The role of prey size in the feeding behaviour and growth of juvenile Atlantic salmon (Salmo Salar L.)

Abstract

The distribution and behaviour of juvenile Atlantic salmon (Salmo salar L.) in a selection of river and stream habitats was studied with particular reference to feeding behaviour. It was concluded that salmon fed predominantly on drifting material. Drift feeding behaviour was studied using a recirculating flume tank simulating a simplified stream environment. The effect of prey particle size on the distance of first reaction, striking, and selectivity was investigated. Reaction distance was maximal on prey whose diameter was 0.025 fish length (fl), and striking distance was maximal on prey of 0.025 to 0.051 fl, and both decreased on larger and smaller prey. 100% of available prey of diameter 0.025 fl and less were ingested, while 90% of 0.051, and 100% of 0.105 fl prey were rejected. Head and jaw movements involved in capture, manipulation, ingestion or rejection of prey were studied, as a result of which mouth breadth and gill raker spacing were proposed as morphometric limitations on the maximum and minimum accessible prey size respectively. On this basis the relative range of accessible prey sizes remained constant at 0.06 fl, irrespective of fish size (2.8 to 20.3 cm length). The effect of prey size on the growth of salmon from first feeding alevins to second year smolts was investigated. Alevins were found to show a good growth rate on all prey of diameters from 0.0115 to 0.090 fl. In contrast all other fish sizes (4.2 to 20.3 cm) showed maximum growth rate on prey of diameter 0.022 to 0.026 fl, although some seasonal variation was indicated. Reaction, strike and selectivity behaviour corresponded with growth response and the proposed morphometric limitations. Results are discussed in relation to vision, behaviour, functional morphology and growth in general and a composite scheme drawn up.