© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
The diel-vertical distribution and characteristics of pre-spawning aggregations of pollock (Pollachius virens) as inferred from hydroacoustic observations: the implications for survey design
a Department of Fisheries and Oceans, Biological Station 531 Brandy Cove Road, St. Andrews, New Brunswick, Canada E5B 2L9
b Department of Fisheries and Oceans, Northwest Atlantic Fisheries Centre East White Hills Road, PO Box 5667, St. John's, Newfoundland, Canada A1C 5X1
*Correspondence to J. D. Neilson; tel: +1 506 529 9954; fax: +1 506 529 5862. e-mail: neilsonj{at}mar.dfo-mpo.gc.ca.
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
Received 29 July 2002; accepted 3 March 2003.
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