Predation and the survival of juvenile Cerastoderma glaucum Bruguière (Mollusca: Bivalvia) in a coastal lagoon

Abstract

The effects of predators on the survival of juvenile Cerastoderma glaucum Bruguière, were examined in a lagoon in Holkham, North Norfolk during 2 years (1993 and 1995) using cages to exclude predators from areas of sediment. A range of cage designs were employed to either control for caging artifacts, exclude all predators or a subset of the predators. Cages were seeded with adult cockles at field densities, and juvenile cockles were counted and measured at regular intervals from the time of spat settlement at 350 μm until reaching an overwintering size of 30 mm, some 4–5 months later. Numbers and sizes of cockles surviving in caged areas were compared to control caged areas and unmanipulated areas of sediment. The evidence from controls in the field experiments suggests that the cages themselves, had no detrimental effect on the survival or size of juvenile cockles. Cage treatments which excluded all predators produced the greatest survival of the cockle spat during the 2 years of experiments. The treatment excluding only epibenthic predators provided a short-term improvement in cockle survival suggesting that this group contributes to the lagoonal cockles mortality. However, there is no firm evidence for one predator playing a more important role than the others over the whole experiment. Additional factors influencing juvenile C. glaucum mortality are discussed and it is concluded that dystrophic crises caused by factors such as decaying algae, anoxia and the presence of hydrogen sulphide could contribute to the mortality found in these juvenile lagoonal bivalves.