Estimation of cod mortality from Irish tagging programme

Mark recapture methods are generally used to study growth and migration. However, under certain conditions these data can also be used to estimate mortality levels. The current Marine Institute cod tagging programme started in November 2003 in a winter nursery area north of the Inishowen Peninsula, Co. Donegal known as the ‘Cape’ ground (ICES Rectangles 39E2 and 39E3). There has been a seasonal closure of this area from mid November to mid February since 2003 and the tagging vessels had derogation during the survey period. Tagging was continued during the same period in 2004, 2005 and 2009 with a total of 14,472 tagged fish being released. A similar programme was instigated during 2007 in the Celtic Sea (VIIa south and VIIg) and occurred annually until 2010 with a total of 9,076 tags being released. In the Celtic Sea two areas were targeted for tagging, Waterford estuary, a known nursery area in VIIa South, and the offshore cod spawning grounds of VIIg. There were 6,721 tagged fish released in Waterford estuary and 2,355 offshore releases. In the current working document the declining rate of tag recapture over time is used to estimate mortality and these figures are discussed in relation to the results from last years working group (WGCSE 2011).