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Stock Identification and Management

Robert A. Skillman

While economic and social evaluations of the unit stock concept. with many authors pro- are today recognized as important posing new or modified definitions. Providing elements in managing fisheries. determining the a historical perspective might be appropriate at biological status of exposed to a and this time. to a management regime remains a basic and Fishery biologists first defined a stock in important element in management. For the status terms of the various tools employed in conduct- of a stock to be assessed. boundaries must be ing an assessment. For example. Cushing (1968) set. If the boundaries actually encompass more said. '*. . . a stock may be defined as a popula- than one stock and management is carried out tion in which the vital parameters of recruitment, as if there were only one stock. the management growth, and mortality are homogeneous." In scheme may perform satisfactorily on average practice, fishery biologists frequently identified for the two stocks. However, the more that popu- stocks based on where the fish were found as lation parameters, such as growth and mortality, larvae, as adults in the fisheries. or while - differ among individual stocks, the more likely ing, as well as by how the fish looked ( it is that the management regime will adversely and morphometrics). Then. the existence of impact one of the stocks. If the boundaries do these putative stocks was evaluated by using not completely encompass a stock, then events estimates of vital (or population) parameters. external to the management area can signifi- With the development of pobulation genetics, cantly affect the stock. the fishery biologists' definition of a stock was This chapter reviews stock identification viewed as vague and not quantitatively rigorous methods as they relate to fisheries management compared with the geneticists' definition of a and. in particular. to billfish management. The population or a subpopulation. Thus. fishery stock concept. itself. will be discussed before biologists began incorporating genetic concepts. reviewing the various methods used to study and new definitions were proposed. For example. stock structure. Then. what is known about ". . . a unit fish stock is one consisting of ran- billfish stock structure will be reviewed. followed domly interbreeding members whose genetic in- by recommendations regarding the most likely tegrity persists whether they remain spatially productive research strategies to follow for and temporally isolated as a group. or whether in the future. they alternately segregate for breeding and otherwise mix freely with members of other unit The Stock Concept stocks of the same " (Kutkuhn 1981). Setting boundaries and identifying stocks is. Along with this definition came numerous in effect. determining what part of the total attempts to identify stocks by using various species population is going to be assessed and genetic techniques. managed. Thus. the term stock or unit stock is The debate on the utility of genetic techniques used. ". . . to connote as well as circumscribe in fisheries stock assessment and fisheries man- the basic grouping on which management needs agement continues today. One problem is whether to locus attention" (Kutkuhn 1981). This initial a population determined to be genetically htep in a5sessment has led to many reviews ot homogeneous meets the needs of fishery bicilo-

207 708 PLANNING THE FUTURE OF BILLFISHES gists to conduct stock assessments: namely, the exchange of individuals among the sites. is unit stock must be homogeneous with respect necessq for genetic separation and could contri- to recruitment. growth, and mortality. A second bute to differences in vital parameters. The dis- problem is whether genetic techniques can re- tribution of larvae and juveniles may also be used solve the structure of a large population inhabit- to identify stocks, particularly when the samples ing an extensive area having vague and changing are collected independently from any fishery. environmental boundaries when biological Thus, using juveniles collected from the stomachs (fishery) information suggests some degree of of commercially-landed predatory should stock structuring. While all the evidence is cer- be done with care. tainly not in. the genetic and fishery biological Popularion Parameters. Estimates of popula- definitions apparently coincide reasonably well tion parameters, such as growth. recruitment. for demersal species and stocks with well-defined and mortality, are used to separate stocks. In physical or environmental boundaries. On the addition, they are commonly used to characterize other hand. they