72
P.P. Molloy et al.
benefi t may be negated by the intensity of modern fi shing. Irrespective of ‘natural’
predation rates, it is likely that fi shing has rendered or will render per capita mortality
higher on exploited aggregations than during non-spawning periods. Fishing
mortality may also reduce spawner numbers to levels where aggregating individuals
no longer gain a dilution of per capita ‘natural’ predation rates.
3.4.2 Other Costs
Individuals may incur other less obvious costs from spawning in aggregations.
Increased predation risk may cause indirect, behaviourally mediated fi tness costs
(Lima and Dill 1990 ) . For example, hogfi sh, Lachnolaimus maximus , are known to
abort spawning attempts when egg predator densities are too high (Colin 1982b ) .
Similarly, group-spawning striped parrotfi sh temporarily reduce their spawning rate
by about 25-fold following attacks by mackerel ( Scomberomorus sp. Colin 1978 ) ,
although spawning rates usually rebound within 10 min (Patrick Colin personal
communication). Releasing gametes at suboptimal locations or times, or reducing
the total number of gametes released as a result of predation risk, could entail reproductive costs for aggregative spawners. Such costs are more likely to be encountered in species that only spawn for a brief period each year, such as some Indo-Pacifi c
surgeonfi sh and some grouper populations (Robertson 1983 ; Montgomery and
Galzin 1993 ; Domeier and Colin 1997 ) – these species have a smaller window of
opportunity to subsequently fully recoup such reproductive costs. Currently there
are no studies addressing the indirect effects of risk of predation on the reproductive
output of fi sh at spawning aggregations. Such studies could test predictions that (1)
the presence of adult or egg predators reduces spawning rates, and (2) the length of
time required for fi sh to resume spawning after predator-induced cessation increases
with the number of predators or their combined attack rate.
Higher population densities may promote parasite or disease transmission.
For example, spawning aggregations of the speckled blue grouper, Epinephelus
cyanopodus , are thought to increase transmission of monogeneans, which would
explain how these parasites, which prefer large hosts, are occasionally found on
smaller individuals (Sigura and Justine 2008 ) . Specifi c observations of isopod
infestations on Cayman Island populations of Nassau grouper immediately after
spawning also suggest that high spawning densities may facilitate transmission or
infestation (Semmens et al. 2006 ) . These examples are somewhat anecdotal. To
demonstrate that spawning in aggregations increases susceptibility to parasitism,
simultaneous comparisons of parasite loads on fi sh spawning in aggregations differing
in densities are needed. The costs of parasite transmission are potentially important
since they may form an integral part of an individual’s decision to visit or avoid an
aggregation to spawn (Semmens et al. 2006 ) .
Various costs are also associated with migrating to spawning grounds. For example,
migration is likely to impose energetic costs for the many species that travel considerable distances to reach spawning aggregations (Crawford et al. 1986 ; Zeller 1998 ;
P.P. Molloy et al.
benefi t may be negated by the intensity of modern fi shing. Irrespective of ‘natural’
predation rates, it is likely that fi shing has rendered or will render per capita mortality
higher on exploited aggregations than during non-spawning periods. Fishing
mortality may also reduce spawner numbers to levels where aggregating individuals
no longer gain a dilution of per capita ‘natural’ predation rates.
3.4.2 Other Costs
Individuals may incur other less obvious costs from spawning in aggregations.
Increased predation risk may cause indirect, behaviourally mediated fi tness costs
(Lima and Dill 1990 ) . For example, hogfi sh, Lachnolaimus maximus , are known to
abort spawning attempts when egg predator densities are too high (Colin 1982b ) .
Similarly, group-spawning striped parrotfi sh temporarily reduce their spawning rate
by about 25-fold following attacks by mackerel ( Scomberomorus sp. Colin 1978 ) ,
although spawning rates usually rebound within 10 min (Patrick Colin personal
communication). Releasing gametes at suboptimal locations or times, or reducing
the total number of gametes released as a result of predation risk, could entail reproductive costs for aggregative spawners. Such costs are more likely to be encountered in species that only spawn for a brief period each year, such as some Indo-Pacifi c
surgeonfi sh and some grouper populations (Robertson 1983 ; Montgomery and
Galzin 1993 ; Domeier and Colin 1997 ) – these species have a smaller window of
opportunity to subsequently fully recoup such reproductive costs. Currently there
are no studies addressing the indirect effects of risk of predation on the reproductive
output of fi sh at spawning aggregations. Such studies could test predictions that (1)
the presence of adult or egg predators reduces spawning rates, and (2) the length of
time required for fi sh to resume spawning after predator-induced cessation increases
with the number of predators or their combined attack rate.
Higher population densities may promote parasite or disease transmission.
For example, spawning aggregations of the speckled blue grouper, Epinephelus
cyanopodus , are thought to increase transmission of monogeneans, which would
explain how these parasites, which prefer large hosts, are occasionally found on
smaller individuals (Sigura and Justine 2008 ) . Specifi c observations of isopod
infestations on Cayman Island populations of Nassau grouper immediately after
spawning also suggest that high spawning densities may facilitate transmission or
infestation (Semmens et al. 2006 ) . These examples are somewhat anecdotal. To
demonstrate that spawning in aggregations increases susceptibility to parasitism,
simultaneous comparisons of parasite loads on fi sh spawning in aggregations differing
in densities are needed. The costs of parasite transmission are potentially important
since they may form an integral part of an individual’s decision to visit or avoid an
aggregation to spawn (Semmens et al. 2006 ) .
Various costs are also associated with migrating to spawning grounds. For example,
migration is likely to impose energetic costs for the many species that travel considerable distances to reach spawning aggregations (Crawford et al. 1986 ; Zeller 1998 ;