Sustaining Fisheries Yields Over Evolutionary Time Scales David O. Conover and Stephan B. Munch

To ensure adequate genetic variation in the founder populations, approximately 700 wild adult M. menidia in spawning condition were collected from Great South Bay, NY on 5 May 1998. Fish were transported to the Flax Pond Marine Laboratory, Old Field, NY and divided evenly among two 1800 L tanks supplied with filtered natural seawater. Between 100,000 to 200,000 eggs from these two spawning groups were collected and pooled over a 2-day period. Each generation was reared under identical environmental and density conditions using previously published protocols (S1). Food was provided ad libitum to allow genetic differences in growth to be fully expressed. Each generation, the six experimental populations were reared in two “phases” consisting of groups of progeny spawned over consecutive intervals each spanning 10-20 days. The phases represented duplicates of each generation and population, and were reared in parallel but separate seawater systems as a precautionary measure in case of system failure or disease (neither occurred). Embryos were initially reared at 21°C. Because M. menidia has temperature-dependent sex determination (S2), larvae between 15 and 90 days old were reared at 15°C to ensure a balanced sex ratio. Larval density was 175 per 19 L container. At 90 days, 550 juveniles from each phase and population were transferred to 700 L cylindrical tanks. Temperatures were then raised to 27°C gradually over a two-week period. When the average age of fish in each phase was 190 days, total lengths of all fish in that phase were individually recorded and sorted by size. The 90^th (small-harvest populations) or 10^th (large-harvest populations ) percentiles of the size distributions were estimated and the appropriate harvest regime was then applied. In control lines, all individuals were also measured but then assigned randomly to either the harvested group or to the spawning stock. The two phases of surviving fish within each population were then combined and photoperiod was altered to induce maturity. About 1.5 months later, eggs were collected from spawning tanks and reared as above, and so on for subsequent generations. Note that because Menidia will generally not mature without the proper sequence of photoperiod cues, and these were applied to all treatments at the same time, age at maturity was unlikely to evolve in these experiments.

References and Notes

S1. D. O. Conover, T. M. C. Present, Oecologia 83, 316 (1990).

S2. D. O. Conover, B. E. Kynard, Science 213, 577 (1981).