Chapter 16: Diets and Energy Flow
James E. Garvey and Steven R. Chipps
Understanding when, where, and what fish eat is central to fisheries management, fish conservation, and aquaculture. Growth and production of fish are typical measures of success in fisheries science and are directly linked to the quantity plus nutritional and energetic quality of food. Thus, information about dietary needs and trophic relationships (i.e., dependencies on energy from prey) is needed to interpret research results as well as make sound management decisions. For example, dietary information can improve population assessment models, identify constraints to fish growth, and aid in managing harvest. Management actions to enhance fishing opportunities such as stocking to support declining populations and augment poor natural recruitment can be guided by the knowledge of dietary needs in the wild (Rand and Stewart 1998). In fish conservation, factors such as invasive species are threatening native organisms (Vander Zanden et al. 2004); research exploring food habits and trophic position can provide insight into competition between invaders and natives for limited food resources (e.g., Bohn and Amundsen 2001) and predation by invaders on endemic fishes (e.g., Pringle 2005). Habitat degradation and loss and its negative effect on fish foraging also may affect conservation efforts. From a culturist’s perspective, a better understanding of dietary preference and nutritional needs in the wild may translate into important innovations that maximize production in the hatchery.
Methods for quantifying food eaten by fishes have focused historically on individual behaviors of both consumers and their prey (Lima and Dill 1990) and the taxonomic identification of diet contents (Gerking 1994; Bowen 1996). These are useful but cumbersome tactics, being particularly arduous if information about daily or seasonal feeding patterns is desired. Foraging behavior of fishes can be complex (Box 16.1), influenced by factors such as water temperature (Kitchell et al. 1977), time of day (Scheuerell and Schindler 2003), light (Vogel and Beauchamp 1999), water clarity (Miner and Stein 1996; Shoup and Wahl 2009), and predation risk (Werner and Gilliam 1984). Thus, capturing individuals and removing diet contents or directly observing foraging behavior in situ is often difficult, laborious, and ultimately expensive. Fortunately, the phrase “you are what you eat” is often true, and the structural composition of fish often reflects the food they consume (Box 16.1). Many analytical techniques are now available to compare the elemental or molecular composition of fish tissues to that of available forage. Although these techniques integrate short- and long-term variation in foraging behavior, they often provide only limited taxonomic resolution (Figure 16.1). Choosing the proper technique ultimately depends on the research question, desired resolution, and available resources (Box 16.1; Figure 16.1; see also Figure 11.2 in Chipps and Garvey 2007).