch rates and abundance depends on gear type for all species, as well as on fish size for cod and haddock and possibly also on fish abundance or stock size for these two species. Differences in bottom-trawl catch rates between day and night hauls are commonly explained by diel vertical migration that introduce differences in the amount of fish available to the trawl (Engås et al., 1988; Walsh, 1991; Engås and Soldal, 1992; Michalsen et al., 1996; Aglen et al., 1997a) and by day/night variations in capture efficiency due to visual herding being absent (or reduced) at night. Most probably both these factors have contributed to the observed results. But why is the day/night ratio for cod and haddock dependent on fish size? And why are the results for the smaller fish so different in the two species? In the Barents Sea in winter cod and haddock of all size groups are commonly found in scattering layers from the bottom up to 50 0 m or more above the

7 290 K. Korsbrekke and O. Nakken Cod Residuals Stock size (millions) Haddock Residuals Stock size (millions) Figure 5. Residuals from an analysis of covariance excluding the stock size term plotted against stock size. bottom, most often in decreasing density with distance from the bottom (Figure 5.4 in Korsbrekke et al., 1995; Mehl, 1998; Ottersen et al., 1998). Aglen (1996) showed that the effective fishing height of the trawl is considerably greater for large sized specimens than for small ones, and it has been observed that large cod dive some 50 0 m between the passage of the vessel and arrival of the trawl (Ona and Godø, 1990). It is to be expected that the avoidance reaction to vessel (propeller noise) and warps is less in small sized individuals so that the effective fishing height becomes less than for large fish. The results obtained by Aglen (1996) indicate an effective fishing height of about m for gadoids of 20 cm in length. Hence, even minor diurnal changes in vertical distribution between cod and haddock, say 20 m, may introduce large differences between day and night catch rates for small sized fish while the day/night ratio for large specimens is little influenced unless their diurnal vertical migrations are quite exclusive. The main cause for the relatively large day/night ratio in the estimates for the redfish (rockhopper gear) is probably due to diurnal vertical migration. This has also been supported by acoustic observations (Aglen et al., 1997a). As for the other species (Figure 2) long rough dab also shows a distinct effect of changing from bobbins to rockhopper groundgear. Other authors (Bowering,

8 Diurnal variation of catch rates ; Shepherd and Forrester, 1987; Walsh, 1991; Neudecker and Breckling, 1992) have found larger catch rates at night for some flatfish species and Walsh (1991) explains this as a result of avoidance under the groundgear or between the gear and the trawl during daytime. Engås et al. (1988) showed that rockhopper groundgear increased the catching efficiency as compared to bobbins groundgear especially for smaller fish of many species and this was a major reason for the change in sampling gear (Godø et al., 1989). The results of the analysis of covariance indicate that the day/night ratio increases with increasing stock size both for cod and haddock; the more fish the larger are the day catches as compared with the night catches. The model used [Equation (3)] assumes that the relative change in the ratio is proportional to the relative change in stock size and this assumption may not hold for all size groups. The proportionality factor λ may change between size groups, but there are too few observations in these data sets to introduce such interaction terms. The increase in ratio with increasing stock size could be caused by density-dependent catching efficiency of the trawl. It has been observed that at high fish densities the fish adopt a schooling behaviour in front of the trawl so that larger proportions are caught than at low densities when fish search actively for escapement and often succeed (Godø, 1995; Aglen et al., 1997b). However, if the diel vertical migrations are density dependent this would have a similar effect on the day night ratio. The values of λ (given in Tables 1 and 2) generate approximately 11% increase in the day/night ratio of estimates when stock numbers are doubled. Hence, for ranges in year-class strength of 2 3 decades as experienced for cod and haddock in the Barents Sea, the effects on the estimates might be substantial. 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