REPORTS Sustaining Fisheries Yields Over Evolutionary Time Scales David O Conover and Stephan B Munch Fishery management plans ignore the potential for evolutionary change in harvestable biomass We subjected populations of an exploited sh Menidia menidia to large small or random size selective harvest of adults over four generations Harvested biomass evolved rapidly in directions counter to the size dependent force of shing mortality Large harvested populations initially produced the highest catch but quickly evolved a lower yield than controls Small harvested populations did the reverse These shifts were caused by selection of genotypes with slower or faster rates of growth Management tools that preserve natural genetic variation are necessary for long term sustainable yield It is well established that wild pest and pathogen populations may evolve in response to anthropogenic forces of mortality 1 but is the same true of fisheries Fishing mortality is highly selective Exploited stocks typically display greatly truncated size and age distributions that lack larger and or older individuals 2 4 This occurs not only because fishers may seek to exploit large individuals but also because regulatory measures often impose minimum size or gear regulations that ensure selective harvest of larger fish Such harvesting practices could favor genotypes with slower growth earlier age at maturity or other changes that would lower population productivity Despite mounting evidence of rapid life history evolution in wild fish populations 5 8 the unexpectedly slow recovery of populations from overexploitation 9 10 and warnings from theorists 3 11 current models and management plans for sustainable yield ignore the Darwinian consequences of selective harvest Failure to consider evolutionary processes in fisheries management continues in part because proof that size selective mortality causes genetic changes in population productivity is lacking Here we present results from experimentally harvested captive populations of a marine fish that demonstrate evolutionary effects of size selective mortality on somatic growth yield and population biomass The Atlantic silverside Menidia menidia is a common marine fish along the North American east coast Although landed commercially mean annual landings in New York from 1996 to 2000 were 20 5 metric tons we chose this species as a model primarily for two other reasons First many of its life history characteristics are similar to those of other harvested marine species e g high fecundity small egg size 1 mm in diameter external fertilization spawning en masse pelagic larvae and schoolMarine Sciences Research Center State University of New York Stony Brook NY 11794 5000 USA To whom correspondence should be addressed Email dconover notes cc sunysb edu 94 ing behavior with one major exception The short generation time of M menidia 1 year coupled with the ease with which large populations can be maintained in captivity enable experimental designs that would otherwise be impossible Second M menidia from different latitudes display clinal adaptive genetic variation in somatic growth rate 12 a geographical pattern common to other harvested species 13 16 Hence a key production trait somatic growth rate appears capable of evolving in the wild in these species We hypothesized that somatic growth rate and population levels of harvest would evolve in directions opposite to the size bias of harvest To test this premise we founded six captive populations of M menidia by sampling randomly from a large common gene pool of embryos produced by mass spawnings of adults collected from the middle portion of the species range After the larval phase was completed 1100 juveniles from each population were stocked in large tanks and reared to the adult stage Allowing for 10 mortality during the juvenile phase this resulted in about 1000 fish available for harvest per population On day 190 postfertilization 90 of each population was harvested on the basis of one of three different size specific rules i in two populations all fish larger than the 10th percentile in length i e the largest 90 were harvested large harvested ii in two other populations all fish smaller than the 90th percentile the smallest 90 were extracted small harvested and iii two populations were controls in which 90 harvest was random with respect to size random harvested Survivors n 100 were induced through photoperiod manipulations to spawn and their embryos were collected and reared under identical conditions over multiple generations see details of our methods in the supporting online material Cross generation trends in yield of the harvested populations strongly supported our hypothesis Fig 1 Large harvested populations initially produced the highest total yield and mean weight of fish but then declined Smallharvested populations started with low yield and then increased By the fourth generation of selection the biomass harvested and the mean weight of harvested individuals in the smallharvested lines was nearly twice that of the large harvested lines Moreover the spawning stock biomass differed even more The mean weight of individual spawners i e the survivors in generation 4 was 1 05 3 17 and 6 47 g in the large random and small harvested populations respectively Hence because fecundity increases with size small harvested lines evolved much higher reproductive potential than did large harvested lines The reason for the opposite shifts in yield among the three treatments was genetic change in somatic growth rate rather than viability Juvenile survival rates differed little among the populations averaging 83 5 84 4 and 87 9 in the large small and random lines respectively Hence size selection did not merely sort fish with generally favorable or unfavorable genes Population level differences in biomass were achieved by increased juvenile growth rates in small harvested populations and decreased juvenile growth in large harvested lines Fig 2 In Fig 1 Trends in average total weight harvested A and mean weight of harvested individuals B across multiple generations of size selective exploitation Closed circles represent smallharvested lines open squares are the random harvested lines and closed triangles are the large harvested lines Each datum is the mean and the vertical lines show the range of two replicate populations per treatment Regression analyses showed that both total weight and mean weight harvested declined
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