Stock structure of southern blue whiting (Micromesistius australis) in New Zealand waters

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1 New Zealand Journal of Marine and Freshwater Research ISSN: (Print) (Online) Journal homepage: Stock structure of southern blue whiting (Micromesistius australis) in New Zealand waters Stuart Hanchet To cite this article: Stuart Hanchet (1999) Stock structure of southern blue whiting (Micromesistius australis) in New Zealand waters, New Zealand Journal of Marine and Freshwater Research, 33:4, , DOI: / To link to this article: Published online: 30 Mar Submit your article to this journal Article views: 128 View related articles Citing articles: 4 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 14 February 2016, At: 19:42

2 New Zealand Journal of Marine and Freshwater Research, 1999, Vol. 33: /99/ $7.00 The Royal Society of New Zealand Stock structure of southern blue whiting (Micromesistius australis) in New Zealand waters STUART HANCHET National Institute of Water & Atmospheric Research Ltd P. O. Box 893 Nelson, New Zealand Abstract Data on the reproduction, growth, and morphometric characteristics of southern blue whiting (Micromesistius australis Norman) were examined to determine its stock structure in New Zealand waters. Consistent differences in the size distribution of fish, and in the timing of spawning were found between spawning grounds on the Bounty Platform, Pukaki Rise, Auckland Islands Shelf, and Campbell Island Rise. Significant differences in morphometric characteristics were found between fish from the Bounty Platform, Pukaki Rise, and Campbell Island Rise and significant differences in growth rates were found between the Bounty Platform and Campbell Island Rise. The results provide strong evidence that fish return to spawn on the grounds to which they first recruit, and suggest that the four areas should be assessed and managed as separate stocks. Keywords stock structure; southern blue whiting; Micromesistius australis; discriminant analysis; morphometrics; growth; reproduction INTRODUCTION Southern blue whiting (Micromesistius australis Norman) is almost entirely restricted to subantarctic waters around New Zealand. Although the fish are dispersed during the summer months, they aggregate M98022 Received 12 May 1998; accepted 6 May 1999 to spawn during August and September on several well-defined spawning grounds on the Bounty Platform and Campbell Plateau. These aggregations form the basis of one of the largest fisheries in New Zealand waters, with average catches of c t since the fishery first developed in 1970 (Hanchet 1997). During the early 1990s the catches increased sharply peaking at t in There was concern over the sustainability of these catches, and a catch limit of t was introduced for The results of preliminary work on stock structure by Hanchet (1991) suggested that there were three stocks on the Bounty Platform, Pukaki Rise, and Campbell Island Rise. Catch limits were therefore set for each area and the fishery has been managed on this basis. Since then a spawning fishery has developed on the Auckland Islands Shelf in 1993, which is currently included with the Pukaki Rise stock, and considerably more biological data have been collected from the other areas. It is therefore important to know for management purposes the relationships between fish from these four areas. Inferences about stock structure of marine fishes have been based on a variety of methods including genetics, tagging, morphometrics, meristics, parasites, growth, and spawning (Smith et al. 1990). Genetics can provide a clear indication of biological separation between stocks. Stocks that are genetically different clearly have little mixing, and should be regarded separately for fisheries management. However, the time scale that gives rise to genetic heterogeneity may be far longer than is important to short-term fisheries management. The analysis of morphometric measurements is one method that has commonly been used to identify and differentiate between fish stocks. Morphometric and meristic studies have been used to identify stocks of blue whiting in the northern hemisphere (Andersen & Jakupsstovu 1978), and to identify stocks of southern blue whiting in Patagonian- Falkland waters (Wiecaszek 1988). These and other methods, such as comparisons of growth rates, parasites, or spawning, have been used to identify

