Reviews in Fish Biology and Fisheries 13: 151163, 2003. 2004 Kluwer Academic Publishers. Printed in the Netherlands. 151

Status of research in stock enhancement and sea ranching

IC. Liao, M.S. Su & E.M. LeaoTaiwan Fisheries Research Institute, 199 Hou-Ih Road, Keelung 202, Taiwan

Contents

Introduction page 151Historical overview 151Status of research 152

Improved techniques for mass production of seedstock and juveniles for releaseRelease strategyProduction of pathogen-free stock and health managementEfficient tagging methodsEcological and genetic studiesMathematical modelingSocio-economic studies

Conclusion 160References 160

Introduction

The growing human population and increasing fooddemand have put pressure on coastal environmentsand fisheries resources. Overfishing and destruc-tion of coastal habitats have resulted in depletion ofcoastal resources, and have threatened the viabilityof the fishing industry. Efforts are being made toprotect the coastal habitats, to restore the popula-tions of aquatic organisms and to create new fishingopportunities. Fisheries management that has beenin place for a fairly long time requires reorienta-tion to meet new challenges (Liao, 1999; Chua,2001). In recent years, stock enhancement and searanching have been recognized for their potential ofincreasing and sustaining coastal fisheries (Oshima,1984; Bartley, 1999; Liao, 1999, 2002). Stockinghas been attempted at some level in over 25 countriesworldwide (Bartley, 1999) with more than 100 speciesof aquatic organisms (fish, crustaceans, molluscs andother invertebrates) (Liao, 1999; Fushimi, 2001).Varying results have been obtained and problemsidentified. A well-planned enhancement or ranchingproject will aspire either to recapturing releases orto promoting inter-breeding between released and

wild populations so that an enhanced self-sustainingpopulation is produced (Cross, 1999). Because manymarine enhancement and ranching projects worldwideare still in the experimental or pilot stages (Bartley,1999) sufficient data is not available to answer agrowing number of questions that are being raised.The purpose of this chapter is to provide a reviewof the status of research and developments on stockenhancement and sea ranching of aquatic animals.

Historical overview

Stock enhancement and sea ranching activities can betraced back more than a century (Liao, 1997a), butsignificant effects were not recognized because littleemphasis was placed on understanding the impact ofstocking on fisheries landings (Leber, 1999). Earlyrecords of stock enhancement activities include therelease of shad (Alosa sapidissima) fry bred from ahatchery in New England in 1867 (Stickney, 1996) andchum salmon (Oncorrhynchus keta) in Japan in 1876(Oshima, 1993). In 18841985, stock enhancementprograms for cod were initiated in USA and Norwaythat lasted for 70 and 90 years, respectively (Liao,1999). However, the approach during this early period

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can be characterized as the production phase whereemphasis and accountability in stocking programswere focused on production and release magnitude,and subsequently, as the denial phase when fisheriesbiologists learned to reject marine stock enhancementdue to lack of evidence of any positive impact (Leber,1999). Apparently, the science of marine stockenhancement began only more than a decade ago whencritical questions about stock enhancement begun toemerge as testable hypotheses (Leber, 1999).

The successful stock enhancement and searanching programs of some countries including Japan(Kitada, 1999, 2001; Inuki, 2002), Taiwan (Liao,1999; Su and Liao, 1999), Norway (Moksness, 2002)and USA (Leber, 2002), have shown the impor-tance of these activities in replenishment of depletingstocks for commercial fisheries and for game fishing.Enhancement programs are undertaken mainly for thepurpose of: mitigation to compensate for somedecrease in a fishery due to another type of devel-opment activity; augmentation to make the fisherymore productive by increasing the number of fishpresent; and, community change involving the useof exotic or introduced species not normally a part ofthe local fish community (Bartley, 1999). However,Lenanton et al. (1999) reported that the inherentdanger of restocking the depleted resources, in theevent that it is successful and catch rates of the targetspecies improve, is the desire to commit the resourcesover the longer term and the subtle insidious changesthat may have contributed to the initial decline willremain unchecked. In time, this situation will causemore serious problems that could be avoided if treatedearlier. It was therefore suggested that stock enhance-ment or sea ranching should not be seen as a substitutefor the long-term conservation and management ofvaluable aquatic resources.

