Stock assessment of hake (Merluccius australis) on the Chatham Rise for the fishing year

Transcription

1 Stck assessment f hake (Merluccius australis) n the Chatham Rise fr the 29 1 fishing year P. L. Hrn R. I. C. C. Francis NIWA Private Bag 1491 Wellingtn 6241 New Zealand Fisheries Assessment Reprt 21/14 June 21

2 Published by Ministry f Fisheries Wellingtn 21 ISSN (print) ISSN (nline) Ministry f Fisheries 21 Hrn, P.L.; Francis, R.I.C.C. (21). Stck assessment f hake (Merluccius australis) n the Chatham Rise fr the 29 1 fishing year. New Zealand Fisheries Assessment Reprt 21/14. This series cntinues the infrmal New Zealand Fisheries Assessment Research Dcument series which ceased at the end f 1999.

3 EXECUTIVE SUMMARY Hrn, P.L.; Francis, R.I.C.C. (21). Stck assessment f hake (Merluccius australis) n the Chatham Rise fr the 29 1 fishing year. New Zealand Fisheries Assessment Reprt 21/14. This reprt summarises the stck assessment f hake in the Quta Management Areas (QMAs) HAK 4 and part f HAK 1, fr the Chatham Rise stck fr the 29 1 fishing year. The reprt presents an analysis f the stck assessment f hake that includes fishery data up t the end f the 27 8 fishing year, plus research survey data frm January 29. Catch-at-age estimates frm resurce surveys and scientific bserver data, cllected frm cmmercial tws f hake in HAK 1 and 4, are revised and updated. Revised landings data fr the three hake stcks (Sub-Antarctic, Chatham Rise, and west cast Suth Island) are presented, and literature published since the previus stck assessment fr hake is summarised. Initial investigatins f the available data indicated that the widely fluctuating and smetimes very high estimates f year class strengths thrughut the late 197s were driven by the errr structure applied t the age data acting n sparse data sets. Subsequently, the 1975 t 1983 year class strengths were smthed. It was als apparent that the sex ratis in the at-age data were incnsistent. Cnsequently, a revised base case mdel structure was develped, excluding sex frm the partitin, and incrprating tw cmmercial trawl fisheries (east and west, separated at E) each with their wn age based selectivity give. (Previus mdels included sex in the partitin, and had either a single fishery r fur fisheries.) The new mdel (called the Single sex mdel) was als encuraged t fit the research survey bimass series well by nt adding any prcess errr t this series. An additinal Tw sex mdel was run as a sensitivity analysis. It was the same as the Single sex mdel except that sex was included in the partitin and at-age data were prvided by sex. The stck assessment f hake n the Chatham Rise has been presented as a Bayesian assessment implemented as a single stck mdel using the general-purpse stck assessment prgram CASAL v2.21. The stck status f hake n the Chatham Rise appears t be relatively clear. The stck has been steadily fished dwn thrughut the 199s, but median B 29 is still estimated t be 47% f B. Strng recruitment in 22 (in cntrast t generally pr recruitment in ther years frm 1995 t 26) has resulted in a slight stck upturn. Hwever, it is likely that an annual catch f abut 115 t ver the next five years will still result in a further stck decline. The stck is prbably being well mnitred by the January trawl survey series, which shwed evidence f a unifrm decline in bimass since 1992, with bimass in 26 at abut ne-third f the riginal level. The sensitivity analysis gave a slightly mre ptimistic estimate f stck status (B 29 was 56% f B ), but still indicated that stck status wuld decline ver five years with an annual catch f 115 t. 3

4 1. INTRODUCTION This reprt utlines the stck assessment f hake in Quta Management Areas (QMAs) HAK 4 and part f HAK 1, fr the Chatham Rise hake stck, with the inclusin f data up t the end f the 27 8 fishing year. The current stck hypthesis fr hake suggests that there are three separate hake stcks (Clman 1998); the west cast Suth Island stck (WCSI, the area f HAK 7 n the west cast Suth Island), the Sub-Antarctic stck (the area f HAK 1 that encmpasses the Suthern Plateau), and the Chatham Rise stck (HAK 4 and the area f HAK 1 n the western Chatham Rise). The stck assessment f hake n the Chatham Rise is presented as a Bayesian assessment implemented as a single stck mdel using the general-purpse stck assessment prgram CASAL (Bull et al. 28). Estimates f the current stck status and prjected stck status are prvided. This reprt fulfils Objective 3 f Prject HAK27-1 T update the stck assessment f hake, including bimass estimates and sustainable yields, funded by the Ministry f Fisheries. 1.1 Descriptin f the fishery Hake are widely distributed thrugh the middle depths f the New Zealand Exclusive Ecnmic Zne (EEZ) mstly suth f latitude 4 S (Andersn et al. 1998). Adults are mainly distributed in depths frm 25 t 8 m althugh sme have been fund as deep as 12 m, while juveniles (+) are fund in shallwer inshre regins under 25 m (Hurst et al. 2). Hake are taken by large trawlers ften as bycatch in fisheries targeting ther species such as hki and suthern blue whiting, althugh target fisheries als exist (Devine 29). Present management practices divide the fishery int three main fish stcks: (a) the Challenger QMA (HAK 7), (b) the Sutheast (Chatham Rise) QMA (HAK 4), and (c) the remainder f the EEZ cmprising the Auckland, Central, Sutheast (Cast), Suthland, and Sub-Antarctic QMAs (HAK 1). An administrative fish stck exists in the Kermadec QMA (HAK 1) althugh there are n recrded landings frm this area. The hake QMAs are shwn in Figure 1. The largest fishery has been ff the west cast f the Suth Island (HAK 7) with the highest catch (17 t) recrded in 1977, immediately befre the establishment f the EEZ. The TACC fr HAK 7 is the largest, at 77 t ut f a ttal fr the EEZ f t. The WCSI hake fishery has generally cnsisted f bycatch in the much larger hki fishery, but it has undergne a number f changes during the last decade (Devine 29). These include changes t the TACCs f bth hake and hki, and als changes in fishing practices such as gear used, tw duratin, and strategies t limit hake bycatch. In sme years, ntably in 1992, 1993, and 26 there has been a hake target fishery in September after the peak f the hki fishery is ver; mre than 2 t f hake were taken in this target fishery during September Bycatch levels f hake early in the fishing seasn in 1995, 1996, 1999, 21, 24, and 25 were relatively high. On the Chatham Rise and in the Sub-Antarctic, hake have been caught mainly as bycatch by trawlers targeting hki (Devine 29). Hwever, significant targeting fr hake ccurs in bth areas, particularly in Statistical Area 44 (HAK 4), and arund the Nrwegian Hle between the Snares and Auckland Islands in the Sub-Antarctic. Increases in TACCs frm 261 t t 3632 t in HAK 1 and frm 1 t t 35 t in HAK 4 frm the fishing year allwed the fleet t increase the reprted landings f hake frm these fish stcks. Reprted catches rse ver a number f years t the levels f the new TACCs in bth HAK 1 and HAK 4, with catches in HAK 1 remaining relatively steady since. Landings frm HAK 4 steadily declined frm t a lw f 811 t in 22 3, but increased t 2275 t in Hwever, frm 24 5, the TACC fr HAK 4 was reduced frm 35 t t 18 t. Annual landings have been markedly lwer than the new TACC since then. Frm 1 Octber 25 the TACC fr HAK 7 was increased t 77 t within an verall TAC f 7777 t. This 4

