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L,Ife History Strategies, Management of the Bonne Bay Fjord Atlantic

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L,Ife History Strategies, Management of the Bonne Bay Fjord Atlantic High Latitude Crabs: Biology, Management, and Economics Alaska Sea Grant College Program ~ AK-SG-96-02, 1996 L,ife History Strategies, Recruitment Fluctuations, and Management of the Bonne Bay Fjord Atlantic Snow Crab Chionoecetes opilio! Cerard Y. Conan Marine Biology Research Center, Dept. of Fisheries and Oceans Canada Universite de Moncton, NB, Canada Michel Starr Ministere des peches et oceans, Institut Maurice-Lamontagne Division de la productivite des oceans, Mont- Joli, Quebec, Canada Michel Cotneau Marine Biology Research Center, Dept. of Fisheries and Oceans Canada Universite de Moncton, NB, Canada Jean-Claude Therriauit Ministere des pecheset oceans, Institut Maurice-Lantontagne Division de la productivite des oceans, Mont Joli, Quebec, Canada Francesc X. Maynou i Hernandez and Guy Robichaud Marine Biology Research Center, Dept. of Fisheries and Oceans Canada Universite de Moncton, NB, Canada Abstract Commercially harvested stocks of Chionoecetes spp. have been observed to fluctuate greatly over the past 20 years. The opinion that these fluc- tuations are fisheries driven and that a practical management goal should be a sustainable yield, is contrasted with a 10-year series of ob- servations on the recruitment and life history of a little harvested stock Currentaddress for GerardY. Conanis Departementde mathematiqueset statistique,Universite de Moncton,Moncton, NB, Canada EIA 3E9.Current address for Niche!Comeauand GuyRobichaud is Departmentof Fisheriesand Oceans,Gulf Region,Box 3030, Moncton, NB, Canada, E1C 9B6. Current addressfor FrancescX. Maynoui Hernandezis ConsejoSuperior de InvestigacionesCientificas, Institut.de Cienciesdel Mar,Passeig Joan de Borbo,s/n 08039,Espana. 60 Conan et al. Life History, Recruitment, and hfanagement of Snow Crab of C. opilio snow crab! isolated at benthic stages in the fjord of Bonne Bay Newfoundland!. The stock was thoroughly monitored from egg production to death of adults by senility. The sampling techniques ranged from underwater television, scuba diving, and mini-submarines to plankton nets for the larvae, trawls, dredges, collectors, traps, tangle nets for the benthic stages. Movements were monitored by acoustic tags. Seasonality and an- nual variations of environmental parameters such as winter ice cover- age, prevailing winds generating Ekman transport, stratification and movements of water masses, and timing of phytoplankton blooms, were recorded and compared with recruitment fluctuations. It is concluded that strong natural fluctuations of recruitment pre- vail in the Bonne Bay snow crab stock in the absence of any important fishing activity. Series of 3 to 4 successive years of good recruitment are followed by series of 5 to 6 years of poor recruittnent. None of the environmental factors monitored appeared to be direct- ly correlated with these fluctuations; rather, a life history regulation fac- tor could be in many casesidentified as a mechanismdampening the environmentaleffects. For instance,the preciseoccurrence of the spring phytoplanktonbloom wasefficiently detectedby the crabsand trig- gered hatching of their eggs and release of their larvae in the plankton at the most opportune time of the year, thereafter negating the possibili- ty that match/mismatch deregulated recruitment. We do not exclude, however,the possibility that the life history regulatingresponses ob- served be at times overwhelmed by particularly unusual environmental conditions. It appears that most of the recruitment variability observed for the Bonne Bay stock results from intra-population effects involving selective cannibalism by predation of older males on early benthic recruits and molting individuals. This "missing cohort dynamics" appears to exist over most harvested stocks of Atlantic snow crab. Further, the success/ failure of cohorts appears to be more or less synchronous in all known locations. In the Gulf of St. Lawrence fishery, there is so far no evidence that setting quotas on the landings has had any effect in regulating recruit- ment success,as good recruittnent years do not correspond with high parental stock. We suggest the existence of natural intra-population ef- fects,based on cannibalism,generating an alternationof good and poor recruitment years, and synchronization between geographic sub-stocks by a superimposedwidely distributed environmentalfactor occurring many years apart, such as the depth oscillations of the shallow and deepthermoclines limiting the depth distribution of Chionoecetesopilio. Symposium on High Latitude Crabs 61 35000 ~ 30000 00 25000 ~ 20000 CO 15000 10000 50000 19651970 1975 1980 1985 1990 1995 Year Figure 1. Commercial landings in metric