Michigan Department of Natural Resources
Fisheries Research Report No. 2024, 1996.
Mechanisms of Recruitment Failure in Bluegill Ponds
James E. Breck
Michigan Department of Natural Resources
Institute for Fisheries Research
212 Museums Annex Building
Ann Arbor, MI 48109-1084
e-mail: breckj@michigan.gov
Abstract.-A
series of eight experiments was conducted to study the factors
influencing year-class failure in high-density bluegill ponds. Ponds
were considered to have successful bluegill reproduction if age-1
fish were present at spring pond draining 1 year after stocking.
Experiment 1 varied total bluegill stocking density to evaluate its
effect on adult relative weight (Wr), gonadosomatic
index (GSI), fecundity of females, and on reproduction. Three ponds
were stocked at each of three densities: 56 ("low"), 134 ("medium")
and 336 ("high") kg/ha; of the specified total, 37 kg/ha was fish
>127 mm total length. There was a year-class failure in all three
high-density ponds, in one medium-density pond, in two low-density
ponds. Of the three successful ponds, two had received the lowest
stocking density of fish <51 mm. A total of 469 adult bluegills
was sampled by hook and line. The sex ratio among fish 152 mm or
greater was 9:1 (males:females). The Wr of males
and females was significantly greater at lower stocking densities.
For fish sampled in June, the 6-fold range of stocking densities
produced a 3-fold range in mean GSI of females, from 3.0% at high
density to 9.3% at low density, but there was no significant effect
of stocking density on male GSI. The June number of eggs per gram
body weight of female varied from 120±11 (mean±SE) at low
density, to 83±11 at medium density, to 40±12 at high
density. This indicates that fewer eggs are produced per female as
density increases. Experiment 2 evaluated the effect on reproduction
of age-1 density. The three treatments were 0 (four ponds), 5.6 (two
ponds), and 17 (three ponds) kg/ha of age 1, each with 56 kg/ha of
small (102-151 mm) adults. Reproduction was successful in all six
ponds with 0 and 5.6 kg/ha of age 1s, but failed in all three ponds
with age 1s at 17 kg/ha. Experiment 3 further evaluated the effect of
age-1 density on reproduction, using age-1 densities of 0, 17, and 50
kg/ha (three ponds at each density); adults were stocked at 56 kg/ha.
Most adults came from experiment 2, and were 1 year older and about
25 mm longer. Reproduction was successful in all six ponds at 0 and
17 kg/ha; reproduction failed in two of three ponds at 50 kg/ha.
Experiment 4 evaluated the ability of adults in poor condition to
reproduce. One pond was stocked with small adults (102-151 mm) with a
very low relative weight (64±6.5%, mean±SD); they came from
ponds stocked the previous year at 336 kg/ha. Though some nests were
observed, no fry were produced; the year class failed. Experiment 5
also tested adults that spent the previous year in ponds stocked at
336 kg/ha. Unexpectedly, not all adults were in poor condition.
Whereas for small adults (102-151 mm) the average
Wr was 70-75%, the Wr of 203-253
mm fish was 82-87%, and three fish 254-278 mm long had a
Wr of 96%. The year class was successful in all
three ponds stocked with these fish. Thus, any carry-over effect of
previous high density is not sufficient to cause year-class failure
if some adults are in good condition. Experiment 6 used a 22
cross-classified design to evaluate effects of both adult size and
age-1 density. Ponds received either only small (102-151 mm) adults
or both small and large (>151 mm) adults, combined with either 0
or 17 kg/ha of age-1 bluegills. Reproduction was successful in 11 of
the 12 ponds, failing only in one of three ponds that received small
adults and 17 kg/ha of age 1s. Experiment 7 evaluated the effect of
age-2 and age-1 density on reproduction. Nine ponds received adults
at 56 kg/ha. Three of these ponds were stocked with no juveniles. Two
ponds were stocked with age 2s at 78 kg/ha and another received 30
kg/ha. The last three ponds received age-1 bluegills at 17 kg/ha; two
of these received age 2s at 78 kg/ha and the other received age 2s at
30 kg/ha (there was a shortage of age 2s at stocking). Reproduction
was successful in all ponds except one receiving age 1s at 17 and age
2s at 78 kg/ha. Experiment 8 was similar to experiment 7, testing the
effect of age 2s and age 1s on reproduction. Nine ponds received
adults at 56 kg/ha. Three of these ponds were stocked with no
juveniles, three ponds with age 1s at 17 and age 2s at 28 kg/ha, and
three ponds with age 1s at 17 and age 2s at 78 kg/ha. Reproduction
was successful in seven of nine ponds, failing in one pond each of
the latter two treatments. Year-class failure in bluegill ponds is
clearly associated with the presence of juvenile bluegills, that is,
age-1 and age-2 bluegills, less than about 100 mm. Of the 61
pond-year occasions in this study, reproduction failed on 16
occasions. For ponds without juveniles, reproduction failed on only 1
of 23 occasions, for a failure rate of 4%; this was the single pond
stocked with small adults in very poor condition. For ponds stocked
with juveniles, reproduction failed on 16 of 38 occasions, for a
failure rate of 39%, not significantly different from the failure
rate of 22 of 45 occasions observed by Clark and Lockwood (1990). The
mechanism of year-class failure does not appear to be starvation of
age 0s after hatching, nor overwinter mortality, nor cannibalism on
fry larger than 20 mm. It does not appear to be adult lack of
adequate energy for reproduction, except in extreme cases. While not
causing failure, these mechanisms can influence year-class strength.
This study and experiments by Gray (1991) suggest that predation by
juvenile bluegills, on eggs and larvae in the nest and on fry soon
after leaving the nest, is the major mechanism producing recruitment
failure in high-density bluegill ponds.