3 600 New Zealand Journal of Marine and Freshwater Research, 1999, Vol. 33 fish stocks in New Zealand (Horn 1993; Colman 1995; Livingston & Schofield 1996). Although such methods do not by themselves provide evidence of genetic differences, they are often used to identify stocks for management purposes. In this paper the term "stock" refers to a fishery stock, comprising a group offish being exploited in a specific geographic area, rather than a biological stock, which is reproductively (and hence genetically) isolated from other intra-specific populations (Smith et al. 1990). The aim of the present research was to review the available data on age and growth, morphometrics, and the timing and location of spawning to determine the number of stocks of southern blue whiting in New Zealand waters. mainly in August and September. By examining ovaries histologically and measuring oocyte diameters they concluded that two oocyte generations were formed by the beginning of the spawning season, and that that this led to two batches of eggs being spawned by females each season. Gonad stages for females were recorded by observers using the gonad staging system given in Table 1. The 5-stage system was accompanied by colour photographs and detailed descriptions, and was chosen to minimise between observer variation. The location of observed tows catching running ripe fish is plotted out to identify the spawning grounds, which may help delineate stock boundaries. There is very little fishing outside the spawning season. METHODS Sampling Data on size distribution and spawning offish from the commercial fishery were obtained from the scientific observer programme. Since 1989, scientific observers on board commercial fishing vessels have collected samples of fish per day during the spawning season on each ground. The samples were taken by collecting the first fish which have passed their sampling point at a random time each day. The observers recorded fork length (down to the nearest centimetre), sex, and for females, gonad stage. The reproductive cycle for southern blue whiting in New Zealand waters was described by Svirskii & Shpak (1977), who noted that spawning occurred Growth Otoliths were collected from the commercial fishery by the scientific observer programme. The number of otoliths read for the period is shown in Table 2. Otoliths were read using the validated procedure of Hanchet & Uozumi (1996). Ageing data are available for the Pukaki Rise only for the years 1990 and 1991, so comparisons of growth between the three areas were restricted to these 2 years. Hanchet & Uozumi (1996) showed significant differences in growth between sexes, so von Bertalanffy growth parameters were fitted to the age length data from each sex and area separately using the NLIN procedure of SAS (SAS Institute 1988). The size distribution of the fish is plotted out for the period , and strong year classes assigned to the dominant modes based on validated otolith Table 1 Description of female southern blue whiting (Micromesistius australis) gonad stages given to scientific observers. (1) Immature/resting Ovary very small and white, no eggs visible. (2) Maturing Ovary creamy white. White eggs barely visible. No clear (hyaline) eggs present in egg mass. Fish that have already spawned this season may have a few clear (ovulated) eggs present in centre of ovary. (3) Ripe At least one clear (hyaline) egg present in egg mass. Ovary considerably enlarged, swollen, and speckled. In fish that have already spawned this season the ovary may become purplish and have a few (<10%) clear (ovulated) eggs in its centre. (4) Running ripe Clear (ovulated) eggs freely extrudible either from vent or cut ovary. Eggs should flow freely and smoothly off the surface of a knife. At least 10% of eggs in the ovary should be in this state. (5) Spent Ovary bloody, flaccid, and dark red/purple. Ovary wall often thickened. Up to 100 residual opaque (white) or ovulated (clear) eggs may be present, depending on size offish.

4 Hanchet Southern blue whiting stock structure 601 Fig. 1 Southern blue whiting {Micromesistius australis) morphometric reference points. Table 2 Number of southern blue whiting (Micromesistius australis) otoliths read by year, sex, and area Bounty Males Platform Females ages reported in Hanchet (1997,1998). Mean lengths at age were compared for males and females separately for the strong 1988 and 1991 year classes between the Bounty Platform and Campbell Island Rise using the /-test. Because multiple tests were carried out the Bonferroni procedure was applied for each sex and year class comparison (Sokal & Rohlf 1981). The P value for a specific test had to be less than or equal to 0.05/rc, where n was the number of tests, to be deemed statistically significant. Morphometrics Morphometric measurements were made from 19 southern blue whiting of each sex collected from the Bounty Platform, Pukaki Rise, and Campbell Island Rise during a trawl survey in summer To minimise bias caused by allometric growth, sampling was restricted to fish of length cm. Campbell Males Island Rise Females Pukaki Rise Males Females Fish of this length are all sexually mature and would be on average c. 10 years old (Hanchet & Uozumi 1996; Hanchet 1998). Between 5 and 10 fish were randomly sampled from each tow, and 13 morphometric measurements (Fig. 1) were taken from each fish, following Andersen & Jakupsstovu (1978): FL, fork length; X6, distance from tip of upper jaw to origin of first dorsal fin; X7, length of first dorsal fin; X8, distance between tip of first dorsal fin and origin of second dorsal fin; X9, length of second dorsal fin; X10, distance between tip of second dorsal fin and origin of third dorsal fin; XI1, length of third dorsal fin; X12, distance from tip of lower jaw to anterior of the vent; X13, length of first ventral fin; X14, length of second ventral fin; X15, distance from tip of upper jaw to origin of pectoral fin; X16, distance from tip of upper jaw to anterior of eye; and XI7, eye diameter. All measurements

5 602 New Zealand Journal of Marine and Freshwater Research, 1999, Vol. 33 Bounty Platform s Auckland-' / Islands Shelf Campbelf IslatidC A Ri^e!I hf (70E Fig. 2 Position of all observed tows catching running ripe female southern blue whiting (Micromesistius australis). Depth contours shown in metres. were taken as the minimum distance between the two points and were made using dial callipers to the nearest 0.05 mm by a single recorder. The data were analysed using multiple discriminant analysis (MDA) with the STEPDISC, DISCRIM, and CANDISC procedures of SAS (SAS Institute 1988). Because of possible bias caused by using fish of different mean sizes from the three areas, both raw data and ratios (raw data/fish length) were used in the analysis. Because of differences in growth between the sexes (Hanchet & Uozumi 1996), the data were first analysed by sex separately and then the sexes were combined. In each analysis an initial stepwise discriminant analysis (STEPDISC) was carried out to determine which variables significantly improved the discrimination between the areas (P<0.05). Then a discriminant function analysis (DISCRIM) was carried out using those variables to determine class rules. Cross validation was used to determine the classification success of the original data (after Livingston & Schofield 1996). This involves removing each fish from the data set in turn and seeing how well it classified into the class rules determined by the remaining fish. The priors indicate the expected classification success by chance if the data are homogeneous. Finally, a canonical discriminant analysis (CANDISC) was carried out using only variables selected above to further examine the relationship of southern blue whiting from each area in discriminant space. Four multivariate statistics (Wilk's Lambda, Pillai's Trace, Hotelling-Lawley Trace, and Roy's Greatest Root with F approximations) were used to test the hypothesis that the class means were equal