5 new catch limit was set equal t average annual catches ver the previus 12 years, a catch level that is believed t be sustainable in the shrt term. Dunn (23a) fund that area misreprting between the WCSI and the Chatham Rise fisheries ccurred frm t 2 1. He estimated that between 16 and 23% (7 1 t annually) f landings were misreprted, predminantly in June, July, and September. Levels f misreprting befre and after 2 1, and between WCSI and Sub-Antarctic, were estimated as negligible, and there is n evidence f significant misreprting since 21 2 (Devine 29). HAK 1 HAK 1 N 35 4 HAK 1 New Zealand EEZ HAK 7 45 HAK 1 HAK 4 5 HAK 1 Depth cnturs at 5 and 1m Scale (N.mile) S 165 E Figure 1: Quta Management Areas (QMAs) HAK 1, 4, 7, & 1; and the west cast Suth Island (light shading), Chatham Rise (dark shading), and Sub-Antarctic (medium shading) hake stck bundaries assumed in this reprt. 1.2 Literature review Previus assessments f hake, by fishing year, are as fllws: (Clman et al. 1991), (Clman & Vignaux 1992), (Clman 1997), (Dunn 1998), (Dunn et al. 2), 2 1 (Dunn 21), 22 3 (Dunn 23b), 23 4 (Dunn 24), 24 5 (Dunn et al. 26), 25 6 (Dunn 26), and 26 7 (Hrn & Dunn 27). The Bayesian stck assessment sftware CASAL (Bull et al. 28) has been used fr all assessments since The mst recent assessments by stck are: Chatham Rise (Hrn & Dunn 27), Sub-Antarctic (Hrn 28), and WCSI (Dunn 1998). 5

6 Since 1991, resurce surveys have been carried ut frm R.V. Tangara in the Sub-Antarctic in Nvember December and 2 26 (Chattertn & Hanchet 1994, Ingersn & Hanchet 1995, Ingersn et al. 1995, O'Driscll et al. 22, O'Driscll & Bagley 23a, 23b, 24, 26a, 26b, 28, 29, Bagley et al. 29), September Octber 1992 (Schfield & Livingstn 1994b), and April June 1992, 1993, 1996, 1998, (Schfield & Livingstn 1994a, 1994c, Clman 1996, Bagley & McMillan 1999). On the Chatham Rise, a cnsistent time series f resurce surveys frm Tangara has been carried ut in January (Hrn 1994a, 1994b, Schfield & Hrn 1994, Schfield & Livingstn 1995, 1996, 1997, Bagley & Hurst 1998, Bagley & Livingstn 2, Stevens et al. 21, 22, Stevens & Livingstn 23, Livingstn et al. 24, Livingstn & Stevens 25, Stevens & O'Driscll 26, 27, Stevens et al. 29a, 29b). Standardised CPUE indices fr the Sub-Antarctic and Chatham Rise stcks were updated fr the perid up t the 27 8 fishing year (Devine 21). These update the indices estimated by Phillips & Livingstn (24), Kendrick (1998), Dunn et al. (2), Dunn & Phillips (26), and Devine & Dunn (28). A descriptive analysis f all New Zealand s hake fisheries up t the 25 6 fishing year was prepared by Devine (29). 2. REVIEW OF THE FISHERY 2.1 TACCs, catch, landings, and effrt data Reprted catches frm 1975 t are shwn in Table 1, and reprted landings fr each QMA since and TACs since are shwn in Table 2. Revised estimates f landings by QMA and by stck fr t 27 8 are prvided in Tables 3 and 4 respectively. West cast Suth Island revised estimates fr and (see Table 4) are taken frm Clman & Vignaux (1992), wh crrected fr under-reprting in and using estimates f landings frm vessel trips with Ministry f Fisheries bservers t crrect catches frm vessel trips that did nt carry Ministry f Fisheries bservers, and nt frm revised estimates f landings based n area misreprting. 6

