SPAWNING, FECUNDITY AND LARVAL SURVIVAL AND GROWTH IN RELATION TO CONTROLLED CONDITIONING IN NATIVE AND TRANSPLANTED POPULATIONS OF PECTEN-MAXIMUS (L) - EVIDENCE FOR THE EXISTENCE OF SEPARATE STOCKS

Five groups of Pecten maximus (L.) were conditioned in a hatchery: indigenous scallops from the Bay of Brest (B), the Bay of St. Brieuc (S) and Iroise (1), France, and scallops of Scottish (ET), and Bay of Brest (BT) origin which had been transplanted as spat to the Bay of St. Brieuc. Prior studies have shown that in the Bay of St. Brieuc native scallops have a very discrete reproductive period in summer whereas native scallops in the other areas have ripe gonads through most of the year. The ability of the five groups to respond to the usual conditioning factors (temperature: 15-degrees-C, high food levels: average 8 x 10(9) cells/animal/day) was investigated in late fall when the gonad index of the St. Brieuc natives (S) is near its minimum value. For each group, 3-year-old scallops caught by diving were conditioned for 2 months. Their reproductive condition was determined by histological examination at the beginning and at the end of the experiment and by periodic visual examination of the gonads. Spawnings obtained by thermal stimulation were followed individually: their performances in terms of normal development, hatching rate, larval growth and survival to the pediveliger stage (23 days old) were estimated. The St. Brieuc native scallops showed little response to conditioning and could not spawn at the end of the conditioning period. In all the other groups the gonad ripened and the scallops responded to spawning stimuli. The Bay of Brest natives (B) responded better than the transplants (BT). Larval rearing performances of the different groups did not differ significantly, but survival at and after metamorphosis was related to the origin of the spawning stock.