The Relationship of Midwater Trawl Catches to Sound Scattering Layers off the Coast of Northern British Columbia

Abstract

In 1964 and 1965 three midwater trawl cruises off the continental slope near the Queen Charlotte Islands were made to study the relationship of the catches to sound scattering layers.During the daytime four layers were distinguished, not all necessarily present at one time. In summer the most intense and persistent layer lay at 220–230 m; others were at 330–400, 185, and 90 m. In fall the main and deep layers were shallower. Vertical migration occurred at dusk and dawn. The main layer frequently split into three parts with different vertical migration patterns. At dawn a layer gradually appeared at about 90 m, intensified rapidly, and descended to the daytime level.Catches show fishes are associated with the sound scattering layers. The largest day catches were from the main layer. At night fish did not appear to be concentrated to the same extent relative to the layers. The largest night catches were usually smaller than the largest day catches and shallower.Myctophids of eight species, four common, made up 80% of the catch, chauliodontids 6%, argentinids 4%, melamphids, rockfish, and larval flatfish 2% each. The remaining 2% comprised 27 species in 21 families. Depth distribution and vertical migration varied considerably with the species.Myctophids with gas-filled swim bladders were apparently dominant in the main layers whereas species without swim bladders — stomiatoids, argentinids, and melamphids — or myctophids with fat-filled swim bladders were dominant below the main layer and in the deep layer. Fish with no swim bladders or with fat-filled ones may make longer vertical migrations than those with gas-filled bladders.