Informe Anual de la Comisión Interamericana del Atún Tropical, 19

24 TUNA COMMISSION
average, however, ifthere is a 1:1 ratio of sexes produced, only 2of all the eggs produced in a lifetime
will survive to maturity, and perhaps only about 501' 10 will survive to recruitment into the fishery. It
is difficult to monitor the changes in the abundance of larval and juvenile fishes of any species
directly by sampling in the ocean, and to do this for yellowfin ayer its entire range would be totally
impracticable. Therefore fisheries scientists try to find statistical relationships between recruits and
the spawners which produced them. These efforts are not usually very successful, as the residual
variance after the statistical model has been fitted is in most cases so large that the results cannot be
used to predict recruitment. Instead the relative abundance ofthe recruits is estimated from fishery
data, and the fishery is managed on that basis. This is how the IATTC staff uses cohort analyses to
assess the condition ofthe yellowfin stock in the eastern Pacific and make recommendations for its
management.
Density-dependent mechanisms must operate at some level of abundance for any species, so
that at levels less than the natural carrying capacity a population must be more likely to increase
than to decrease, and at levels greater than the natural carrying capacity the opposite must be the
case. These mechanisms ameliorate an otherwise stochastic environment: when the population is
reduced the chances of survival for each individual are improved, which reduces the probability that
the population will become extinct; similarly, at high population levels the growth ofthe population is
curtailed so that it is unlikely to increase further. Density dependence is usually most apparent in
the fertility ofthe adults 01' the survival ofthe young. Adults may forego 01' reduce reproduction when
the density is great, while the the young tend to have fewer reserves to buffer them against the
environment. When populations of animals are modeled mathematically it is often found that
density dependence, particularly when associated with delays which are inherent in reproduction,
can cause cycles in their abundance. This has been most clearly demonstrated for insect populations.
Fish populations, in most cases, are relatively stable because several year classes are
important contributors to egg production. (Pacific salmon, however, spawn only once, and then die,
and the populations of these are subject to wide f1uctuations and persistent cycles.) It might be
expected that, since the mortality rates of fish are generally high, the youngest age group ofmature
fish would contribute by far the greatest numbers ofeggs, and when the abundance ofthat age group
was especially low 01' especially high the production of eggs, and subsequently of recruits, would be
reduced 01' increased. Fish, however, unlike most animals which have been studied, continue to grow
after they reach maturity. Since the fecundity is approximately proportional to the weight ofthe fish,
the older fish have much greater fecundities than the younger ones. Ifover several years the loss of
individuals to a cohort roughly equals the gain in weight of the individuals which survive the
production of eggs of that cohort remains roughly constant ayer that periodo This results in more
stable recruitment than would be the case if the fish ceased to grow after reaching maturity. The
estimated biomasses of yellowfin of the various ages which are caught by the fishery are shown in
Figure 32. Age-1, -2, and -3 fish contributed the greatest biomasses in 14,5, and 2cases, respectively,
and age-4 and -5 fish are significant contributors in many years. Reproduction is thus not almost
exclusively dependent on the abundance of a single age group, which might create instabilities, and
the population is stable enough that any relationship between stock and recruitment is masked by
other factors which are not yet understood.
If spawner-recruit studies are to be usefui a long series of data on the total weights of
spawners and the resulting numbers ofrecruits must first be assembled to obtain an understanding
of density dependence. Then, after an adequate understanding of the spawner-recruit relationship
has been obtained, the amount of recruitment can be predicted from the total weight of spawners,
provided the relationship is not highly variable. Itis necessary for spawner-recruit studies, ofcourse,
to have reasonably accurate estimates ofthe total weight of spawners and of the numbers of recruits
produced. These can be obtained from catch-per-unit-of-effort data, provided catch per unit of effort
provides a suitable measure of abundance and maturity occurs after recruitment. If, however, the