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UCSD: Scripps Oceanography News Release: Fishing Throws Targeted Species Off Balance


William Silvert <[log in to unmask]>


Scientific forum on fish and fisheries <[log in to unmask]>


Thu, 17 Apr 2008 10:53:24 +0100





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Given the relative quiet on these lists, I feel that a fairly long posting
will not overload your mailboxes. A colleague sent me this message and while
the view that we should conserve the older fish rather than catching them
and just letting the younger fish escape has been posted frequently, this is
one of the more interesting arguments that I have seen. It does not address
the points of larval quality, but introduces the concept of stability
against environmental fluctuations. It will be interesting to see how these
ideas affect fisheries management -- not only whether the concept is
accepted, but how it will be implemented. Mesh size regulations are designed
to let the smaller fish escape, but how do we let the large firsh escape?

Bill Silvert

----- Original Message -----
From: Scripps News <[log in to unmask]>
Date: April 16, 2008 12:15:38 PM CDT (CA)
Subject: [Ucomm_sio-l] UCSD: Scripps Oceanography News Release: Fishing
Throws Targeted Species Off Balance, Scripps Study Shows

The following news release and any accompanying images can be found at:

Scripps Contacts: Mario Aguilera or Annie Reisewitz
858-534-3624; [log in to unmask]

Fishing Throws Targeted Species Off Balance, Scripps Study Shows

Researchers say fishing disrupts age structure, making regulation difficult

Scripps Institution of Oceanography/UC San Diego

Fishing activities can provoke volatile fluctuations in the populations they
target, but it’s not often clear why. A new study published in the journal
Nature by scientists at Scripps Institution of Oceanography at UC San Diego
and colleagues has identified the general underlying mechanism.

Research led at Scripps with a distinguished team of government and
international experts (including two chief scientific advisors to the United
Kingdom) demonstrates that fishing can throw targeted fish populations off
kilter. Fishing can alter the “age pyramid” by lopping off the few large,
older fish that make up the top of the pyramid, leaving a broad base of
faster-growing small younglings. The team found that this rapidly growing
and transitory base is dynamically unstable—a finding having profound
implications for the ecosystem and the fishing industries built upon it.

“The data show that fished species appear to be significantly more nonlinear
and less stable than unfished species,” said Professor George Sugihara of
Scripps. “We think the mechanism involves systematic alteration of the
demographic parameters—and especially increases in growth rates that magnify
destabilization in many ways—which can happen as fishing truncates the age

Imagine a container of water with a 500-pound fish. With food, it grows a
little bigger. Without food it gets a bit smaller. Imagine the same
container with 500 one-pound fish. They eat, reproduce and the resulting
thousands of fish boom, quickly outstripping the resources and the
population crashes. These many smaller fish—with the same initial “biomass”
as the larger fish—can’t average out the environmental fluctuations, and in
fact amplify them through higher turnover rates that promote boom and bust

The study that included academic and government scientists from Alaska, Asia
and Great Britain is based on data from the California Cooperative Oceanic
Fisheries Investigations (CalCOFI), a program based at Scripps that has
monitored fish and oceanographic activities of the California Current for
more than 50 years. To arrive at their results, the researchers compared the
CalCOFI records of larvae, a key indicator of adult populations, of both
fished and non-fished species in the California Current.

Fishing typically extracts the older, larger members of a targeted species
and fishing regulations often impose minimum size limits to protect the
smaller, younger fishes.

“That type of regulation, which we see in many sport fisheries, is exactly
wrong,” said Sugihara. “It’s not the young ones that should be thrown back,
but the larger, older fish that should be spared. Not only do the older fish
provide stability and capacitance to the population, they provide more and
better quality offspring.”

Thus the danger, according to Sugihara, is that current policies that manage
according to current biomass targets (without significant forecast skill)
while ignoring fish size pose risks that can further destabilize the
population. This instability can in principle propagate systemically to the
whole ecosystem, much like a stock market crash or a domino effect, and
magnify risk for the fishing industry itself as well as those of
ecologically related fisheries.

This is especially true when trying to rebuild fish stocks, Sugihara says.

“This may be the most important implication of this work, as we attempt to
rehabilitate fisheries,” said Sugihara. “Regulations based solely on biomass
harvest targets are incomplete. They must also account for age-size
structure in the populations,” he said. “Current policies and industry
pressures that encourage lifting bans on fishing when biomass is
rehabilitated—but where maximum age and size are not—contain risk.”

This is currently the case with Atlantic swordfish, for which industry
pressures to resume fishing are based on the restoration of historic biomass
levels, even though the swordfish are clearly undersized.

“In the extreme case, the danger of such unstable dynamics for certain
populations for management is that harvest targets may lag the population,
potentially making things worse,” said Sugihara. “A high harvest target may
be set after an especially abundant period when the population may be poised
to decline on it’s own. Likewise future abundant periods may represent
missed opportunities, despite current low abundances. As senior officials
of the Canadian Department of Fisheries and Oceans have said, ‘we are often
a year behind in our stock projections.’”

Sugihara cautioned that nonlinearity is not unique to fished species.
Nonequilibrium overshooting and undershooting occurs in unexploited stocks,
but to a lower extent. Therefore, classical single-species population models
that require equilibrium are unlikely to be very successful in stock
forecasts, except perhaps in the very short term.

"Other methods that do not rely on these assumptions may be more promising,"
suggests Christian Anderson, paper co-author.

In addition to Sugihara and Anderson, the study included Scripps
Oceanography alumnus Chih-hao Hsieh (now a professor at National Taiwan
University); Stuart Sandin of Scripps; Roger Hewitt of the National Marine
Fisheries Service, Southwest Fisheries Science Center; Anne Hollowed of the
National Marine Fisheries Service, Alaska Fisheries Science Center; Sir John
Beddington of Imperial College London (current Chief Science Advisor to the
United Kingdom) and Lord Robert May of Oxford (a former Chief Scientific
Advisor to the UK).

The research was supported by NOAA Fisheries and the Environment program,
The MacQuown Chair of Natural History, The Deutsche Bank – Jameson
Complexity Studies Fund, the Sugihara Family Trust and the Kyoto University
grant for Biodiversity Research of the 21st Century.

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