Informe Anual de la Comisión Interamericana del Atún Tropical, 19

22 TUNA COMMISSION
otolith primordium, or point of original growth, to the mark, and from the mark to the edge of the
otolith, the lengths at tagging and at recapture can be estimated from a growth curve relating the
otolith length to the length ofthe fish. For each fish, the corresponding values of otolith growth rate
(G o ) and G i can be estimated from these measurements. The benefits of estimating growth rates of
the fish by this procedure are: (l) errors associated with measuring live fish are avoided, (2) the
values of G i are based on a consistent standard of measurement (frozen and thawed fish) via the
otolith length-fish length relationship, and (3) all values of G i are obliged to positive because G o
cannot be negative. Under these conditions, the variance of G i should be smaller and more reliable
than the corresponding variance of G t prepared from tagging data. This statement is supported by
comparisons derived from the 1976 and 1980-1981 experiments listed in Table 15. Negative values of
G t are not included in this tableo
Eastern Pacific skipjack
In contrast to the daily deposition rate ofincrements in yellowfin, the 1976 experiment showed
that skipjack deposited, on the average, only 0.76 lid. Originally it was thought that this departure
from what was expected may have been due to the onset ofmaturity ofthe females, and to explore this
possibility further the sexes ofthe fish recovered from the 1980-1981 experiments were identified by
gross examination oftheir gonads. Apreliminary analysis indicates that the deposition rates for the
sexes are similar and stillless than 1lid, i.e., males: 0.73 lid (n = 14); females: 0.70 lid (n = 23).
These rates are based on the maximum increment count obtained from three different counting
positions. Initially, the counts were made on the distal surfaces ofthe anterior otolith projections, as
in the 1976 experimento Subsequently counts were also obtained from the distal surface of
projections in the ventral-anterior quadrant, and from both surfaces of the centrally-Iocated
posterior projections. Frequently, the counts from these three positions did not agree, and the
possibility exists that sorne increments may be too small to discern with a light microscope (1200x).
Otolith sections are therefore being prepared for examination at increased levels of magnification,
using a scanning electron microscope.
Eastern Atlantic yellowfin
The experience gained in identifying and counting increments in the aboye experiments was
applied to a small sample of otoliths from eastern Atlantic yellowfin. The fish were injected and
recaptured during 1979-1983, and their otoliths were sent to the Commission by the Centre de
Recherches Océanographiques, Abidjan, Ivory Coast. The number ofincrements from the mark to
the otolith edge could be counted reliably on only five fish. In the relationship between increments
and time, i.e.,
1 = - 6.95 + 1.05d (n = 5)
The coefficient 1.05 ( ±: 0.15) does not differ significantly from 1.0. Therefore, yellowfin in the eastern
Atlantic may also deposit increments daily, or at least the sample size is too small to reject this
hypothesis. This result is encouraging, despite the small sample size.
Reproductive biology ofyellowfin
Knowledge ofthe reproductive biology ofyellowfin tuna is important for understanding ofthe
population dynamics of this species. Various aspects of the reproductive biology of yellowfin,
including locations and times of spawning, size and age at maturity, and spawning frequency, are
still not well understood. There is also a need to determine whether there is diversity with respect to
reproductive characteristics of fish in different parts of the eastern Pacifico Research on yellowfin
reproductive biology should contribute to an understanding of the link between spawning and
recruitment and make it possible to evaluate the effect offishing on the reproductive potential ofthe
populatíon.