259
8 Fishery and Biological Implications of Fishing Spawning Aggregations…
Sex ratio effects are also relevant to sex-changing species, which tend to have
naturally male-biased (protandrous) or female-biased (protogynous) adult sex ratios.
In certain protogynous groupers that form spawning aggregations (e.g. red hind,
Shapiro et al. 1993 , gag grouper Koenig et al. 1996 ) , sex ratio assessment during
aggregation periods may be an important cue for sex change, since this is the only
time when adult males and females are known to come together in signifi cant numbers; in all sex-changing species studied, the incidence and timing of sex change is
mediated at the level of social groups in response to behavioural cues (Muñoz and
Warner 2003 ; Munday et al. 2006 ) . Particularities in fi sher behaviour can also skew
sex ratios. In an exploited aggregation of tiger grouper, Mycteroperca tigris , sex ratios
in catches were skewed towards males because spearfi shers actively selected males
to “protect” females and the eggs they bear (Sadovy et al. 1994a, b ; Matos-Caraballo
et al. 2006 ) . The selective male removals could potentially cause problems with
mate choice, mate encounter rates, or other reproductive behaviours, depending on
the mating system. In another example, a male-biased sex ratio in an aggregation of
squaretail coralgrouper in Palau was associated with greater harassment (i.e. chasing)
of the relatively smaller number of ripe females moving around the aggregation site
in Palau compared to sites with less male bias (Johannes et al. 1999 ).
Heavy fi shing pressure on spawning aggregations could be a major factor in
reducing the average length and size range of fi sh, although this may be a general
effect of fi shing rather than one specifi cally related to aggregation fi shing. In leopard coralgrouper in Australia (Adams et al. 2000 ) , Nassau grouper in Belize and
Mexico (Carter et al. 1994 ; Aguilar-Perera 2007 ) , red hind in the United States
Virgin Islands (USVI) (Beets and Friedlander 1999 ) , and in leopard grouper,
M. rosacea in the eastern Pacifi c (Sala et al. 2003 ) sizes of fi sh taken from aggregations have declined relative to past baselines (Figs. 8.11, 8.12 ). In a few species, such
as Atlantic cod, stripey seaperch ( Lutjanus carponatus ), gag grouper and scamp,
reduced body size is associated with reductions in ages of sexual maturity and in sex
change, or in decreases in egg size, larval survivorship, and batch fecundity of
females (Coleman et al. 1996 ; Olsen et al. 2004, 2005 ; Evans et al. 2008 ) . This is a
major concern for fi sheries, given the relative reproductive value of large females
due to the association of high fecundity with large size and, in some species, high
egg and larval quality (Berkeley et al. 2004 ; Birkeland and Dayton 2005 ) . While
declines in body size have several possible causes, aggregation-fi shing could be a major
contributing factor if it is particularly intensive or size-selective. Conversely, reduction of fi shing pressure can result in larger and more plentiful fi sh (Nemeth 2005 ) .
8.4.2 Allee and Other Mating Behaviour Effects
Allee Effects, expressed by positive relationships between various fi tness components (e.g. number of matings or eggs fertilized) and population densities, are related
to mate-fi nding factors that can be infl uenced by fi shing (Allee 1931 ; Stephens et al.
1999 ) . In birds, the once abundant passenger pigeon, Ectopistes migratorius, became
extinct, at least in part, because pairs no longer bred once colony sizes were reduced
8 Fishery and Biological Implications of Fishing Spawning Aggregations…
Sex ratio effects are also relevant to sex-changing species, which tend to have
naturally male-biased (protandrous) or female-biased (protogynous) adult sex ratios.
In certain protogynous groupers that form spawning aggregations (e.g. red hind,
Shapiro et al. 1993 , gag grouper Koenig et al. 1996 ) , sex ratio assessment during
aggregation periods may be an important cue for sex change, since this is the only
time when adult males and females are known to come together in signifi cant numbers; in all sex-changing species studied, the incidence and timing of sex change is
mediated at the level of social groups in response to behavioural cues (Muñoz and
Warner 2003 ; Munday et al. 2006 ) . Particularities in fi sher behaviour can also skew
sex ratios. In an exploited aggregation of tiger grouper, Mycteroperca tigris , sex ratios
in catches were skewed towards males because spearfi shers actively selected males
to “protect” females and the eggs they bear (Sadovy et al. 1994a, b ; Matos-Caraballo
et al. 2006 ) . The selective male removals could potentially cause problems with
mate choice, mate encounter rates, or other reproductive behaviours, depending on
the mating system. In another example, a male-biased sex ratio in an aggregation of
squaretail coralgrouper in Palau was associated with greater harassment (i.e. chasing)
of the relatively smaller number of ripe females moving around the aggregation site
in Palau compared to sites with less male bias (Johannes et al. 1999 ).
Heavy fi shing pressure on spawning aggregations could be a major factor in
reducing the average length and size range of fi sh, although this may be a general
effect of fi shing rather than one specifi cally related to aggregation fi shing. In leopard coralgrouper in Australia (Adams et al. 2000 ) , Nassau grouper in Belize and
Mexico (Carter et al. 1994 ; Aguilar-Perera 2007 ) , red hind in the United States
Virgin Islands (USVI) (Beets and Friedlander 1999 ) , and in leopard grouper,
M. rosacea in the eastern Pacifi c (Sala et al. 2003 ) sizes of fi sh taken from aggregations have declined relative to past baselines (Figs. 8.11, 8.12 ). In a few species, such
as Atlantic cod, stripey seaperch ( Lutjanus carponatus ), gag grouper and scamp,
reduced body size is associated with reductions in ages of sexual maturity and in sex
change, or in decreases in egg size, larval survivorship, and batch fecundity of
females (Coleman et al. 1996 ; Olsen et al. 2004, 2005 ; Evans et al. 2008 ) . This is a
major concern for fi sheries, given the relative reproductive value of large females
due to the association of high fecundity with large size and, in some species, high
egg and larval quality (Berkeley et al. 2004 ; Birkeland and Dayton 2005 ) . While
declines in body size have several possible causes, aggregation-fi shing could be a major
contributing factor if it is particularly intensive or size-selective. Conversely, reduction of fi shing pressure can result in larger and more plentiful fi sh (Nemeth 2005 ) .
8.4.2 Allee and Other Mating Behaviour Effects
Allee Effects, expressed by positive relationships between various fi tness components (e.g. number of matings or eggs fertilized) and population densities, are related
to mate-fi nding factors that can be infl uenced by fi shing (Allee 1931 ; Stephens et al.
1999 ) . In birds, the once abundant passenger pigeon, Ectopistes migratorius, became
extinct, at least in part, because pairs no longer bred once colony sizes were reduced