Research Document 1998/27

Acoustic surveys for cod in the Trinity Bay and Bonavista Bay (NAFO Div. 3L) during spring 1997

By D.R. Porter, J. Brattey, and J. Anderson

Abstract

Acoustic surveys for cod were conducted in western Trinity Bay during April 1997, and in southern Bonavista Bay during June 1997. Severe ice conditions in early May prevented more extensive survey coverage. Integrated density estimates for cod were obtained using a dual-beam 38Khz transducer, and a Biosonics model 102 echosounder, in conjunction with a Femto digitizer. A series of equidistant parallel transects with a random starting point was run. A total of 116 transects were surveyed and targets classified as cod were detected on 70 transects (60.3%). Average densities of cod on most transects were generally low (<0.20 gm/m²), but high densities (>300 gm/m²) were observed in four adjacent transects across deep (>120 m) water in Smith Sound, Trinity Bay. This large school of pre-spawning and spawning cod extended over an area 6 nm long by 0.5 to 1.3 nm wide, was up to 70 m deep and the densest part of the school was 25-30 m up off the bottom. Total estimated biomass for the surveyed portion of Trinity Bay was 21,783 mT, with 96.3% observed in Smith Sound. A total of 1,516 mT was estimated for southern Bonavista Bay. Biological sampling, conducted with a Yankee 36 otter trawl and hand-lines equipped with a jigger and feathered hooks revealed cod ranging in size from 35 to 80  cm (mean 58.1) in Trinity Bay and 35-72 cm (mean 53.6) in Bonavista Bay. In sampled cod, ages 5 and 7 yr (1992 and 1990 yr classes) were most strongly represented in Trinity Bay with age 5 predominating in Bonavista Bay. In Smith Sound a small proportion (6.8%) of sampled females had hydrated eggs or were spent; the remaining mature females had mostly ripe gonads (Stage Mat AP) suggesting that spawning was just beginning and that spawning occurred later in 1997 than in 1996. In Bonaviata Bay, which was sampled in June, only 10% had ripe gonads (Stage Mat AP) and there were many females with hydrated eggs or spent gonads (50%) suggesting that peak spawning had already occurred.

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