Comparative growth of cultured blue mussels, Mytilus edulis, M-trossulus and their hybrids, in naturally occurring mixed-species stocks

In Atlantic Canada, both Mytilus edulis and M. trossulus occur naturally in sympatric populations and readily hybridize forming genotypically complex assemblages that vary widely in relative species composition throughout the region. We monitored growth in single year-class cohorts of rope-cultured Mytilus spp. during a 14-month period from sleeving of seed to harvest at three commercial mussel farms in Notre Dame Bay, Newfoundland, Canada. Electrophoretic variation at the mannose phosphate isomerase (Mpi) enzyme locus was used to categorize the two species (M. edulis and M. trossulus ) and their hybrids. Seed populations varied significantly among sites in relative proportions of each species and hybrids. Growth in shell length, wet weight and shell weight varied significantly with Mpi genotype at all three farm sites. Growth in flesh weight varied significantly with Mpi genotype at two of the three sites. Growth rates for all weight variables were consistently significantly greater in M. edulis than in sympatric M. trossulus . Growth in shell length was significantly different between sympatric M. edulis and M. trossulus at only one farm site. Growth rates of hybrids were typically intermediate between sympatric M. edulis and M. trossulus , but were often statistically similar to one or other of M. edulis or M. trossulus . We conclude that the production of rope-cultured blue mussels in Atlantic Canada is significantly influenced by genotype-dependent growth variation in naturally occurring mixed-species stocks. Our results also suggest that farm site production may be increased and intrasite variability reduced at farm sites currently using indigenous mixed-species stocks by substitution and growout of transplanted seed from unispecific M. edulis stocks.