Biological Oceanography

(^) Usually much of the reproductive stock has been removed in fisheries that are well
down the right side of the curve. In that sense the yield vs. effort curve is the recruit
vs. spawner stock curve plotted backward.
(^) While MSY is part of every fishery manager’s mental machinery, it is no longer
explicitly used in many fisheries. It is there, however, always working just under the
surface, sometimes reappearing as “reference points” in international agreements
about limits to catch or effort. The yellowfin tuna (Thunnus albacares) fishery of the
eastern tropical Pacific was managed for an interval by an explicit MSY model, and
Schaefer’s (1967) analysis of catch and effort served as the basis for that
management. The fishery has had several major eras. A California coastal fishery for
albacore failed in the middle 1930s, so canneries supported development of some
refrigerated boats for a distant-water yellowfin fishery in the eastern tropical Pacific.
After World War II the fishery extended farther and farther south, joined in those
years by boats from Latin American nations. From about 1930 to 1958 the system of
fishing was to drive a boat next to a school sighted by a masthead lookout, release
some silvery bait fish from a tank, then fish in the ensuing feeding frenzy with silvery
hooks. Tuna were heaved into the boat by muscle power. Such boats were called “tuna
clippers”, and the unit of effort recorded by managers was the “standard clipper day”,
obtained from a comparison of the fishing effectiveness of clippers of different sizes.
In the second era, from 1958 on, the fishery switched fairly quickly to purse seines,
enormous curtains of net strung around a tuna school by a small boat dropped off the
fishing vessel. When the free end is returned to the ship, the net is closed beneath the
school by pulling in a pursing cable. Next the net is pulled back aboard with a
hydraulic power block, concentrating the tuna alongside. From there they are lifted
out of the water and frozen in the holds. Seiners and clippers were both working for
enough years to get a comparison of their fishing power, so the unit of effort remained
“standard clipper days”.
(^) Schaefer’s plot (Fig. 17.5a) of CPUE vs. effort (thousands of clipper days) is
reasonably linear. He employed several approximate corrections, trying to overcome
the fact that the fishery age structure, recruitment, and yield never had a chance to
come to equilibrium with any given level of fishing effort. Rather, effort increased
steadily through the years. All that argument can be studied in the original paper. The
plot was readily converted to a yield vs. effort curve (Fig. 17.5b), which suggested an
annual MSY around 200,000 pounds of tuna. The Inter-American Tropical Tuna
Commission eventually enforced this as an annual quota for the fishery. Ships would
radio their catches to headquarters where they were summed. When the quota was
reached, a message was sent back to cease fishing and return to port.
(^) The third era began in 1967, when the fishery reached its quota far ahead of its
usual schedule. The fishers had discovered that setting their seines around schools of
porpoises would capture submerged schools of yellowfin tuna that live beneath them.