Status of research

For stock enhancement and sea ranching to besuccessful, the biology and ecology of target animalsmust be thoroughly understood from productionof seed stocks through environmental requirements,carrying capacity of the habitat, and all factors thatcontribute to mortality, to monitoring and assessingthe efficiency of release (Rothlisberg et al., 1999;Fushimi, 2001). Undoubtedly, stock enhancementand sea ranching are applied sciences but variousfundamental studies are required for their implementa-

tion. Liao (1999) has summarized such basic studies(Table 1). Supporting roles of managers, concernedorganizations and the fish farming community are alsoneeded for effective and responsible restocking (Bell,2002). Some of these roles should be: independentassessment of the effects of releases on stock levelsand the ecosystem; appropriate institutional arrange-ments including co-management where resource userssupport the release of juveniles and assume responsi-bility for management; compliance with fishing regu-lations; limiting effort so that any new entrants donot disadvantage existing participants; measures toincrease carrying capacity (for example, addition ofartificial habitat, removal of predators); and, modifica-tion of regulations in response to demonstrated waysto improve production and sustainability.

The varying levels of success in stock enhance-ment programs around the world have resulted invarious research concerns. Scientific informationgenerated from stock enhancement has emphasizedthe importance of physiological, morphological, andecological attributes of the hatchery-reared juvenilesin effectiveness of the release operations (Fushimi,2001). Consequently, the current research thrust is on:improved techniques for mass production of seedstockand juveniles for release; release strategy; produc-tion of pathogen-free stocks and health management;efficient tagging methods including genetic markers;ecological observations on carrying capacity and itseffects on success rate; mathematical modeling; andsocio-economic analysis.

Improved techniques for mass production of seedstockand juveniles for release

The success of stock enhancement and sea ranchingprograms will depend on technological innovationsin the mass production of seedstocks and juve-niles for release. In the history of aquacultureresearch, artificial propagation of finfish and shell-fish (including broodstock selection and maintenance,induced spawning, and larval rearing) has resulted inestablishment of technologies for large-scale produc-tion of stocking material. Although this was donemainly for aquaculture, the technologies are usedfor stock enhancement and sea ranching programsaround the world. In China, reseeding of fleshy prawn(Fenneropenaeus chinensis) is a by-product of theaquaculture industry, where a surplus of post-larvaeare produced and reared at a little marginal costto pre-supplementary feeding stage for subsequent

153Table 1. Fundamental studies on stock enhancement and sea ranching (Liao, 1999)

Coastal fisheries survey (1) Collection of coastal fisheries statistics(2) Investigation of coastal environment

(a) Primary productivity(b) Fishing ground survey

Life history of released species (1) Distribution(2) Sex ratio(3) Fecundity(4) Biological minimum size(5) Feeding habits(6) Recruitment(7) Growth(8) Life span

Mass production techniques of relatedspecies

(1) Broodstock cultivation(2) Larval rearing(3) Facility automation for production

(a) Healthy larvae(b) Live food(c) Formulated feeds

Methods of live transport of released species Short, medium and long periods

Nursery techniques of released species (1) Optimum size(2) Optimum season(3) Optimum density(4) Suitable feeds(5) Predator control

Environmental improvement for releasingareas

(1) Artificial reefs(2) Protective zones(3) Fish paths and fish ladders

(a) Structures and shapes(b) Materials(c) Casting methods(d) Algae grounds(e) Eel grass zones(f) Substrates(g) Predators

Release and recapture investigation (1) Biodiversity(2) Assessment methods

(a) Population genetics(b) Tagging experiments(c) Catch data

Management and regulation (1) Prohibited fishing periods(2) Prohibited fishing zones(3) Prohibited fishing gears and methods(4) Defined penalty

Social matters (1) Dissemination of stock enhancement and sea ranching(2) Education of professional fishermen and anglers(3) Establishment of associations and centers

International cooperation (1) Exchanges of techniques and information(2) Migratory fish resource enhancement and profit sharing

release (Rothlisberg et al., 1999). In South Africa, thedevelopment of an abalone (Haliotis midae) farmingmethod prompted the release of hatchery-producedjuveniles in an area beyond the natural range ofthe species in an attempt to assess the potential forranching and enhancing stocks of this species (Cookand Sweijd, 1999).