apparently do not coincide well putative stocks and evaluate whether their ex- for species inhabiting, as adults or larvae. the istence seems plausible; that is. whether the pelagic ecosystem with poorly defined physical parameters are homogeneous over the range of and environmental boundaries. the reputed stock. While demonstrating that parameter estimates are different among poten- Stock Identification Methods tially separate stocks does not necessarily prove As indicated above, the study of population the existence of separate stocks. stock structur- structure in fisheries consists of two basic ap- ing is supported when geographical clumping proaches. The traditional biological approach or clinal variation of parameters occurs. Such consists of a number of alternative techniques results are consistent with a lack of genetic mix- for identifying stocks. These stocks meet the ing (subpopulation formation) and a lack of ex- fishery biologists' stock asessment needs but change of fish among fishing grounds or mixing may not be the same as genetic subpopulations. of individuals in nursery areas (stock formation) The newer genetic approach consists of a few because separation. from either perspective, but growing number of techniques for identifying could lead to different population parameters. subpopulations that may or may not be helpful Given the poor ability to estimate mortality even for stock assessment and management purposes. for well-studied stocks, however. estimates of billfish mortality are not likely to provide as Bioloqical much information as growth estimates. On the Disrriburion. Probably the first method technical side. comparison of the growth esti- employed was to examine the geographical dis- mates is complicated because the commonly tribution of fishery catches. which often consist used von Bertalanffy growth parameters. of adults. Thus. this method has direct applica- maximum size (Lint) and rate of approaching bility to defining the boundaries of a fishery, that maximum (k). are correlated. Simple cluster and possibly the boundaries of management analysis conducted by plotting LInrversus k for action. but it may not be satisfactory for deter- samples from different areas has proven produc- mining the boundaries of a stock unless the dis- tive in the identification of stocks. Other popu- tribution of fishing effort is extensive. For ex- lation parameters (e.g., the age at first maturity ample. if the fishing effort is directed at the and gonadal index) are also used. Age at first species being examined. it is not likely to occur maturity is strongly affected by stock size. which beyond the region where the species is commer- is. in turn. affected by the size of the fishery. cially or recreationally important. In such cases. Gonadal development is seasonal but may differ stocks may be distributed more extensively than among stocks. indicated. Compiling the distribution of catches Parasires. Using the occurrence of fish para- by size classes may help substantiate the ex- sites as a means of studying stock structure is istence of putative stocks when recruitment can based on the concept that parasites are naturally he shown to vary. occurring "tags": that is. the incidence of para- Determining the distribution of spawning sites sitic species varies peographically and these may rewlt in more reliable identlfication of natural tags are not lost. The method is not \tacks. The cnistt'ncc of gcographically or applied frequently to large pelagic species tor a temporally separate spawning sites. wlth no number of reasons. Fishery biologists reco,'mze STOCK IDENTIFICATION & MANAGEMENT 209 that parasitology is not a tool in the sense that when released and retaken, recaptures may in- morphometric and multivariate statistical methods dicate movement but provide little information are. but is an additional level of biological com- on stock structuring. plexity requuing research itself. This complexity involves the identification of relatively bizarre : use of different sampling, preservation. and preparation techniques: and. most im- Morphomerrics and Meristics portantly, familiarity with a different set of life- Fish exhibit greater observable (phenotypic) history strategies involving host-parasite relation- variation than other , apparently be- bhips. Parasitologists have tended to work more cause they are poikilothermic (cold-blooded) in freshwater and coastal marine habitats where and have a capacity for indeterminate growth fish stock sizes and parasite-to-host linkages are (e.g., Allendorfet al 1987). Thus, early workers more amenable to reseach. Consequently, the were able to recognize stocks on the basis of life histories of parasites occurring on large appearance. and later workers