There is an interaction between porpoises and tuna that is still not fully understood,
but the fishers learned to exploit it. Moreover, the tuna beneath porpoises are much
larger than the average in surface schools, so the catch rate, the CPUE, increased
dramatically, ruining the analysis and basis for management. The Commission more
or less threw up its hands and let fishers fish until their boats were submerged to the
Plimsoll line. In fact, the entire eastern Pacific yellowfin tuna fishery was subject to
no limits from 1980 to 1998. At present, there is again a nominal MSY process for
managing this fishery, since the treaty establishing the Inter-American Tropical Tuna
Commission (IATTC) requires it. However, entry of Korean and Japanese long-liners,
and a much more complex scheme of fishing effort from the Americas, have made
effort estimation very complex. A yield vs. effort graph for the period 1968 to 1998
(Fig. 17.15; IATTC 2000) has been developed on the basis of just the CPUE for large
(“class-6”) seiners. Since those account for most of the eastern Pacific catch, they can
represent the whole fishery for our purposes.
Fig. 17.15 Yield vs. effort curves for yellowfin tuna in the eastern tropical Pacific.
Separate curves apply for each of two eras, 1968–1984 and 1985–1998. Distinctive
production regimes apparently applied in these periods.
(After IATTC 2000.)
(^) Clearly, the graph breaks into two eras, before and after 1985. IATTC analysts fitted
separate yield–effort parabolas for the two periods. The earlier period was clearly one
of increasing effort, and while total yield remained relatively constant, CPUE went
down. Once CPUE got low enough, boats were sold or moved to the Atlantic or
western Pacific where fishing was better. The stock rebounded, and yields improved
dramatically, soon far exceeding those of the era up to 1984, with less effort
expended. Examination of the catch showed higher recruitment rates and faster
growth in the later era. This change is not the least unusual; the success of fish stocks
changes over time, and usually we cannot confidently identify the causes. It makes
obvious the difficulty of managing by this sort of MSY analysis, which assumes that
the biomass of the stock will respond in the same way over very long periods to more