Sevlgny et a\: Identification and distribution of larvae or Sebastes fasaatus and S mentdia 



383 



were observed. As noted in 1991, heterozygous indi- 

 viduals co-occurred with those of the MDH'AIAI 

 genotype. The frequency of allele MDW'Al varied 

 from 0.794 to 0.953 (Table 2). Allelic frequency of 

 1.0 was observed at stations 27 (MDH*A1) and 33 

 (MDH*Al2) only. Deviations from Hardy-Weinberg 

 expectations caused by a deficit in the number of 

 heterozygous individuals were obser\'ed at 14 sam- 

 pling stations (Table 2). When data for the entire 

 study area were pooled, a significant departure from 

 the Hardy-Weinberg equilibrium, also due to a defi- 

 cit in heterozygotes, was noted for both years and for 

 the eastern and western sectors of the Gulf in 1991 

 (Tables 1 and 2). Such deviations from Hardy-Wein- 

 berg equilibrium at any given station, within the dif- 

 ferent sectors of the Gulf or over the entire sampling 

 area, indicate that redfish larval populations do not 

 form a panmictic group in the Gulf of St. Lawrence 

 but rather consist of a mixture of at least the two 

 most frequent species, S. mentella and S. fasciatus. 



Astrong spatial heterogeneity in the distribution of 

 genotypes was observed. The genotype MDH*A2A2 

 dominated or was more frequent at stations located 

 in the northern part of the Gulf whereas MDH*A1A1 

 dominated in the southern part of the Gulf The fre- 

 quency distribution of the genotypes over the entire 

 area in 1991 differed significantly between the eas- 

 tern and the western Gulf of St. Lawrence (x'^=53.0; 

 P<0.0001), indicating important spatial variation on 

 a large scale. 



ler size of the MDH*A1A1 and MDH*A1A2 larvae 

 in 1992 compared with those in 1991. The size dis- 

 tribution of the homozygous MDH*A2A2 larvae was 

 similar in both years. 



In 1991, the genotype MDH*A2A2 was more fre- 

 quently represented in recently extruded larvae (<8. 5 

 mm) and the genotype MDH*A1A1 was most fre- 

 quent for larvae >8.5 mm. In 1992, the genotype 

 MDH*A1A1 was the most frequent genotype in all 

 size classes except for larvae <6.5 mm which were 

 dominated by the genotype MDH'^A2A2. In both 

 years, the genotypic composition varied between the 

 size classes of larval redfish, indicating that a larval 

 cohort may be dominated by individuals belonging to 

 a single species (Fig. 6). These differences are most 

 likely associated with a difference in the extrusion 

 time between S. fasciatus and S. mentella. 



Relative proportion of each genotype 

 In the overall larval population 



Larvae of the MDH*A1A1 genotype accounted for 

 61.8% and 77.6'7f of all collected larvae in 1991 and 

 1992 respectively. The contribution of the MDH*A2A2 

 genotype was 14.4% in 1991 but dropped to 5.6% in 

 1992. The heterozygote genotype accounted for 23.8% 

 in 1991 and 16.8% in 1992. The combined contribu- 

 tions of the genotypes MDH*A1A1 and MDWA1A2 

 thus represented 85.6% and 94.47c of the larval popu- 

 lation in 1991 and 1992, respectively. 



Genotype and larval-size distribution 



In 1991, the size of the larvae varied significantly be- 

 tween the three genotypes ( Kruskal-Wallis, P<0.000 1 ); 

 the MDH'^A2A2 larvae were smaller than those of 

 the other two genotypes (Fig. 5). The size distribution 

 of the larvae was not significantly different between 

 the genotypes MDH'AIAI and MDH*A1A2. The size 

 distribution of larvae of each genotype did not differ 

 between the eastern and the western sectors of the 

 Gulf The obvious bimodal distribution of larval size 

 (all genotypes confounded) observed on the west side 

 of Anticosti Island can thus be explained by the diffe- 

 rence in size of larvae of the different genotypes. 



Similar results were obtained in 1992, where 

 the size of the homozygous larvae MZ)//*A2A2 was 

 significantly smaller than those of the genotypes 

 MDH*A1A1 and MDH*A1A2. There was again no 

 significant difference in the standard length between 

 larvae of the MDH'^AlAl and MDH*A1A2 genotypes 

 (Fig. 6). 



A significant difference in the size of larvae between 

 1991 and 1992 (Kruskal-Wallis, P<0.0001; Fig. 6) 

 was however noted and can be attributed to the smal- 



Discussion 



Our results clearly show that the larval redfish popu- 

 lation in the Gulf of St. Lawrence is not genetically 

 homogeneous but rather is multispecific and com- 

 posed of at least two genetically distinct species cur- 

 rently identified as S. mentella and S. fasciatus. As 

 previously indicated in the introduction, S. norvegi- 

 cus may also occur in the Gulf but at very low abun- 

 dance and cannot be distinguished from S. mentella 

 at the MDH* locus. The co-existence and the larval 

 extrusion of the two species of redfish, identified by 

 the number of anal-fin rays, have been inferred for 

 Gulf species in previous studies based on the pre- 

 sence of gravid adults (Ni and Sandeman, 1984) and 

 on the presence of sexually mature females in spawn- 

 ing and postspawning condition for both species (St- 

 Pierre and de Lafontaine, 1995). We report here the 

 first evidence that the two species are clearly identi- 

 fied in the larval population. 



The taxonomic status of the heterozygous indivi- 

 duals is however problematic. Rubec et al. ( 199 1 J sug- 

 gested that the presence of heterozygotes was best 