built on this by pelagic species and the effect of environmental analyzing measured characters (morphometrics) changes on infestation are not well known. and or analyzing counted characters (meristics). interpretation of results has been open to ques- With the development of analytical tools in the tion. Research is needed on the possibility of fields of and statistics, this approach tag loss (selective or differential loss of parasitic flourished and provided a means for rigorously \pecies or parasite size classes) with movement identifying stocks. However, morphometric and of fish into areas having different environments. meristic characters were shown to be strongly and then reinfestation by other parasitic species. influenced by the environment, and their use Also, application of the method to date has as- fell into disfavor with the development of elec- sumed that each natural tag (each parasitic trophoresis for studying population genetics. species) is made up of one population, but I see Experience has shown that stocks identified by no reason why they could not be divided into using morphornetrics or meristics are in general subpopulations as well. Nonetheless, such agreement with subpopulations identified studies can provide a fishery-independent deter- genetically (e.g.. see Ihssen et ai 1981). How- mination of stock structure, and additional long- ever, a greater degree of stock structuring is term work on the basic biology of often found by using morphometric and meristic parasites is likely to improve the utility of the methods than by using genetic procedures, be- method as a stock identification tool. cause of the superior power of the statistical Tugging. Tagging fish for the study of stock tests associated with the former methods and the StNCNR is very popular because it provides direct. use of environmentally influenced characters. if not irrefutable. evidence of fish movement Thus. amon_emany biologists. morphometric and the lack of stock separation. Movement is and meristic methods are still viable means of the mechanism for exchange of individuals and investigating stock structuring. With the de- the mixing of genetic products. However. the velopment of new techniques for studying method has several problems. and care must be shape, the statistical power of the methods has taken in analyzing the results (Hilborn and Sibert increased even more. Truss analysis (e.g., see 1988). First. the method is expensive if large Winans 1987) makes use of recently developed numbers of tagged fish are to be released. and powerful multivariate analysis techniques. this is certainly desirable if not a requirement Showing great promise is the use of planar shape with the large populations that exist for billfishes analysis. first used in the study of the evolution and other pelagic species. Also. a preat deal of of ostracods (e.g.. see Benson 19671 and re- international promotion and cooperation are re- cently making its way into fisheries fBird et al quired to conduct such experiments successfully. 1986). This method is based on pattern recogni- Second. results are meaningful only if fisheries tion techniques using mathematical tools such exist that have the potential of recapturing the as Fourier analysis and discriminant analysis. tagged fish. Third. fishing effort statistics from The collection of data for analysis using the these tishenes are required for quantitative evalu- truss and planar shape methods can be performed ations of the recaptures. Fourth. without additional on microcomputer-based iniape processing bys- information about the biology of the !ish and rems. rhus reducrng the cost of collecting data wch specifics as the size and maturity ofthe fish in the field. ?IO PLANNING THE FUTURE OF BILLFISHES

Generics suggests. DNA sequence determination involves The following historical perspective is drawn the resolution of genes directly on the DNA in part from Utter et al (1987). Although great molecule. which is located in the cell nucleus. strides in the inheritance of observable characters The method is sufficiently sensitive that related- have been made from the time of the pioneering ness of progeny and ancestors of a generation work of Mendel (1866) up to the 1930s. this or two can be detected: beyond this. variability work was not transferable to fish because of the becomes so great that all individuals appear to magnitude of environmental influences on their be different (Wetton et al 1987). Thus, the name observable characters. About this time. the in- "DNA fingerprinting" has been used to describe heritance of blood groups had been resolved. this method. It is not likely to be of much benefit and attempts were made later to use the for the study of billfish stock