Informe Anual de la Comisión Interamericana del Atún Tropical, 19

ANNUAL REPORT 1987
stopped fishing this net gain would cause the population to increase gradual1y to its original size. As
the population approached that size the rates of recruitment, growth, andlor natural mortality
would gradual1y approach their previous levels until they were the same as before the fishery began.
The population would thus be restored to equilibrium at its maximum size. If the boats continued to
fish at the same rate, however, the population would eventual1y come into equilibrium with different
rates ofrecruitment, growth, andlor natural mortality and a population size which was lessthan the
original population size. The catch per unit of effort (CPUE) would still be proportional to the
population size. Ifthere were only afew boats the population size would be only slightly reduced and
the CPUE would be relatively high, but if there were many boats the population would be
considerably reduced and the CPUE would be much lower.
The total catch would be the product ofthe CPUE and the total effort. Ifthe fishing effort were
relatively low, modest increases in effort would more than offset the corresponding decreases in
CPUE, resulting in higher catches, but at higher levels ofeffort such would not be the case. At sorne
intermediate level ofeffort the product would be at its maximum. Ifthe objective ofmanagement is to
obtain the average maximum sustainable yield (AMSY) offish (which is the objective stated in the
IATTC's convention) the fishing effort should be maintained at that level. If the object is maximum
profit the effort should be somewhat less(assumingthat the price remains constant), and ifthe object
is maximum employment offishermen the effort should be somewhat higher. Ifthe effort is less than
whatever optimum is selected, the population is said to be underfished, and if the effort is greater
than that optimum, the population is said to be overfished.
It is easiest to consider the interrelationships of effort, catch, and CPUE at equilibrium
conditions, but it must be recognized that such conditions are not always presento For example, ifthe
population is at equilibrium near its maximum and the effort is suddenly increased the CPUE will
gradual1y decrease for a period before a new equilibrium point with a lower CPUE is reached. The
catches wil1 be greater during the period oftransition than at the new point ofequilibrium. Likewise,
ifthe population is at equilibrium at a low level and the effort is suddenly decreased the CPUE will
gradual1y increase for a period before a new equilibrium point is reached with a higher CPUE. The
catches wil1 be less during the period of transition than at the new point of equilibrium.
It is appropriate at this point to examine the applicability and usefulness of production
models for the yel10wfin resource ofthe eastern Pacific Ocean. The previous discussion has dealt with
"a population of fish." Production models are not applicable unless the amount of interchange
between the fish in the area ofstudy and adjacent areas is relatively low. The extent ofinterchange of
yel10wfin between the CYRA and the area outside the CYRA (Figure 37) has been studied, and the
results are discussed in previous Annual Reports ofthe IATTC. In summary, the results indicate that
there is sorne intermixing of fish between the two areas, but not enough to invalidate the use ofthe
model.
Assuming that production models are applicable, that fact can be detected only if(1) there are
catch, effort, and CPUE data available for awide range oflevels offishing effort, preferably including
levels on the overfishing side of the curve and (2) the perturbations caused by fiuctuations in
environmental conditions are not so great as to mask the changes in apparent abundance caused by
fiuctuations in fishing effort. It will be seen below that data are available for a wide range oflevels of
fishing effort. The effects of environmental conditions have been studied and reviewed in previous
Annual Reports ofthe IATTC. As far as could be determined, these effects usual1y balanced out over
fairly short periods oftime, so it was decided that they could be ignored without invalidating the use
ofthe model. The period oflate 1982 to early 1984 includes the strongest El Niño conditions which
have been recorded, however, so the effects ofthis phenomenon have been given special attention in
assessing the fishery during and subsequent to this periodo
Effort has been mentioned, but so far no mention has been made of the fact that there are
different kinds ofeffort, i.e., baitboat, purse-seine, and longline effort, and effort by different sizes of
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