| Contents Notes |
Sixteen populations of guppies (Poecilia reticulata) were established in laboratory aquariums. Eight populations were exposed to dieldrin at a mean concentration of 0.525 ppb in the water and 2.11 ppm in the food, tubificid worms. Eight populations received control water and food. All populations were fed three grams of tubificid worms per day. Exploitation rates of 0.60, 0.45, 0.30, and 0.15 were selected. Each rate was assigned to two control populations and two populations exposed to dieldrin. All fish except the young (less than nine millimeters) in each population were exploited at their designated rate according to a systematic exploitation schedule every 28 days. All populations were exploited fifteen times. The number of fish in the exploited phase of populations exploited at 0.60 remained stable during most of the experiment. Populations exposed to dieldrin maintained a greater number of fish than control populations through increased recruitment. Apparently, dieldrin reduced the cannibalistic behavior of adult guppies and allowed more young to survive to recruitment size. The biomass and yield in weight of control populations and populations exposed to dieldrin was similar. Increased recruitment in populations exposed to dieldrin was therefore compensated by reduced growth. Reductions in recruitment resulting from increased cannibalism on young fish caused the number of fish in the exploited phase of populations exploited at 0. 45, 0. 30, and 0. 15 to decline. Populations exposed to dieldrin maintained the same number of fish and followed the same trends as control populations. These populations maintained a high density of adult fish and intraspecific competition for food may have overcome the tendency of dieldrin to reduce cannibalistic behavior. Although population number declined, the biomass and yield in weight of these populations remained stable. Reductions in recruitment had not yet caused a major decline in the number of adults and may have been compensated by increased fish growth. As population number declined, the average length and weight of the catch increased. Populations exploited at 0. 45, 0. 30, and 0. 15 were apparently beginning to undergo periodic cycles. Mean equilibrium yield in weight declined as exploitation rate was reduced. Yield in weight of each population was located on the descending limb of a surplus-yield curve. Age, growth, production, and yield in weight of females in populations exploited at 0.60, 0. 45, and 0.30 were determined from the length-frequency distributions of the catch. At each exploitation rate, during the younger age classes, when the production of a cohort of females was greater than its yield, the biomass of the cohort increased. During the older age classes, when yield became greater than production, the biomass of the cohort declined. As exploitation rate increased, the mean individual weight of each age class and the relative growth rate of fish of a particular weight also increased. Increased growth enabled fish from high exploitation rates to maintain greater production and yield than fish from low exploitation rates. |
