260
Y. Sadovy de Mitcheson and B. Erisman
by hunting and habitat losses (Blockstein and Tordoff 1985 ). Allee Effects may be
important if they constrain population growth and recovery in overfi shed fi sh species. Mate-fi nding Allee Effects may be important in aggregating species if the primary purpose of aggregation formation is to fi nd mates and avoid the problem of
low density (which may occur in large species that disperse widely during nonaggregation times), or if there is some positive reproductive effect of groups of
animals coming together such as mate choice and fertilization rate (see Chap. 3 ).
Important factors might involve mate encounter rates and mate attraction, gamete
density and sperm limitation, physiological stimulation of courtship, female choice,
sexual selection, and reproductive investment (Gascoigne et al. 2009 ) . It is relevant
to consider whether there is evidence that such factors might be associated with
aggregation-spawning, or with different types of mating systems (i.e. transient
versus resident aggregations, group- versus pair-spawning) within aggregations.
Species for which Allee Effects have been demonstrated or hypothesized range
from Pacifi c sardine, Downs herring, scallops, abalone, sea urchins, and giant clams,
to muskrats, fl our beetles, sheep ticks, and passenger pigeons (Chap. 3 , Frank and
Brickman 2000 ) . Examples of Allee Effects include diffi culty in fi nding a mate,
and a breakdown in social structure and migration patterns. Species that have
characteristic social behaviour such as group-mating, group defense, and schooling
may be at increased risk of extirpations under heavy exploitation due to Allee
Effects (Frank and Brickman 2000 ) . While there are few examples of clear Allee
0
20
40
60
80
100
120
140
160
180
200
80
100
120
140
160
180
200
220
240
260
280
300
320
No. of individuals
Standard Length (mm)
Fig. 8.11 Size frequency distribution by sex (female circle , male square ) data for Nassau grouper,
Epinephelus striatus , from catches taken at a (formerly) non-exploited aggregation site (Northern
Two Caye, solid symbols ) and an exploited aggregation site (Cay Glory, open symbols ) in Belize.
Data show that both sexes are smaller at the exploited site (Carter et al. 1994 )
Y. Sadovy de Mitcheson and B. Erisman
by hunting and habitat losses (Blockstein and Tordoff 1985 ). Allee Effects may be
important if they constrain population growth and recovery in overfi shed fi sh species. Mate-fi nding Allee Effects may be important in aggregating species if the primary purpose of aggregation formation is to fi nd mates and avoid the problem of
low density (which may occur in large species that disperse widely during nonaggregation times), or if there is some positive reproductive effect of groups of
animals coming together such as mate choice and fertilization rate (see Chap. 3 ).
Important factors might involve mate encounter rates and mate attraction, gamete
density and sperm limitation, physiological stimulation of courtship, female choice,
sexual selection, and reproductive investment (Gascoigne et al. 2009 ) . It is relevant
to consider whether there is evidence that such factors might be associated with
aggregation-spawning, or with different types of mating systems (i.e. transient
versus resident aggregations, group- versus pair-spawning) within aggregations.
Species for which Allee Effects have been demonstrated or hypothesized range
from Pacifi c sardine, Downs herring, scallops, abalone, sea urchins, and giant clams,
to muskrats, fl our beetles, sheep ticks, and passenger pigeons (Chap. 3 , Frank and
Brickman 2000 ) . Examples of Allee Effects include diffi culty in fi nding a mate,
and a breakdown in social structure and migration patterns. Species that have
characteristic social behaviour such as group-mating, group defense, and schooling
may be at increased risk of extirpations under heavy exploitation due to Allee
Effects (Frank and Brickman 2000 ) . While there are few examples of clear Allee
0
20
40
60
80
100
120
140
160
180
200
80
100
120
140
160
180
200
220
240
260
280
300
320
No. of individuals
Standard Length (mm)
Fig. 8.11 Size frequency distribution by sex (female circle , male square ) data for Nassau grouper,
Epinephelus striatus , from catches taken at a (formerly) non-exploited aggregation site (Northern
Two Caye, solid symbols ) and an exploited aggregation site (Cay Glory, open symbols ) in Belize.
Data show that both sexes are smaller at the exploited site (Carter et al. 1994 )