A FIELD METHOD FOR DETERMINING PREY PREFERENCE OF 



PREDATORS 



N. B. Hargreaves^ 



ABSTRACT 



A new field method for determining prey preferences of fish that feed on juvenile salmon is described. 

 The basic elements of this method consist of capturing, tagging or marking, and releasing prey with 

 known characteristics, and comparing these characteristics with those of tagged prey subsequently 

 recovered from the stomachs of predators. The feasibility of this approach is illustrated by two experiments 

 conducted in 1985, designed to assess prey size preferences of predators feeding on juvenile pink salmon 

 during the early sea-life period. The results indicate that yearling coho salmon, Oncorhynchus kisutch, 

 were size selective when feeding on juvenile pink salmon, 0. gorbuscha, preferring the smaller prey. 

 A major advantage of this method is that it eliminates the need to determine the abundances of various 

 prey t\T)es in the field. It also allows the investigator to control or precisely measure many of the variables 

 that are known to affect the availability of prey to predators. 



Predation plays an important role in shaping the 

 ecological structure of many biological systems. One 

 aspect of predation that has attracted considerable 

 interest is the observation that, when offered a 

 choice of prey types, predators typically show a pref- 

 erence for one of them. The result is that more of 

 the preferred prey are consumed than would be ex- 

 pected, based on the relative abundances of the 

 various prey types. 



There have been many attempts to quantify the 

 food or prey preferences of predators (e.g., Hess and 

 Swartz 1940; Ivlev 1961; Schneider 1981) using a 

 wide variety of mathematical indices of preference 

 (reviewed by Cock 1978; Pearre 1982). In some 

 situations, however, this approach clearly is not 

 suitable. For example, in many systems the relative 

 abundances and species composition of the prey can 

 vary substantially over the normal feeding range of 

 the predators. This is particularly evident in fish- 

 eries, where piscivorous predators and their prey 

 are often very mobile, and can travel considerable 

 distances during even a single feeding period. In 

 such cases, the proportions of the various prey found 

 in the stomachs of predators may result from varia- 

 tions in the relative concentrations or availability 

 of prey over the extensive area searched by the 

 predator, rather than from any prey preference. It 

 is typically very difficult to determine the concen- 



^Department of Fisheries and Oceans, Fisheries Research 

 Branch, Pacific Biological Station, Nanaimo, B.C. V9R 5K6, 

 Canada. 



trations and species composition of prey over such 

 large areas, so indices of preference may not pro- 

 vide much insight into the predation process. 



The purpose of this paper is to describe a new 

 method for determing the prey preference of pred- 

 ators in the wild. This method consists of a field 

 experiment in which a number of potential prey with 

 known characteristics are released. These prey are 

 tagged prior to release to allow positive identifica- 

 tion even if they mix with other prey of the same 

 type after they are released. The prey preference 

 of the predators is assessed directly by comparing 

 the characteristics of the tagged prey found in the 

 stomachs of predators with those of the prey that 

 were released. The major advantage of this ap- 

 proach is that the relative abundances and other 

 characteristics (species ratios, size composition, etc.) 

 of the various prey types are known in advance and 

 additional field measurements are not required. In 

 addition, if only tagged prey are compared, there 

 is much less ambiguity in assessing the preference 

 of predators for each type of prey because the major 

 alternative explanations are eliminated. Although 

 applicable to a wide variety of predator-prey inter- 

 actions, the details and potential utility of this 

 method are illustrated by two experiments con- 

 ducted in Masset Inlet and Masset Sound, B.C., 

 Canada. In both cases the goal was to test the 

 hypothesis that natural fish predators were size 

 selective when feeding on juvenile pink salmon, 

 Oncorhynchus gorbuscha, during the early sea-life 

 period. 



Manuscript accepted July 1988. 



FISHERY BULLETIN: VOL. 86, NO. 4, 1988. 



763 



