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.
This schematic outlines variability on exploited and unexploited.
"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 structure."
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 cycles.
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
The schematic outlines variability on exploited and unexploited.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.
Photo Credit: Charles Smith
"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
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 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).