Declining European seabass (Dicentrarchus labrax) numbers in the Irish Sea, Celtic Sea, English Channel and southern North Sea required detailed assessments of stocks to be carried out. This led to a collaborative project between Ifremer, CLS (Space Oceanography Division), Institut Telecom/Telecom Bretagne, Parc Naturel Marin d’Iroise in the Iroise Natural Marine Park, a marine protected area around Brittany, western France. The project aimed to provide biological evidence to support the implementation of conservation measures to save the declining stocks.
A total of 246 G5 Long-Life Data Storage Tags were deployed in the summers of 2010, 2011 and 2012. Fish were caught by professional fishers in either Ushant or Raz de Sein waters but, due to difficulties in navigating the Iroise Sea, two methods of tagging were practiced. The first involved a support vessel where seabass were tagged and released on the same day. The second method transported fish to a shore-based location where the fish would be kept overnight before being released the next day.
In order to increase the return rate of the tags, they were supplied housed in a bright orange flotation collar (Fig. 1). The tags were surgically implanted into the peritoneal cavity with an external Floy Tag® for identification purposes.
Fig 1. A G5 Long-Life fitted with a flotation collar
Until the end of 2016, 36 individuals were recovered. 38.9% of returns were as a result of the tags drifting to the shore and being found (made possible by the flotation collars). The remainder of the recoveries were caught by fisherman or through the sale channel.
Researchers used the hidden Markov model for fish geolocation to infer the seabass’ movement from release to recapture. Analysis of the data revealed a spatial structure to the stocks, with individuals exhibiting either migration or residency behaviours.
The study demonstrated how archival loggers can be used to investigate species’ behavioural patterns in the long-term and could therefore influence conservation actions.
References:
de Pontual, H., Lalire, M., Fablet, R., Laspougeas, C., Garren, F., Martin, S., Drogou, M., Woillez, M. (2019), ‘New insights into behavioural ecology of European seabass off the West Coast of France: implications at local and population scales’, ICES Journal of Marine Science, Volume 76, Issue 2, pp. 501-515