6 Hanchet Southern blue whiting stock structure 603 (SAS Institute 1988). Class means and individual observations were plotted for the two canonical axes to illustrate the relationship between the areas. RESULTS Location of spawning females The location of all observed tows catching running ripe females suggests four distinct spawning areas on the Bounty Platform, Pukaki Rise, Auckland Islands Shelf, and Campbell Island Rise (Fig. 2). Smaller spawning grounds within the larger areas appear to occur on the north and south-east of the Campbell Island Rise. Timing of spawning The percentages of running ripe females recorded by observers for the four main areas is shown in Fig. 3. Unfortunately, coverage of the entire season is incomplete for most areas and for most years. Interpretation of the data is further complicated by the variability in the timing of spawning between years and a particular spawning peak may be the result of either the first or the second batch of eggs being spawned. The year with the best coverage of all four areas was Spawning occurred first on the Bounty Platform, then on the Pukaki Rise, next on the Auckland Islands Shelf, and last on the Campbell Island Rise. Data from the other years follow a similar pattern with spawning on the Bounty Platform always starting 3-4 weeks before spawning on the Campbell Island Rise. There are too few data to be certain of the timing of spawning on the Pukaki Rise and Auckland Islands Shelf in other years. Size and age distribution of the commercial catch The length-frequency distribution of fish in the commercial fishery from 1993 to 1997 is shown for males in Fig. 4. (The length-frequency distribution of females is not shown here but is very similar.) The length frequencies are different between areas but are relatively consistent within an area between years. There are two important differences: (1) the relative strength of the year classes between areas; and (2) the modal length of the year classes between areas. The size distribution from 1994 to 1997 has been dominated by a single mode (the 1991 year class) on the Campbell Island Rise, by two modes (the 1988 and 1991 year classes) on the Auckland Islands Shelf, and by three modes (the 1986, 1990, and 1991 year classes) on the Pukaki Rise. The size distribution on the Bounty Platform is less clear, but since 1995 has been dominated by a single mode, which is smaller and wider than that on the Campbell Island Rise, and which comprises the 1991 and 1992 year classes. These differences may have come about through either differences in recruitment and/or differential fishing mortality, but that they are consistent between years suggests little mixing between these areas. The differences in mean length at age between the Campbell and Bounty fish are examined in more detail below. Growth The von Bertalanffy growth parameters were not significantly different between the three areas for either sex (/-test,,p>0.05) (Fig. 5). Although overall growth was similar between areas, there appeared to be differences in growth of particular year classes between areas. Mean lengths at age of the strong 1988 and 1991 year classes are shown in Fig. 6. The mean length at age of the 1988 year class has been significantly greater on the Campbell Island Rise than on the Bounty Platform for most ages in both sexes (/-test, P<0.008). In the last 2 years the growth rate of the Campbell Island fish has declined slightly and the mean lengths have become closer, and were not significantly different. In contrast the mean length at age of the 1991 year class has been significantly greater on the Bounty Platform than on the Campbell Island Rise for most ages in both sexes (/-test, PO.017). Morphometrics The mean length of fish sampled was very similar between areas and is given, together with the means and coefficients of variation of the morphometric measurements, in Appendix 1. Two significant discriminant functions were generated (P<0.0\; Likelihood ratio test). The multivariate tests indicated significant differences in the class means between areas (P<0.05). The classification success is shown in Table 3. The results were similar between raw data and ratios, and the variables entering the model were the same. The most significant variables entering the model were the head measurements X6, XI5, XI6, and XI7. Overall classification success was between 54 and 70% which is considerably higher than the 33% expected by chance. Bounty Platform and Campbell Island Rise fish tended to be misclassified as Pukaki Rise fish, whereas Pukaki Rise fish were misclassified equally into the other two areas.

7 604 New Zealand Journal of Marine and Freshwater Research, 1999, Vol T 55 BP PR AI Clj I > I i I I I I I I I I I I I I I I I I I I I I T^H-H I i I I I 15Aug 20Aug 25Aug 30 Aug 4 Sep 9 Sep 14Sep 19Sep 24Sep 29Sep 1989 I'T'T'T'1 I t-h I y~~^ 35 - y K I \ I \ I I I I I \ i I I I \ Fig. 3 Percentage running ripe female southern blue whiting (Micromesistius australis) by area and year. Each point represents a median sample size of 250 fish. (BP, Bounty Platform; PR, Pukaki Rise; AI, Auckland Island Shelf; CI, Campbell Island Rise.) (Note: the* axis has been set below zero to show when there was coverage but no running ripe fish.)

8 Hanchet Southern blue whiting stock structure 605 Campbell Bounty Pukaki Auckland - JLA o L L, Fork length (cm) Fig. 4 Weighted length-frequency distribution of male southern blue whiting (Micromesistius australis) in the catch by area. Results of the canonical discriminant analysis showed that fish from the Campbell Island Rise and Bounty Platform were separated along canonical variate 1 (CAN 1), whereas fish from the Pukaki Rise were separated from the other two areas along canonical variate 2 (CAN 2) (Fig. 7). Proportionally, CAN 1 explained 65% and CAN 2 35% of the total variation. DISCUSSION The similar mean length of fish used between areas (Appendix 1), and the similar growth rates offish between areas, suggests that the differences in morphometrics between areas are the result of stock differences rather than growth differences. The morphometric data provide evidence that adult southern blue whiting from the Bounty Platform, Pukaki Rise, and Campbell Island Rise areas differ significantly from each other. This is consistent with other morphometric studies carried out on blue whiting in the North Atlantic (Andersen & Jakupsstovu 1978), and southern blue whiting in the South Atlantic (Wiecaszek 1988), which also found considerable variation in morphometric characteristics between areas. However, it is not clear whether the morphological differences reflect reproductive isolation or environmental factors. The analysis of biological data also suggests differences in adult southern blue whiting between the study sites. First, there are four geographically distinct spawning grounds each associated with a bathymetric high. Second, there are consistent differences in timing of spawning between the four areas. Fish from the Bounty Platform spawn at least 3 weeks earlier than fish from the Pukaki Rise which in turn tend to spawn a week earlier than fish from the Campbell Island Rise. Third, there are consistent differences in the size distribution of the fish between 0

