Age validation and growth of southern blue whiting, Micromesistius australis Norman, in New Zealand

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  • This article was downloaded by: [University of Pennsylvania]On: 08 October 2013, At: 13:45Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

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    Age validation and growth of southernblue whiting, Micromesistius australisNorman, in New ZealandS. M. Hanchet a & Y. Uozumi ba National Institute of Water & Atmospheric Research Ltd ,P.O.Box 14901, Kilbirnie, Wellington, New Zealandb National Research Institute of Far Seas Fisheries , 71 Orido 5Chome, Shimizu, Shizuokashi, 424, JapanPublished online: 30 Mar 2010.

    To cite this article: S. M. Hanchet & Y. Uozumi (1996) Age validation and growth of southernblue whiting, Micromesistius australis Norman, in New Zealand, New Zealand Journal of Marineand Freshwater Research, 30:1, 57-67, DOI: 10.1080/00288330.1996.9516696

    To link to this article: http://dx.doi.org/10.1080/00288330.1996.9516696

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  • New Zealand Journal of Marine and Freshwater Research, 1996: Vol. 30: 57-670028-8330/96/3001-0057 $2.50/0 The Royal Society of New Zealand 1996

    57

    Age validation and growth of southern blue whiting,Micromesistius australis Norman, in New Zealand

    S. M. HANCHETNational Institute of Water & AtmosphericResearch LtdP.O.Box 14-901, KilbirnieWellington, New Zealand

    Y. UOZUMINational Research Institute of Far Seas Fisheries7-1 Orido 5 Chome, ShimizuShizuoka-shi 424 Japan

    Abstract Over 3000 otoliths and 100 000 lengthand sex records were examined of southern bluewhiting caught from commercial fishing groundson the Campbell Island Rise, south of New Zealand,during August and September between 1981 and1989. Ages of juveniles were validated by followingmodes in length-frequency data over a 13-monthperiod between September 1981 and 1982. Agesof adults were validated up to at least 10 years byfollowing strong year classes both from otolith-based age frequency distributions and from length-frequency data from 1981 through to 1989.Independent analysis of the length-frequency datausing MULTIFAN further supported the adultotolith ages. Above age 10 there was less confidencein otolith ages, less agreement between readersand a greater degree of bias between readers. Whenusing the data in catch-at-age models it isrecommended that ages greater than 10 be groupedas a single plus group.

    Keywords Micromesistius australis; southernblue whiting; age validation; growth; otoliths;MULTIFAN; length-frequency analysis

    M95031Received 16 May 1995; accepted 10 October 1995

    INTRODUCTIONSouthern blue whiting (Micromesistius australisNorman) is largely confined to subantarctic watersoff the coasts of South America and to the south ofNew Zealand. Its centres of abundance in NewZealand are on the southern Campbell Plateau tothe east of Campbell Islands, Pukaki Rise, andBounty Platform, in depths of 300-600 m (Fig. 1).A fishery developed for this species in the early1970s, and since then annual landings have rangedfrom 3000 to 76 000 tonnes (Hanchet 1993). From1981 to 1989 the fishery focused on an area to thenorth of the Campbell Island Rise (Fig. 1) wherefish gather each August and September to spawn.Annual catches averaged about 15 000 tonnes,increasing in 1989 to about 25 000 tonnes, when itwas New Zealand's fourth largest fishery. Despitethe relatively large size of the fishery little wasknown about the stock size or sustainable yields.During this period otoliths and length-frequencydata were collected from the fishing grounds foruse in catch-at-age models, such as cohort analysisor virtual population analysis (Pope 1972). Severalauthors have recently emphasised the importanceof using reliable (preferably validated) ages whenapplying this technique (e.g., Kimura 1989).

    There are several methods of age validationboth direct and indirect (Beamish & McFarlane1983). Direct methods such as captive rearing, tagrecapture studies, and the capture of known agefish are probably not applicable to southern bluewhiting, which are difficult to catch alive and losescales easily on capture. Indirect methods includeanalysis of length-frequency modes, monitoring ofstrong year classes, examination of marginalincrements on hard parts, growth analysis, andcomparison of the different techniques (Beamish& McFarlane 1983).

