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.I. Roldan et al. J. Exp. Mar. Biol. Ecol. 253 2000 63 –74 71
4. Discussion
The genetic and morphologic data reveal the clear existence of two groups, Mediterranean and Southern Atlantic, and some evidence of stock separations between
´ ´
Rıo de la Plata and El Rincon samples Fig. 2. The magnitude of Nei’s genetic distance Nei, 1972; 0.003 between Pals and Argentinian chub mackerel populations does not
support the subspecific status S . japonicus marplatensis proposed by Lopez 1959 for
the Southwestern Atlantic group. However, both data sets support genetic and mor- phological distinctions between Mediterranean and Southwestern Atlantic populations.
The distinction between the two Argentinian collections is less clear. The presence or absence of alleles at three loci in one or the other location is consistent with some
subpopulation structures. However, alternatives such as localized reflections of transitory favorable survivals of individual matings Hedgecock, 1994 or other sampling effects
remain possibilities. We interpret the genetic differences to isolation within perhaps the last 100,000 years, with little or no migration. The relatively high number of alleles per
locus 2.6 is consistent with large effective population sizes Chakraborty et al., 1980 which would retard the divergence from genetic drift of the two demographic units. This
conclusion is supported by the morphological data Fig. 2. Despite the unknown environmental and genetic influences of these morphological distinctions, the presence
of two discrete groups of individuals is consistent with the limited contemporary migration in two distinct environments. Further understanding of these relationships
must await broader samplings throughout the species range, collections of potentially more discriminating molecular genetic data such as microsatellites with higher mutation
rates Grant et al., 1999, and physical tagging programs designed to measure long distance movements of chub mackerels.
Morphologic results provided details of chub mackerel shape, showing the importance of head size as a principal factor affecting differentiation between the samples examined.
´ ´
The El Rincon mackerels showed more posterior development than Rıo de la Plata mackerels, perhaps attibutable to growth responses to the differing habitats arising from
the oceanographical and ecological conditions. The greater zooplankton productivity of ´
the Rıo de la Plata area could account for the adaptive trend towards the development of a larger head to enhance feeding activity. The progressive increase in the length of the
body correlated with a decreasing head size would appear to be more closely related to a migration feeding strategy. This consideration is based on the findings of diet studies on
´ chub mackerel Pajaro, 1993; Perrotta et al., 1999.
Results of both approaches along with water circulation data and previous biological information on S
. japonicus suggest one plausible scenario with regard to chub mackerel stocks in Argentinian waters Fig. 1. At this moment, only one spawning area has been
detected, located from October to January off the Mar del Plata coast coincident with the Mar del Plata fishing ground. Massive spawns in the area with four or five
successive settings are associated with temperatures ranging from 16 to 188C Perrotta and Christiansen, 1993. We propose that the larvae are transported near the coast in a
northerly direction, where they develop into the early juveniles stages age classes 0 and 1. The coastal current serves as an entrainment system for the young. As these young
´ become older and larger, they are able to arrive at and utilize the Rıo de la Plata area for
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M .I. Roldan et al. J. Exp. Mar. Biol. Ecol. 253 2000 63 –74
´ feeding. The Rıo de la Plata area is abundant with juvenile stages throughout the year.
These juveniles then recruit at the end of February to the adult stock caught in the Mar del Plata area during the fishing season age classes 1 to 8; Perrotta, 1992.
´ Does the El Rincon area have an independent spawning population that utilizes the
´ southern currents for its life cycle? The San Matıas Gulf area Fig. 1 is somewhat of an
enigma. Only during November and December are chub mackerel detected in the northern area when the temperature ranges from 16 to 188C Perrotta, unpublished data.
The large size of the individuals suggests they are adults, but there is no information on their gonadal development. The existing evidence especially environmental factors
´ ´
suggests that San Matıas Gulf is not independent. A connection with the El Rincon area ´
is highly likely if we consider that adults living in the El Rincon area start gonadal development in August Perrotta and Forciniti, 1994. Thus, the area may be occupied
´ ´
by individuals from El Rincon. Further investigations, especially in the San Matıas Gulf or based on a complete adult tagging program, would answer this question.
The absence of genetic data from the Mar del Plata spawning area restricts further genetic details, however the results endorse the present policy of managing northern and
southern stocks of S . japonicus as two separate units. Since 1996, two fishing stocks of
chub mackerel have been designated based on morphometric characteristics, Mar del ´
Plata and El Rincon north and south of latitude 398009S Fig. 1. A two-stock hypothesis provides a more conservative approach to harvest management since it
focuses attention on the maintenance of local population levels in the short term and on the preservation of local adaptations in the long term.
These fishing stocks need to be further investigated in order to be elevated to the status of genetic stocks i.e., completely isolated reproductive populations of the same
´ species. Sampling of additional localities Mar del Plata and San Matıas Gulf is
required to more firmly resolve the questions raised by this study. Meanwhile, the present data provide a necessary foundation for determining the population structure of
chub mackerel in the South West Atlantic.
5. Conclusions