9 606 New Zealand Journal of Marine and Freshwater Research, 1999, Vol x Males BP j Females 0.1 -I (- Cl PR BP Fig. 5 Estimated values of male and female southern blue whiting (Micromesistius australis) L jnf and k (with 95% confidence intervals). (BP, Bounty "platform; CI, Campbell Island Rise; PR, Pukaki Rise.) the four areas, and these differences have persisted over the past 5 years of the fishery. Last, there are significant differences in growth rates of the strong year classes between the Bounty Platform and Campbell Island Rise. Collectively, these biological and morphometric data suggest that there is little mixing of adult fish once they have recruited to a particular spawning ground, and suggests that fish from these four areas are essentially acting as separate stocks. These findings support preliminary work by Hanchet (1991), who also concluded that fish on the Bounty Platform, Pukaki Rise, and Campbell Island Rise formed separate stocks. His study was carried out before the development of the spawning fishery on the Auckland Islands Shelf, and so did not consider fish from this area in his analysis. Although by no means conclusive, the results of the current study suggest that the Auckland fish are separate from those on the Pukaki Rise and as a precautionary approach should be assessed and managed separately. Without genetic studies it has not been possible to determine whether some mixing occurs at the early life history stages. If mixing occurs between areas in the early life history stages then reduction in the biomass in one of the four areas may be replenished quickly. However, if there is little mixing or no mixing of the early life history stages then biomass levels in one of the four areas would take longer to recover after heavy fishing pressure. The Bounty Platform is separated bathymetrically, and possibly hydrologically, from the rest of the Campbell Plateau. Depths between the two topographic features exceed 1000 m, which may Table 3 Percentage of southern blue whiting {Micromesistius australis) correctly classified using the cross validation procedure of linear discriminant analysis into each area, (n, sample size; Van, number of variables; CI, Campbell Island Rise; PR, Pukaki Rise; BP, Bounty Platform, (priors = 33%).) n Var. CI PR BP Total Raw data Males Females Total Ratios Males Females Total

10 Hanchet Southern blue whiting stock structure r Males, T Males, T- 50 T- Females, 1991 Fig. 6 Mean southern blue whiting (Micromesistius australis) lengths at age of the 1988 and 1991 year classes by sex and area, with 95% confidence intervals. (BP, Bounty Platform; CI, Campbell Island Rise.) A A X A A A X X X -x X X x x X a a o a - a n D x X J\ D D CI ft a a D CI X PR A BP CAN1 Fig. 7 Class means and canonical variate scores for southern blue whiting (Micromesistius australis) from the Campbell Island Rise (CI), Pukaki Rise (PR), and Bounty Platform (BP).