Technologies developed in Taiwan for larvicultureof finfish (Liao, 1993, 1996; Liao et al., 2001) andcrustaceans (Liao, 1985) also led to many enhance-

ment programs (Su, 1988; Liu and Su, 1993; Liaoet al., 1994; Liao, 1997b; Liao, 1999; Su andLiao, 1999). The successful sea ranching programs inJapan for several species of fish, mollusk and crus-tacean, including scallop (Patinopecten yessoensis),kuruma prawn (Marsupenaeus japonicus), swimmingcrab (Portunus trituberculatus), abalone (Nordotissp.), red sea bream (Pagrus major), black sea bream(Acanthopagrus schlegeli), salmon (Oncorhynchusspp.) and Japanese flounder (Paralichthys olivaceus),

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have resulted from development of hatchery techno-logies for mass production of larvae and juvenilesof these species (Kaeriyama, 1999; Kitada, 1999;Fushimi, 2001). In Taiwan, Japanese eel (Anguillajaponica) was introduced to increase production (Liaoet al., 1994; Liao, 2001). Supply of elvers or glass eels,however, still depends on the wild that is continuouslydecreasing and is unstable. Investigations are beingundertaken for developing suitable techniques amongeel producing countries, particularly Japan and Taiwan(Tanaka, 1999; Liao and Chang, 1999; Tanaka et al.,2000; Liao and Chang, 2001).

Techniques of artificial propagation and larvalrearing of donkey-ear abalone (Haliotis asinina),window-pane shell (Placuna placenta), top shell(Trochus niloticus), sea urchin (Tripnuestes gratilla),and seahorse (Hippocampus spp.) are being developedin the Philippines for stock-enhancement purposes(Junio-Meez et al., 1998, 2002; Madrones-Ladjaand dela Pea, 2000; SEAFDEC Asian Aquaculture,2001). Follow-up studies are being made on rearingof larvae and juveniles of giant clam (Tridacna spp.)in many countries (Mingoa-Licuanan and Gomez,1996; Bell, 1999). In Norway, hatchery productionof European lobster (Homarus gammarus) has pavedthe way for experimental enhancement of this species(Scovarrichi, 1999; Agnalt et al., 1999). Mass produc-tion of hatchlings of giant cuttlefish (Sepia latim-anus) in Japan was instrumental in stocking operationsbetween 1992 and 1999. This is the first enhancementactivity to demonstrate stocking effect in a cephalopodspecies (Oka et al., 2002). Sea urchin aquaculturein Japan is part of a multi-species fisheries enhance-ment effort organized by local fishery cooperatives(Hagen, 1996). Under this program, the methods ofhatchery production and nursery rearing of the greensea urchin (Strongylocentrotus intermedius) juvenilesfor both enhancement and closed cycle cultivationwere developed to meet the increasing market demand.Stock enhancement of sea cucumber (Holuthuriascabra) is now being assessed in many countriesincluding Ecuador, India, Kiribati, Maldives, theMarshall islands and the Solomon islands (Battagleneand Bell, 1999).

Studies on mass production of other candidatespecies for sea ranching are on-going with greater zealand zest.

Release strategy

One of the factors affecting the success of any searanching program is the size of the animals to bereleased. Many studies on stock enhancement and searanching have reported the performance of aquaticorganisms released in the sea at different sizes. Roth-lisberg et al. (1999) reported that the benefits ofreseeding different sizes of penaeid shrimp are largelydetermined by the cost of producing their seed and thedifference in survival rates during rearing in captivityand in the wild. Considering the mortality rates ofdifferent stages of shrimp in the wild, it can bepredicted that in a cohort of one million zoea restockedin the wild, only 100 adults will survive. If the samecohort is kept in ponds and released in the wildas 10 mm CL (carapace length) juveniles, survivalwill increase a thousand-fold and 100,000 adults willbe obtained. If this cohort is released even later as20 mm CL sub-adult, 200,000 adults will be obtained.However, the balance between increased survival andincreased cost of producing stocks for release willdetermine the overall benefit of stock enhancement ata particular stage. This example can also apply to otheraquatic animals selected for enhancement or ranching.

In an investigation conducted by Kristiansen(1999) on Atlantic cod (Gadus morhua), small-sized specimens measuring 1920 cm released duringwinter were observed to have low survival comparedto larger ones (2030 cm) released in summer. Atag-releasere-capture study was carried out to eval-uate size-at-release impact on recruitment of culturedstriped mullet (Mugil cephalus) juveniles in Hawaii(Leber, 1995). Five size groups (45 to 120 mmlength) were used and results showed that recapturefrequencies were clearly skewed in favor of largersize at release after nine weeks. Smaller fish (lessthan 70 mm) when released were rare or absent after18 weeks. These results confirmed that fish size-at-release has a major impact on the success of hatcheryreleases in marine habitats. The idea of releasingjuvenile fish, instead of larvae or fingerling, is toprotect the fish during their vulnerable early stagewhen mortality in nature is expected to be high,and release them at a larger size when their survivalchances have significantly improved (Ottera et al.,1999). Fitness of reared individuals for release in thewild is also one of the key elements in a successfulstocking program (Svsand, 2002). Yamamoto andMorioka (2002) noticed that a change in the rearingcondition (for example, low rearing density, reduced

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light density, and modified feeding frequency) led toimprovement in seedling behavior of red sea bream(Pagrus major) within a few days, and to biochemicalbody composition within 1020 days. Physiological,morphological and behavioral conditioning as part ofa pre-release strategy are also crucial in survival ofranched marine fish (Mahnken et al., 2002). Condi-tioning could include manipulation of diet, stockingdensity and growth rate, use of enriched rearingenvironment, predator recognition training and foragetraining.

