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Subject: Re: death by fishing
From: "Mia J. Tegner" <[log in to unmask]>
Reply-To:Academic forum on fisheries ecology and related topics <[log in to unmask]>
Date:Tue, 8 Apr 1997 11:52:43 -0700

text/plain (60 lines)

"I keep hearing pro-fishing lobbyists saying "sure we fish 'em down
pretty hard, but there's never actually been a species that was fished
to extinction."

California is very close the dubious distinction driving an invertebrate to
biological extinction through fishing.  White abalones are highly prized
shellfish which are found in predictable and accessible locations near
stands of their algal food.  These large snails are benthic, movement is
minimal, and as adults most adopt non-cryptic habitat.  Abalones generally
are slow growing, long-lived, and have unpredictable recruitment; as a
result size-frequency distributions tend to be skewed to the right with an
accumulation of older individuals.  Whites are the deepest living of the
seven abalone species in California, but their tender meat yielded premium
value. After harvest of the shallower living species declined, there was a
short but intense commercial fishery on whites; 95% of commercial landings
took place in just nine years, 1969-77.  Recent surveys found two to three
order of magnitude drops in abundance from the 1970s and 1980s.  Only three
white abalones were found in 3.06 ha of prime habitat in 1991-3, habitat
that had supported 6120-30600 animals 20 yrs earlier.  Size analysis of the
shells and live animals suggested that that the last major recruitment to
the population was in the late 60s, early 70s, and that the population is
slowly dying of old age.

A combination of fishing pressure and biology seems to have created this
situation.  The high economic return to fishermen (abalone meat currently
retails at $85/lb) has justified extensive commercial search effort for
many years; recreational diving pressure is also very intensive in Southern
California and under no economic constraints.  Management was based almost
entirely on a size limit designed to allow for several years of spawning;
their deep (26-85m) and patchy distributions largely precluded fishery
independent stock assessment.  What we now know is that these dioecious
broadcast spawners require high concentrations of sperm for successful
fertilization, and thus aggregations of adults are required.  Basically,
fishing reduced the density of adults below the minimum viable population
and reproduction failed.

Because these animals were predominantly found on offshore islands, there
is no evidence that pollution or habitat alterations have affected the
stock.  Nor is there evidence of a role for introduced species or
hybidization in this story.

There are no doubt a few dozen white abalones left in the wild, but it is
highly unlikely that reproduction will be successful without human
intervention.  There are less than a dozen white abalones in captivity and
currently no funds for aquaculture.  The challenge for California is to
incorporate this lesson into management of its other abalone species:
protection of natural stocks will require going beyond dependence on
minimum size limits to ensure survival of brood stock at appropriate
densities through periods of poor recruitment.  Marine reserves are an
obvious suggestion, but will only work if education and enforcement are
adequate to make them functional.  A brood stock transplantation experiment
to the coastline of Los Angeles County actually produced recruitment of
green abalones, but the adults were poached and abalones are essentially
absent from this habitat today.

Mia J. Tegner                                             Phone:  (619) 534-2059
Scripps Institution of Oceanography             Fax:  (619) 534-6500

University of California, San Diego            Internet:  [log in to unmask]
La Jolla, California 92093-0201

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