Journal Article

The diel-vertical distribution and characteristics of pre-spawning aggregations of pollock (<i>Pollachius virens</i>) as inferred from hydroacoustic observations: the implications for survey design

John D Neilson, Donald Clark, Gary D Melvin, Peter Perley and Chris Stevens

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 60, issue 4, pages 860-871
Published in print January 2003 | ISSN: 1054-3139
Published online January 2003 | e-ISSN: 1095-9289 | DOI: https://dx.doi.org/10.1016/S1054-3139(03)00068-7
The diel-vertical distribution and characteristics of pre-spawning aggregations of pollock (Pollachius virens) as inferred from hydroacoustic observations: the implications for survey design

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The characteristics of pollock (the synonymous European common name is saithe) pre-spawning aggregations were described at two locations with contrasting bathymetric features on the Scotian Shelf, off the Canadian Maritimes. The data were collected using a split-beam echosounder onboard a research vessel, augmented with periodic, bottom-trawl samples. Pollock form aggregations each fall that persist at the same location over time. Such aggregations appeared to be associated with spawning. Hydroacoustic information indicates that pollock become more densely aggregated at night. This could reflect movement away from the study area during the day, or changes in the proportion of pollock in the acoustic dead zone over the 24-h period. The hydroacoustic information indicates that while pollock can occur up to 30 m off bottom, the greatest proportion remains within 1–5 m off bottom during both day and night. The length composition of the pollock aggregations differed between the two sites, with larger fish found at the site further offshore. Within an aggregation, there was spatial heterogeneity with respect to fish size, with larger fish found primarily within the core area of aggregation as shown by the hydroacoustics. An appropriate survey design for obtaining an index of abundance for pollock would reflect both the contagious (patchy) distribution as they prepare to spawn, and the diel differences in the availability of the fish to the hydroacoustic-sampling gear.

Keywords: acoustic surveys; groundfish; aggregations; diel; bathymetry

Journal Article.  5190 words.  Illustrated.

Subjects: Environmental Science ; Marine and Estuarine Biology

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