A photographic study of the recolonization of cleared patches in a dense population of Mytilus edulis in the northern Baltic proper

Abstract

Recolonisation of cleared patches (15 × 15 cm) on a Mytilus edulis-dominated rocky bottom was followed by photography from June 1992 to October 1994. An experiment was also performed to study movement of newly recruited mussels on ropes in cleared areas. Mussels recolonised the rock surfaces by moving in from the perimeter of the clearings. Juvenile recruitment directly to the cleared surfaces was insignificant for the recolonisation process. However, a large number of juvenile mussels was observed in filamentous algae over and around the cleared patches. The speed of recolonisation was 4.5 cm² of cleared surface per month. The rate of recolonisation was somewhat higher at 6 m than at 12 m water depth. On ropes, downward movement of newly recruited mussels (from 3 up to 12 months old) was small but significantly larger than upward or horizontal movement. There was a positive correlation between movement of mussels to cleared parts of the ropes and the density of mussels on uncleared rope parts. These findings indicate that the recolonisation of disturbed patches by M. edulis in the northern Baltic proper is a relatively slow process driven primarily by a redistribution of already established mussel individuals.