Research Document - 2013/104
Harvest rates and movements of cod (Gadus morhua) in NAFO Divs. 3KL from tagging and acoustic telemetry
By J. Brattey
Abstract
Results from analyses of recaptures of cod tagged and released in the Northwest Atlantic Fisheries Organization (NAFO) Divisions (Divs.) 3KL during 2004-12 are reported along with updated analyses of acoustic telemetry data from cod released offshore during March 2008. During 2004-11, approximately 25,000 tagged cod have been released and approximately 1,370 recaptured. Mean annual exploitation rates (percent harvested) are estimated for cod released in various regions (unit areas); the methods incorporate estimates of initial tagging mortality, tag loss (from double tagging) and reporting rate (from high-reward tags) and a range of assumed values for the instantaneous rate of natural mortality (M). Annual tag reporting rates were also estimated; the time-series average was 70% and estimate for 2012 was 65%. Separate reporting rates were also estimated for recreational and commercial fishers and these indicated a lower reporting rate for recreational fishers (49% in 2012), compared to commercial fishers (71%). Recreational fisher tag returns amounted to 73% of commercial fisher tag returns suggesting that landings during recreational fishery, though not directly measured, are a substantial fraction of total removals. Average annual exploitation rates (percent harvested) for 2010-12 estimated from tagging were low in all regions (range 2‑6%). These results indicate that fishing is a small component of total mortality rates (Z=0.46 or 37% mortality per year during 2010-12) estimated from cohort analyses of Fisheries and Oceans Canada (DFO) autumn trawl survey data. However, overall productivity of the stock is low and management actions should focus on promoting further increases in spawning biomass and subsequent recruitment as the stock is currently (2012) at 15% of the limit reference point. Cod released offshore in Div. 3K during March 2008 with implanted acoustic transmitters migrated inshore during summer and were detected widely throughout the inshore of 3KL during summer and fall 2008-10; these cod disappeared during intervening winters and presumably migrated back offshore, indicating that the traditional seasonal migration pattern observed prior to the moratorium was taking place. Migratory offshore cod first arrived inshore in late June and through July, approximately 3-4 weeks later than in those tagged offshore in the 1980s.
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