In striped jack (Pseudocaranx dentex), acclimationfor 2 days prior to release and provision of decoys(similar size as the fish in cage) prevented loss offish within 10 days following release (Kuwada et al.,2002). Reducing handling stress that prevents loss ofstamina also improved survival of the seeded stock.Worth mentioning in this connection is a seedingdevice developed for transplantation of abalone juve-niles, which minimized handling stress and protectedabalone from predation during and after placement onthe ocean floor (McCormick et al., 1994). Protectionduring the initial hours following placement allows theabalone to recover from handling and transport stressand become acclimated to the marine environment.McComick et al. (1994) have suggested that develop-ment and improvement in planting should be incorpo-rated into additional studies on the cost-effectivenessof ranching.

Production of pathogen-free stock and healthmanagement

Intensification of aquaculture operations has resultedin emergence of disease problems that have causeddevastating effects on production. Degradation ofenvironment and proliferation of pathogenic microor-ganisms are the main causes. Because some of theinfectious diseases of aquatic animals are verticallytransmitted from parents to offspring, the possi-bility of mass production of infected stocks is amatter of concern. Many viral and bacterial diseasesof finfish and other aquatic organisms are identifi-able, and molecular diagnostic procedures are nowavailable to detect these infections in broodstock aswell as seedstock. Efforts have also been under-taken to produce specific pathogen-free (SPF) stocksfor aquaculture and ranching. Moreover, the impor-tance of increasing disease resistance through theuse of vaccines and immunostimulants, and diseaseprevention by hygienic practices and proper water

management during the hatchery and nursery phasesof production, are increasingly emphasized.

Health management of hatchery stocks is recog-nized as high priority research for future sea ranchingprograms. Specific areas of urgent attention are prac-tically feasible measures of disease control in seedproduction phase, evaluation of health standard andwell-being of broodstock, and presence of pathogensin wild stocks. In Japan, technological developmentof seed production has shifted its focus from quantityto quality, since health and adaptability of hatchery-reared seed are the most important factors determiningthe recovery rate (Fushimi, 2001).

Research studies on the production of SPF stocksof fish and crustacean have been carried out with theaim of controlling disease problems in aquacultureoperations and preventing the transmission of infec-tious diseases among wild stock. In USA, produc-tion of SPF white leg shrimp (Litopenaeus vannamei)broodstocks was highly successful, resulting in thecommercial production of healthy post-larvae forgrow-out (Wyban et al., 1993). Trials conductedso far have shown that shrimp derived from SPFbroodstock dramatically outperformed those thatcarried pathogens. In Japan, selection of virus-freespawners of marine fish (for viral nervous necrosis,VNN) and penaeid shrimp (for white spot syndromevirus, WSSV) together with disinfection of eggs hasbeen used with good effects in hatchery produc-tion (Mushiake, 2002). Hygienic practices includingdisinfection of live diets are also recommended toprevent bacterial diseases. When wild populations ofshrimp are infected with pathogen and the infectionis transmitted to the larvae, the disease multiplies andcauses mortality in released stock. Specimens that areresistant to diseases (either with innate or acquiredimmunity) perform better and give a higher yield.Development of vaccines against major infectiousdiseases and the use of immunostimulants to enhancenon-specific immune response of aquatic organismswill, therefore, contribute significantly to the resist-ance of hatchery bred SPF as well as mass-producedseedstock to infectious diseases once released intothe wild. The example of vaccination of sea-ranchedAtlantic salmon (Salmo salar) using a polyvalentvaccine that resulted in a significant increase in recap-ture rate (25% in injected group) compared to bathvaccinated fish (14.7%) and control (non-vaccinated)group (16.8%) (Buchmann et al., 2001) suggests theimportance of vaccination in a mass seed releaseprogram.

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Efficient tagging methods

To assess the effectiveness of stock enhancement andsea ranching programs, it is necessary to identifythe released stock from the wild. Tags and markersare, therefore, indispensable for evaluating stockingeffectiveness (Kitada, 1999). Tagging studies or theappearance of natural tags have provided some indi-cation of the contribution of hatchery fish to fishery(Bartley, 1999).

