Research Document - 2008/047
Northern cod (Gadus morhua) 16 years after the moratorium: new information from tagging and acoustic telemetry
By J. Brattey, B. Healey, and D. Porter
Abstract
A combined tagging and acoustic telemetry approach was used to investigate the survival and migratory behaviour of a coastal population of northern cod, with emphasis on over-wintering cod in Smith Sound (SS), Trinity Bay, Newfoundland (NL). The home range, seasonal movements, fidelity to over-wintering areas, and survival (mortality) rates, of these cod were investigated. Movements patterns inferred from telemetry results were also compared with those based on recaptures of conventionally tagged cod. Following a pilot scale study, we released large numbers of cod (100 per year) with surgically implanted coded transmitters (Vemco V16, 69kHz) and external (Floy) t-bar tags and deployed a “counting fence” of receivers (Vemco VR2) at the mouth of SS to provide detailed information about daily movements. We also deployed arrays of receivers along the northeast coast of NL to investigate migration patterns and dispersal, and determine if these cod were subsequently over-wintering in other inshore areas. Small numbers of cod (<20) with implanted transmitters were also released at other sites along the north east coast and their movements monitored. Survival of telemetred and tagged cod following release was only 66% for trawled cod from deep (190-225 m) water, compared to 96.4% for those caught with hand-lines in shallow (10-82 m) water. There was a clear seasonal pattern in movements that was repeated in three consecutive years (2005-07); most cod left SS in spring (March-June), remained outside SS during summer, dispersed mainly northward in Trinity Bay and Bonavista Bay, and returned during late autumn and winter (November-January); a small proportion of telemetred cod (0-20%) remained in SS throughout the year. Cod released in SS showed strong over-wintering site fidelity and return rates were: 9 of 9 (100%) in 2005, 64 of 77 (83%) in 2006, and 65 of 99 (65%) in 2007. Less than 10% of telemetred cod showed other behaviours, including overwintering elsewhere in subsequent years, and returning to and leaving SS repeatedly during summer and fall. Ten percent of telemetred SS cod were captured in the fishery in 2006 and 9% in 2007, from reported landings of only a few thousand tons. Direct estimates of the minimum survival rate of two groups of telemetred cod were 80% (from 19 May 2006 to 29 January 2007) and 68% (from 31 May 2007 to 29 January 2008). Some cod released off Twillingate and in Newman Sound over-wintered in the deep inlets of inner Bonavista Bay, whereas those released in southern 3L stayed in the local area or moved south and some were captured in NAFO Subdiv. 3Ps the following winter. The results support the revised stock structure used in assessments of northern cod since 2005, and indicate a resident component in the central inshore region of 3KL, and a migratory component in southern 3L.
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