    Previous aging studies of southern blue whitingin New Zealand waters have not successfullyvalidated age estimates (Paul 1992). During the1970/71 cruise of Kaiyo Maru, a maximum age of12 years was recorded using whole otoliths

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  • 58 New Zealand Journal of Marine and Freshwater Research, 1996, Vol. 30

    mmmm PUKAKI RISE

    CAMPBELL ISLAND RISE

    180

    BOUNTY PLATFORM

    Fig. 1 Main distribution area and sampling locations of southern blue whiting.

    (Anonymous 1972). Shpak (1978) recorded amaximum age of 14 years from whole otoliths andscales. Van den Broek (1983) prepared and readthin sections of over 500 otoliths and recorded amaximum age of 23 years. Nakaguchi &Shimomura (1989) examined cross sections of over1000 otoliths and recorded a maximum age of 18years. Barrera-Oro & Tomo (1988) examined thincross setions of over 1000 otoliths fromM australisin the south Atlantic around the Falkland Islandsand recorded a maximum age of 23 years. Theyalso published photographs identifying annuli(defined as narrow translucent zones when observedunder reflected light), check rings, and regionswhere annuli could not be clearly discerned.

    The objective of the present paper is to validatethe ages of southern blue whiting from New Zealand

    waters. This is carried out by following theprogression of modes in juvenile length-frequencydata, and by tracking strong year classes in adultfish using both length-frequency data and annulicounts in otoliths.

    MATERIAL AND METHODSLength-frequency data and otoliths for 1982-86were obtained during cruises of the Japanese MarineResearch Centre's research vessel, Shinkai Maru.Samples were also collected by observers on boardSoviet and Japanese trawlers fishing southern bluewhiting commercially between 1986 and 1989,and by MAF Fisheries staff on board the Sovietcommercial fishing vessel MYS Kuznetsova in 1981(MAF Fisheries unpubl. data). All vessels used

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  • Hanchet & UozumiAge validation of southern blue whiting 5960 mm mesh cod-ends, which equates to a 50%selection length for southern blue whiting of about30 cm (Anonymous 1978).

    Adult fish were collected during late Augustand September of each year, from the main fishinggrounds north-east of Campbell Island Rise indepths of 400 to 600 m (Fig. 1). Samples of juvenilefish were collected by Shinkai Maru in the samegeneral area throughout much of the year betweenSeptember 1981 and September 1982. Additionalsamples of juvenile otoliths were collected onShinkai Maru in December 1982 and 1984, byobservers in September 1988 and by MAP Fisheriesstaff on board Amaltal Explorer during a researchtrawl survey in October 1989.

    Each day up to 300 randomly sampled fishwere measured and sexed, and between 5 and 20pairs of non-randomly sampled otoliths were taken.Sex and fork length (FL, measured to the nearestcm below) were recorded for each fish. No otolithswere collected from the commercial fishery in1987.

    Length-frequency dataLength-frequency data were collected by scientistson board Shinkai Maru from 1982 to 1986 and byobservers from 1986 to 1989. Length-frequencydata for the year of overlap of the Shinkai andobserver data sets were near-identical (Hanchet1991). The data sets were therefore combined foranalysis.

    Length-frequency data were analysed to deriveestimates of the growth curve parameters andlengths at age independent of the otolith ages, vonBertalanffy growth curves were fitted to the maleand female length-frequency distributions separ-ately using the MULTIFAN model Version 32(Fournier et al. 1990). This model simultaneouslyanalyses multiple sets of length-frequency samples,using a maximum-likelihood method to estimatethe proportions of fish in each age class, and thevon Bertalanffy growth parameters Lx, K, and to-As southern blue whiting are known to spawnduring September of each year (Hanchet 1993),the fish were assigned a birthdate of 1 August, andthis was used as the birth date to calculate ?o fromthe age of the first year class in the first sample, towas estimated using the equation:

    tc, ="K

    where t\ is the estimated age (in years from thebirth date) of the first age class and m\ is the mean

    length of that age class, as estimated byMULTIFAN.