strucmnng. although technique on (Cushing 1956. Sprague et it may be the genetic "marker" so long sought ai 1963. Fujino 1970). This technique was un- for hatchery-reared fish released into the wild. productive because of the fragility of fish ery- The second technique involves the analysis throcytes and problems in making and preserving of DNA occumng in mitochondria located in the discriminating antisera. Lundstrom ( 1987) the cell cvtoplasm. Because mitochondrial DNA indicates that the use of monoclonal antibodies (mtDNA) is maternally inherited. each individual shows promise, but other genetic procedures has an exact copy (except for rare mutations) of probably show more potential for success with the mtDNA genotype from its mother. Therefore. billfishes. the operational taxonomic unit is the individual The determination of the structure of the DNA rather than the population. as with nuclear gene molecule by Watson and Crick (1953) led analytical techniques such as electrophoresis. quickly to the determination of the relationship Thus, in practical terms. the statistical require- between genes and intracellular, enzymatic pro- ments are quite different. and sample sizes can teins. called isoenzymes or isozymes for short. be substantially less than with electrophoretic By 1955. electrophoretic techniques were de- work (Fems and Berg 1987). For example, re- veloped that allowed easy examination of the ported sample lot sizes per species have varied variation (polymorphism) of these proteins and. from I to 12 (Avise et al 1979. Berg and Ferris thereby. the genes that make them. Thus was 1984, Graves et al 1984. Wilson et al 1985, 1987) born the most powerful and productive tech- with species totals in 2 studies reaching 36 (Avise nique to date for studying population genetics. et al 1979) and 87 (hsman et al 1981). This dominating since the mid-1960s. Examples even conmasts with sample lot sizes of 50 to 100 (Fujino in the fisheries literature are too numerous to and Kang 1968) or even 200 (IA'ITC 1978) and mention. A serious problem with the technique total sample sizes in the thousands for elec- is that separation of stocks depends on statisti- trophoretic studies of fish. Although the technique cally testing of what are called the Castle-Hardy- apparently holds great promise for resolving stock Weinberg gene proportions. This test is statisti- structuring (Avise et al 1979). little experience cally weak and. with the typical genotypic ratios has been gained with pelagic species. Avise ( 1987) involved. requires large sample sizes to obtain could not find any evidence of stock structuring meaningful results. After many years of work in the American (Anguilla rostraia) which on skipjack (Kursuwonus pelamis) in the spawns in the pelagic environment. whereas stock Pacific (Fujino and Kang 1968. Argue structuring was evident for several demersal or et al 1986). some dewe of population structuring estuarine species. Graves et al ( 1984) did not find is indicated. but distinct subpopulations have any difference between Atlantic and Pacific not been resolved. Grant et al (1987) state that samples of shpjack tuna however. the specimens marine fish. in general. exhibit little genetic were frozen. which considerably reduces the population structuring because of the passive amount of mtDNA recovered for analysis dispersal ot' larvae and active movement of (Lansman et al 1981: Ferris and Berg 1987). adults (Ryman et al 1984: Shaklee 1984). Given the life history of most billfishes. electrophoretic Billfiih Stock Structure \tudy of isozvme variabilitv is not likely to be In the following sections. information on the very productive. htock structure of billfishes is summarized from Recently. two additional DNA-based tech- a varietv of sources. Papers presented at the first niques have been developed. As the name billfish symposium in 1072 (Shomura and STOCK IDENTIFICATION & MANAGEMENT 'I 1

Williams 1974) discuss. for Atlantic. Indian. and and black (M.indica) suegest multiple Pacific . the distribution of adults based on stocks. In addition. the workshop participants tuna longline catches, the distribution of larvae (FA0 1980) believed that movement of the latter and ostensible spawning areas based on research species could occur between the Pacific and Indian data. and results from tagging. For the Atlantic Oceans although no tag recaptures had been made Ocean, I am not aware of any single source sum- to support the contention. marizing information on billfish stock structure. so 1 have used the way billfish stocks are assessed Puclfic Ocean and reported in the biennial reports of the Interna- For the Pacific Ocean. stock structure informa- tional Commission for the Conservation of At- tion is available for six