Use of an index based on the blue mussel ( Mytilus edulis and Mytilus trossulus) digestive gland weight to assess the nutritional quality of mussel farm sites

Abstract

An index based on the digestive gland (DGI) of transplanted (Gaspé, Havre-St-Pierre and Grande-Rivière) and indigenous (Gaspé and Magdalen Islands) 1-year-old blue mussels ( Mytilus edulis and Mytilus trossulus) from eastern Québec (Canada) was used to assess the nutritional quality of mussel farm sites. To understand variations of this index, the effects of temperature and phytoplankton concentrations were examined on growth, gametogenesis, and immunocompetence of transplanted ( M. trossulus, Gould 1850) and cultivated mussels ( M. trossulus and M. edulis, Linnaeus 1758) in Eastern Quebec mussel farm sites. DGI was influenced by the gametogenic cycle and was decreased before spawning with the utilization of the lipids and glycogen reserves stocked in the digestive gland. DGI patterns from all sites showed variations linked to food supply that contributed to the storage of energy reserves. Protein concentrations showed an inverse relationship with DGI, suggesting that structural proteins were not used as an energy source during spawning. Indigenous mussels from the Magdalen Islands site showed a stable DGI, suggesting both the utilization of energy reserves for reproduction and storage of energy reserves due to the abundant food supply in the Magdalen Islands. Transplanted mussels showed a clear seasonal pattern of the condition index closely related to the gametogenic cycle. Differences in soft tissue and shell growths were observed suggesting a nonsynchronous growth. In all sites, phytoplankton cells <2 μm were more abundant than the larger-size classes, and concentrations seemed sufficient for mussel growth. Phagocytosis varied according to gametogenesis and decreased during spawning. Due to the high energetic cost of spawning, which mainly influenced DGI, a direct influence of environmental parameters could not be clearly detected.