Sevigny et a\ Identification and distribution of larvae or Sebastes fasaatus and 5. mentetia 



387 



1985). This finding suggests that the larvae of S. 

 fasciatus were extruded shghtly later than those of 

 S. mentella in the Gulf of St. Lawrence. Once again 

 assuming that larval gi'owth rates can vary between 

 0.10 mm and 0.15 mm/day, one can conlude that the 

 peak in larval extrusion as determined by the modal 

 size groups of each homozygote genotype (7.5 mm for 

 MD//*A2A2 and 10.2 mm for MDH'AIAI in 1991) 

 would be approximately 15-25 days apart. From 

 age estimates of larvae from otolith microstructure, 

 Penney (1987) concluded that extrusion time of S. 

 mentella was earlier than that of S. fasciatus in Fle- 

 mish Cap area. Previous larval collections made over 

 three consecutive summers at a fixed site along the 

 Gaspe coast in the Gulf of St. Lawrence also showed 

 that larvae extruded earlier tend to be larger than 

 those extruded later in the season (Jean, 1955). Such 

 a sampling strategy combined with genetic identifi- 

 cation of the larvae (as presented our paper) would 

 permit a more precise description of the seasonal 

 variability in the extrusion of the two redfish species 

 in the Gulf of St. Lawrence. The significantly smal- 

 ler larvae collected in 1992, compared with 1991, is 

 probably due to the earlier sampling time in 1992. 



Sebastes mentella dominated the redfish larval 

 population in the Gulf of St. Lawrence, contributing 

 61.8% and 77.6% of all larvae collected in 1991 and 

 1992, respectively. If we assume that the hetero- 

 zygous individuals belong to S. mentella. the propor- 

 tion of S. mentella increases to 85.6% and 94.4% for 

 these years, respectively. Furthermore, if the hete- 

 rozygous individuals belong to S. mentella, it might 

 be expected that a small proportion of homozygous 

 MDH'*A2A2 individuals were actually S. mentella, 

 although misclassified as S. fasciatus. In which case, 

 we would have slightly underestimated the abun- 

 dance of S. mentella. The presence of these indivi- 

 duals in the MDH'A2A2 group might correspond to 

 the largest individuals of this gi'oup (Figs. 5 and 6). 

 In any case, the values we obtained for larval stages 

 are very close to those (90.3% ) for adult S. mentella 

 from samples in the Gulf of St. Lawrence in 1989 

 and 1990 (St-Pierre and de Lafontaine, 1995). 



These results contrast with those of Sevigny and 

 de Lafontaine (1992) who showed that S. fasciatus 

 dominated the population of juveniles sampled in the 

 northeastern sector of the Gulf in summer 1990 and 

 1991. Those juveniles mostly belonged to the 1988 

 cohort whose origin is not known, and Sevigny and 

 de Lafontaine ( 1992 ) did not rule out the possibility 

 that these fish may have originated from outside the 

 Gulf. This rather distinct variation in the genotypic 

 frequency among the various life stages of redfish 

 in the Gulf of St. Lawrence remains unexplained in 

 the light of present knowledge of the ecology of red- 



fish over the entire distribution area. Change in the 

 relative proportion and the geographic distribution 

 of these two species (both at the larval and juvenile 

 stage) may result from variable annual recruitment 

 or from different larval drift and retention patterns 

 among species. The mechanisms responsible for these 

 recruitment patterns are not known but are consis- 

 tent with the view that the Gulf of St. Lawrence 

 does not form a homogeneous environmental unit 

 but rather may consist of distinctly different pelagic 

 ecosystems in the eastern and western sectors (de 

 Lafontaine et al., 1991). 



Acknowledgments 



This study would not have been made possible with- 

 out the support, technical skill and expertise of sev- 

 eral people to whom we wish to express our sincere 

 gratitude: Pierre Joly, Genevieve Ross, Eric Parent, 

 and Yves Morin for assistance during work at sea 

 and in the laboratory. We also acknowledge the cap- 

 tain and crew of the CSS Alfred Needier, for techni- 

 cal help during sampling cruises. A particular and 

 very sincere thank you to M. Jean-Franfois Saint- 

 Pierre who participated in all aspects of this study 

 and who suggested and developed the video camera 

 system for filming larvae at sea. Dominique Gascon 

 and Jean-Denis Dutil made useful comments on an 

 early version of the manuscript. We also want to 

 thank three anonymous reviewers for their useful 

 comments. This study was supported by the Depart- 

 ment of Fisheries and Oceans, Canada. 



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