|Abstract or Summary
- 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.