Comparison of the burrowing response of undersized cockles (Cerastoderma edule) after fishing disturbance caused by hand dredge and harvesting knife

Abstract

In the Ria Formosa lagoon (southern Portugal), the common cockle (Cerastoderma edule) has been commercially exploited using the harvesting knife (HK), an artisanal rudimentary tool used over the generations by local fishermen. More recently, the hand dredge (HD) has gained popularity among harvesters, because it is easier to operate and allows higher fishing yields per unit of effort (harvesting time). The damage caused by the gear on the shell and the way the gear is operated can affect post-discarding survival of undersized cockles, i.e. faster burrowing may determine higher chances of survival, thus reducing indirect mortality due to desiccation or predation. In this study, the burrowing response of cockles discarded after HD and HK harvesting were investigated in situ, during fishing surveys. After sorting a sample of undersized cockles (shell length<25mm) was collected from the bycatch and placed into previously harvested areas which, according to fishermen, is common practice. No difference was found between the burrowing response of cockles after harvesting with HD and HK. Independently of the gear, 83% of the cockles burrowed within 15min and only 10% of the individuals remained on the sediment surface 1h after being discarded. The fast burrowing response indicates that neither harvesting method affects the physical condition of cockles. These results suggest that, contrary to what has been argued by local fishermen using HK, post-discarding survival is not directly linked with the different modus operandi of the gear.

Keywords:

• Burrowing
• Cerastoderma edule
• Cockle
• daily quotas
• hand dredge
• harvesting knife

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

We would like to thank all fishermen that collaborated in the fieldwork and Tibério Simões (IPIMAR technical staff) for his collaboration in collecting the samples. We are particularly grateful to Dr Jorge Ramos for reviewing a draft version of the manuscript. Thanks are also due to the anonymous referees, whose suggestions improved the revised version of the manuscript.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark