11 .7. Q uestions and Tasks
227
respectively. As the exploitation rate increases , horses hoe crab and shorebird populations show declines. Since the eel industry harvests only female
crabs, their numbers are naturally lower than the males, and decline more
sharply from the outset.
11.6. Conclusions
A host of model conditions yields horseshoe crab and migratory shorebird
populations in the Delaware Bay that are stable in the long run . Since
shorebird populations depend on horseshoe crab spaw ning for energy to
complete their migrations and successfully reprodu ce, any decrease in
horseshoe crab spawning noticeably affects bird popul ations. The opposite,
however, is not true . Fluctuations in bird predation activity do not noticeably effect horseshoe crab populations. Commercial exploitation of adult
horseshoe crabs has a profound impact on crab and shorebird populations.
Total exploitation levels of 0 to 0.11 crabs per crab per season for both the
eel and conch fisheries impact crab population levels, but do not take the
system out of steady-state. Rates above 0.12 send crab and bird populations
into permanent decline.
Commercial exploitation of horseshoe crabs is on the rise throughout
much of the Atlantic coast. In 1996, an estimated 2,000,000 lbs. of horseshoe crabs were rep orted caught in the Delaware Bay (Virtualbirder 2000).
Assuming each crab we ighs an average of 3.5 lbs, and a high count of
3,700,000 crabs spaw ned that year , fishe rmen took 15% of the total population . Given the results of our model , we argue that current trends are not
sustainable.
11.7. Questions and Tasks
1. This model assumes a constant physical environment. However, natur al
destruction and anthropogenic conversion of bea ches may impact horseshoe crab habitat.
Introduce a parameter into the model that reflects land conversion,
or random impacts of tide fluctuations on the availability of horseshoe
crab spawning area. How does such a land conversion affect the model's
conclusions?
2. Introdu ce a exploit ation rate for juvenile crabs, keeping all other values
constant. How does this affect sustainable exploitation levels of adults?
3. One of the key assumptions in the model is that migratory shorebirds
only eat horseshoe crab eggs. How could other food sources for migrating shorebirds be introduced into the model?
4. Make exploitation rates an endogenous part of the model to reflect adjustments in effort as a function of past (and anticipated future) catch.
227
respectively. As the exploitation rate increases , horses hoe crab and shorebird populations show declines. Since the eel industry harvests only female
crabs, their numbers are naturally lower than the males, and decline more
sharply from the outset.
11.6. Conclusions
A host of model conditions yields horseshoe crab and migratory shorebird
populations in the Delaware Bay that are stable in the long run . Since
shorebird populations depend on horseshoe crab spaw ning for energy to
complete their migrations and successfully reprodu ce, any decrease in
horseshoe crab spawning noticeably affects bird popul ations. The opposite,
however, is not true . Fluctuations in bird predation activity do not noticeably effect horseshoe crab populations. Commercial exploitation of adult
horseshoe crabs has a profound impact on crab and shorebird populations.
Total exploitation levels of 0 to 0.11 crabs per crab per season for both the
eel and conch fisheries impact crab population levels, but do not take the
system out of steady-state. Rates above 0.12 send crab and bird populations
into permanent decline.
Commercial exploitation of horseshoe crabs is on the rise throughout
much of the Atlantic coast. In 1996, an estimated 2,000,000 lbs. of horseshoe crabs were rep orted caught in the Delaware Bay (Virtualbirder 2000).
Assuming each crab we ighs an average of 3.5 lbs, and a high count of
3,700,000 crabs spaw ned that year , fishe rmen took 15% of the total population . Given the results of our model , we argue that current trends are not
sustainable.
11.7. Questions and Tasks
1. This model assumes a constant physical environment. However, natur al
destruction and anthropogenic conversion of bea ches may impact horseshoe crab habitat.
Introduce a parameter into the model that reflects land conversion,
or random impacts of tide fluctuations on the availability of horseshoe
crab spawning area. How does such a land conversion affect the model's
conclusions?
2. Introdu ce a exploit ation rate for juvenile crabs, keeping all other values
constant. How does this affect sustainable exploitation levels of adults?
3. One of the key assumptions in the model is that migratory shorebirds
only eat horseshoe crab eggs. How could other food sources for migrating shorebirds be introduced into the model?
4. Make exploitation rates an endogenous part of the model to reflect adjustments in effort as a function of past (and anticipated future) catch.