11 608 New Zealand Journal of Marine and Freshwater Research, 1999, Vol. 33 restrict the mixing of adult fish between the two regions. Although there is a prevailing eastward flow of water along the southern and eastern flanks of the Campbell Plateau towards the Bounty Platform, ocean circulation models predict the presence of "squirts" which force water northwards between the eastern edge of the Campbell Plateau and the Bounty Platform (NIWA unpubl. data). These squirts may deflect any eggs and larvae spawned on the Campbell Plateau north towards the Chatham Rise, and hence restrict movement of larvae between the two regions. The hydrology and circulation patterns over the Campbell Plateau itself are currently being researched. Preliminary results have suggested a weak westward flow of water along the northern flank of the Campbell Plateau (M. Morris, NIWA pers. comm.). Water flows over the Campbell Plateau also appeared weak, and were suggestive of weak anticyclonic circulation over the Pukaki Rise and Campbell Island Rise. Such weak gyres may help retain eggs and larvae adjacent to these bathymetric highs. In conclusion, biological and morphometric studies have identified differences between fish from the main fishing grounds on the Bounty Platform, Pukaki Rise, Auckland Islands Shelf, and Campbell Island Rise. These differences show that these fish return to spawn on the grounds to which they first recruit, and suggest that the fish form four separate stocks, and should be assessed and managed separately. More research is needed to fully understand the stock structure of southern blue whiting, and in particular the amount of mixing in the early life history stages. Only then can fully informed decisions be made regarding the long-term sustainability of the various stocks. ACKNOWLEDGMENTS Thanks to Peter Smith, Ross Daley, and two anonymous referees for comments on earlier drafts of the manuscript. This project was funded by the Ministry of Fisheries, Project Number MDBW02. REFERENCES Andersen, K. P.; Jakupsstovu, S. H. 1978: Sexual dimorphism and morphological differences in blue whiting (Micromesistius poutassou). ICES CM/H p. Colman, J. A. 1995: Regional morphometric variation in ling (Genypterus blacodes Ophidiidae) in New Zealand waters. New Zealand Journal of Marine and Freshwater Research 29: Hanchet, S. M. 1991: Southern blue whiting fishery assessment for the fishing year. New Zealand Fisheries Assessment Research Document 91/7. 48 p. Hanchet, S. M. 1997: Southern blue whiting (Micromesistius australis) fishery assessment for the and fishing years. New Zealand Fisheries Assessment Research Document 97/ p. Hanchet, S. M. 1998: Southern blue whiting (Micromesistius australis) fishery assessment for the Bounty Platform and Pukaki Rise for 1998 and New Zealand Fisheries Assessment Research Document 98/ p. Hanchet, S. M.; Uozumi, Y. 1996: Age validation and growth of southern blue whiting, Micromesistius australis Norman, in New Zealand. New Zealand Journal of Marine and Freshwater Research 30: Horn, P. L. 1993: Growth, age structure, and productivity of ling, Genypterus blacodes (Ophidiidae), in New Zealand waters. New Zealand Journal of Marine and Freshwater Research 27: Hurst, R. J.; Bagley, N. W.; Chatterton, T. D.; Hanchet, S. M.; Schofield, K. A.; Vignaux, M. 1992: Standardisation of hoki/middle depth time series trawl surveys. MAF Fisheries Internal Report No p. Livingston, M. E.; Schofield, K. A. 1996: Stock discrimination of hoki (Macruronus novaezelandiae, Merlucciidae) in New Zealand waters using morphometrics. New Zealand Journal of Marine and Freshwater Research 30: SAS Institute 1988: SAS/STAT user's guide, release 6.03 edition. Cary, NC, SAS Institute Inc p. Smith, P. J.; Jamieson, A.; Birley, A. J. 1990: Electrophoretic studies and the stock concept in marine teleosts. Journal du Conseil 47: Sokal, R. R.; Rohlf, F. J. 1981: Biometry. Second edition. San Francisco, California, United States, W. H. Freeman. Svirskii, V. G.; Shpak, V. M. 1977: Oogenesis, sexual cycle, and fertility of the southern blue whiting Micromesistius australis Norman (1937) from the south-west part of the Pacific Ocean. Transactions of the Pacific Scientific Research Institute of Fisheries and Oceanography (TINRO) 101: (In Russian, Translation held in NIWA library, Wellington.) Wiecaszek, B. 1988: Morphometry of southern blue whiting Micromesistius australis (Norman, 1937) from the region of Burdwood Bank. Acta Ichthyologica et Piscatoria 18 (2): 3-18.

12 Hanchet Southern blue whiting stock structure 609 Appendix 1 Mean morphometnc measurements (mm) and CV (%) of southern blue whiting (Microinesistius australis) by area. Variable Bounty Mean Platform CV Pukaki Mean Rise CV Campbell Mean Island CV Length X6 X7 X8 X9 X10 Xll X12 X13 X14 X15 X16 X17 X18 X19 X20 X21 X22 X