Research Document - 2009/099

An acoustic-trawl survey of offshore over-wintering northern cod, February-March 2008

By L.G.S. Mello and G.A. Rose

Abstract

An acoustic-trawl survey was conducted during the winter 2008 off eastern Newfoundland and southern Labrador (47° to 54°N), in NAFO Div. 2J3KL. Survey goals included assessing the distribution, the abundance and the biological characteristics of northern cod (Gadus morhua) overwintering on the outer shelf and upper slope. As observed during the 2007 winter survey, cod were found highly aggregated primarily in the region adjacent to the Bonavista Corridor (NAFO Div. 3K) and to a lesser extent in the inner region of Hawke Channel (NAFO Div. 2J). However, major differences in abundance, distribution and behavioral patterns were observed in 2008, notably in the Bonavista Corridor. The detected biomass (-/+ 95% CI) in 3K amounted to 101,162 t (94,272-108,050 t), of which > 90% was located within an area of 18.5 x 37 km at depths <400 m in the Bonavista Corridor, including a spawning component of 43,000 t (37,000-50,000 t). Aggregations ranged from demersal layers (5-25 m from the seafloor) to vertical columns (up to 100 m from the seafloor), the latter typically associated with cod courtship and spawning behavior. Aggregations also displayed marked diel migrations; in 2007 cod were found only demersally and no diel migrations were observed. Catch data indicated that most fish ranged between 35-65 cm in length (4-7 years-old). Approximately 45% of the sampled fish were mature or maturing. The biomass estimates in 2J amounted to 4,760 t (4,596-4,924 t), with 771 t (758-784 t) in 3L (mostly of fish <35 cm and immature and/or maturing; target areas in 3L were not completed surveyed). When compared to the 2007 survey, the results of this study indicate that the biomass estimate of cod overwintering offshore in 3K has increased several fold, age and length structures have expanded; and behavioral and distribution changes related to spawning and migratory cycles are now evident.

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