tons of snow crab harvested in the Gulf of St. Lawrence. The variability has been threefold from 1970 to 1995, Introduction Snow crab Chionoecetes opilio! is a major commercial species in Eastern Canada. Landings are in the order of 25,000 tons per year, for a value of Can. $100,000,000. A full-scale commercial fishery developed only over the past 15 years during a decline of Alaskan king crab resources. Re- search emphasis has been set only recently on the understanding of the life history of the species. In the Southwestern Gulf of St. Lawrence, from 1966 to 1990, the landings first rose with fishing effort from an average of 7,000 tons 969-1977! to 33,000 tons in 1982. The landings then dropped abrupt- ly to 7,000 tons in 1990 Figure 1! and have recently risen again to 25,000 tons in 1994. Such strong harvestable stock fluctuations could not be explained by the prevailing interpretation of the life history of snow crab. Up to 1986, recruitment was believed to be fully protected by a legal minimal harvesting size of 95 mm carapace width. It was generally accepted at that time that all males matured and had the opportunity to mate sever- al times before reaching this legal minimal size, It was believed that fe- males were fully protected from fishing throughout their life span since they achieved a terminal molt at onset of maturity at sizes much smaller than the legal minimal size. The females would mate only once in their life, shortly after the terminal molt, thereafter depending on sperm 62 Conanet ai. Life History,Recruitment, and Managementof SnowCrab stored in their spermathecafor further broods.Based on laboratoryob- servations, it was thought that one male would naturally serve several females during the same mating season. lt was also believedthat adult maleswere much larger than females because they kept on growing after reaching maturity. As males used lit- tle energyfor reproduction,they wereexpected to havean asymptotic growth curve based on increasing time intervals between successive molts, as it is known for lobsters, and eventually reduced increments in size at molt. Femalesstopped growing at onset of maturity, because they always carried eggs throughout the year and never had the chance to molt, therefore diverting all their energy to reproduction rather than growth. Under this interpretation of the snow crab life cycle, it seemed that a yield per recruit model would have ideally served the purpose of man- aging a snow crab fishery, which benefited from an almost unlimited natural protection of recruitment from fisheries effects. The direct im- plication would havebeen to set a veryhigh fishing effort allowing to crop down male snow crab cohorts as soon as they reached a legal mini- mal size set ! abovethe size at which all malesreach first maturity, and ! at the size at which the male component of a cohort reached its maximalweight prior to the ageat which individual growth slowsdown and no longercompensates for biomassdecrease by natural mortality. This would have allowed all males to successfully mate, and would have maximized yield, In practice, the two major elements required for yield calculations: ! the growth curve of males, through increments in size at tnolt and reducedfrequencies of molting with size, and ! the natural mortality, were not sufficiently well known.The fishery was thereforepreemptive- ly managedby settinga maximum"exploitation rate" catch/standing harvestable biomass! which would have allowed about 40% of harvest- able males to survive the annual fishing season. The methodfor calculatingannual standingfishable biomass was retrospective and based on the fit of an anticipated linear relationship betweenthe catch over equalsuccessive time intervals during a fishing season and the catch cumulated up to each of these interval. The tech- nique was referred to as "Leslie analysis" Mohn and Elner 1987 for re- view! and had underlying assumptionsof no recruitmentto the fishery, negligiblenatural mortality, and constantcatchability throughoutthe fishing season.It neglectedthe dynamicchanges affecting the spatial distributions of effort and resourceabundance during a fishing season. Under these simple life history and tnanagement assumptions, re- cruitment was thought to be fully protected, and catch had first been thought to be altnost inexhaustible,A closeinspection of the sequence of landing statistics Figure1!, and of the data on catchand effort Fig- ure 2! reveals,however, that the 1986sharp drop in landingswas most likely due to a failure in recruitment. The catch per unit of effort values 64 Conanet ai. Life History,Recruitment, and Managementof SnowCrab were low right at the start of the 1987 fishing season and remained as such until 1989.Simultaneously, landings of large amountsof "white," low commercial quality, and apparently recently molted crabs were re- ported
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