billfishes (Shomura and lantic Tunas (ICCAT) as an indication of the state Williams 1974. Shomura 1980). Based on the of knowledge (e.g., see ICCAT 1987). For the distribution of tuna longline catches. tagging Indian Ocean. participants at a 1979 tuna and returns. and the distribution of spawning, the billfish stock assessment workshop summarized lndo-Pacific blue marlin is believed to consist the state of knowledge for billfish stock structure of a single Pacific-wide stock (Shomura 1980). (FA0 1980). Likewise. for the Pacific Ocean. whereas the other billfishes exhibit some evidence participants at the 1977 billfish stock assessment of stock structuring. In an electrophoretic survey workshop (Shomura 1980) summarized what was of blue marlin captured in Hawaii. sufficient known about the stock structure of billfishes as a polymorphism to conduct a population genetic prerequisite to assessing the status of the presumed study was found (Shaklee et a1 1983). The only stocks. suggestion of population structuring (mixing of subpopulations) in this small sample collected Atlantic Ocean at one site in 1 month, however, wasadeficiency Among the several billfishes occurring in the of heterozygotes for one of seven polymorphic Atlantic Ocean. the ICCAT biennial reports con- gene systems surveyed. For , tain material on separate stocks only for participants at the 1977 stock assessment work- (Isriophonu platyrenu), namely eastern and shop (Shomura 1980) indicated that separate western stocks. The basis for this separation Seems eastern Pacific and two western Pacific stocks to be the distribution of longline catches and probably exist. Skillman (1989) assumed one tagging results (Mather. Tabb. Mason, and Clark Pacific-wide stock centered off Australia, based 1974). While tagging results support the existence on the distribution of tuna longline catches. of separate northern and southern stocks of the Nakamura (1983) indicates two spawning areas. (Mokaira nigricans; Mather. one off Australia and a second in the north Mason. and Clark 1974). the ICCAT reports con- Pacific between the Caroline and Marshall tain discussions of stock assessment results for the Islands. Northern and southern stocks of striped entire Atlantic Ocean but present figures showing marlin may exist, but the evidence is contradictory surplus production models for separate stocks. or inconclusive. For . the area of (Tetrupturus albidus) assessments high tuna longline catches in the eastern tropical are presented for one Atlantic-wide stock. For Pacific appears to join or merge seasonally in (Xiphias gladius), researchers (ICCAT many years with high catch areas in the middle 1987) now believe that separate eastern and west- latitudes of both the north and south Pacific. In ern stocks exist. as well as a separate Mediterra- contrast. the annual distribution of spawning nean stock. based on longline catch distributions. stretches from comparable northern and southern tagging results. and the distribution of spawning central Pacific areas across the Equator in the activities and larvae. western tropical Pacific (Nakamura 1983). Swordfish population is generally believed to Indian Ocean consist of a single Pacific-wide stock. but mav Information on the stock structure of five consist of separate stocks in the northwest. billfishes is available for the Indian Ocean (FA0 southwest. and eastern Pacific. Spawning 1980). The distribution of loneline catches and grounds are located in warm. tropical waters in information on spawning suggest that there is a the western and central Pacific (Nakamura single stock of Indo-Pacific blue marlin (M.ma:- 1983). Sailfish are believed to be divided into ura) in the Indian Ocean. Similar kinds of data eastern and western stocks based on tuna lor suiped marlin (T.t~~llar). swordfish. sailfish. longline catch rates and distribution of spawning 212 PLANNING THE FUTURE OF BILLFISHES

sites. Distribution of tuna longline catches vital parameters, such as growth. age at suggests separate northern and southern stocks first maturity, recruitment, and mortality, of (T.angusrirosrris). and the to ensure that valid stocks are identified distribution of spawning activity supports this and that models used to assess the status conclusion. of stocks are applied correctly. 