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Two size groups (20-30 and 45-55 mm) of Mytilus edulis and Mytilus taossulus were assessed from May to December 1992 at two locations in Nova Scotia. The two species were classified using electrophoresis based on mannose phosphate isomerase, and a discriminant function based on shell morphornetry. The discriminant function misclassified less than 5% sf the mussels and of these, 1% of the M. edulis were rnisclassified as M. taossulus, and 10% of the Me tros.sulus were rnisclassified as M. sdulis. In general, both size groups of M edulis tended to have higher shell growth rates than M. trossulus. Overall mortality was 6% and occurred primarily from mid-August to mid-September; M. edulis exhibited a slightly lower survival and for a given shell length had a higher average tissue and shell weight, as well as a greater shell height, than M. trossulus. Mytilus tasssulus had a significantly lower tissue weight during the summer and was eliminated at a significantly higher rate during commercial grading trials than M. edulis. We estimated from the information on individual weight, survival, and grading losses that the economic value of M. edulis was 8.7 times higher than that of M. tasssulus. RCsnmC: Nous avons Cvaluk deux groupes de taille (20-30 et 45-55 mm) de Mytilus edulis et de Mytllus trmsulus, de mai h dkcembre 1992, B deux endroits de la ~ouvelle-lhosse. Nous avons sCparC les deux espkces par Clectrophorisse & partir de la phospho-mannose-isomerase et par une fonction discriminante fondCe sur la morphomCtrie de la coquille : la fonction discriminante a ma8 class6 moins de 5% des moules et, parmi celles-ci, 1% des M. edralis ont Ct6 classkes ii tort comme les M. trossulus, tandis que 18% des M. trossulus ont CtC classCes comme les M. sdulis. De fagon gCnCrale, dam lies deux groupes Be taille, M. edulis presentait un taux de crsissance de la coquille plus ClevC que M. taossulus. La mortalit6 Ctait globalement de 6%; elle se produisait surtout de la mi-aoiat B la mi-septembre; Mytilus edulis prksentait un taux de survie lCg2rernent plus bas et, pour une longueur de coquille QsnnCe, avait un poids moyen de tissus et de coquille plus klevC, ainsi qaa'une hauteur de coquille plus grande que Ma trossulus. Mytilus trossulus avait pendant l'Ct6 un poids de tissus significativement plus bas et a 6tC 6liminCe pendant les essais de tri commercial 2 un rythme significativement plus ClevC que M. edulis. A partir de I'information sur le poids individuel, la survie et les pertes au tri, nous estimons que la vaBeur Cconomique de M* edulis est E,7 fois plus ClevCe que celle de M. trossulus. [Traduit par la Redaction] Several studies have established the presence of two species of blue mussels, Mytilus edulis and Mytilus trsssulus, in certain regions of Atlantic Canada (Varvio et al. 1988; McDonald et al. 1992; Gosling 1993). Specifically, mixed populations of the two species with varying proportions are a common occurrence along the Atlantic coast sf Nova Scotia, in Newfoundland, and dong the upper reaches of the Gulf of St. Lawr...

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We combined extensive water sampling with monthly growth measurements of juvenile sea scallops held in cages 0 to 200 cm above the bottom to (1) construct predictive empirical models of shell and soft-tissue growth based on oceanographic variables, and (2) determine whether scallops on or near the bottom can derive a food supplement from resuspended sediment when seasonal phytoplankton production is low. Variation in growth was strongly dependent on depth, but this relationship was not consistent over time or tissue type. In late fall, when phytoplankton biomass was generally low (-1 pg chl I-'), the adductor muscle of scallops on the bottom lost mass (-1.5 mg dry wt d.'), but for scallops held only 20 cm higher in the water column, growth was 2.5 mg d.'. During the winter, softtissue growth on the bottom was significantly lower than that of scallops held above the sediment surface. At this time, there was no variation in shell growth with respect to depth. At the end of the study, soft-tissue weight (excluding muscle tissue) of scallops on the bottom was -40% less than that of scallops growing 250 cm above bottom. Rather than providing an energetic benefit, results suggest that high concentrations of seston near the bottom inhibit growth. Empirical regression models of scallop growth using data from water sampling every 2 wk accounted for up to 68% of growth variation, with temperature and seston quality being the most important predictor variables. Marginal improvements to the model uslng data collected hourly with In situ probes suggest that estimates of food supply should be corrected, i.e. reduced, when high flows or high seston concentrations limit filtration rates. In addition, results indicate that attention to the magnitude and variation of predictor variables without consideration of their seasonal coherence may be a primary factor limiting the ability to construct truly predictive models of bivalve growth.

To investigate environmental and seasonal patterns of mortality among natural populations of the blue mussel, Mytilus edulis, we transplanted 11 stocks of juvenile mussels to nine sites along the coast of Nova Scotia, Canada. Stock, Stock by Site, and Stock by Season together accounted for 53% of the variance in mortality whereas Site and Season together explained less than 1% of the variance. Genetic variability among stocks was inferred from differences in their cumulative mortality averaged over all environments. Several stocks, however, exhibited similar levels of mortality over all sites and were apparently insensitive to the environmental gradient. This implies physiological flexibility or the ability to tolerate a wide range of environmental conditions. We conclude that the blue mussel adapts to variable environments by maintaining both physiologically flexible and genetically variable populations. In addition, we noted that stocks originating from more stressful environments tended to exhibit lower mortalities than those originating from less stressful environments.

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