ACOUSTIC OBSERVATIONS OF PATTERNS OF AGGREGATION IN HADDOCK (MELANOGRAMMUS-AEGLEFINUS) AND THEIR SIGNIFICANCE TO PRODUCTION AND CATCH

A dual-beam acoustic system was used to collect detailed information on density and body size over a 24-h period in an area of the Scotian Shelf occupied by a population of large spawning haddock (Melanogrammus aeglefinus). The acoustic sampling showed a persistent gradient of decreasing density with distance from bottom with 50% of the fish < 2.5 m off bottom. Horizontally (scale of hundreds, of metres), there was a high degree of local aggregation which changed dramatically over 24 h. Highest aggregation was in daytime, with fish more dispersed throughout the water column at night. The differences in the degree of aggregation with time of day appear to be sufficient to explain diel changes in net catch rates. On larger geographic scales (tens of kilometres), fish density was relatively uniform. Acoustics provide a method for studying the relative importance of aggregation on these various spatial scales to estimates of abundance. Haddock biomass density was also related to organism body size, as has been observed for other species in both aquatic and terrestrial situations, suggesting that haddock population density is determined by trophic interactions similar to those that underlie production of other populations of organisms.