7 Table 1: Reprted hake catches (t) frm 1975 t Data frm 1975 t 1983 frm Ministry f Agriculture & Fisheries (Fisheries); data frm t frm Fisheries Statistics Unit; data frm t frm Quta Management System. New Zealand vessels Freign licensed vessels Fishing year Dmestic Chartered Ttal Japan Krea USSR Ttal Ttal N data available Calendar year 2. 1 April t 31 March 3. 1 April t 3 September 4. 1 Octber t 3 September Table 2: Reprted landings (t) f hake by QMA frm t 28 9 and actual TACs (t) fr t Data frm t frm Fisheries Statistics Unit; data frm t 28 9 frm Quta Management System ( indicates that the data are unavailable). QMA HAK 1 HAK 4 HAK 7 HAK 1 Ttal Landings TAC Landings TAC Landings TAC Landings TAC Landings TAC

8 Table 3: Revised landings (t) by QMA t 27 8 frm Devine (21). Fishing QMA Ttal Year HAK 1 HAK 4 HAK Table 4: Revised landings frm t 27 8 (t) fr the Sub-Antarctic (Sub-A), Chatham Rise (Chat), and west cast Suth Island (WCSI) stcks. Fishing year Sub-A Chat WCSI Fishing year Sub-A Chat WCSI

9 2.2 Recreatinal and Mari custmary fisheries The recreatinal fishery fr hake is believed t be negligible. The amunt f hake caught by Mari is nt knwn, but is believed t be negligible. 2.3 Other surces f fishing mrtality Clman & Vignaux (1992) cmpared hki and hake catches frm vessels carrying Ministry f Fisheries bservers with thse nt carrying bservers, and suggested that the catch f hake was nt always fully reprted in HAK 7 between and They cncluded that the actual catch f hake was significantly under-reprted in HAK 7 in sme years, and they estimated the actual hake catch in HAK 7 by multiplying the ttal hki catch (which was assumed t be crrectly reprted by vessels bth with and withut bservers) by the rati f hake t hki in the catch f vessels carrying bservers. Reprted and estimated catches fr were respectively 6835 t and 8696 t; fr , 493 t reprted and 8741 t estimated; and fr , 6189 t reprted and 8246 t estimated. Mre recently, the level f such misreprting has nt been estimated and is nt knwn. Dunn (23a) revised the estimates f the ttal landings by stck, accunting fr area misreprting, between and 2 1. He estimated that the level f hake ver-reprting n the Chatham Rise (and hence under-reprting n the west cast Suth Island) had been between 16 and 23% (7 1 t annually) f landings between and 2 1, predminantly in June, July, and September. Prbable levels f misreprting befre and between the west cast Suth Island and Sub-Antarctic were prbably negligible. There is n evidence f significant misreprting since 21 2 (Devine 29). There is likely t be sme mrtality assciated with escapement frm trawl nets, but the level is nt knwn and is assumed t be negligible. 3. BIOLOGY, STOCK STRUCTURE, AND RESOURCE SURVEYS 3.1 Bilgy Data cllected by bservers n cmmercial trawlers and frm resurce surveys suggest that there are at least three main spawning areas fr hake (Clman 1998). The best knwn area is ff the west cast f the Suth Island, where the seasn can extend frm June t Octber, pssibly with a peak in September. Spawning als ccurs t the west f the Chatham Islands during a prlnged perid frm at least September t January. Spawning fish have als been recrded ccasinally near the Mern Bank. Spawning n the Campbell Plateau, primarily t the nrtheast f the Auckland Islands, may ccur frm September t February with a peak in September Octber. Spawning fish have als been recrded ccasinally n the Puysegur Bank, with a seasnality that appears similar t that n the Campbell Plateau (Clman 1998). Hrn (1997) validated the use f tliths t age hake. New Zealand hake reach a maximum age f at least 25 years. Males, which rarely exceed 1 cm ttal length, d nt grw as large as females, which can grw t 12 cm ttal length r mre. Readings f tliths frm hake have been used t develp age-length keys t scale length frequency distributins fr hake cllected n resurce surveys and frm cmmercial fisheries n the Chatham Rise, Sub-Antarctic, and west cast Suth Island. The resulting age frequency distributins were reprted by Hrn & Suttn (29). The relative bserved prprtins-at-age data frm resurce surveys f the Sub-Antarctic and Chatham Rise stcks are als shwn in Figures 2 and 3 respectively. 9

