15.5. Discussion
353
ment rate remains significant for juvenile survivorship. However, the magnitude of the influence is reduced as the size of the MPAs encompasses a
greater portion of the possible range of a juvenile cod .
Practical limitations imposed on MPA designation by real-world constraints
in the management process indicate that we must look at the lower half of
the response surfaces for juvenile cod survivorship patterns as we might expect to see them in the field. It is unlikely that large areas of a given system
will be closed for the purposes of fishery management. Rather, smaller areas
(e.g. 10-20% area protected) are far more likely to be implemented (Plan Development Team 1990). Moreover, the export of fish from MPAs to surrounding harvested populations is of particular importance to the fishing community and managers alike. Although our model does not deal explicitly with
recruitment to adult populations harvested outside MPAs, the flexibility of the
model would permit easy simulation of this phenomenon.
With respect to juvenile cod density, results from the model indicate that
survivorship of juvenile cod was highest for populations at low densities, particularly where MPA size was large, with survivorship significantly lower at
higher population densities and for smaller MPAs. This basic relation generally
holds across the three forms of density-dependent mortality. The location of
the peak of each response surface confirms a priori expectations with the minimum modeled density of 1 m? and 100% of the habitat. Atlantic cod populations in the Gulf of Maine and on Georges Bank currently represent a fraction
of historical population levels (Clark 1998). The utilityof habitat-specific MPAs
for the protection of cod at low population densities is a particularly appropriate subject of concern for both conservation and fisheries management.
From a management perspective, these results suggest that the designation of habitat-specific MPAs for areas identified as location s of juvenile cod
settlement may produce significant increases in survivorship for populations at low densities. Again, when one looks at the lower half (MPA < 50%
total area protected) of the response surfaces for guidance in identifying the
role of habitat for improving juvenile cod survivorship , survivorship in
MPAs that protect 20% of hard-bottom sea floor habitat range from 0.5 to
19% higher than cases in which no protection exists.
The ultimate objectives of modeling studies such as ours are to explore
fish population responses to habitat alteration , and to identify the role
MPAs can serve in conservation of biodiversity and fishery management.
Modeling also provides the opportunity to identify needed laboratory and
field research that can be used to further specify key parameter values in
the model for more accurate prediction of population responses. The use of
nonextractive and habitat-specific MPAs for fisheries management that we
modeled does not exhaust the management tools available to conservationists and fishery managers. In the face of continued uncertainty regarding
post-settlement ecological processes on the sea floor, such MPAs represent
a precautionary or risk-averse approach to management (Ludwig et al.
1993; Auster et al. 1997) designed to protect diversity in general, and in fish
communities in particular, while research continues.
353
ment rate remains significant for juvenile survivorship. However, the magnitude of the influence is reduced as the size of the MPAs encompasses a
greater portion of the possible range of a juvenile cod .
Practical limitations imposed on MPA designation by real-world constraints
in the management process indicate that we must look at the lower half of
the response surfaces for juvenile cod survivorship patterns as we might expect to see them in the field. It is unlikely that large areas of a given system
will be closed for the purposes of fishery management. Rather, smaller areas
(e.g. 10-20% area protected) are far more likely to be implemented (Plan Development Team 1990). Moreover, the export of fish from MPAs to surrounding harvested populations is of particular importance to the fishing community and managers alike. Although our model does not deal explicitly with
recruitment to adult populations harvested outside MPAs, the flexibility of the
model would permit easy simulation of this phenomenon.
With respect to juvenile cod density, results from the model indicate that
survivorship of juvenile cod was highest for populations at low densities, particularly where MPA size was large, with survivorship significantly lower at
higher population densities and for smaller MPAs. This basic relation generally
holds across the three forms of density-dependent mortality. The location of
the peak of each response surface confirms a priori expectations with the minimum modeled density of 1 m? and 100% of the habitat. Atlantic cod populations in the Gulf of Maine and on Georges Bank currently represent a fraction
of historical population levels (Clark 1998). The utilityof habitat-specific MPAs
for the protection of cod at low population densities is a particularly appropriate subject of concern for both conservation and fisheries management.
From a management perspective, these results suggest that the designation of habitat-specific MPAs for areas identified as location s of juvenile cod
settlement may produce significant increases in survivorship for populations at low densities. Again, when one looks at the lower half (MPA < 50%
total area protected) of the response surfaces for guidance in identifying the
role of habitat for improving juvenile cod survivorship , survivorship in
MPAs that protect 20% of hard-bottom sea floor habitat range from 0.5 to
19% higher than cases in which no protection exists.
The ultimate objectives of modeling studies such as ours are to explore
fish population responses to habitat alteration , and to identify the role
MPAs can serve in conservation of biodiversity and fishery management.
Modeling also provides the opportunity to identify needed laboratory and
field research that can be used to further specify key parameter values in
the model for more accurate prediction of population responses. The use of
nonextractive and habitat-specific MPAs for fisheries management that we
modeled does not exhaust the management tools available to conservationists and fishery managers. In the face of continued uncertainty regarding
post-settlement ecological processes on the sea floor, such MPAs represent
a precautionary or risk-averse approach to management (Ludwig et al.
1993; Auster et al. 1997) designed to protect diversity in general, and in fish
communities in particular, while research continues.