Pacific Coastal Biome
415
stress in El Niño years when it has been observed to retreat to water >100 m deep.
Although it has been assumed that anchoveta recruitment was poor in these years, the
recruitment failure of 1971, which led to the subsequent population collapse, occurred
prior to the 1972–73 El Niño. This occurred very shortly after the period 1965–1970 when
the stock biomass was variously estimated at 12–20 million tons. Almost immediately, the
very small biomass of the sardine stock began to increase so that catches of ∼6 million
tons were obtained in the early 1980s. Meanwhile, the anchoveta stock was reduced to less
than 1 million tons by the end of the 1982–83 El Niño but subsequently exploded (the
word is taken from Alheit and Bernal) by an as-yet unsatisfactorily explained recruitment
process to reach well above 6 million tons by 1985.
What to make of all this? Certainly, as Garth Murphy pointed out long ago (to
the chagrin of the Peruvian fishery managers) the truncation of the year-classes of the
anchoveta stock by the fishery, so that each fish could count on only a single spawning
event, must logically lead to population collapse. This simple truth, denied by some
theoretical ecologists, reflects the fact that specific longevity in fish must evolve as a
function of uncertainty of recruitment success. Industrial fisheries inevitably truncate
longevity, with predictable results.
Nevertheless, over the relatively short period that the stocks have been observed, it
is clear that while El Niño conditions reduce the survival of all life stages of anchoveta,
the same conditions have a positive effect on sardines. This despite the fact that sardines
perform extensive migrations during these events, apparently to avoid the intrusion of
warm waters from the north.
Regional Benthic and Demersal Ecology
The benthic phenomenon that has attracted most attention in this province is the generation of anoxic bottom water off Peru, with formation of dissolved H 2 S. This is the
consequence of the very heavy deposition of POM, including unconsumed phytoplankton cells, onto the continental shelf, there to undergo transformation by the filamentous
sulfur bacteria Thioplica (Rosenberg et al., 1983). Disturbance of bottom water, as by the
1828 earthquake at Callao, may release a high concentration of H 2 S into the atmosphere.
This phenomenon is often confused with the red water caused by massive growth of
Gymnodinium splendens, termed the “Callao painter” by mariners, thought to be stimulated by masses of decaying organic material on the seabed. This phenomenon occurs
commonly enough here, and in similar places—as on the SW African coast.
There is some evidence, reviewed by Rosenberg et al. (1983), that the probability of
the release of H 2 S is related to the biomass of planktivorous pelagic fish in the superjacent
water column. After the 1971 collapse of the anchoveta stocks there was a decade-long
increase in sediment organic matter and in sulfate reduction on the sea floor associated
with declines in nitrate and oxygen content of near-bottom water. The relative distribution
of anoxic conditions remained relatively unchanged, so that from the surface to 50 m,
oxygen concentration was progressively reduced from 5.0 to 05 ml liter
−1 leading down
to minimal concentrations (< 03 ml liter
−1 ) at 400–600 m, below which concentrations
again increase. There is little seasonality in this pattern, which persists from northern
Peru to central Chile.
Both benthic invertebrates and demersal fish populations respond more directly to
oxygen tension than to any other factor; a “classical” benthos survey with van Veen
grabs at 65 stations on the Peruvian shelf by Rosenberg et al. clearly showed that here
the Thorson-Petersen benthic communities are essentially absent, except in the surf zone
along the beach and just below the beach, where a perfectly normal benthic assemblage was
found: Donax, Emerita, Nepthys, Callinassa, Paguridae, and so on. This study confirmed
the relative distribution of dissolved oxygen discussed earlier, showing that percentage
415
stress in El Niño years when it has been observed to retreat to water >100 m deep.
Although it has been assumed that anchoveta recruitment was poor in these years, the
recruitment failure of 1971, which led to the subsequent population collapse, occurred
prior to the 1972–73 El Niño. This occurred very shortly after the period 1965–1970 when
the stock biomass was variously estimated at 12–20 million tons. Almost immediately, the
very small biomass of the sardine stock began to increase so that catches of ∼6 million
tons were obtained in the early 1980s. Meanwhile, the anchoveta stock was reduced to less
than 1 million tons by the end of the 1982–83 El Niño but subsequently exploded (the
word is taken from Alheit and Bernal) by an as-yet unsatisfactorily explained recruitment
process to reach well above 6 million tons by 1985.
What to make of all this? Certainly, as Garth Murphy pointed out long ago (to
the chagrin of the Peruvian fishery managers) the truncation of the year-classes of the
anchoveta stock by the fishery, so that each fish could count on only a single spawning
event, must logically lead to population collapse. This simple truth, denied by some
theoretical ecologists, reflects the fact that specific longevity in fish must evolve as a
function of uncertainty of recruitment success. Industrial fisheries inevitably truncate
longevity, with predictable results.
Nevertheless, over the relatively short period that the stocks have been observed, it
is clear that while El Niño conditions reduce the survival of all life stages of anchoveta,
the same conditions have a positive effect on sardines. This despite the fact that sardines
perform extensive migrations during these events, apparently to avoid the intrusion of
warm waters from the north.
Regional Benthic and Demersal Ecology
The benthic phenomenon that has attracted most attention in this province is the generation of anoxic bottom water off Peru, with formation of dissolved H 2 S. This is the
consequence of the very heavy deposition of POM, including unconsumed phytoplankton cells, onto the continental shelf, there to undergo transformation by the filamentous
sulfur bacteria Thioplica (Rosenberg et al., 1983). Disturbance of bottom water, as by the
1828 earthquake at Callao, may release a high concentration of H 2 S into the atmosphere.
This phenomenon is often confused with the red water caused by massive growth of
Gymnodinium splendens, termed the “Callao painter” by mariners, thought to be stimulated by masses of decaying organic material on the seabed. This phenomenon occurs
commonly enough here, and in similar places—as on the SW African coast.
There is some evidence, reviewed by Rosenberg et al. (1983), that the probability of
the release of H 2 S is related to the biomass of planktivorous pelagic fish in the superjacent
water column. After the 1971 collapse of the anchoveta stocks there was a decade-long
increase in sediment organic matter and in sulfate reduction on the sea floor associated
with declines in nitrate and oxygen content of near-bottom water. The relative distribution
of anoxic conditions remained relatively unchanged, so that from the surface to 50 m,
oxygen concentration was progressively reduced from 5.0 to 05 ml liter
−1 leading down
to minimal concentrations (< 03 ml liter
−1 ) at 400–600 m, below which concentrations
again increase. There is little seasonality in this pattern, which persists from northern
Peru to central Chile.
Both benthic invertebrates and demersal fish populations respond more directly to
oxygen tension than to any other factor; a “classical” benthos survey with van Veen
grabs at 65 stations on the Peruvian shelf by Rosenberg et al. clearly showed that here
the Thorson-Petersen benthic communities are essentially absent, except in the surf zone
along the beach and just below the beach, where a perfectly normal benthic assemblage was
found: Donax, Emerita, Nepthys, Callinassa, Paguridae, and so on. This study confirmed
the relative distribution of dissolved oxygen discussed earlier, showing that percentage