A wide range of physical (artificial) tags hasbeen used in different aquatic organisms selected forranching. Coded wire tags, anchor tags, and fluore-scent dyes are among the important ones. Recentinnovations in physical tags include acoustic tags,magnetic microtags, and metal tags. In one study, thered drum (Sciaenops ocellatus) was implanted withcoded acoustic tag to assess the feasibility of enhance-ment. Tracking this fish provided better means oflocating the schools, and information on dispersal,habitat preferences, and release site effect. The obser-vations also enhanced our ability to monitor growth, toestimate their stock strength, to determine their long-term survival and to estimate the rate of recovery.Using acoustic tag, Neidig et al. (2002) reported73% recovery in red drum. In Norway, juveniles ofEuropean lobster were tagged with magnetic binary-coded microtag injected into the muscle (Agnalt etal., 1999) and some 125,000 specimens were releasedduring 19901994. Different binary codes were usedto separate groups from different hatching years,release times and localities. Results obtained from thisstudy showed that enhancement of depleted lobsterpopulation was feasible with as high as 8% recoveryrate for a single year-class. However, the tag loss washigh in some release groups. A tag in the form ofinternal visible implant elastomeres (VIE) was alsotested in red snapper (Lutjanus campechamus) byinjecting the implants (of different fluorescent colors)under the skin of the nose bridge or between the skinlayers of the caudal fin. Tag retention was promi-sing 100% after 6 weeks and 98% after 6 months(Brennan et al., 2002a). VIE implanted in the caudalfin using red and orange fluorescent colors was morevisible among tagged snappers. In an experimentalrelease aimed at comparing the performance of VIE-and T-bar-tag in common snook (Centropomus unde-cimalus), it was noticed that recapture and growth rateof VIE tagged snook were significantly higher than T-bar-tagged fish at four and eight months post-release(Brennan et al., 2002b). If used in different colors and

body locations, VIE can help in identifying varioustreatments in situ.

Since the loss of physical tags is inevitableonce the tagged organisms are released in the wild,the use of biological tags is a better alternative.Biological tags include some visible characters andmolecular markers. Natural tags are found in someaquatic organisms. In Japanese flounder the appear-ance of permanent black pigment on the blind sideof hatchery-reared seedstock is effectively used as anatural tag for stock assessment studies (Yamashitaand Yamada, 1999). The fused nares of hatchery-reared red sea bream are also used as a natural tag(Kitada, 1999). In the Philippines, a bluish-greenband on the shell of donkey-ear abalone and reddishbanding pattern on shell of top-shell maintained ona formulated diet are regarded as biological tags forstock enhancement use (Gallardo et al., 2002; Platonand Yap, 2002; Caron and Marte, 2002). In kurumaprawn, a tagging method was developed by tail-cutting(Miyajima and Toyota, 2002). In this method, whole(for specimens of > 30 mm body length) or lateralhalf (for specimens of > 70 mm body length) ofthe ramus (tail fans) was cut and allowed to regene-rate. Regenerated ramus had less pigmentation thatwas visually recognizable, and retention rate was 91100%. These marking techniques were applied to alarge field sample and the recovery rate of releasedprawn was estimated to be 18%. Mass marking ofsockeye (Oncorhynchus nerka) and chinook salmon(O. tshawytscha) using thermal marking of otolithhas been done in Russia since 1995 (Kudzina andChebanov, 2002). The otolith marks were inducedduring the eye pigmentation stage in sockeye salmonand in passive phase (chinook salmon) under thetemperate thermal regime in seasons characterizedwith water temperature decrease.

The advent of molecular techniques in popula-tion genetics of aquatic animals has resulted inthe development of different molecular markers forstock enhancement and sea ranching studies. Geneticmarking makes use of specific genes that distinguisha released fish from the native stock. The use ofthese genetic variants (marks) requires selection of arare DNA variant from the natural populations and abreeding program to produce the required number ofindividuals for reseeding (Rothlisberg et al., 1999).The genetic marks remain intact throughout life andin the offspring (Nvdal, 1994). The most usefulmolecular techniques currently available involve mini-and microsatellite DNA, mitochondrial DNA and

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allozyme electrophoresis. Nuclear DNA assay canbe performed using small tissue samples and ampli-fying the DNA by the help of polymerase chainreaction. Protein (allozyme) electrophoresis is lessexpensive than DNA analysis and is still a widely usedmolecular tool in fisheries biology because of its ease,power and cost-effectiveness (Carvalho and Hauser,1995). Allozymes, as genetic markers, provide mostof the information commonly obtained using artificialand other biological marking methods (i.e., growth,movement, survival, population estimation, and stockcontribution) and have additional advantages (King etal., 1993).

