15 .5. Discussion
349
for increasing values of v for all MPA sizes of 100% habitat protected or less.
The decline was more pronounced at MPA sizes of 50% or less and for values of v ranging from 0.1 to 0.5. Survivorship for the type II response curve
ranged from a maximum of approximately 4.5% to a minimum of 1%, while
ranging from 9.5% to 3.9% for the type III response curve .
15.4.2. Post-Settlement Juvenile Cod Density
and MPA Size
Now let us investigate the influence of juvenile cod densities on the optimal
sizing of MPAs. The number of fish settling to the sea floor in the first period, S, was varied from 1 m ? to 10 m ? in subsequent runs. The size of an
MPA was varied from 0 to 100% of a habitat patch protected. Juvenile
movement, v, was held constant at 0.5 for all model runs to isolate the influence of fish density . For each treatment, cells in a 4 X 4 matrix were converted randomly from unprotected to MPA over the course of multiple (n =
100) model runs . Juvenile cod settled to the sea floor in the first month and
were immediately subjected to density-dependent natural mortality, specified by type I, type II and type III predator functional response curves.
Juvenile cod survivorship in areas protected by an MPA reflected similar
patterns for each of the three functional response curves (Figures 15.4
a.b,c), The uniform slope of the response surface for the type I response
(Figure15.4 a) reflected the dominance of MPA size over fish density in determining survivorship in a system wherein predation exhibits a type I functional response. For type II and III response curves, the response surfaces
(Figures 15.4 b & c) indicate the population responses to low fish density
and the corresponding increase in survivorship .
15.5. Discussion
Habitat disturbance by fishing activities is the dominant anthropogenic affect
on the marine environment (Dayton et al. 1995; Watling & Norse 1998). Mobile fishing gear in particular is estimated to affect sea floor habitat with an intensity and spatial extent that is orders of magnitude greater than other disturbances to the same environment (Watling & Norse 1998). Nevertheless, the
effect of habitat alteration on associated fish species in the northwest Atlantic,
and the rate of recovery of those species in the absence of fishing, have yet to
be sufficiently quantified for the development of strategic management decisions (Auster et al. 1997). Lindholm et al. (999) quantified relative differences
in habitat-mediated predation on juvenile cod at the microhabitat scale. In the
model present here, we used the results of this small-scale study to predict the
effects of habitat-mediated processes on population responses at the landscape scale. Such an exercise requires the extrapolation of trends identified at
the scale of microhabitats to the scale of kilometers or more, and thus can invite errors of extrapolation. However, our approach relies on the use of rela-
349
for increasing values of v for all MPA sizes of 100% habitat protected or less.
The decline was more pronounced at MPA sizes of 50% or less and for values of v ranging from 0.1 to 0.5. Survivorship for the type II response curve
ranged from a maximum of approximately 4.5% to a minimum of 1%, while
ranging from 9.5% to 3.9% for the type III response curve .
15.4.2. Post-Settlement Juvenile Cod Density
and MPA Size
Now let us investigate the influence of juvenile cod densities on the optimal
sizing of MPAs. The number of fish settling to the sea floor in the first period, S, was varied from 1 m ? to 10 m ? in subsequent runs. The size of an
MPA was varied from 0 to 100% of a habitat patch protected. Juvenile
movement, v, was held constant at 0.5 for all model runs to isolate the influence of fish density . For each treatment, cells in a 4 X 4 matrix were converted randomly from unprotected to MPA over the course of multiple (n =
100) model runs . Juvenile cod settled to the sea floor in the first month and
were immediately subjected to density-dependent natural mortality, specified by type I, type II and type III predator functional response curves.
Juvenile cod survivorship in areas protected by an MPA reflected similar
patterns for each of the three functional response curves (Figures 15.4
a.b,c), The uniform slope of the response surface for the type I response
(Figure15.4 a) reflected the dominance of MPA size over fish density in determining survivorship in a system wherein predation exhibits a type I functional response. For type II and III response curves, the response surfaces
(Figures 15.4 b & c) indicate the population responses to low fish density
and the corresponding increase in survivorship .
15.5. Discussion
Habitat disturbance by fishing activities is the dominant anthropogenic affect
on the marine environment (Dayton et al. 1995; Watling & Norse 1998). Mobile fishing gear in particular is estimated to affect sea floor habitat with an intensity and spatial extent that is orders of magnitude greater than other disturbances to the same environment (Watling & Norse 1998). Nevertheless, the
effect of habitat alteration on associated fish species in the northwest Atlantic,
and the rate of recovery of those species in the absence of fishing, have yet to
be sufficiently quantified for the development of strategic management decisions (Auster et al. 1997). Lindholm et al. (999) quantified relative differences
in habitat-mediated predation on juvenile cod at the microhabitat scale. In the
model present here, we used the results of this small-scale study to predict the
effects of habitat-mediated processes on population responses at the landscape scale. Such an exercise requires the extrapolation of trends identified at
the scale of microhabitats to the scale of kilometers or more, and thus can invite errors of extrapolation. However, our approach relies on the use of rela-