4. Billfish larval surveys should be designed Recommendations and implemented. Results from ichthyo- The following research recommendations plankton tows. taken by different kinds of would, in my opinion. most likely contribute to gear, at different depths, in different our understanding of the stock stmcture of seasons. and. for the most part, without billfishes. any experimental design (along with charts I . Tag-and-release programs, particularly for of the distribution of billfish catches by young fish. should be continued and tuna longline gear), have been the most extended to other tournaments and commonly used means of identifying fisheries. Recaptures provide valuable in- stocks. Enough is now known. however. formation on movement and the relation- about the location of spawnins grounds ship among fisheries. information not and seasonal occurrence of spawning for easily gotten by any other means. Al- most billfishes. to formulate testable though the number of billfish that could hypotheses about their stock structure. be released from most fisheries is small, Because of the large areas involved. such and indeed probably smaller than desirable experimentation would benefit from inter- for estimating such parameters as fishing national collaboration among national mortality, growth rate, and rate of exchange fisheries agencies and universities. between fisheries, qualitative information 5. Seiected genetic studies using mtDNA or on movement is still useful. Returning vi- electrophoresis should be continued but able billfish to the sea without tagging not on a large scale. I suggest that selecting them, or retaining all nontrophy or nonpoint- species such as sailfish, generally believed contributing billfish for tournaments, is, to consist of multiple stocks, would be in effect. throwing away a valuable re- more productive than choosing a species search opportunity. Support of tagging thought to consist of one panmictic stock. operations is particularly amenable to Also, regarding mtDNA analysis. the de- cooperative arrangements among national velopment of standard tissue collection fishery agencies and recreational interests and preservation procedures for situations and. indeed. could provide the basis for where the samples cannot be immediately further cooperation. processed would benefit billfish research. 1. More effort should be expended on shape analysis. especially the newer truss and planar shape methods. but possibly on Summary traditional morphometric and meristic The identification of separate biological units methods as well. Although the newer or stocks is required for determining the status methods require or would benefit from ac- of the resources and for conserving them via cess to microcomputer-based image fisheries management. A number of biologically processing equipment and some based methods for separating stocks are availa- 5pecialized computer software (though not ble: using such methods on billfishes is likely very sophisticated or costly). the tradi- to be more productive than using population tional methods can be used by small genetic methods. The stock structures of bill- fisheries offices with measuring devices. fishes are. in general. poorly known. The largest a microcomputer. and standard statistical billfishes. blue and black . appear to con- \oftware. Such studies would benefit from sist of single ocean-wide stocks. while there is intergovernmental cooperation and. of some evidence for east and west or north and course. col Inboration with fishermen. south stocks of the smaller striped marlin and tournament organizers. and fish proces- swordfish. Evidence tor multiple stocks for the wrs. smallest species (spearfish and sailfish) is 3. Studies zhould he conducted to estimate stronger. STOCK IDENTIFICATION & MANAGEMENT 213

Literature Cited IATTC. 1978 Annual repon ofthe Inter-Amencan Tropical Tuna Commission. 1977 La Jolla. Calif.. 155 P.. IIn Allendorf. F.W.. N. Ryman. and F.M. Uner. 1987. Genetics Enel. and Span. I and fishery management. Past. present. and future. In ICCAT. 1987. Repon tor biennial penod. 1986-87. Pt I N. Ryman. and F. Utter leds.). Population genetics (1986). Int. Comm. Consew.. Atl. Tunas. Madnd. and fishery management. p. 1-19. Washington Sea Spain, 310 p. Grant Prog.. Univ. Washington Press. Seattle. Ihssen. P.E.. H.E. Booke. J.M. Casselman. J.M. Argue. A.W.. P. Kleiber. R.E. Kearney. and J.R. Siben. Mffi1ade.N.R. Payne. and F.M. Utter. 1981. Stocks 1986. Evaluation of methods used by the South Pacific identification: Matenals and methods. Can. J. Fish. Commission for identification of skipjack population Aquat. Sci. 38:1838-1855. structure. In P.E.K. Symons. P.M. Miyake. and G.T. Kutkuhn. J.H. 1981. Stock definition as a necessary basis Sakagawa (eds.). 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NOAATsch Memo. NMFS. SOAA- 13101 iHerl 70 315.31~ Thl-NhlFS-SWFC-5. 58 p Hilhorn. K.. and J. Sihen. IYXX I\ iniernaiionnl manugement Shomura. R S . and F Williamb Id.) lY7J Proceedings $11 tun2 nucehwrv ' Ilar. I'~ilic> 121 I ):3 1-39 (11 the lniernaiional Billfish Svmposium. Kailua-lima. 2 14 PLANNING THE FUTURE OF BILLFISHES

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