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Fishbyte Section

From the Editor

This is the Hrst issue of Naga with a Fishbyte section - we are still
in touch!

The papers presented here are in part Fishbyte backlog papers -
some selected to match the "tuna" theme of this issue of Naga - in
part papers written to inaugurate this section. This applies especially

tothatof LM. Vakilywhichdocuments aprojectwhich we hope will
Serveasamodeltoother Countriesin West AHicaandelsewherein
the developing world. Feedback on this and on the Otherpapers are
welcome. D. Paufy

Relationship Between Sea Surface Temperatures and
Dolphin-Associated Fishing Activities by the
Mexican Tuna Fleet

VICTOR M. GOMEZ-MUNOZ

Abstract

Data from the Mexican purse seiner fleet operating in the eastern
tropical Pacific, for the year 1985-1990, are used to show that the fraction
of surface schools of yellowfin tuna TTiunnus albacores associated with
dolphins (Stenella attenuate and others) increases with sea surface
temperature (P<0.01). Possible reasons for this correlation are briefly
discussed.

Introduction

The Mexican purse-seine fishers in the eastern tropical
Pacificdetectschoolsofyellowfin tuna ( Thunnus albacares)
in several ways: floatingobjects (commonly called "logs"),
surface disturbances ("brisa") caused by tuna and their
prey and the presence of birds or marine mammals. In
1985-90, more than half the sets were associated with
dolphins, which suggests that tuna schools are best
detected through dolphin sightings. However, nocorrelation
has been found between tuna catch and the nurhber of
dolphins sighted, nor between the catch and the number
of dolphins confined in the net. Hence the success of
fishing does not seem to depend on density of dolphin
schools.

Surface schooling of tuna with dolphins occurs when
a shallow thermocline limits the vertical distribution of
the two species to the surface layer (Au and Perryman
1985). In 1981, staff of the Inter-American Tropical Tuna
Commission performed a discriminant function analysis
of several physical factors to determine which were
related to high and low catches of yellowfin tuna. In the
551 Casesexamined Whereyellowfin tuna Wereassociated
with dolphins, the gradient of thermocline was the only
statistically significant discriminator (Anon. 1983). In
fact a shallow thermocline probably acts as a vertical
aggregating mechanism for the squids and fishes on
which dolphins prey, along with tuna (Reilly 1990).

Association between Tunas and Dolphins

The spotted dolphin Stenella attenuata is the species
most often associated with Mexican tuna fishing, being
represented in almost 90% all purse-seine sets. The historical
range of this species in the eastern tropical Pacific, as
reported by Perrin et al. (1983), is similar to the operating
range of the tuna fleet, as can be seen, e.g., in the maps of
Ortega-Garcia (1989).

Blackburn (1965) and Nakamura (1969) have shown
that the overall limits Ofdistribution of most tuna species
can be clearly defined by sea surface temperature (SST).
Variations in dolphin sets showed maxima in zones of
higher SST. Also, significant differences in SST and
success of fishing found between the dolphin and the
nondolphin associated sets: dolphin-associated Setsoccurred
more frequently ata mean of 28 *C, with an average of 13
t per set; while most nondolphin sets occurred at a mean
of 23 *C, with not quite 91 per set (Fig. 1). Au et al. (1979)

Fig. 1 • Percentage, In the Mexican (eaatom Pacific) tuna fleet,
of dolphin and nondolphin seta as a function of SST 1985∙90.

NAGA₁ THE ICLARM QUARTERLY

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