10 Clman (1998) fund that hake reach sexual maturity between 6 and 1 years f age, at ttal lengths f abut cm (males) and cm (females); he cncluded that hake reached 5% maturity at between 6 and 8 years in HAK 1, and 7 8 years in HAK 4. In assessments befre 25, the maturity give fr the Chatham Rise and Sub-Antarctic was assumed frm a cmbinatin f the estimates f Clman (1998) and mdel fits presented by Dunn (1998) t the west cast Suth Island stck. Frm 25 t 27, maturity gives fr the Chatham Rise and Sub-Antarctic stcks were fitted within the assessment mdel t data derived frm resurce survey samples with infrmatin n the gnsmatic index, gnad stage, and age (Hrn & Dunn 27, Hrn 28). Individual hake were classified as either immature r mature at sex and age, where maturity was determined frm the gnad stage and gnsmatic index (GSI, the rati f the gnad weight t bdy weight). Fish identified as stage 1 were classified as immature. Stage 2 fish were classified as immature r mature depending n the GSI index, using the definitins f Clman (1998) i.e., classified as immature if GSI <.5 (males) r GSI <.15 (females), r mature if GSI.5 (males) r GSI.15 (females). Fish identified as stages 3 7 were classified as mature. Mdel fits indicated that Chatham Rise hake reach 5% maturity at abut 6 years fr males and 7 years fr females, and Sub-Antarctic hake reach 5% maturity at 6 years fr males and 7.5 years fr females (Figure 4). Frm 29, fixed gives (derived frm the fitted curves in Figure 4) were used in the assessment mdels, with values listed in Table 5. Vn Bertalanffy parameters were previusly estimated using data up t 1997 (Hrn 1998). The parameters fr all three stcks were updated using all data available at February 27 (Hrn 28). Plts f the fitted curves n the raw data indicated that the vn Bertalanffy mdel tended t underestimate the age f large fish (Figure 5). Cnsequently, the grwth mdel f Schnute (1981) was fitted t the data sets (Table 5). This mdel appeared t better describe the grwth f larger hake (Figure 5), and the resulting parameters can be used in the CASAL stck assessment sftware. Mst aged hake have been 3 years r lder. Hwever, juvenile hake have been taken in castal waters n bth sides f the Suth Island and n the Campbell Plateau. It is knwn that they reach a ttal length f abut 15 2 cm at 1 year ld, and abut 35 cm ttal length at 2 years (Hrn 1997). Estimates f natural mrtality (M) and the assciated methdlgy were given by Dunn et al. (2); M was estimated as.18 y -1 fr females and.2 y -1 fr males. Clman et al. (1991) estimated M as.2 y -1 fr females and.22 y -1 fr males using the maximum age methd f Henig (1983) (where they defined the maximum ages at which 1% f the ppulatin survives in an unexplited stck as 23 years fr females and 21 years fr males). These are similar t the values prpsed by Hrn (1997), wh determined the age f hake by cunting znes in sectined tliths and cncluded frm that study that it was likely that M was in the range.2.25 y -1. 1

11 Year class (a) Males Year class (b) Females Figure 2: frequencies (ages 1 t 2+) by year class and year (symbl area prprtinal t the prprtins-at-age within sampling event) in the Sub-Antarctic resurce surveys, 3 8 m strata. Zer values are represented by a dash, and hrizntal brken lines indicate the earliest (1974) and latest (25) year class strengths that wuld be estimated within the stck assessment mdel. Year class (a) Males Year class (b) Females Figure 3: frequencies Fishing (ages year 1 t 2+) by year class and year (symbl Fishing year area prprtinal t the prprtins-at-age within sampling event) n the Chatham Rise resurce surveys, 2 8 m strata. Zer values are represented by a dash, and hrizntal brken lines indicate the earliest (1975) and latest (26) year class strengths estimated within the stck assessment mdel. 11

12 Table 5: Estimates f bilgical parameters fr the three hake stcks. Estimate Surce Natural mrtality Males M =.2 (Dunn et al. 2) Females M =.18 (Dunn et al. 2) Bth sexes M =.19 (Current study) Weight = a (length) b (Weight in t, length in cm) Sub-Antarctic Males a = 3.95 x1-9 b = 3.13 (Hrn 1998) Females a = 1.86 x1-9 b = (Hrn 1998) Chatham Rise Males a = 2.49 x1-9 b = (Hrn 1998) Females a = 1.7 x1-9 b = (Hrn 1998) Bth sexes a = 2.12 x1-9 b = (Current study) vn Bertalanffy grwth parameters Sub-Antarctic Males k =.295 t =.6 L = 88.8 (Hrn 28) Females k =.22 t =.1 L = 17.3 (Hrn 28) Chatham Rise Males k =.33 t =.9 L = 85.3 (Hrn 28) Females k =.229 t =.1 L = 16.5 (Hrn 28) WCSI Males k =.357 t =.11 L = 82.3 (Hrn 28) Females k =.28 t =.8 L = 99.6 (Hrn 28) Schnute grwth parameters (τ 1 = 1 and τ 2 = 2 fr all stcks) Sub-Antarctic Males y 1 = 22.3 y 2 = 89.8 a =.249 b = (Hrn 28) Females y 1 = 22.9 y 2 = 19.9 a =.147 b = (Hrn 28) Chatham Rise Males y 1 = 24.6 y 2 = 9.1 a =.184 b = (Hrn 28) Females y 1 = 24.4 y 2 = a =.98 b = (Hrn 28) Bth sexes y 1 = 24.5 y 2 = 14.8 a =.131 b = 1.7 (Current study) WCSI Males y 1 = 23.7 y 2 = 83.9 a =.278 b = 1.38 (Hrn 28) Females y 1 = 24.5 y 2 = 13.6 a =.182 b = 1.51 (Hrn 28) Maturity gives (prprtin mature at age) Sub-Antarctic Males Females Chatham Rise Males Females Bth Miscellaneus parameters Steepness (Bevertn & Hlt stck-recruitment relatinship).9 Prprtin spawning 1. Prprtin f recruits that are male.5 ing errr c.v..8 Maximum explitatin rate (U max ).7 12

13 Sub-Antarctic Chatham Rise M a tu rity Figure 4: Estimated gives f prprtins mature by age fr Sub-Antarctic and Chatham Rise hake males (slid lines) and females (brken lines). Figure 5: Raw age-length data, by sex, fr hake frm Chatham Rise (CHAT), west cast Suth Island (WCSI), and Sub-Antarctic (SUBA), with fitted vn Bertalanffy curves (slid lines) and Schnute curves (brken lines). 3.2 Stck structure There are at least three hake spawning areas: ff the west cast f the Suth Island, n the Chatham Rise, and n the Campbell Plateau (Clman 1998). Juvenile hake are fund in all three areas, there are differences in size frequency f hake between the west cast and ther areas, and differences in 13

