| In
all types of fishing, we almost always assume that the biggest
fish are the best. We target the biggest fish with lures
and bait and with nets that catch only larger individuals,
and we often make regulations that require the smaller fish
to be released. There are unquestionably positive results
from some of these regulations, such as allowing individuals
of a species to achieve spawning age before they can be
taken. But many recent studies are showing that this approach
is also having unintended consequences.
These
studies have ranged from laboratory experiments in aquaria,
to studies of cod across expansive ocean fishing grounds,
to wide-ranging compilations of multiple studies on salmon
and trout, to controlled studies of carefully stocked fish
in lakes. The results are giving a very clear picture. Fishing
pressure on the biggest fish often selects against the traits
we favor: fast growth, active feeding behavior and large
size. In essence, we are seeing that many fishing strategies
are inducing evolutionary responses in fish populations,
such as early maturation at small size and reduced fecundity.
In one
study on North Atlantic cod, analysis revealed probable
genetic changes in growth in this population in response
to size-selective fishing. The main question being investigated
was why cod stocks have not rebounded in areas where fishing
pressure has been reduced. Results indicated that there
had been genetic changes in growth rates in this population
in response to size-selective fishing, accounting for the
continued small size-at-age despite good conditions for
growth and little fishing for over a decade.
In another
revealing experiment, two small Canadian lakes were stocked
with equal densities of two types of rainbow trout. One
genotype was selected for its fast growth and aggressive
feeding behavior (fast/bold) and the other for traits of
slow growth and cautious behavior (slow/shy). The fish were
the same size at stocking. Then both lakes were fished intensively,
but evenly, with gill nets. Fifty percent of the fast/bold
fish were captured, but only 30 percent of the slow/shy
fish were taken. The authors stated that: “Given that
growth is heritable in fishes, we speculate that evolution
of slower growth rates attributable to behavioral vulnerability
may be widespread in harvested fish populations. Our results
indicate that commonly used minimum size-limits will not
prevent overexploitation of fast-growing genotypes and individuals
because of size independent growth-rate selection by fishing.”
The
next step will be to apply this knowledge to real-world
fisheries management. Some research has already begun on
how to best avoid these problems. Each fishery will require
a somewhat different approach, since response to selective
harvesting depends on specific life histories, varying environments
and different community structures. In the future, managers
will need to consider these factors of fisheries-induced
evolution. Better genetics data will be required, along
with sound understanding of existing ecological processes
and changing aquatic environments.
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