Download Figure 1. Marine fish are a major food source for humans—as well

Figure 1. Marine fish are a major food source for humans—as well as for gulls. Moreover,
fisheries, especially small operations, provide employment for more than 200 million
people worldwide. This continuing need for protein and income, however, will likely be
impossible to meet in the future without the development of new ecosystem-based fisheries
management practices. The authors have used data from one of best understood marine
ecosystems, the Baltic Sea, to model how various approaches to managing catch might
influence the future health of the world’s fisheries. They conclude that a model based on
ecologically sustainable yield will be most effective in balancing the needs of current and
future generations. This herring boat was photographed near Ahvenanmaa (the Åland
Islands), Finland.
Nik Wheeler/Corbis
Figure 2. Based on the logistic population growth model, populations have a modest
capacity for growth (top) when they are small and attain the greatest capacity when they
reach an intermediate size. When the number of organisms in a population becomes so
large that the level of resources available to each member becomes limiting, the capacity
for growth is reduced once more. In turn, because modeling suggests that the sustainable
yield of a fish species is maximal when that species’ population is intermediate in size
(bottom), optimal population size can be maintained by harvesting the species at a rate
equal to the annual growth rate. The same general result holds for more complex models.
These models, which have been widely applied for more than half a century, do not take
into account the ecologically important effects that impacts on nontarget populations can
have on target populations.
Figure 3. Marine organisms interact in numerous ways, transferring energy through various
trophic levels in the marine food web. Thus fishing can affect entire ecosystems as well as
target species, often disturbing several trophic levels. By removing cod, sprat and herring,
for example, fishing decreases the amount of food available to seals. Conversely, it favors
organisms that normally serve as prey for cod, sprat and herring. When populations of
organisms at this second trophic level increase, they consume more organisms at the first
trophic level, reducing the number of primary producers, such as phytoplankton, that bring
energy into the ecosystem.
Figure 4. Computer modeling with Ecopath/Ecosim predicts how populations of sprat, herring and cod (a)
might respond if fishing in the Baltic Sea were prohibited between the years 2000 and 2100 (unfished) or
permitted at 20 percent, 40 percent, 60 percent, 80 percent or 100 percent of the year-2000 level (the status
quo level, or SQ). One current recommendation, precautionary fishery management (PFM), would reduce cod
fishing by 35 percent and herring fishing by 65 percent. Various scenarios for fishing levels also influence the
prevalence of species at lower trophic levels (b), particularly benthic macrofauna, which profit from reduced
numbers of predators at higher fishing levels. These models suggest that changes in predator populations
affect the rest of the ecosystem. They also predict that changes in fishing pressure affect seal biomass (c).
Because humans compete directly with seals for fish, even a low level of fishing (20 percent of SQ) reduces
seal populations far below the predicted unfished level.
Figure 5. Models suggest that fishing decreases an ecosystem’s average trophic level by
removing predatory secondary consumers, such as cod, and enabling populations of
primary consumers, such as herbivorous fish, to increase. This shift occurs at low fishing
pressures, resulting in a curve shaped like a decay function.
Barbara Aulicino
Figure 6. Data demonstrate that marine fisheries landings have, at best, remained level
since the late 1980s. Further, the amount of primary productivity required to sustain these
yields (as much as 35 percent on temperate coastal shelves) is quite high. It is unlikely,
therefore, that improvements in efficiency or management will increase harvests. Instead,
the authors propose that future fisheries management be based on ecologically sustainable
yield, a concept that will support marine ecosystems and therefore maintain productivity for
generations to come.
Barbara Aulicino