ESTIMATING FOOD CONSUMPTION

5.9 ESTIMATING FOOD CONSUMPTION

Estimation of food consumption by fish popula- tions has widespread uses in ecological and fish- eries research. Such estimations are required for the investigation of predator–prey interactions and predation mortality, for assessment of produc- tion dynamics of fish populations, and for the development of multispecies stock assessment models (Ney 1990; see also Shepherd and Pope, Chapter 7 and Pauly and Christensen, Chapter 10, both Volume 2). For accurate estimation of popula- tion food consumption many different types of data are required. These include information about population density and age structure, as well as quantitative assessments of food consumption rates of fish of different sizes under different envi- ronmental conditions, and information about dietary composition with respect to the relative proportions of the different prey types consumed.

Studies of the dietary composition of wild fish are almost invariably based on the analysis of stomach contents, unless one is interested in over- all trophic structure, in which case stable isotopes provide a complementary method. Polunin and Pinnegar (Chapter 14, this volume) provide an ex- tensive review of the use of stable isotopes. Here I review three approaches that are usually adopted when carrying out analyses of stomach contents: numerical analysis, volumetric analysis and gravi- metric analysis (Windell and Bowen 1978; Bowen 1996). Each approach provides different types of in- formation relating to prey selection and dietary composition, but none gives any quantitative as- sessment of the amount of food consumed. Of the numerical analyses the frequency of occurrence gives an estimate of the proportion of the popula-

tion that has fed on a particular prey type. This pro- vides an indication of the uniformity with which fish within a population select their diet, but it does not give any information about the relative importance of the different prey types. The calcu- lation of percentage composition by number gives the latter sort of information because it provides an estimate of the relative abundance of a particu- lar food item in the diet. Percentage composition by number has its equivalents in percentage com- position by volume and percentage composition by weight, obtained from the results of volumetric and gravimetric analyses respectively (Windell and Bowen 1978; Bowen 1996). There are weak- nesses associated with all these forms of analysis, and in an attempt to overcome the problems there have been several attempts to develop indices that combine the information obtained using the dif- ferent analyses (Bowen 1996).

In many cases it may be deemed desirable to have estimates of the size, or weight, of the differ- ent prey items at the time of ingestion. Sizes of par- tially digested prey can be estimated by measuring the dimensions of some digestion-resistant hard part such as an otolith, operculum, head capsule, carapace or chitinous element of a compound appendage. The initial size of the prey can then be estimated by reference to regression equations established for the relationships between prey size and the dimensions of the particular digestion- resistant hard part in question (e.g. Table 5.1). However, care must be exercised when interpret- ing the data obtained when using the method of reconstructed sizes or weights, because it is very easy to draw incorrect conclusions about die- tary composition and the relative contributions of different prey organisms to the diet of the predator (for discussion see Jobling and Breiby 1986; Jobling 1987; Juanes et al., Chapter 12, this volume).

It is not usually practicable to quantify rates of food consumption of wild fish by direct observa- tion, and two different indirect approaches have been adopted for the estimation of food intake by natural fish populations (Windell 1978; Adams and Breck 1990; Bromley 1994). The first approach is based on calculations of the energy budget and

Rates of Development and Growth

115

Chapter 5

bioenergetics modelling; the second relates to the combination of stomach contents data ob- tained in field surveys with information about gastric evacuation rates obtained from laboratory experiments.

Bioenergetics models can be used to estimate food consumption from growth, or vice versa, by incorporating information about fish body size, water temperature, diet composition and the energy densities of predators and prey. Model out- puts are, however, subject to errors related to un- certainties about the accuracy of input variables, insufficient information about the metabolic costs of activity of wild fish, and possible problems with the extrapolation of physiological rate functions, developed in the laboratory, to the field situation. Models that vary in complexity have been devel- oped for the estimation of food consumption or growth of natural fish populations, but all have their basis in the energy budget equation (Windell 1978; Weatherley and Gill 1987; Adams and Breck 1990; Hewett and Johnson 1992; Elliott 1994; Brett 1995; Forseth et al. 2001).

A bioenergetics model based on inputs derived from a series of laboratory experiments may pro- vide good predictions of growth and/or food con- sumption of fish exposed to conditions towards the centre of their environmental tolerance range (Whitledge et al. 1998; Wright et al. 1999). How- ever, such models may give erroneous estimations at environmental extremes or when fish are undergoing repeated cycles of food deprivation and refeeding. For example, Wright et al. (1999) reported that the models could overestimate the overwintering costs of largemouth bass (Mi- cropterus salmoides ) by 20% or more. Further, in tests made on fish undergoing cycles of depriva- tion and refeeding, model predictions were in error by 25–35% in some instances (Whitledge et al. 1998). Given that fish in the wild are unlikely to feed and grow at constant rates and that substan- tial variations are likely in both the short and long term, these test results may have important impli- cations for field applications of bioenergetics models.

The alternative to the bioenergetics model for estimation of food consumption of natural fish

populations involves examination of the rate at which food is digested and evacuated from the stomach. Several models are available for the esti- mation of food consumption of wild fish from gas- tric evacuation data, each model incorporating a set of assumptions about meal frequencies and the form of the curve that describes the evacuation of food from the stomach (Windell 1978; Adams and Breck 1990; Bromley 1994; Elliott 1994). Few of the models have, however, been rigorously tested under controlled conditions to check their predic- tive accuracy (Elliott and Persson 1978; dos Santos and Jobling 1995). In one approach it is assumed that stomach contents decline exponentially with time and that feeding is continuous between sam- pling times. At the opposite extreme the assump- tion is made that a constant amount of food is evacuated from the stomach per unit time (i.e. stomach contents decline linearly with time) and that digestion times are long relative to the timing of meals. Both approaches may have merit, since they may apply to fish that have different feeding habits. For example, fish which feed on small prey organisms or on food that is nutritionally poor (planktivores, herbivores, detritivores) may feed almost continuously, and the pattern of evacua- tion of these species is often found to be exponen- tial. On the other hand, other species, such as many piscivores, consume large meals at infre- quent intervals, and the evacuation of food from the stomachs of these fish can often be modelled using a simple linear relationship (Jobling 1986; Bromley 1994).

Irrespective of the model applied, gastric evacu- ation rate data must be accurate if food consump- tion estimates are to be reliable. Both the rate of evacuation and the form of the curve (e.g. exponen- tial vs. linear) will be influenced by the type of prey, and, as with other physiological processes, gastric evacuation rates will be affected by water tem- perature (Jobling 1986; Adams and Breck 1990; Bromley 1994; dos Santos and Jobling 1995). A model that has been used extensively in field studies of food consumption by fish is the one developed by Elliott and Persson (1978):

C (5.17) F F t Et t Et Et = - ( ) - ( ) - - 0 e 1 e C (5.17) F F t Et t Et Et = - ( ) - ( ) - - 0 e 1 e

exponential). Daily food consumption (C 24 ) is then

calculated as the sum of the C t values obtained for each of the periods within the 24 hours. This model seems to be applicable when feeding is more or less continuous within sampling periods, the amounts of food in the stomach at the start and end of a period are not the same, and when gastric evacuation is adequately described by an expo- nential function.