The feeding ecology and behaviour of inshore fishes

Abstract

The food and feeding relationships of 21 species of inshore fish from Loch Etive were examined over a 13 month period from October 1971 to October 1972. Some aspects of the feeding behaviour of two species Sninachia spinachia and Pholis gunnellus were investigated experimentally under laboratory conditions. With the exception of three species which were zooplankton feeders, the majority of the fish examined were benthos feeders. Amphipods were the most heavily preyed upon benthic food group. Most fishes depended on relatively few prey types as their major food, the number of principal food types were commonly between 2 to 5 and there was often an overlap between the main food types of both related and unrelated fish species. The majority of the fishes utilized similar sources of food, but the proportions of the most commonly occurring food types eaten by different fish species were significantly different. The types of main food groups in the diet of individual species usually did not alter throughout the year, but their importance in relation to one another frequently changed. The quantity of food in most fishes diets, however, exhibited a marked seasonal variation. In the diet of some fishes there was evidence for a clearly defined size-related feeding heterogeneity, in others prey size selection was less marked. The effects of some visual cues utilised in prey selection by S. spinachia and P. gunnellus were tested. S. spipachia showed a marked prey size selection under experimental conditions when the catchability of prey was strictly controlled. Selectivity was found to be related to hunger level. There was an optimal prey size for which the fish showed greatest selection. This optimal prey size was much smaller than the maximum prey size the fishes could swallow, but it coincided with the critical prey size, defined as the size at which the prey handling time began to increase drastically. F . gunnellus exhibited less clear prey size selection pattern and no optimal prey size was found. The fish took prey of a wide size range, showing preference for the larger prey within the preferred size range. Both fishes showed a marked preference for moving prey, selection for movement overruling the selection for size. P. gunnellus had a narrower preferred speed range of prey then S. spinachia. Further experiments with Spinachia in which the influence of shape and shade were tested, suggested that the order of importance of the visual stimuli presented by a prey organism was: movement > size > colour > shape.