Mitochondrial DNA data is more sensitive thanthe nuclear DNA analysis but it is mainly used fordetecting maternal inheritance. A molecular markerrecently used for marking of penaeid shrimp is basedon amplified fragment length polymorphism.

The above molecular markers are applied forassessing the success of many stock enhancementand sea ranching activities and in evaluating thegenetic changes, including the possible loss of geneticvariability in both hatchery bred and wild stocks.However, deciding on the appropriate technology toanswer any particular question may not always bestraightforward (Ward and Grewe, 1995). The choiceof a particular molecular tool should not be motivatedonly by its novelty, complexity or a quest to uncoveran ever-increasing level of diversity, but by the need toaddress stated management objectives (Carvalho andHauser, 1995). Despite tediousness of the techniquesand some impediments, the genetic marking remainsa powerful tool for determining the fate of releasedanimals and their interaction with the wild stocks overgenerations (Nvdal, 1994).

The application of genetic markers in screeningwill depend on what is available for the species ofinterest. In Norway, a genetically marked strain of cod,based on a rare allele in the white muscle tissue (GPI-1(30)), was developed and offspring from this strainwere used in some of the large-scale releases (Jrstad,2002). In Japan, mitochondrial DNA of Japaneseflounder which is characterized by extremely highsequence variability in the control region, has beenused to assess the genetic variability of the hatchery-produced stocks as well as the migration of releasedflounder (Fujii, 2002).

Ecological and genetic studies

Marine resources management, including stocking,is often underpinned by largely untested ecologicalassumptions or hypotheses (Miller and Walters, 2002).Environmental concern about the effect of releasedhatchery bred stocks on the aquatic ecosystem, espe-cially on the genetic diversity of wild populations,is currently an important factor being consideredin sea ranching. It is a widely accepted theoret-ical or conceptual argument that the direct effects ofhatchery fish on wild population could be substan-tial and potentially detrimental, but the empiricaldata supporting such views are either lacking orlargely circumstantial (Campton, 1995). Kitada (1999)reported that although genetic differences betweenhatchery-produced fish and wild populations havebeen investigated, genetic effects of released fish onthe wild stocks have not been properly documented.Nonetheless, genetic factors are important and deserveserious consideration in producing and monitoringthe aquatic organisms for stock enhancement and searanching. The genetic identification and discrimina-tion of aquaculture stocks is a fundamental require-ment in any culture program including the productionof fish/shellfish for rehabilitation of natural population(Ferguson, 1995).

Managing hatcheries for genetic conservationrequires that natural reproduction in the wild and arti-ficial propagation in the hatchery be integrated as onegene pool (Campton, 2002). Such integrated hatcheryprograms should: (1) maintain genetic continuitybetween hatchery and wild components of a singlegenetic population, (2) serve as demographic buffersagainst future natural population declines, (3) main-tain existing levels of genetic diversity by maximizingeffective population sizes, (4) potentially increase thenumber of naturally-reproducing animals, and (5) usehatcheries as genetic repositories for future resto-ration of naturally spawning population (Campton,2002). However, because of the high fecundity of mostmarine fish, there has been a tendency to perform arti-ficial propagation using a small number of breeders inthe hatchery, and this has resulted, to some degree, inthe loss of genetic variability (Taniguchi, 2002). Manyrecent publications have emphasized the need to main-tain genetic diversity through broodstock manage-ment. Improvement of egg quality and fertilizationrate is also important in enhancing genetic variabilityin the cultured stocks (Yamashita and Yamada, 1999).

