Harvest management - Ecological Applications: Toward a Sustainable World - page 196

It is easy to conceive some potential benefi ts of marine protected areas for harvest
management. First, for species that don't move too much, the individuals in the
reserve are likely to be spatially protected from fi shing and achieve higher density.
Second, elevated density within the reserve may result in net migration to fi shed
areas, either by random dispersal or via density-dependent spillover. And third the
unfi shed population in the reserve can be expected to consist of larger individuals,
which are more fecund and thus might contribute disproportionately to recruitment
in the fi shed area (Willis et al., 2003).
When marine protected areas are established, increases in densities and sizes of
target species are commonly noted. For example, after 10 years of protection from
fi shing in a reserve in the French Mediterranean, target fi sh species were two to
three times more abundant inside the reserve than outside, larger species were ten
times more abundant, and the mean size of some of the species was about 50%
greater (Harmelin-Vivien et al., 1995). Such results are promising, lending weight
to the hypothetical benefi ts of reserves. But, as Sale et al. (2005) point out, while
there are now a fair number of cases where density and size increases have been
documented, the changes are often very slight, and very rarely has the acid test been
applied - to show that benefi cial effects are felt outside the reserve. The crucial gaps
in our knowledge relate to the distance and direction of dispersal, particularly of
larval stages, but also of juveniles and adults. These gaps will need to be fi lled, and
more clearcut demonstrations of benefi t to fi sheries made, before marine protected
areas can be rigorously incorporated into harvest management strategies. Sale and
his colleagues anticipate that marine protected areas will become part of the
fi sheries manager's armory but they bemoan what has often been uncritical advocacy
by scientists, which may erode confi dence in marine science and marine
scientists.

7. 2 . 5 Evaluation of
the MSY approach -
the role of climate

To under st a nd wh at underlies the concept of a maximum sustainable yield is to
understand a lot about how to manage a harvest. Theory tells us when we plot net
recruitment against population size (or fi shing effort), the curve will be hump-
shaped with a maximum that gives harvesters something to aim for. However,
putting that understanding into practice, or in other words 'fi nding the top of the
curve', is far from straightforward. Indeed, as Hilborn and Walters (1992) wryly
point out, 'You cannot determine the potential yield from a fi sh stock without over-
exploiting it'. After amassing data for a number of years, managers may think they
have found the top of the curve, when in fact they have not (Figure 7.5 - yellowfi n
tuna) or the graph they produce may not have a curve that can be discerned at all -
usually the result of year-to-year vagaries in climatic conditions that infl uence the
production of eggs or survival of eggs or juveniles in the population. If climate were
actually constant we might see a hump-shaped curve for the Pacifi c whiting fi shery
in Figure 7.4, but instead there is no sign of it. In the face of this uncertainty, the
message that management by constant effort (or constant escapement) is safer than
by constant quota is even more important.
Fisheries collapses often result from straightforward overfi shing and human
greed, but climate may also be implicated. Without doubt fi shing pressure exerts a
strain on the ability of a population to recruit suffi ciently to counteract losses. But
the immediate cause of a collapse, in one year rather than any other, may be unusu-
ally unfavorable environmental conditions. For example, prior to the major crash of