14 grwth parameters between all three areas (Hrn 1997). There is reasn, therefre, t believe that at least three separate stcks may exist in the EEZ. Analysis f mrphmetric data (J.A. Clman, NIWA, unpublished data) shwed little difference between hake frm the Chatham Rise and frm the east cast f the Nrth Island, but highly significant differences between these fish and thse frm the Sub-Antarctic, Puysegur, and n the west cast. The Puysegur fish are mst similar t thse frm the west cast Suth Island, althugh, depending n which variables are used, they cannt always be distinguished frm the Sub-Antarctic hake. Hwever, the data are nt unequivcal s the stck affinity is uncertain. Fr stck assessment mdels, the Chatham Rise stck was cnsidered t include the whle f the Chatham Rise (HAK 4 and the western end f the Chatham Rise that frms part f the HAK 1 management area). The Sub-Antarctic stck was cnsidered t cntain hake in the remaining Puysegur, Suthland, and Sub-Antarctic regins f the HAK 1 management area. The stck areas assumed fr this reprt are shwn earlier, in Figure Resurce surveys In the Sub-Antarctic, three resurce surveys were carried ut by Tangara with the same gear and similar survey designs in Nvember December 1991, 1992, and 1993, but the series was then terminated as there was evidence that hake, in particular, might be aggregated fr spawning at that time f the year and that spawning aggregatins had a high prbability f being missed during a survey. Hwever, research interest in hki in the Sub-Antarctic resulted in a return t the Nvember December survey annually since 2. Surveys by Tangara in April 1992, May 1993, April 1996, and April 1998 frmed the basis fr a secnd series, with hake appearing t be mre evenly distributed thrugh the survey area at that time f year. A single survey in September 1992 by Tangara was als cmpleted. The bimass estimates frm the Sub-Antarctic Tangara and 1989 Amaltal Explrer surveys are shwn in Figure 6. The distributins f catches frm these surveys are given in Appendix A. Sub-Antarctic surveys were cnducted by Shinkai Maru (March May 1982 and Octber Nvember 1983) and Amaltal Explrer (Octber Nvember 1989, July August 199, and Nvember December 199). Hwever, these vessels used different gear and had different perfrmance characteristics (Livingstn et al. 22), s cannt be used as a part f a cnsistent time series. Resurce surveys have been carried ut at depths f 2 8 m n the Chatham Rise since 1992 by Tangara with the same gear and similar survey designs (see Appendix A). While the survey designs since 1992 have been similar, there was a reductin in the number f statins surveyed between 1996 and 1999, and sme strata in the survey design used between 1996 and 1999 were merged (see Bull & Bagley 1999). The surveys since 2 used a revised design, with sme strata being split and additinal statins added. In additin, fur surveys (2, 22, 27, and 28) included deepwater strata (i.e., 8 1 m) n the nrthern Chatham Rise. The deepwater strata were excluded frm the Tangara data used in this analysis t maintain cnsistency in the time series. Chatham Rise surveys were cnducted by Shinkai Maru (March 1983 and June July 1986) and Amaltal Explrer (Nvember December 1989). Hwever, these surveys used a range f gear, survey methdlgies, and survey designs (Livingstn et al. 22), and cannt be used as a cnsistent time series. The bimass estimates frm Chatham Rise resurce surveys are shwn in Figure 7. The distributins f catches frm these surveys are given in Appendix A. 14

15 6 5 Nv/Dec: 3-8 m strata Nv/Dec: 1991 area Nv/Dec: 1996 area Apr/May: 3-8 m strata Apr/May: 1991 area Apr/May: 1996 area Sep: 3-8m strata Bimass (' t) Figure 6: Hake bimass estimates frm the Amaltal Explrer (Octber Nvember 1989) and Tangara ( including the Nvember December, April May, and September series) surveys f the Sub- Antarctic, with apprximate 95% cnfidence intervals. (See als Appendix A.) m strata 2-1 m strata 5 Bimass (' t) Figure 7: Hake bimass estimates frm the Amaltal Explrer (Nvember December 1989) and Tangara ( fr the January series) f the Chatham Rise, with apprximate 95% cnfidence intervals. (See als Appendix A.) 3.4 Observer length and age data Chatham Rise The fishery n the Chatham Rise was stratified using a tree-based regressin n mean lengths f hake in tws where bservers had measured five r mre hake (Hrn & Dunn 27). The defined strata are shwn in Figure 8. Mean fish length tends t increase frm west t east, and with increasing depth. Area 44 is a knwn spawning grund. Because landings and intensity f bserver effrt varied 15

16 markedly ver the fur fisheries between years it was cnsidered necessary t mdel the Chatham Rise stck with fur separate fisheries, each with its wn selectivity gives. Cnsequently, catch-atlength and catch-at-age series were develped separately fr each fishery in the last assessment f this stck (Hrn & Dunn 27). West deep Area West shallw East (excl. 44) 45 S 53 m E 175 E 18 Figure 8: Fishery strata defined fr the Chatham Rise hake fishery. The stratum bundary defined by depth (53 m) is shwn nly apprximately. Isbaths at 1, 5, and 25 m are als shwn..8.7 Mean prprtin at age (years) West shallw West deep East (excl. 44) Area 44 Figure 9: -frequency distributins, by fishery, cmbining data frm all years. See Figure 8 fr definitins f the fishery areas. Hwever, it was apparent that the tw western fisheries have quite similar age-frequency distributins (Figure 9). Hake caught in the eastern fisheries are, n average, lder than thse frm the west. Althugh the age-frequency distributins frm the eastern fisheries are different (i.e., Area 44 fish tend t be lder), these series are data pr. It was pssible t calculate sufficiently precise catch-at-age distributins fr the Area 44 fishery in nly tw years, and nly three years fr East (excl. 44) fishery (Hrn & Suttn 29). This cmpared t eight and nine years f data fr the West shallw and West deep fisheries, respectively. We cnsidered that 2 3 years f data were insufficient t characterise the eastern fishery age distributins, and that these fisheries shuld be cmbined. We als cnsidered it was wrth investigating the effects f cmbining the tw western fisheries because their catch-at-age distributins were s similar. 16

