Fig. 17.17 Catch per set (CPUE) vs. sets (effort in baskets of hooks) for North Pacific
halibut on the “southern grounds”. The curve represents a constant total yield of 24.4
million pounds.
(^) (Based on data of the International Pacific Halibut Commission.)
The MSY argument leads the unwary to assume that a prudent exploiter would seek
to establish a level of fishing effort, X, such that the MSY would be realized. There
are reasons for fishing at somewhat lower rates, and there are severe difficulties in
realizing either MSY or the X appropriate to MSY. That there should be some MSY
against which to design a fishery is a clear ideal. However, because there is
oversimplification in arriving at the analysis, it is not a sufficient management tool. It
does have many problems; prominent is the assumption that Y/X is proportional to B,
which works better for some stocks and fishing situations than for others. Another is
that we cannot study Y at large X without endangering the stock, as pointed out above.
Then there is the assumption that the biomass of the stock is all the same, just a
variable responding in a deterministic way to fishing effort. Of course, fishing usually
rearranges the age, size, and reproductive structure of the stock, and substantial lag
times are involved in its response to changes in fishing mortality. Reducing a stock
also gives advantage to its ecological competitors, which may increase. Reversing this
replacement may not occur quickly or at all if fishing is scaled back. Finally, MSY