Environmental Engineering Reference
In-Depth Information
In the case study, Crouse and colleagues systematically manipulated each
survival probability individually by 50 percent and estimated population
growth rate. Such an improvement is within the realm of possibility.This
analysis reveals that for average baseline conditions observed in natural pop-
ulations at the time of analysis, loggerhead sea turtles should have a nega-
tive population growth rate, implying that their numbers should dwindle to
extinction if there is no management action taken to rescue the population.
The analysis shows, however, that improving egg and hatchling survival by
50 percent
will not
reverse the decline. (See figure 5.5.) However, a 50 per-
cent increase in the survival of small
juveniles, large juveniles, and mature
breeders will reverse the trend and
lead to positive gains in population
size. In essence, the analysis reveals
that the pay off for concentrating ef-
forts on improving egg and hatchling
survival will not rescue the popula-
tion, but improving survival of older-
aged individuals may.
The analysis reveals that the pay off
for concentrating efforts on improv-
ing egg and hatchling survival will
not rescue the population, but
improving survival of older-aged
individuals may.
Rescuing Sea Turtle Populations
The simulation results beg the question: How does one improve the sur-
vivorship of individuals that spend most of their life unseen in the vast
ocean? The key here is to identify the source of their mortality. It turns out
that one important source of loggerhead sea turtle mortality in the ocean
is being inadvertently caught in fishing trawler nets (Crouse et al. 1997). Un-
like fish, which can extract oxygen from water through gills, sea turtles have
lungs and so they must swim to the ocean surface regularly to breath.Tur-
tles are unable to do this when they are caught in trawler fishing nets.The
solution, therefore, is to figure out how to prevent turtles from being caught
by fisheries.The ingenious technological solution is called a turtle excluder
device, known by the acronym TED.This device selectively harvests fish and
other seafood but prevents turtles from becoming entangled in the nets.
However,TEDs also decrease the capture efficiency of nets and thus lower
the income rate of the fishermen.Thus, there is a trade-off between an eco-
nomic gain and conservation.
Further analysis illustrates how a structured modeling approach can eval-
uate different ways of reconciling this trade-off through different regulatory
