Sustaining Fisheries Yields OverEvolutionary Time ScalesDavid O. Conover* and Stephan B. MunchFishery management plans ignore the potential for evolutionary change in har-vestable biomass. We subjected populations of an exploited fish (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-dependentforce of fishing mortality. Large-harvested populations initially produced the high-est catch but quickly evolved a lower yield than controls. Small-harvested popu-lations did the reverse. These shifts were caused by selection of genotypes withslower or faster rates of growth. Management tools that preserve natural geneticvariation are necessary for long-term sustainable yield.It is well established that wild pest and pathogenpopulations may evolve in response to anthro-pogenic forces of mortality (1), but is the sametrue of fisheries? Fishing mortality is highlyselective. Exploited stocks typically displaygreatly truncated size and age distributions thatlack larger and/or older individuals (2–4). Thisoccurs not only because fishers may seek toexploit large individuals but also because regu-latory measures often impose minimum size orgear regulations that ensure selective harvest oflarger fish. Such harvesting practices could fa-vor genotypes with slower growth, earlier age atmaturity, or other changes that would lowerpopulation productivity. Despite mounting evi-dence of rapid life history evolution in wild fishpopulations (5–8), the unexpectedly slow recov-ery of populations from overexploitation (9, 10),and warnings from theorists (3, 11), currentmodels and management plans for sustainableyield ignore the Darwinian consequences of se-lective harvest.Failure to consider evolutionary processesin fisheries management continues in part be-cause proof that size-selective mortality causesgenetic changes in population productivity islacking. Here, we present results from experi-mentally harvested captive populations of a ma-rine fish that demonstrate evolutionary effectsof size-selective mortality on somatic growth,yield, and population biomass.The Atlantic silverside, Menidia menidia,isa common marine fish along the North Ameri-can east coast. Although landed commercially(mean annual landings in New York, from 1996to 2000, were 20.5 metric tons), we chose thisspecies as a model primarily for two other rea-sons. First, many of its life history characteris-tics are similar to those of other harvested ma-rine species [e.g., high fecundity, small egg size(1 mm in diameter), external fertilization,spawning en masse, pelagic larvae, and school-ing behavior], with one major exception. Theshort generation time of M. menidia (1 year)coupled with the ease with which large popula-tions can be maintained in captivity enable ex-perimental designs that would otherwise be im-possible. Second, M. menidia from differentlatitudes display clinal adaptive genetic varia-tion in somatic growth rate (12), a geographicalpattern common to other harvested species (13–16). Hence, a key production trait (somaticgrowth rate) appears capable of evolving in thewild in these species.We hypothesized that somatic growth rateand population levels of harvest would evolve indirections opposite to the size bias of harvest. Totest this premise, we founded six captive popu-lations of M. menidia by sampling randomlyfrom a large, common gene pool of embryosproduced by mass spawnings of adults collectedfrom the middle portion of the species’ range.After the larval phase was completed, 1100juveniles from each population were stocked inlarge tanks and reared to the adult stage. Allow-ing for 10% mortality during the juvenile phase,this resulted in about 1000 fish available forharvest per population. On day 190 postfertil-ization, 90% of each population was harvestedon the basis of one of three different size-spe-cific rules: (i) in two populations, all fish largerthan the 10th percentile in length (i.e., the largest90%) were harvested (large-harvested); (ii) intwo other populations, all fish smaller than the90th percentile (the smallest 90%) were extract-ed (small-harvested); and (iii) two populationswere controls in which 90% harvest was randomwith respect to size (random-harvested). Survi-vors (n ⬇ 100) were induced through photope-riod manipulations to spawn, and their embryoswere collected and reared under identical con-ditions over multiple generations (see details ofour methods in the supporting online material).Cross-generation trends in yield of the har-vested populations strongly supported our hy-pothesis (Fig. 1). Large-harvested populationsinitially produced the highest total yield andmean weight of fish but then declined. Small-harvested populations started with low yieldand then increased. By the fourth generation ofselection, the biomass harvested and the meanweight of harvested individuals in the small-harvested lines was nearly twice that of thelarge-harvested lines. Moreover, the spawningstock biomass differed even more. The meanweight of individual spawners (i.e., the survi-vors) in generation 4 was 1.05, 3.17, and 6.47 gin the large-, random-, and small-harvestedpopulations, respectively. Hence, because fe-cundity increases with size, small-harvestedlines evolved much higher reproductive poten-tial than did large-harvested lines.The reason for the opposite shifts in yieldamong the three treatments was genetic changein somatic growth rate rather than viability. Ju-venile survival rates differed little among thepopulations, averaging 83.5, 84.4, and 87.9% inthe large, small, and random lines, respectively.Hence, size selection did not merely sort fishwith generally favorable or unfavorable genes.Population-level differences in biomass wereachieved by increased juvenile growth rates insmall-harvested populations and decreased juve-nile growth in large-harvested lines (Fig. 2). InMarine Sciences Research Center, State University ofNew York, Stony Brook, NY 11794–5000, USA.*To whom correspondence should be addressed. E-mail: [email protected]. 1. Trends in aver-age total weight har-vested (A) and meanweight of harvestedindividuals (B) acrossmultiple generationsof size-selective ex-ploitation. Closed cir-cles represent small-harvested lines, opensquares are the ran-dom-harvested lines,and closed trianglesare the large-harvest-ed lines. Each datum isthe mean, and the vertical lines
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