64
P.P. Molloy et al.
per capita risk of acquiring non-contagious parasites may be reduced: with more
conspecifi cs around, the probability that any given individual will be parasitized is
reduced (Côté and Gross 1993 ) . Within aggregations, locations that confer protection
from predation or yield higher mating success may be hotly contested, leading to
agonistic interactions among competitors and risk of injury. Finally, temporary breeding aggregation sites are often separate from the territories or home ranges held by
individuals during non-breeding periods. Travel to these breeding sites may entail
additional energetic expenditure and place the traveller under higher risk of predation.
Territories left unprotected while the owners are breeding can be raided or usurped.
Although the two major explanations for breeding aggregations, i.e. habitat
limitation and individual choice, have been presented here as mutually exclusive
alternatives, this is not necessarily the case for all reproductive groupings. Breeding
in an aggregation could yield more than one benefi t or entail more than one cost.
A major challenge for behavioural ecologists has been, and remains, to convert all
benefi ts and costs to a comparable currency to evaluate the ‘bottom line’.
3.3 Benefi ts and Costs of Spawning in Fish Aggregations
We now examine evidence for or against some of the reproductive costs and benefi ts
outlined in the previous section, focussing specifi cally on fi sh aggregations from a
range of habitats. Since reproductive success is such a critical fi tness component,
understanding the reproductive consequences and dynamics of spawning aggregations may help to shed light on their evolutionary signifi cance.
3.3.1 Increased Mate Encounter Rate
Mate encounter rates have never been tested explicitly in aggregative spawners;
however, given that densities at spawning aggregations are higher than in nonreproductive populations, it seems likely that mate encounter rates should be higher
at spawning aggregations. Increased encounter rates are likely to be particularly
signifi cant for species such as the red hind, Epinephelus guttatus , (Shapiro et al.
1994 ) and gag grouper, Mycteroperca microlepis (Coleman et al. 1996 ) , which
exhibit some spatial segregation of the sexes during the non-breeding season, and
for species such as the blob sculpin, Psychrolutes phrictus (Drazen et al. 2003 ) , and
permit, Trachinotus falcatus (Graham and Castellanos 2005 ) , which occur at low
densities during the non-reproductive season, possibly owing to the general scarcity
of non-reproductive resources such as food and/or shelter. In these species, interactions with potential mates apparently occur almost exclusively on spawning aggregations. Conversely, increased mate encounter rates are unlikely to be important in
species that occur in high densities away from aggregations. For example, the blue
tang, Acanthurus coeruleus , which forms spawning aggregations of several thousand
P.P. Molloy et al.
per capita risk of acquiring non-contagious parasites may be reduced: with more
conspecifi cs around, the probability that any given individual will be parasitized is
reduced (Côté and Gross 1993 ) . Within aggregations, locations that confer protection
from predation or yield higher mating success may be hotly contested, leading to
agonistic interactions among competitors and risk of injury. Finally, temporary breeding aggregation sites are often separate from the territories or home ranges held by
individuals during non-breeding periods. Travel to these breeding sites may entail
additional energetic expenditure and place the traveller under higher risk of predation.
Territories left unprotected while the owners are breeding can be raided or usurped.
Although the two major explanations for breeding aggregations, i.e. habitat
limitation and individual choice, have been presented here as mutually exclusive
alternatives, this is not necessarily the case for all reproductive groupings. Breeding
in an aggregation could yield more than one benefi t or entail more than one cost.
A major challenge for behavioural ecologists has been, and remains, to convert all
benefi ts and costs to a comparable currency to evaluate the ‘bottom line’.
3.3 Benefi ts and Costs of Spawning in Fish Aggregations
We now examine evidence for or against some of the reproductive costs and benefi ts
outlined in the previous section, focussing specifi cally on fi sh aggregations from a
range of habitats. Since reproductive success is such a critical fi tness component,
understanding the reproductive consequences and dynamics of spawning aggregations may help to shed light on their evolutionary signifi cance.
3.3.1 Increased Mate Encounter Rate
Mate encounter rates have never been tested explicitly in aggregative spawners;
however, given that densities at spawning aggregations are higher than in nonreproductive populations, it seems likely that mate encounter rates should be higher
at spawning aggregations. Increased encounter rates are likely to be particularly
signifi cant for species such as the red hind, Epinephelus guttatus , (Shapiro et al.
1994 ) and gag grouper, Mycteroperca microlepis (Coleman et al. 1996 ) , which
exhibit some spatial segregation of the sexes during the non-breeding season, and
for species such as the blob sculpin, Psychrolutes phrictus (Drazen et al. 2003 ) , and
permit, Trachinotus falcatus (Graham and Castellanos 2005 ) , which occur at low
densities during the non-reproductive season, possibly owing to the general scarcity
of non-reproductive resources such as food and/or shelter. In these species, interactions with potential mates apparently occur almost exclusively on spawning aggregations. Conversely, increased mate encounter rates are unlikely to be important in
species that occur in high densities away from aggregations. For example, the blue
tang, Acanthurus coeruleus , which forms spawning aggregations of several thousand