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When sea ranching programs are considered, astudy of natural sub-specific structure of the targetspecies of animal should be undertaken (Cross, 1999),and genetic degradation on account of inbreeding,founder effect and genetic drift be minimized. Reddrum is an important example that deserves mentionhere. Long-term genetic studies on this species inTexas, USA have revealed that significant hetero-geneity occurs among geographic samples, obvi-ously due to the existence of genetically definedstocks (Gold, 2002). The distribution of genetic vari-ation in red drum, however, follows an isolation-by-distance model, where substantial gene flow betweengeographically proximate localities is inferred tooccur. This suggests that mixing adult broodstockfrom different but geographically proximate estuariescan be considered in sea ranching of this species. InAustralia, tiger prawn (Penaeus monodon) popula-tion from the west coast was found to differ geneti-cally from the east coast population using mito-chondrial DNA analysis and allozyme electrophoresis(Benzie et al., 1993). Examination of parentage andgenetic diversity of different batches of the giant clam,Tridacna gigas also gave interesting results (Benzieand Williams, 1996). The data indicated that geneticdiversity was lower and the gene frequencies markedlyskewed in all the hatchery-produced batches relativeto those in the wild populations from which theywere derived. Genetic differences between successiveyear classes of two farmed rainbow trout (Onco-rhynchus mykiss) in Ireland, were observed usingenzyme electrophoresis (Butler and Cross, 1996). Theheterogeneity between the cohorts was attributed toeither broodstock maintenance practices or to bottle-neck effect. Slight genetic differentiation was noticedamong samples of Japanese eel from Taiwan andChina using microsatelline DNA analysis (Tseng etal., 2001).

Another area of concern in sea ranching is theecological impact on natural environment. Assess-ment of carrying capacity of the habitats to whichhatchery bred stocks are to be released, including thepresence of potential predators deserves due impor-tance. Reseeding will not be successful if the naturalproductivity of the rearing habitat cannot sustain thehigher density of released larvae or juveniles (Rothlis-berg et al., 1999). Studies conducted by Salvanes etal. (1992) that focused on carrying capacity for cod inNorwegian fjord have demonstrated that the primaryproductivity of the ecosystem was driven by solarradiation, temperature, freshwater runoff, and deep

water convection. Considering these factors as of vitalsignificance in carrying capacity of the area, authorsproposed a new method for obtaining estimates ofthe half-saturation parameter and derived two mainimplications of extensive cod farming: (1) possibilityof obtaining optimal cod production if the sum ofreleased and wild recruits is within the range of thecarrying capacity of juveniles, and (2) distinct fluc-tuation in cod production despite release of the samenumber of juveniles in several years related to inter-annual variations in the magnitude of advection thatprofoundly influences zooplankton production. Thedynamic ecosystem model (DYNECOMAS II) forfjord cod population in Norway (Salvanes and Balio,1998) shows that the key factors in carrying capacityfor cod in the fjord are: availability of zooplankton,density-dependent predation and cannibalism.

Mathematical modeling

The evaluation of effectiveness of seedling release oftarget species is a main consideration in sea ranching.Stocking effectiveness depends directly on survivalrate of the released juveniles. Marking and taggingexperiments, as described above, have been widelyundertaken to estimate the post-release survival rates.The mathematical models for estimating rates ofinstantaneous natural mortality and fishing mortalitybased on tag recoveries have been developed sincethe 1950s (Gulland, 1955; Paulik, 1963; Farebrother,1985; Hearn et al., 1987; Farebrother, 1988). Unfortu-nately, the problems of over-dispersion and correlationhave not been treated in these models. Kitada et al.(1994) proposed a statistical method that involvedconsideration of these problems. In addition, estima-tions based on fishery independent samples withdifferent intervals and catching efforts (Kitada et al.,1992) and on the data pertaining to sighting surveywere also included (Kitada, 1997).

To obtain an unbiased estimation of the releasedfish landed, a mathematical model using a two-stagesampling method has been proposed based on selec-tion of markets as the primary sampling unit andthe selection of survey dates as the secondary unit(Kitada et al., 1992). Estimators formulated with theirrespective variances were the total number of releasedfish landed, the sum of the total number of releasedfish and wild fish landed, the ratio of the released fishto the number of landings, the return rate, and thetotal amount recovered (Kitada et al., 1992; Kitada,1999).

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Because mathematical modeling is one of themethods to efficiently assess the effectiveness of stockenhancement or sea ranching, continued research isneeded in this field. To date, very few mathematicalmodels are available for use. Kitada et al. (1992)suggested a model for the estimation and evaluation ofprecision of stock enhancement program with regardto total landings and the ratio of marked fish inthose landings. Kitada (1999) also proposed a modelfor quantitative assessment of stocking effectivenessthrough direct estimation of released fish landed andevaluation of post-release survival rates. A mathema-tical model based on Atlantic cod enhancement experi-ment was formulated by Giske and Salvanes (1999) toexplore the potential for increasing local population bymass release of juveniles. With the growing interestin sea ranching research for a better understandingof the environmental, ecological and socio-economiceffects, the biological assessment methods shouldgo hand-in-hand with the mathematical assessmentmethods or models in order to fully understand howeffective such activities are in enhancing the continu-ously depleting populations of commercially exploitedmarine animals.

