14.4. Results
323
The relative sizes of the age classes vary dramatically over time in response to fishing. Figures 14.3 and 14.4 show changes in the relative size of
cod populations by age cohort in one of the areas (Region 1) captured by
the model. The trends of the changes in the age structure of the population
in Region 2 and of the other species in each of the regions are in essence
identical to each other, exhibiting a long-term eradication of older age
classes and an increased predominance of juveniles. This trend is in accordance with currently observed population dynamics on Georges Bank (US
Department of Commerce 1996). The model further shows relatively steady
overall system behavior which is accompanied by significant changes in individual population sizes and age distributions, results that are in accordance with recent literature on multispecies fisheries (Sherman & Alexander
1986). However, in contrast to Wilson et al. C1991a,b), no erratic population
dynamics are found at the individual species level, in part because, in the
model presented here , fishermen are unable to reallocate effort at will to individual species, but continue indiscriminately to raise fishing-induced mortality through the use of otter trawls.
As a result of the shift in the age structure to younger fish, overall egg
production is reduced. Not modeled here, but also of significant relevance
for the groundfisheries, is the change in fecundities and mean weight of
fish in individual age classes as the mean age of fish in the population decreases . Too little empirical work has been done, however, to incorporate
such dynamics into the model at this point.
The previous scenarios assume continuation of current management practices under alternative rates of technical change . We assume enactment of effort controls similar to those used by the New England Fisheries Management
i
i
i
I
1.00 ---- l- -- -I
..
I .> I
=
o
] 0.50
I I"J
,...;J-v]'
I
r----.- - ---1
I
'
.
I I
I
I
I
; V
''I I' V
- r N e,v
it!
0.00 1990
I
- 3__
2
Year
FIGURE 14.3. Relative size of age cohorts of cod in Region 1 (numbers on curves correspond to age classes 1 to 5) at 3% annual technological improvement.
'I
I
I
2030
323
The relative sizes of the age classes vary dramatically over time in response to fishing. Figures 14.3 and 14.4 show changes in the relative size of
cod populations by age cohort in one of the areas (Region 1) captured by
the model. The trends of the changes in the age structure of the population
in Region 2 and of the other species in each of the regions are in essence
identical to each other, exhibiting a long-term eradication of older age
classes and an increased predominance of juveniles. This trend is in accordance with currently observed population dynamics on Georges Bank (US
Department of Commerce 1996). The model further shows relatively steady
overall system behavior which is accompanied by significant changes in individual population sizes and age distributions, results that are in accordance with recent literature on multispecies fisheries (Sherman & Alexander
1986). However, in contrast to Wilson et al. C1991a,b), no erratic population
dynamics are found at the individual species level, in part because, in the
model presented here , fishermen are unable to reallocate effort at will to individual species, but continue indiscriminately to raise fishing-induced mortality through the use of otter trawls.
As a result of the shift in the age structure to younger fish, overall egg
production is reduced. Not modeled here, but also of significant relevance
for the groundfisheries, is the change in fecundities and mean weight of
fish in individual age classes as the mean age of fish in the population decreases . Too little empirical work has been done, however, to incorporate
such dynamics into the model at this point.
The previous scenarios assume continuation of current management practices under alternative rates of technical change . We assume enactment of effort controls similar to those used by the New England Fisheries Management
i
i
i
I
1.00 ---- l- -- -I
..
I .> I
=
o
] 0.50
I I"J
,...;J-v]'
I
r----.- - ---1
I
'
.
I I
I
I
I
; V
''I I' V
- r N e,v
it!
0.00 1990
I
- 3__
2
Year
FIGURE 14.3. Relative size of age cohorts of cod in Region 1 (numbers on curves correspond to age classes 1 to 5) at 3% annual technological improvement.
'I
I
I
2030