Multifactor stress for large cockle emergence in the Limfjord, DenmarK?
Carsten Fomsgaard1, Camille Saurel1*, Mia Gommesen1, K. Thomas Jensen2, Paula Canal-Vergés1, Jens Kjerulf Petersen1
1Technical University of Denmark, National Institute of Aquatic Resources (DTU Aqua), Danish Shellfish Centre Øroddevej 80, 7900 Nykøbing Mors, Denmark.
2Department Bioscience, Aarhus University, Ole Worms Allé, Bygning 1135, Denmark
E-mail: cs@skaldyrcenter.dk
Mussel fishermen have reported increases in the amount of cockles dredged for the past 10 years in the Limfjorden, Denmark, a micro tidal system. Cockles are normally found buried in the 5 first cm of the sediment and the surfacing ones can be caught as by-catch in mussel dredging operations. Emerged cockles are known for their limited capacity to bury back into the sediment and are consequently affected by predation, dislocation or death from weakness within a short period of time. Several events of high mortalities in the surfacing cockles - similar to observations in European cockle aquaculture sites - have been recorded in the Limfjorden.
Experiments in the laboratory together with monitoring of a wild population at 10 m water depth were conducted to understand the mechanisms responsible for the occurrence of large and dense surfacing cockle beds, and to inform and advise decision makers as part of the management of cockle fisheries. It was hypothesised that several factors could be linked to this phenomenon: 1) High densities of cockles competing for food and space; 2) Presence of parasites weakening the cockle and preventing their ability to dig back into the sediment; 3) Oxygen depletion at the bottom of the eutrophic Limfjorden; 4) A combination of adverse and stressful environmental conditions together with loss of condition from spawning.
Monitored cockle populations did not present higher densities than found elsewhere. Laboratory experiments comparing burrowing abilities from cockles collected simultaneously at the surface (S) and buried (B) in the sediment in the Limfjorden showed that depending on the sampling month, emerged cockles (S) presented less success than previously buried ones (B) in burrowing back into the sediment. This result was further amplified when cockle density was high. The consequence of oxygen depletion simulations in the laboratory was a reduction of the cockle burrowing capacity for both S and B. The reverse was also tested: after S and B cockles were buried in the sediment, they emerged during oxygen depletion. Both S and B cockles were infected at 20% by the digenean trematode Monorchis parvus. It is the first time that M. parvus is recorded in Danish waters. In contrast to other studies, no neoplasia due to the infection was recorded. All of the factors studied could not individually explain the surfacing of the wild cockle population. Further experiments with multifactorial analysis will complement this study.