    There is a danger that parameter estimates madeby MULTIFAN will represent only a localminimum. To overcome this problem, we startedby making a systematic search, using all combin-ations from a matrix of lvalues (0.20, 0.25, 0.30,0.35, 0.40, 0.45) and plausible age classes (usually10 to 16). For the analysis of each sex the procedurewas to start with the simplest model and then togradually increase the complexity by introducing(i) variable standard deviation and (ii) constraintson mean lengths of ages 1 and 2. At each stage theparameters were estimated, with K always beingthe last parameter to be estimated. Log-likelihoodratio tests were carried out to determine whetherthe inclusion of extra parameters significantlyimproved the model. Following Fournier et al.(1990), we used a significance level of 0.10 fortesting whether there is any gain in introducing anadditional age class in the length-frequencyanalyses; all other tests were carried out with asignificance level of 0.05.

    OtolithsOne of each pair of otoliths was burnt over analcohol flame, and embedded in a resin block whichwas then cut through the core using a low-speeddiamond saw. The cut surface of the section of theotolith was viewed under reflected light.

    Age estimates were determined by countingthe number of annuli on the cut surface of theotolith. The annulus is defined as the translucentzone, or the zone of slower growth, that appearedas a dark zone under reflected light. Zone countswere usually made from at least two different areasof the otolith:(1) from the core out to the dorsal edge, and(2) from the core out to the proximal edge (areas I,

    II, and IV in Fig. 2), following Chilton &Beamish (1982).Most otoliths collected during August and

    September had wide opaque zones at the edge ofthe otolith, although occasionally a narrowtranslucent zone had begun to form. Because thebirthdate was assigned 1 August one was added tothe age estimate of all fish caught during Augustand September which had an opaque zone at theedge of the otolith.

    Otolith appearance and structure was verysimilar to that reported by Barrera-Oro & Tomo(1988), and so interpretation of the annuli followed

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  • 60 New Zealand Journal of Marine and Freshwater Research, 1996, Vol. 30

    Distal edge

    Checks and annuliCore

    First annulus

    Fig. 2 Generalised cross sectionof an otolith showing selectedcounting areas (after Chilton &Beamish 1982).

    m iv

    Proximal edge

    their methodology. The first annulus was oftenabsent or poorly defined so back-calculation wasapplied to predict the likely position of the firstand second annuli. In back-calculation proceduresthe diameter of the zone was measured in thetransverse plane from the ventral to the dorsaloutside edge. Care was also taken to correctlydistinguish annuli from the check rings whichoccurred occasionally between the core and thefirst, second, and third annuli. In general, thesecheck rings were identified as being narrower thanthe annuli and usually could not be traced aroundthe entire core.

    A readability category ranging from 1 to 5 wasrecorded for each otolith:1Clear and unambiguous;2Little doubt;3Reasonably confident but some doubt;4Considerable uncertainty;5No confidence in age estimate.

    A total of 3218 otoliths were read. The sex andlength of the fish that the otolith came from wasnot known to the reader. No attempt was made torandomise the samples by year because the agereaders did not know that strong year classeswere present. A random sample of about 40otoliths from the years 1983 to 1989 was read by asecond reader to examine "between-reader"variability.

    Growth curves were fitted to the otolith agesusing the non-linear multivariate secant parameterestimation procedure (SAS Institute 1988).

    Differences in growth between the sexes wereexamined using the log-likelihood ratio test.

    Age data were scaled up to catch-at-age becauseotoliths were not always collected in a randommanner. As the majority of the catch was taken inAugust and September (Hanchet 1993), catch-at-age estimates for each year were derived bycombining the age-length key for that year withthe weighted length-frequency for that year. Thesexes were treated separately but combined toproduce the final age-frequency distributions. Theweighted length-frequency data were obtained inthe following way. First, the data were stratifiedinto weekly time periods. Then the length-frequencysamples taken aboard the vessels were scaled up tothe total weight of the sampled catch. These werescaled up to the weight of the catch taken by thewhole fishery for that week. Results from eachweek were then summed to give the weighted length-frequency distribution of the total catch for theyear.

    RESULTSJuvenile length-frequency distribution andotolithsThe monthly progression of modes in l...

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