of intense fishing shifts the age structure to smaller, younger, usually less-fecund fish.
Moreover, once a fishery is established and the larger age groups are gone, there is
tremendous economic pressure (from people whose jobs and boat payments depend
upon the fishery) to allow capture of younger and younger fish. That may simply
result from continued fishing. For, example, size composition of the Peruvian
anchoveta (Engraulis ringens) stock progressively emphasized smaller, younger fish
during the intense fishery of the late 1960s and early 1970s. By 1971, most of the
stock was made up of small individuals. Their egg output was not great enough to
overcome the low larval survival caused by the 1972 El Niño, and the fishery crashed.
The stock finally came back in the late 1990s and is again being heavily fished. An
attempt by managers to close the fishery in response to the 1998 El Niño held for
about 10 days before political pressure reopened it.
Natural Mortality (ΔB/Δt = . . . − M(B,A) ...)
(^) This is usually the least well known of the variables of a fishery, and the least
accessible to management. It can sometimes be estimated by tagging studies, and
from time to time lots of energy goes into the effort. If we tag T fish of the recruitment
age (tag part of a “cohort”), we can assume the following proportionality over the
remaining life span of the fish of that age class:
(^) where:
TR = the number of returned tags over the whole life of the cohort,
TL= the number of tags never seen again,
N = the number of individuals in the cohort,
Y = the total yield of the cohort to the fishery (number),
M = the number in the cohort that died uncaught.
(^) We have the following necessary relations:
(^) From the proportionality assumed:
(Eqn. 17.4)
(^) Hence:
(Eqn. 17.5)
(^) Evaluation of population sizes, N, by marking and recapture is called the Lincoln
Index method of population estimation. It seems easy. We get an estimate of actual
population and of mortality at the same time. It is never so easy. The problems are: (i)
both tagged and recaptured samples tend to be trap-prone individuals; (ii) the statistics