Electrophoretic analysis of stock structure in Northern Mediterranean anchovies, Engraulis encrasicolus

Abstract

Starch gel electrophoresis of allozymes was used to investigate the genetic stock structure of European anchovies (Engraulis encrasicolus L.) in the northern Mediterranean area. Twenty-four putative enzyme-coding loci were examined in 634 fish, constituting 13 samples caught in the Adriatic, Tyrrhenian, Ionian, and Aegean seas, the Sicilian Channel and the Bay of Biscay, between March 1993 and May 1994. Eight loci were monomorphic in all samples, eight were weakly polymorphic (p=0.99), and a further eight exhibited common allele frequencies <0.95 (p=0.95). Over 99% of tests indicated that genotypic proportions were in accordance with Hardy-Weinberg predictions. Average mean unbiased heterogeneity per locus (H_L) was 0.055 and the effective mean number of alleles per locus was 1.17. Genetic distance between samples was low (maximum Nei's D=0.009).

An outgroup sample from the Bay of Biscay was easily distinguishable from Mediterranean fish, due to fixation at a number of polymorphic loci. Chi-square analyses revealed significant differences (p<0.05) between Mediterranean samples at six loci (G3PDH-1, G3PDH-2, IDHP-2, mMEP, GAPDH-1 and GPI), and GAPDH-1 and GPI in particular showed obvious heterogeneity between seas. Gene diversity analysis indicated that 96.6% of variation was within samples, with only 3.4% between samples (G_ST). 77% of this between-sample variation was partitioned between seas, the remainder accounting for spatial and temporal variation within seas.

The results are discussed in relation to the hydrographic and physical barriers to migration in the study area, and compared with those obtained by other workers. The possible consequences of stock integrity in clupeoid fish with regard to resilience and recovery from collapse are discussed.