216
11 . Horseshoe Crabs and Shorebirds
their ocean hab itat along the continental shelf and approach the beach in
orde r to spawn. Males attach themselves to females, fertilizing the eggs as the
female lays them in a burrow in the dry sand. Studies have shown that female
horseshoe crabs can lay up to 88,000 eggs per season (Virtualbirder 2000).
The larvae hatch and enter the open ocea n. Juvenile crabs move farther and
farther away from the sho re with increasing age until they are sexua lly mature (Shuster 1982). Although there are notabl y more males than females on
the beach , surveys conducted in deep er water show an oppos ite trend (Shuster 1982). While many accounts vary, the model below assumes a 1 : 1 sex
ratio. Adult crabs return to their yearly hab itat on the bay floor after spawning. In general, "high fecundity, high egg and larval mortality, and low adult
mortality" characterize crab population s (Botton and Loveland 1989).
Horseshoe crab s live to be 17-1 9 years old , but do not reach sexu al maturity for 9-11 years. Juvenile and adult horseshoe crabs experience mortality du e to predators and the harshness of the aquatic environment. Juvenile
crabs are most susceptible to predation from other marin e organisms. Adult
crabs are susceptible to predation, but also stranding, which occurs when
spaw ning crabs becom e "lost" and fail to return to the open sea after
spaw ning. Reported stranding rates range from 0.84% of male crabs (Brockmann and Penn 1994) to 10% of all spaw ning crabs, resulting in death (Botton and Loveland 1989). Comme rcial exploitation, as discussed ea rlier, is
increasingly becoming the major threat to adult crab populations.
11.2.2. Shorebirds
Botton et al. (994) estimate 425,000 to 1,000,000 sho reb irds use Delaware
Bay as a staging area eac h May and June, though other reports estimate as
man y as 1.5 million birds or more . Because of its ecological importan ce, the
bay has been the center for horseshoe crab and sho rebird population studies in recent yea rs.
Traveling as many as 30,000 km each year, shorebirds require stopover
sites to build fat reserves (Clark et al. 1993). During stopo ver periods, shorebirds in the Delaware Bay rep ortedly double their mass with crab eggstheir prim ary food resource . Each bird consumes as many as 135,000 eggs
per season (Virtualbirder 2000). Castro and Myers (993) suggest that approximately 3,640,000 adult horse shoe crabs are required to sustain all
sho rebirds stopping-over in the Delaware Bay. Man y different species of
sho rebirds utilize the bay as a staging area. Clark et al. (993) rep ort the four
most abunda nt species to be the Semipa lmated Sandpiper ( Calid ris pusilld),
Ruddy Turnstone (A renaria iruerpres), Red Knot (Calidris canutus) , and
Sande rling ( Calid ris alba). In the model below, these species are gro uped
into a single category of shorebirds, effectively assuming that their impa ct on
horseshoe crabs and the effects of crab egg availability are uniform across
the different species. Specifically, we assume here for simplicity that the
birds have similar body masses, and similar birth and death rates.
11 . Horseshoe Crabs and Shorebirds
their ocean hab itat along the continental shelf and approach the beach in
orde r to spawn. Males attach themselves to females, fertilizing the eggs as the
female lays them in a burrow in the dry sand. Studies have shown that female
horseshoe crabs can lay up to 88,000 eggs per season (Virtualbirder 2000).
The larvae hatch and enter the open ocea n. Juvenile crabs move farther and
farther away from the sho re with increasing age until they are sexua lly mature (Shuster 1982). Although there are notabl y more males than females on
the beach , surveys conducted in deep er water show an oppos ite trend (Shuster 1982). While many accounts vary, the model below assumes a 1 : 1 sex
ratio. Adult crabs return to their yearly hab itat on the bay floor after spawning. In general, "high fecundity, high egg and larval mortality, and low adult
mortality" characterize crab population s (Botton and Loveland 1989).
Horseshoe crab s live to be 17-1 9 years old , but do not reach sexu al maturity for 9-11 years. Juvenile and adult horseshoe crabs experience mortality du e to predators and the harshness of the aquatic environment. Juvenile
crabs are most susceptible to predation from other marin e organisms. Adult
crabs are susceptible to predation, but also stranding, which occurs when
spaw ning crabs becom e "lost" and fail to return to the open sea after
spaw ning. Reported stranding rates range from 0.84% of male crabs (Brockmann and Penn 1994) to 10% of all spaw ning crabs, resulting in death (Botton and Loveland 1989). Comme rcial exploitation, as discussed ea rlier, is
increasingly becoming the major threat to adult crab populations.
11.2.2. Shorebirds
Botton et al. (994) estimate 425,000 to 1,000,000 sho reb irds use Delaware
Bay as a staging area eac h May and June, though other reports estimate as
man y as 1.5 million birds or more . Because of its ecological importan ce, the
bay has been the center for horseshoe crab and sho rebird population studies in recent yea rs.
Traveling as many as 30,000 km each year, shorebirds require stopover
sites to build fat reserves (Clark et al. 1993). During stopo ver periods, shorebirds in the Delaware Bay rep ortedly double their mass with crab eggstheir prim ary food resource . Each bird consumes as many as 135,000 eggs
per season (Virtualbirder 2000). Castro and Myers (993) suggest that approximately 3,640,000 adult horse shoe crabs are required to sustain all
sho rebirds stopping-over in the Delaware Bay. Man y different species of
sho rebirds utilize the bay as a staging area. Clark et al. (993) rep ort the four
most abunda nt species to be the Semipa lmated Sandpiper ( Calid ris pusilld),
Ruddy Turnstone (A renaria iruerpres), Red Knot (Calidris canutus) , and
Sande rling ( Calid ris alba). In the model below, these species are gro uped
into a single category of shorebirds, effectively assuming that their impa ct on
horseshoe crabs and the effects of crab egg availability are uniform across
the different species. Specifically, we assume here for simplicity that the
birds have similar body masses, and similar birth and death rates.