Pacific Coastal Biome
403
in recent decades, here and in other similar species pairs, is natural or is induced by the
inevitable modification of the population characteristics of the target species by fishing.
A scan of recent volumes of the CalCOFI Reports will convince you that we have a long
way to go before consensus is achieved; the essay by Smith et al. (1992) would be a good
entry point to these discussions.
Regional Benthic and Demersal Ecology
The California Current is also home to a major population of Pacific hake (Merluccius
productus) that dominates the demersal fish community of the outer shelf, although these
gadoids behave as midwater pelagics during their migrations between feeding areas off
Oregon-Washington and spawning areas off southern California; larvae and juveniles
return to the northern region in the undercurrent.
The demersal fish fauna of the inner shelf, and especially of the coastal regions, is
dominated by flatfish (Citharichthys spp. and Microstoma spp.) that form about 45%
of all trawl-caught fish in the Southern California Bight; next in abundance are large
sciaenids (Genyonymus spp.) and rockfish (Sebastes spp.). Large populations of decapods
occur here also. There are major differences in the fish and benthic fauna with depth
across the shelf; where the thermocline encounters the shelf (20–60 m) these changes are
most profound and rapid, whereas from 60 m out to the shelf edge, changes are more
gradual; this region is dominated by rockfish, hake, and grenadiers.
The dynamic circulation pattern of the California Current has major consequences for
demersal—as well as for pelagic—fish; seasonal and episodic changes in the upwelling
pattern cause shifts in the pattern of anoxic bottom water on the shelf and consequent
shifts in the distribution of demersal fish (Mearns and Smith, 1975). But more than that,
the specific patterns of reproduction of species of fish and invertebrates are matched to
the exigencies of circulation so as to ensure closure of life cycles (Shanks and Eckert,
2005). Because drift in the California Current is unidirectional for long periods, organisms
with planktonic larvae potentially risk becoming extinct at the population level: this is,
as the authors say, “a marine equivalent of the drift paradox in streams.” Probability of
the closure of life cycles is enhanced by the evolution of suitable duration of planktonic
larval drift, and by the abandoning of planktonic larvae in certain situations. Shanks and
Eckert analyzed the reproductive stratagems of 154 fish and 50 benthic crustaceans and
found three patterns: (i) long drift duration of c. 135 d and high fecundity in long-lived
shelf/slope species, (ii) short drift duration of c. 45 d and lower fecundity in nearshore
species having near-benthic larvae, and (iii) short drift duration of c. 48 d and high
fecundity in coastal species having planktonic larvae.
These patterns appear to have evolved so as to maximize life-cycle closure: the
long-lived larvae of offshore species below the mixed layer experience northward flow in
winter and are returned southward in summer, while the short-lived planktonic larvae
of coastal species maximize the probability of being retained within eddies, as in the
Southern California Bight. The near-shore fishes may experience a reversal of coastwise
flow during and after individual upwelling events. Furthermore, a higher proportion of
nearshore species than others have evolved live bearing of larvae that directly enter the
demersal habitat. These adaptations to the requirement for closure of life cycles, while
exploiting the abundant and suitably scaled food offered to larval fish by the zooplankton,
are probably of general occurrence in other regions influenced by a major coastal current
system.
Synopsis
Case 6—Intermittent production at coastal divergences—Seasonality of pycnocline depth,
meaned over the entire province, is consistent with both boreal winter mixing (60 m,
403
in recent decades, here and in other similar species pairs, is natural or is induced by the
inevitable modification of the population characteristics of the target species by fishing.
A scan of recent volumes of the CalCOFI Reports will convince you that we have a long
way to go before consensus is achieved; the essay by Smith et al. (1992) would be a good
entry point to these discussions.
Regional Benthic and Demersal Ecology
The California Current is also home to a major population of Pacific hake (Merluccius
productus) that dominates the demersal fish community of the outer shelf, although these
gadoids behave as midwater pelagics during their migrations between feeding areas off
Oregon-Washington and spawning areas off southern California; larvae and juveniles
return to the northern region in the undercurrent.
The demersal fish fauna of the inner shelf, and especially of the coastal regions, is
dominated by flatfish (Citharichthys spp. and Microstoma spp.) that form about 45%
of all trawl-caught fish in the Southern California Bight; next in abundance are large
sciaenids (Genyonymus spp.) and rockfish (Sebastes spp.). Large populations of decapods
occur here also. There are major differences in the fish and benthic fauna with depth
across the shelf; where the thermocline encounters the shelf (20–60 m) these changes are
most profound and rapid, whereas from 60 m out to the shelf edge, changes are more
gradual; this region is dominated by rockfish, hake, and grenadiers.
The dynamic circulation pattern of the California Current has major consequences for
demersal—as well as for pelagic—fish; seasonal and episodic changes in the upwelling
pattern cause shifts in the pattern of anoxic bottom water on the shelf and consequent
shifts in the distribution of demersal fish (Mearns and Smith, 1975). But more than that,
the specific patterns of reproduction of species of fish and invertebrates are matched to
the exigencies of circulation so as to ensure closure of life cycles (Shanks and Eckert,
2005). Because drift in the California Current is unidirectional for long periods, organisms
with planktonic larvae potentially risk becoming extinct at the population level: this is,
as the authors say, “a marine equivalent of the drift paradox in streams.” Probability of
the closure of life cycles is enhanced by the evolution of suitable duration of planktonic
larval drift, and by the abandoning of planktonic larvae in certain situations. Shanks and
Eckert analyzed the reproductive stratagems of 154 fish and 50 benthic crustaceans and
found three patterns: (i) long drift duration of c. 135 d and high fecundity in long-lived
shelf/slope species, (ii) short drift duration of c. 45 d and lower fecundity in nearshore
species having near-benthic larvae, and (iii) short drift duration of c. 48 d and high
fecundity in coastal species having planktonic larvae.
These patterns appear to have evolved so as to maximize life-cycle closure: the
long-lived larvae of offshore species below the mixed layer experience northward flow in
winter and are returned southward in summer, while the short-lived planktonic larvae
of coastal species maximize the probability of being retained within eddies, as in the
Southern California Bight. The near-shore fishes may experience a reversal of coastwise
flow during and after individual upwelling events. Furthermore, a higher proportion of
nearshore species than others have evolved live bearing of larvae that directly enter the
demersal habitat. These adaptations to the requirement for closure of life cycles, while
exploiting the abundant and suitably scaled food offered to larval fish by the zooplankton,
are probably of general occurrence in other regions influenced by a major coastal current
system.
Synopsis
Case 6—Intermittent production at coastal divergences—Seasonality of pycnocline depth,
meaned over the entire province, is consistent with both boreal winter mixing (60 m,