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Fishery Bulletin 95(4), 1997 
were removed from the population, smaller fish could 
occupy the now vacant territory and undergo transi- 
tion. As new males became increasingly smaller, the 
size of the territory they could successfully hold might 
also become smaller, freeing territory for additional 
males. However, as the size of the fish declined even 
further, it is possible that the males would be unable 
to compete against other species, thereby further 
reducing the available habitat for red porgy males 
and reducing the sex ratio to the same level as that 
found in 1979-81. 
It has been hypothesized that the selective removal 
of individuals predisposed to rapid growth and 
greater size may cause a genetic shift resulting in a 
population of slower growing, smaller individuals 
than those found in the unfished population 
(Bohnsack, 1990; Sutherland, 1990). Edley and Law 
( 1988) found that individuals of a population of Daph- 
nia magna subjected to size-selective mortality of its 
large individuals for 10 generations did not return 
to the size and growth rates of a control population, 
even after the size selective pressure was removed. 
Changes in the life history of red porgy over the last 
two decades strongly confirm the hypothesis of 
Bohnsack (1990) and Sutherland (1990). Although 
exploitation may not last long enough to result in a 
permanent genetic shift to slower growing, smaller 
individuals, phenotypic changes have already oc- 
curred. Failure to consider the potential evolution- 
ary changes that could be induced in a population 
through fishing mortality could result in a reduction 
of the maximum potential yield of that stock (Law and 
Grey, 1989). In addition, a reduced population of smaller 
red porgy could have implications in reef fish commu- 
nity structure, i.e. the role of smaller red porgy in a 
reef habitat may be different, or smaller red porgy may 
be less able to compete for more desirable habitat. 
Current management strategies only enhance the 
impact of the size selective mortality associated with 
fisheries. In 1992, Amendment 4 of the SAFMC snap- 
per-grouper management plan was enacted, requir- 
ing a minimum size of 12 inches (305 mm TL) for red 
porgy in catches (SAFMC 1 ). Fishermen tend to tar- 
get larger fish because these bring the largest re- 
turn (economic for commercial fishermen, aesthetic 
for recreational fishermen). Size-at-maturity for red 
porgy females was 270 mm TL in 1972-74; 200-225 
mm TL in 1979-81; and 175-200 mm TL in 1991- 
94. However, 100% maturity occurred at 350 mm TL 
during 1972-74 and >300 mm TL in 1991-94. Thus, 
many faster growing individuals may reach legal size 
before they are sexually mature or when they have 
only had the opportunity to spawn once or twice. 
Slower growing individuals would take longer to reach 
the size limit and have a greater chance to spawn be- 
fore becoming available to the fishery, thus further ex- 
acerbating the effect of size-selective mortality. 
The SAB population of red porgy has undergone 
significant changes in life history, presumably in re- 
sponse to sustained, long-term size-selective 
overexploitation. Individuals in the population are 
smaller, have reduced growth rates, a reduced theo- 
retical maximum size, and undergo sexual maturity 
and transition at smaller sizes now than 20 years 
ago. The selective pressure of fishing mortality may 
be causing a genetic shift towards a slower growing, 
smaller population. Unless appropriate management 
measures are taken, sustained overfishing could re- 
sult in a permanent genetic shift in the fish or a to- 
tal collapse of the stock (or both). 
Acknowledgments 
The authors wish to thank the crews and scientific 
parties of the research vessels that made collection 
of these data possible. Bill Roumillat and Wayne Waltz 
aged, sexed, and identified maturity stages of red porgy 
collected between 1979 and 1987, and Oleg Pashuk 
sexed and staged all red porgy collected since 1987. 
Three anonymous reviewers provided numerous com- 
ments that improved the manuscript immeasurably. 
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