262
Y. Sadovy de Mitcheson and B. Erisman
designed to test the effect of fi shing on reproductive activity of gag predicted that
mean fertilization rate and the number of fertilized eggs per recruit are markedly lower
in small mating groups (<50 individuals) than in larger aggregations (100–1,000 individuals, Alonzo and Mangel 2004 ) . A study that examined the results of several independent captive-breeding studies in Atlantic cod found that fertilization rates declined
and had higher variance as the number of males and breeding aggregation size
decreased (Rowe et al. 2004 ) . For both species, reductions in fertilization rates were
attributed to sperm limitation associated with a low number of males and insuffi cient
sperm to fertilize the eggs of all females during group-spawning events.
Mobile invertebrate examples show that high densities of eggs and sperm can be
critical to fertilization success and presumably account for the clumping behaviour
of adults for gamete release in some species. Sperm limitation can be severe unless
numerous individuals spawn simultaneously (e.g. Levitan and Petersen 1995 ) ,
although Yund ( 2000 ) suggests that sperm limitation may not be as severe as initially thought in marine free-spawners. Many commercially over-exploited nonsedentary invertebrate species are recruitment-limited or display density-dependent
population dynamics and the effect of possible sperm limitation has sometimes been
included in fi shery models (Yund 2000 ) . In the bluehead wrasse sperm numbers and
fertilization rates are lower in single-male matings than during multiple-male matings (Shapiro et al. 1994 ; Marconato et al. 1997 ) (Chap. 3 ).
Fishing spawning aggregations could reduce reproductive output through effects
on courtship and mate choice. Assortative mating, whereby individuals tend to mate
with fi sh of similar size, is found in several aggregating species (bluehead wrasse
Shapiro et al. 1994 , bucktooth parrotfi sh Sparisoma radians Marconato and Shapiro
1996 , Atlantic cod Rowe and Hutchings 2003 , leopard grouper Erisman et al. 2007 ) ,
while in others, females show preferences for larger males (e.g. Chap. 3 Rasotto
et al. 2010 ) . If preferred mate phenotypes are removed during or just prior to spawning periods by fi shing, the choosier sex may respond by releasing fewer gametes or
performing fewer spawning rushes, thereby reducing reproductive output (Shapiro
et al. 1994 , Marconato et al. 1997 ).
Fishing could potentially lower mating frequency or fertilization rates through
disruption of courtship in other ways. For example, sound production plays an
important role for attracting individuals at aggregation sites, in mate competition,
during courtship, and for stimulating or synchronizing gamete release or maturation
in several species of croakers (spotted weakfi sh Cynoscion nebulosus Gilmore 2003 ,
white weakfi sh Atractoscion nobilis Aalbers and Drawbridge 2008 , the goliath
grouper Mann et al. 2008 ) and in the Atlantic cod (Rowe and Hutchings 2006 ) .
At reduced aggregation sizes, acoustic intensity may be insuffi cient to attract all
individuals to spawning sites or may decrease fertilization rates due to reduced
synchronization of gamete release (Rowe and Hutchings 2003 ) .
Stress can affect reproduction in fi shes although there is little evidence to suggest
that this ultimately results in reduced annual reproductive output. In the common
snook, Centropomus undecimalis , silver seabream and red gurnard, Chelidonichthyes
kumu , stress in captivity can cause changes in hormone levels, fecundity, egg size
and development, and egg survival (e.g. Morgan et al. 1999 ) . However, the extent to
Y. Sadovy de Mitcheson and B. Erisman
designed to test the effect of fi shing on reproductive activity of gag predicted that
mean fertilization rate and the number of fertilized eggs per recruit are markedly lower
in small mating groups (<50 individuals) than in larger aggregations (100–1,000 individuals, Alonzo and Mangel 2004 ) . A study that examined the results of several independent captive-breeding studies in Atlantic cod found that fertilization rates declined
and had higher variance as the number of males and breeding aggregation size
decreased (Rowe et al. 2004 ) . For both species, reductions in fertilization rates were
attributed to sperm limitation associated with a low number of males and insuffi cient
sperm to fertilize the eggs of all females during group-spawning events.
Mobile invertebrate examples show that high densities of eggs and sperm can be
critical to fertilization success and presumably account for the clumping behaviour
of adults for gamete release in some species. Sperm limitation can be severe unless
numerous individuals spawn simultaneously (e.g. Levitan and Petersen 1995 ) ,
although Yund ( 2000 ) suggests that sperm limitation may not be as severe as initially thought in marine free-spawners. Many commercially over-exploited nonsedentary invertebrate species are recruitment-limited or display density-dependent
population dynamics and the effect of possible sperm limitation has sometimes been
included in fi shery models (Yund 2000 ) . In the bluehead wrasse sperm numbers and
fertilization rates are lower in single-male matings than during multiple-male matings (Shapiro et al. 1994 ; Marconato et al. 1997 ) (Chap. 3 ).
Fishing spawning aggregations could reduce reproductive output through effects
on courtship and mate choice. Assortative mating, whereby individuals tend to mate
with fi sh of similar size, is found in several aggregating species (bluehead wrasse
Shapiro et al. 1994 , bucktooth parrotfi sh Sparisoma radians Marconato and Shapiro
1996 , Atlantic cod Rowe and Hutchings 2003 , leopard grouper Erisman et al. 2007 ) ,
while in others, females show preferences for larger males (e.g. Chap. 3 Rasotto
et al. 2010 ) . If preferred mate phenotypes are removed during or just prior to spawning periods by fi shing, the choosier sex may respond by releasing fewer gametes or
performing fewer spawning rushes, thereby reducing reproductive output (Shapiro
et al. 1994 , Marconato et al. 1997 ).
Fishing could potentially lower mating frequency or fertilization rates through
disruption of courtship in other ways. For example, sound production plays an
important role for attracting individuals at aggregation sites, in mate competition,
during courtship, and for stimulating or synchronizing gamete release or maturation
in several species of croakers (spotted weakfi sh Cynoscion nebulosus Gilmore 2003 ,
white weakfi sh Atractoscion nobilis Aalbers and Drawbridge 2008 , the goliath
grouper Mann et al. 2008 ) and in the Atlantic cod (Rowe and Hutchings 2006 ) .
At reduced aggregation sizes, acoustic intensity may be insuffi cient to attract all
individuals to spawning sites or may decrease fertilization rates due to reduced
synchronization of gamete release (Rowe and Hutchings 2003 ) .
Stress can affect reproduction in fi shes although there is little evidence to suggest
that this ultimately results in reduced annual reproductive output. In the common
snook, Centropomus undecimalis , silver seabream and red gurnard, Chelidonichthyes
kumu , stress in captivity can cause changes in hormone levels, fecundity, egg size
and development, and egg survival (e.g. Morgan et al. 1999 ) . However, the extent to