Predation of shore crabs (Carcinus maenas (L.)) and starfish (Asterias rubens L.) on blue mussel (Mytilus edulis L.) seed from wild sources and spat collectors

In The Netherlands, several pilot projects are carried out on the use of spat collectors as an additional supply of seed for bottom culture of mussels (Mytilus edulis). The method proves to be successful in yielding substantial amounts of seed. One of the conditions for successful application of collector seed on bottom plots is a good yield of the seed on bottom plots. Mussel seed of different origin (from wild littoral and sublittoral beds or from collectors) was offered to predators (crabs Carcinus maenas and starfish Asterias rubens) and seed survival was monitored. In addition, the effect of density and size of collector seed on predation was studied. Circular cages containing predators and seed were placed in a basin with running seawater, or suspended from a jetty in a harbour. Two size classes of predators and three size classes of seed were used. Survival was monitored. Consumption of mussel seed by starfish was much lower than by crabs. Maximum observed consumption rates were 23 seed/day/crab and 1 seed/day/starfish. Consumption rates increased significantly with decreasing seed size. Seed larger than 20 mm were consumed at a significantly lower rate. Seed density did not affect survival. Collector seed was not consumed at higher rates than wild littoral or sublittoral seed. In conclusion, collector seed can be a promising additional source of seed for bottom culture of mussels in The Netherlands.