17 Observer data frm each fishery stratum were cnverted int catch-at-age distributins if there were at least 4 length measurements (frm western strata) r 32 length measurements (frm eastern strata), and the mean weighted c.v. ver all age classes was less than 3%. Table 6 summarises the quantities f available data. The tw western fisheries have been generally well sampled, but bth eastern fisheries (and particularly the Area 44 fishery) have been prly sampled. Althugh the bserver length data frm each year were partitined int fisheries, the age data frm each year were nt (i.e., a single age-length key was cnstructed fr each year and applied t all available sets f length data frm that year). Hrn & Dunn (27) shwed that mean age at length did nt differ between fisheries, s the use f a single age-length key per year has prbably nt biased the age distributins. Table 6: Numbers f measured hake available fr analyses f catch-at-age, by fishery n the Chatham Rise (i.e., after remval f data frm tws where fewer than five hake were measured)., insufficient data t calculate catch-at-age. West cmbined is a cmbinatin f data frm the West deep and West shallw fisheries. East is a cmbinatin f data frm the East (excl. 44) and Area 44 fisheries. Fishery Year West shallw West deep West cmbined East Sub-Antarctic The Sub-Antarctic hake bserver data were fund t be best stratified int the fur areas shwn in Figure 1 (Hrn 28). Mst f the hake target fishing, and mst f the catch (average 94% per year), is assciated with the Snares-Pukaki area. Puysegur is the next mst imprtant area with abut 3% f the catch. Available bserver data are als cncentrated in the Snares-Pukaki regin, but it is clear that the smaller fisheries (particularly the Campbell Island area) can ften be ver-sampled in mst years. Cnsequently, the Sub-Antarctic bserver data are analysed as ne majr and three very minr fisheries, with a single fishery give. Hwever, because f clear differences in mean fish length between the fisheries (Hrn 28), it is imprtant t use the fur fishery strata when calculating catch-at-age distributins. Withut stratificatin, the frequent ver-sampling in the minr fisheries culd strngly bias the catch-at-age distributins. Hwever, it is satisfactry t apply a single agelength key t the scaled length-frequency distributins fr each fishery t prduce the catch-at-age data. Catch-at-age distributins frm the Sub-Antarctic trawl fishery are available frm all but three years frm t 27 8 (Hrn & Suttn 29). 17

18 Puysegur Bank Snares-Pukaki Campbell Island Auckland Island S 165 E Figure 1: Fishery strata defined fr the Sub-Antarctic hake fishery. Numbers shw latitudes r lngitudes f fishery bundaries. Isbaths at 1, 5, and 25 m are als shwn. 3.5 CPUE indices Standardised CPUE indices were calculated by Devine (21) frm daily prcessed summary data up the end f the 27 8 fishing seasn. Series were prduced fr each f the fur separate fisheries n the Chatham Rise, fr the tw eastern Chatham Rise fisheries cmbined, and fr the entire Sub- Antarctic fishery (Table 7). 18

19 Table 7: Hake CPUE indices (and assciated c.v.s) fr the fur fisheries n the Chatham Rise, the tw Chatham Rise eastern fisheries cmbined, and the Sub-Antarctic. Chatham Rise West shallw West deep East (excl. 44) Area 44 Year Index c.v. Index c.v. Index c.v. Index c.v Chatham Rise Sub-Antarctic East cmbined All areas Year Index c.v. Index c.v

20 4. MODEL STRUCTURE, INPUTS, AND ESTIMATION 4.1 Intrductin An updated assessment f the Chatham Rise stck nly is presented here. In the mst recent previus assessment f this stck (Hrn & Dunn 27) the assessment mdel partitined the ppulatin int tw sexes and age grups 1 3, with the last age class cnsidered a plus grup. The partitin als included maturity, with gives being estimated within the mdel. The mdel s annual cycle was based n a year beginning n 1 September and divided the year int three steps (Table 8). Nte that mdel references t year within this dcument are labelled as the mst recent calendar year, i.e., the year 1 September 1998 t 31 August 1999 is referred t as Sme previus assessments f the Chatham Rise stck have been based n fishing year, i.e., years starting n 1 Octber. Hwever, landings peaks tend t ccur frm September t January (Figure 11), s it is lgical t include the September catch with landings frm the five mnths immediately fllwing it, rather than with catches taken abut seven mnths previusly. Table 8: Annual cycle f the Chatham Rise stck mdel, shwing the prcesses taking place at each time step, their sequence within each time step, and the available bservatins. Fishing and natural mrtality that ccur within a time step ccur after all ther prcesses, with half f the natural mrtality fr that time step ccurring befre and half after the fishing mrtality. Observatins Step Perid Prcesses M 1 2 Descriptin %Z 3 1 Sep Feb Fishing, recruitment, & spawning January resurce survey 1 2 Mar May Nne Jun Aug Increment age M is the prprtin f natural mrtality that was assumed t have ccurred in that time step. 2. is the age fractin, used fr determining length at age, that was assumed t ccur in that time step. 3. %Z is the percentage f the ttal mrtality in the step that was assumed t have taken place at the time each bservatin was made. 2

