13 - Diet

Summary

Introduction

Estimation of food consumption is usually approached by combining dietary data with information on metabolic energy requirements. Bioenergetic considerations are dealt with in chapter 14; here we consider the main methods for investigating seal diets. Knowledge of their diet is obviously fundamental to studies of the ecology of Antarctic seals and especially to any understanding of their role as predators in the Southern Ocean marine ecosystem. Unfortunately, while we have acceptable qualitative information on the diet of most species (well summarized by Øritsland, 1977), there is insufficient quantitative data to characterize adequately the diet of any species, even in summer. This chapter describes the best methods available at present for the collection, preservation and analysis (including species' identification) of various types of food sample. Suggestions are also provided for appropriate ways in which to present the results of such studies.

Nowadays, the first main objective of dietary studies of seals must be to determine the quantitative composition of the diet. There is no method for acquiring and analysing prey eaten by seals that is free from bias of one kind or another. Research to estimate the extent of these biases and to provide correction factors which may be applied to appropriate compositional data is therefore an important additional requirement.

Collection of food samples

There are three main sources of material suitable for studies of seal diets:

1. complete stomachs (and intestines) from killed seals;

2. partial samples of stomach contents obtained from live seals either by using emetics, stomach pumps or other stomach flushing techniques or by natural regurgitation;

3. collection of faecal droppings (scats).