| The
collapse of the North Atlantic cod fishery is widely described
as the perfect example of overfishing and mismanagement.
Analyses of the causes focus on “a range of social
and political factors that were implicated in the collapse
of the stocks, including overfishing, government mismanagement
and the disregard of scientific uncertainty.”
However,
a new comprehensive analysis of the cod populations of the
North Atlantic has taken another approach – looking
for similarities and differences in the various discrete
stocks throughout the North Atlantic, while measuring differences
in growth and reproduction in each stock. When commercial
fishery effects are added to this study, a surprising new
conclusion becomes apparent: Environmental factors have
been an overwhelming influence all along.
A study
by Brian Rothschild, published in the Transactions of the
American Fisheries Society, finds that a strong negative
environmental signal, probably associated with plankton
dynamics, was a likely suspect in the cod’s disappearance.
Spawning stock biomass (SSB - total weight of mature fish)
tracked similar trends in all 11 cod stocks: declining from
the 1960s through 1975, then increasing through 1985, then
falling drastically through the 1990s and remaining low
afterwards. Both the population increase after 1975 and
the later decline occurred while mortality from fishing
was low. Fishing mortality began to increase as the stocks
declined post-1985. During this period, relative growth
rates also fell, indicating a problem quite different than
simple fishing mortality. Cod stocks did not recover even
after a fishing moratorium was implemented.
Rothschild
suspects that several factors interacted to cause the population
crash, and that this was almost certainly not simply a case
of overfishing. That the population effects were similar
across the different stocks of the North Atlantic most likely
indicate problems at the base of the food web – plankton
dynamics.
"These
environmental changes were probably as important in influencing
declines in cod abundance as the effects of fishing,”
said Rothschild. “The standing stock biomass and weight-at-age
statistics for various stocks tend to follow the same pattern.
However, when fishing is superimposed on top of an unfavorable
environment, it appears to accelerate the negative effects
of the environment in bringing about a decline.”
Rothschild
noted these observations have important implications for
ocean fishery management.
All
of the measurements used in fishery management, such as
production,
yield-per-recruit, SSB and indices of recruitment tend to
measure the effects of fishing while ignoring environmental
effects and ecosystem interactions. The current method of
rebuilding depleted fish stocks by reducing fishing mortality
alone may be extremely simplistic.
The
study further points to our disturbing level of ignorance
of large-scale ocean ecology. Although we have gathered
bits of information about various aspects of aquatic systems,
it is obvious that, in many cases, what we know is a fraction
of what we need to know to make the best management decisions.
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