Relationships between herring school distribution and seabed substrate derived from RoxAnn

Abstract

A series of three fine-spatial-scale acoustic studies was carried out to study the relationships between herring-school distribution and the seabed substrate. The study was carried out on three separate bank areas east of Shetland in July 1993. The study areas were characterised respectively as: mainly mud with some hard ground; mixed mud, sand and hard ground; and mostly hard ground. Herring schools were identified from the echo-sounder record. The substrate was mapped using the RoxAnn seabed classification system interfaced to the same echo-sounder. The relationship between herring distribution and substrate was examined at two different levels; map-based, using contour plots of the important variables, and track-based, using individual herring schools in relation to the specific substrate type found on the survey track under the schools. Data are presented which show a strong tendency for schools to be found preferentially over hard seabeds, particularly in the track-based analysis. There also appears to be a strong relationship between herring aggregations and particular topographic features. This is discussed in relation to specific features, a low ridge and two escarpments, identified in two of the study areas.