21 Catch(t) Catch(t) Catch(t) Catch(t) Catch(t) Catch(t) O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S Figure 11: Estimated daily catch (t) f hake n the Chatham Rise by mnth and target species, t 25 6, frm Devine (29). Black lines indicate hake targeted tws and grey lines are hki targeted tws. Fr all subsequent mdels, estimates f fixed bilgical parameters used in the assessments are given in Table 5. A Bevertn-Hlt stck-recruitment relatinship, with steepness.9, was assumed. Variability in the Schnute age-length relatinship was assumed t be lgnrmal with a cnstant c.v. 21

22 f.1. The maximum explitatin rate was assumed t be.7 fr the stck. The chice f the maximum explitatin rate has the effect f determining the minimum pssible virgin bimass allwed by the mdel, given the bserved catch histry. This value was set relatively high as there was little external infrmatin frm which t determine it. The mdel s annual cycle was as described in Table 8. Bimass estimates frm the resurce surveys were used as relative bimass indices, with assciated c.v.s estimated frm the survey analysis. The survey catchability cnstant (q) was assumed t be cnstant ver all years in the survey series. Catch-at-age bservatins were available fr each research survey (see Figure 3), and frm cmmercial bserver data fr the fishery. Lgnrmal errrs, with knwn c.v.s, were assumed fr all relative bimass and prprtins-at-age bservatins. ing errr was assumed t ccur fr the bserved prprtins-at-age data, by assuming a discrete nrmally distributed errr with c.v..8. The c.v.s (fr bservatins fitted with lgnrmal likelihds) are assumed t have allwed fr sampling errr nly. Additinal variance, assumed t arise frm differences between mdel simplificatins and real wrld variatin, was added t the sampling variance fr mst bservatins in all mdel runs. The additinal variance, termed prcess errr, was estimated in MPD runs f each mdel. Hwever, the ttal errr assumed in each run fr each bservatin was nt always the sum f prcess errr and bservatin errr (see details fr individual mdels belw). Year class strengths were assumed knwn (and equal t ne) fr years befre 1975 and after 25, when inadequate r n catch-at-age data were available. Otherwise, year class strengths were estimated under the assumptin that the estimates frm the mdel must average ne. The Haist parameterisatin fr year class multipliers is used here (see Bull et al. (28) fr details). The catch histry assumed in all mdel runs was derived as fllws. Using the grming algrithms f Dunn (23a), landings f hake reprted n TCEPR and CELR frms frm t 27 8 were allcated t mnth and fishery (based n reprted date, lcatin, and depth). Annual ttals fr each fishery were btained by summing the mnthly ttals, but, fr reasns described abve, using a September t August year. Thus, catch histries fr mdel years 199 t 25 were prduced. At the same time, catch histries fr FMA 3 and FMA 4 were als prduced. Fr each year frm 199 t 25, the prprtins f the FMA 3 catch made up by the west shallw and west deep fisheries were calculated, as were the prprtins f the FMA 4 landings made up by the east fishery. Means ver all years indicated that the west shallw and west deep fisheries accunted fr landings f 99% and 75% respectively f the FMA 3 ttal, and that the east fishery tk landings equivalent t 83% respectively f the FMA 4 ttal. [Nte that the percentages fr west and east d nt equate t 1% because the western fisheries include an area greater than FMA 3, and the eastern fishery cmprises an area smaller than FMA 4.] Dunn et al. (26) had prduced estimates f ttal Chatham Rise hake catch frm 1975 t 1989, and the FMA 4 catch frm 1984 t Estimates f FMA 4 catch befre 1984 were btained primarily frm Clman & Livingstn (1988). Hence, estimates f hake catch frm FMA 3 and FMA 4 frm 1975 t 1989 were available r culd be derived. T estimate catch by fishery frm 1975 t 1989, the percentages presented abve were applied t the FMA 3 r FMA 4 landings. The catch in 28 9 was estimated based largely n patterns f catch frm the previus year. Catch histries by fishery are presented in Table 9. 22

23 Table 9: Estimated catch (t) by FMA (3 and 4) frm the Chatham Rise stck, and ttal catch, by fishing year, and estimated catch (t) by fishery fr the mdel years. Nte that frm ttals by fishing year and mdel year differ because the September catch has been shifted frm the fishing year int the fllwing mdel year. Landings frm 28 9 are estimated assuming catch patterns similar t the previus year. Fishing year FMA 3 FMA 4 Ttal Mdel year West shallw West deep East Ttal Develping a base mdel It was nted abve (Sectin 3.4.1) that sme amalgamatin f the fisheries defined frm the tree regressin analysis f the bserver catch-at-age data wuld be desirable, i.e., cmbining the tw western fisheries as ne, and cmbining the tw eastern fisheries as ne. It was als apparent that the previus Chatham Rise assessment mdel did nt fit the nly fishery-independent relative abundance series (i.e., the summer trawl survey series) particularly well (Hrn & Dunn 27). Because this series exhibits a relatively smth trend ver time, and s is prbably a reasnable index f relative abundance, we believe that any gd assessment mdel shuld fit it well. Cnsequently, sme initial investigatins were cmpleted t develp a new base mdel. Mdel parameters were 23

