or less fishing pressure. Evaluation of the eastern Pacific yellowfin tuna fishery and
its stock have become so complex, the terminology so arcane (e.g. Maunder &
Watters 2001), that explaining it goes beyond our goal in this chapter.
(^) A few other examples of fisheries managed on the basis of MSY may be instructive.
Hunter et al. (1986) developed two yield vs. effort graphs (Fig. 17.16) for Atlantic
yellowfin tuna. One was for data up to 1975, and a fitted parabola suggested an MSY
around 50,000 metric tonnes. However, the growth of effort did not stop at the
roughly 60,000 fishing days producing that yield, it rapidly increased to over 200,000
days by 1983, leading to a yield curve peaking over 100,000 metric tonnes, and still
not obviously trending downward. Hilborn and Walters (1992) draw the excellent
caveat from this that (at least in the absence of a detailed population dynamic
analysis) “you cannot determine the potential yield from a fish stock without
overexploiting it”. Once that is done, of course, the damage to the stock, even its place
in its ecosystem, may well not be repaired simply by reducing X to the level at the
apparent MSY. It is possible that no MSY will be evident from effort and yield data.
In the northeast Pacific halibut fishery, effort is measured as baskets of baited hooks
deployed on the bottom for a day. For many fishing regions, the graph of annual
CPUE vs. effort (e.g. Fig. 17.17) is not linear, but has an obvious curve. It turns out
that the data in Fig. 17.17 can be fitted with a curve by assuming a constant catch of
24.4 million pounds and dividing it by the effort in thousands of sets. In other words,
it doesn’t matter how many sets are put out above about 300,000, the catch is about
the same. Of course, the CPUE vs. effort function is well defined, even if curved, so it
can be converted to a yield vs. effort plot, which for this range of effort is a flat
plateau, always about 24 million pounds. The time sequence of effort was, in fact,
generated by a regulatory agency, which saw the decline in CPUE from 1925 and had
the authority after about 1931 to reduce effort. In this case, CPUE was a useful
management tool even when no particular MSY was specified. The agency was also
concerned about the age structure of the stock, which had shifted to younger fish.
Fig. 17.16 Two yield vs. effort curves for yellowfin tuna in the eastern Atlantic
Ocean. Curve (a) was fitted by the management commission after 1973, predicting
that MSY had been reached. Increased fishing to 1983 supported (b), a different curve
and higher MSY.
(^) (After Hilborn & Walters 1992, citing results of Hunter et al. 1986.)