Socio-economic studies

In the early history of sea ranching, the lackof evidence of any positive environmental impactsresulted in aborted programs, and the efforts wereconsidered a waste of money. In recent programs,however, management strategies include not onlyimproved biological systems but also a better manage-ment of the investments. Managing investmentessentially involves management of both social andeconomic systems (Hallenstvedt, 1999). Thus, socio-economic studies have become an integral part ofstock enhancement and sea ranching aimed at tacklinglegal, and social, as well as financial problems. Socio-economic uncertainties and requirements that mustbe handled in the management of enhanced stocksinclude: the relationship between investment andthe economic return; the transformation of scientificknowledge into economic enterprise; institutions tohandle questions of access and property rights; and,in some models of sea ranching, the complex issue ofdistribution (Hallenstvedt, 1999).

Stock enhancement and sea ranching have raiseddifficult issues concerning rights of ownership overranched fish and the legality of the methods used tosecure recapture (Howarth and Lera, 1999). It has

been suggested that the cornerstone of an appropriatelegal regime for ranching is an effective mechanismto prevent or restrict ranching in circumstances whereit is likely to have an unacceptable impact uponexisting fisheries or the aquatic environment. Carefuldecisions also need to be made, in consultation withresource users, about whether these interventions arelikely to improve the existing management regime forthe fishery (Bell, 2002). There is also a need for anew paradigm in fishery management concept and fordeveloping new methodologies by strengthening thesector on fishery management within a broader andintegrated management framework (Chua, 2001). Thechallenge in most projects and programs practicingintegrated management lies in its ability to achievepolicy and functional integration especially at thelocal level where implementation is expected to occur(Chua, 1993). For restocking, the imperative is toensure that the released individuals, and their progeny,are protected until replenishment occurs. This willinvolve a moratorium on fishing, use of closed areas,and maybe even translocations of the progeny ofsedentary species with limited larval dispersal. Forstock enhancement, measures must be taken to safe-guard the juveniles and regulate a size at capture thatmaximizes economic return (Bell, 2002).

In New Zealand, the Southern Scallop Fisheryhas developed a management framework underpin-ning the sustainable utilization of this fishery that ischaracterized by a successful enhancement program(Drummond, 2002). The program comprised captureof scallop (Pecten novaezelanidae) spat in dedic-ated collector bags suspended from a rope back-bone, and direct seeding onto the seafloor. Oncescallop are reseeded onto the beds, they becomepart of the wild resources that are managed underthe Fisheries Act 1996 which establishes a broadframework for managing the harvest of all fisheriesresources.

The economic feasibility and profitability ofreleasing the stocks are of great importance in successof any ranching program. Ranching in Japan, Taiwanand China have significantly increased the harvestof shrimp and fish species, consequently resulting inhigher profit for the fishers. However, few studies oncost-benefit analysis of stock enhancement and searanching have been undertaken because of lack ofsufficient data on the total cost of production, and forthe fact that seedstocks are released in open water.Moksness and Stle (1997) reported that the profitabi-lity of ranching marine fish heavily depends on cost

160

of producing juveniles, return rate and market price ofthe recaptured fish.

In Taiwan, a cost-benefit analysis on stock-enhancement of black sea bream showed that stockingis likely to be a cost-effective management techniquefor recreational fishery (Liao and Liao, 2002). Basedon a realistic analysis, less than 1% recapture rateis needed to cover the cost of sea bream stockingprogram. The same result (< 1% recapture rate) wasobtained by Russel and Rimmer (2001, 2002) usingcost-benefit analysis on barramundi (Lates calcarifer)for the recovery of direct cost of stocking. In the caseof lobster, using the simulation model LOBST.ECO,it has been shown that a recapture rate above 23%is needed to make private sea ranching profitable(Borthen et al., 1999). For abalone (Haliotis midae),recapture rates of greater than 10 to 15% are desirablefor a modest profit (Cook and Sweijd, 1999).

Conclusion

The science of stock enhancement and sea ranchingis new. Sea ranching has benefited a great dealfrom developments in hatchery production of aquacul-ture animals, appropriate release strategies, taggingand marking, and understanding of the ecosystem.However, the many problems that still remain tobe resolved suggest the need for more intensifiedand focused research leading to what Blakenshipand Leber (1995) have defined for a responsibleapproach to sea ranching. Side by side with biolo-gical and environmental research, legal and socio-economic aspects, education, training and outreachshould receive more emphasis. There is a dire needfor developing a comprehensive and integrated searanching framework in countries where it does notcurrently exist, and for review of the contemporarypractices where it exists.

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