24 estimated using Bayesian estimatin implemented using the CASAL sftware. Hwever, nly the mde f the jint psterir distributin (MPD) was estimated in these initial runs. (Full details f the CASAL algrithms, sftware, and methds were detailed by Bull et al. (28).) In develping a base mdel a series f eight mdels was cnsidered, with each new mdel typically differing frm previus mdels in nly ne key assumptin (Table 1). Table 1: Brief descriptin f the assumptins that differed amngst the eight mdels that were cnsidered in develping a base case mdel (see text fr mre detail). Fr each mdel, the underlined assumptin(s) is the main ne that distinguished it frm preceding mdels. Mdel number Assumptin Include prcess errr fr survey bimass Y N N N N N N N Duble prcess errr fr at-age data N Y Y Y Y Y Y N Number f western fisheries ing errr assumed Y Y Y N Y Y Y Y Smth year-class strengths N N N N Y Y Y Y All selectivities dmed Y Y Y Y Y N Y Y CPUE data used N N N N N N Y N Sex in partitin and data Y Y Y Y Y Y Y N 1 Referred t in Sectin 5 as the tw sex mdel; 2 Base case mdel fr the assessment An initial mdel (mdel 1) was set up, partitining the ppulatin int tw sexes and age grups 1 3, with the last age class cnsidered a plus grup. The partitin did nt include maturity. The mdel used eight selectivity gives: male and female survey selectivities fr the January Tangara resurce survey series, and male and female survey selectivities fr each f the three cmmercial fisheries (i.e., west shallw, west deep, east). Female selectivity was always estimated relative t male selectivity. Selectivities were assumed cnstant ver all years in the fisheries r the survey series. All selectivity gives were estimated using the duble-nrmal parameterisatin. Prcess errr f.2 was added t survey bimass indices fllwing the recmmendatin f Francis et al. (23). Prcess errr fr all the catch-at-age series was estimated in the mdel. N catch-at-length data r CPUE series were incrprated. The MPD fit t this initial mdel prduced the fllwing estimates f prcess errr fr the catch-atage series: research survey,.1; west shallw fishery,.31; west deep fishery,.61; east fishery,.19. Stck status in 29 was estimated t be 61% f B. Hwever, the survey bimass series was prly fitted, with clearly unbalanced residuals (Figure 12). Cnsequently, this mdel was nt cnsidered t be satisfactry. 24

25 Bimass (t) Survey bservatins Mdel 1 Mdel 2 Mdel Figure 12: MPD mdel fits (lines) t the trawl survey bimass indices (dts) fr mdels 1 3. T encurage a better fit t the bimass indices, mdel 2 included the survey series with n prcess errr, and apprximately dubled the prcess errr fr the catch-at-age series, i.e., prcess errrs were: research survey,.1; west shallw fishery,.6; west deep fishery, 1.2; east fishery,.4. This mdel prduced a smewhat better fit t the survey bimass series (Figure 12), and changed the stck status t be 54% f B. Mdel 3 examined the effect f cmbining the tw western fisheries. It was identical t mdel 2 except that there was a single catch histry, and a single set f catch-at-age distributins, fr the previusly separate western fisheries. Prcess errr f the western fishery catch-at-age was entered as 1., i.e., abut twice the estimated value. The results frm mdel 3 run were virtually identical t thse frm mdel 2 (see fit t bimass in Figure 12); estimated stck status in 29 was again 54% f B. Because there was little difference between results frm mdels 2 and 3, and because the catch-at-age distributins are s similar fr the tw western fisheries (see Figure 9), we chse t use a single western fishery in all fllwing mdels. A striking feature f mdel 3 is that the spawning bimass was estimated t have increased by 49% in the 198s, befre the survey series started, and this increase was driven primarily by extremely strng year classes in 1977 and 198 (Figure 13). Spawning stck bimass Fit t survey bimass Year class strength SSB (' t) Bimass (t) YCS Figure 13: Estimated spawning stck bimass and year class strengths, and fits t the research survey bimass, frm mdel 3. 25

26 The age data were examined t see what infrmatin existed t indicate that the 198 and 1977 year classes were particularly strng (Figure 14). There was sme indicatin frm the earlier part f the trawl survey series (i.e., the 199 and surveys) that the 198 year class is strng. Hwever, the cmmercial fishery data seldm indicated that the 198 year class was exceptinal. The 1977 year class seldm appears t be strng in any data set; nly the 1994 survey and 1992 fishery distributins suggest that this year class might be strnger than average. It was suspected that these estimated strng year classes were an artefact, the cnsequence f a tendency fr mdels which assume ageing errr t estimate high variability in year-class strength in perids with few data. Cnsequently, tw additinal mdels were run t investigate year class strength estimatin: in mdel 4, the assumptin f ageing errr was drpped; mdel 5 retained this assumptin but smthed the year class strengths frm 1975 t The new mdels bth prduced early year class strength estimates that were markedly different frm thse f mdel 3, as well as large changes in estimated spawning stck bimass (Figure 15). It was clear that the extreme estimates f year class strength (bth high and lw) are artefacts f the applicatin f ageing errr t age classes with few data. All three mdels prduced similar patterns f year class strengths frm 1984 t 25, where the data were mre abundant. The tw new mdels were similar ver their entire range, and they still prvided a clear indicatin f sme strnger than average year classes in the late 197s. Hwever, the magnitude f the pre-survey rise in bimass is markedly reduced in the tw new mdels (frm 49% in mdel 3 t 26% and 32% in mdels 4 and 5), as are the estimates f B (mdel 3, t; mdel 4, t; mdel 5, t). But stck status (B 29 as %B ) varies little between all three mdels, i.e., mdel 3, 54%; mdel 4, 55%; mdel 5, 56%. 26