Experimental Manipulation of Suspended Culture Socks: Growth and Behavior of Juvenile Mussels (Mytilus spp.)

Suspended mussel culture entails loading high densities of juvenile mussels into mesh socks, and hanging them from floating longlines, often resulting in intraspecific crowding, reduced growth, and mussel yield. Despite this potential bottleneck in culture, there are few data on behavioral mechanisms that regulate juvenile density and growth rate. A field experiment was conducted with culture socks to examine the effects of stocking density (High ∼800 mussels/30.5 cm; Low ∼400 mussels/30.5 cm), blue mussel species (Mytilus edulis and M. trossulus) and environment on early development of the culture population. In situ photography and direct sampling were used to generate time series of mussel size, valve gape, siphon area, shell orientation, and emergence in experimental socks at a commercial farm in Ship Harbour, Nova Scotia, Canada. Moored CTD-current meters and water sampling were used to characterize the sites. Emergence from the culture socks required 1–2 mo, with faster initial emergence in M. trossulus. High densities generally did not affect mussel emergence rate or orientation, but higher current speeds produced a negative effect on M. edulis emergence rate. At the experimental densities used, emergence was complete and few mussels remained inside the culture sock. Nonetheless, interior mussels showed mechanical stress such as shell distortion and reduced growth compared with emerged individuals. Mussels exhibited largely horizontal orientation presumably in response to horizontal currents. Siphon area displayed a unimodal response with an optimum at the middle of the current speed range. No density-dependent effect on valve gape was detected, but a significant interaction between siphon area and stocking density suggests that optimal hydrodynamic response is sensitive to crowding. Growth of shell and tissue showed little negative density-dependence. Mussels are proposed to alleviate crowding via behavioral sorting. Although there is concern that growth studies of cultured mussels are often deficient because they use cages or other containers rather than socks, this study suggests that conditions of the sock environment such as crowding may be less important than macroenvironmental factors (e.g., current speed) in determining growth.

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