Food HabitsSharks are among the most common large predators in the sea and play an important role in energy flows through marine food webs. Knowledge of food habits and feeding behavior of sharks allows us to determine the effect sharks have on other organisms through predation and competition. It can be used in management of shark fisheries, by determining the energy needs of sharks and how changing biological and physical conditions in the sea affect them. It tells us how changes in shark populations may affect populations of their prey and their competitors, and how changes in prey and competitor populations and behavior may affect sharks.
Food habits are determined by examination of the stomach contents. For small sharks such as sandbar sharks or smooth dogfish, the stomach can be manually everted and the stomach contents obtained without harm to the animal, which is then released.
For larger sharks, stomach contents are retrieved from sharks taken in sportfishing tournaments, from commercial and research fishing vessels, and from recreational anglers. Generally, the stomach is removed from the shark and the individual items in it are identified to the lowest possible taxonomic category. A gravimetric measure is then taken (either weight or volume) for each prey item. Prey lengths are taken when possible, and the stage of digestion is noted.
The relative importance of a prey type in the shark's diet can be expressed in several different ways. Percentage by number (%N) is the proportion of the number of a specific prey item to the total number of prey items in all the stomachs examined. Percentage by weight or volume (%W or %V) is the proportion of the weight (or volume) of one prey item to the weight (or volume) of all of the stomach contents. Percentage frequency of occurrence (%O) is the proportion of shark stomachs containing a specific prey item. Each of these indices give information about the feeding behavior of the shark, but each also has limitations. A high value for %N or %O can give information about feeding behavior but may imply a great importance to the diet of very small but numerous prey which contribute little to the shark's energy needs. A high value for %W or %V can indicate a prey item of great nutritional value, but may overestimate the importance of a very large prey item rarely encountered or taken by the shark. All indices are subject to error if prey of different sizes and types are digested at different rates, giving the impression that larger and harder to digest items are more important then they really are. A compound index such as IRI or %IRI (Pinkas et al. 1971, Cortés 1997) is therefore also used to provide a more balanced, comprehensive view of food habits. A compound index combines information from the component indices above, and contains information on the contribution of each prey type to nutrition of the predator population as a whole and the likelihood of that prey type occurring in the stomach of an individual predator (Liao et al. 2001).
To ascertain whether there is enough data to adequately describe the diet of a species, a cumulative prey curve is constructed (Ferry et al. 1997). A curve that plateaus indicates that sampling of additional stomachs will not add significant new information to the diet description. Diets are often described as generalist, showing no real preference for any specific prey, or specialist, showing a strong preference for one or a few prey types. Most sharks tend to be generalists to some degree, but some have a far more generalized diet than others, and a robust knowledge of a shark's diet indicates where is lies in this range. For example, blue sharks have a very diverse diet with 51 families found in the stomach contents, and the most important groups by number, volume and occurrence are a wide range of teleosts and cephalopods (Kohler 1987). Shortfin makos, by contrast have a more specialized diet with 24 families represented, but by far the most important item by number, volume and occurrence, is bluefish (Stillwell and Kohler 1982). Diets often change as a shark grows and also may vary by season and location. Monitoring shark's diets over a long period of time may give some indication of whether they can easily shift to different prey as the abundance of prey species changes over time (due to fishing pressure, alterations in environmental conditions due to global climate change, or natural cyclical variations in environmental conditions). Sharks with a more generalized diet may be better equipped to tolerate such changes. Food habits data for some species of sharks from the northwest Atlantic extends from the 1960's to present, and combining this data with abundance estimates for their prey may tell us how well these different shark species adapt to changing conditions.
Examination of stomach contents (both amount and stage of digestion) over the course of a day can yield information on feeding behavior, such as whether the shark feeds continuously or is a sporadic feeder. If the stage of digestion of stomach contents of a shark varies, it probably feeds continuously throughout the day. If all the prey items are at about the same stage of digestion, the shark is probably a sporadic feeder. Some species tend to have a large proportion of empty stomachs and many individuals with one or few well digested prey items. This may indicate sporadic feeding, but may also be an artifact of capture methods (bait versus non-bait methods). Other sharks, particularly coastal and demersal species, rarely have empty stomachs and frequently contain many different items at all stages of digestion.
Food habits data can be used to derive an estimate of consumption, or daily ration for sharks. Knowledge of daily ration and conversion efficiency can be used to determine the sharks' energy needs and potential for growth and reproductive success, which is important for management of shark fisheries. Consumption estimates can also be used to understand potential impacts on prey populations and on competitors (including fisheries). Daily ration is generally found either by using a bioenergetics approach, or by using the weight of stomach contents and gastric evacuation rates. For the bioenergetics approach, energy expenditures (routine metabolism, metabolic cost of activity, losses to excretion and egestion) are directly measured, or are estimated using measurements from other shark species, and scaling these to the environmental temperatures typical for the species being examined. The amount of food necessary to cover these energetic costs is the maintenance ration, the lowest amount the shark can consume. Any consumption over this amount can be spent on growth and reproduction. If growth and reproductive costs are known, the amount ingested can be deduced.
The second method requires knowledge of the weight of stomach contents over the course of a day and an estimate of gastric evacuation rate (GER). Mean stomach content weights are found at fixed time intervals over a 24-hour period, or an average for the entire 24-hour period is used. The GER is generally derived in a laboratory for each prey type, although the GER for some sharks has been estimated from field experiments (Kohler 1987, Medved 1985). The equations used (from Elliot and Persson 1978) to find daily ration are based on the assumption that the amount of food in the stomach at any time is a function of the amount consumed, the time since consumption, and the evacuation rate.
Since at least some equation parameters for either method of determining daily ration must be estimated from other species, both approaches are often used to find the best estimate for daily ration.
Cortés, E. 1997. A critical review of methods of studying fish feeding based on analysis of stomach contents: application to elasmobranch fishes. Canadian Journal of Fisheries and Aquatic Sciences 54: 726-738.
Elliot, J.M. and L. Persson 1978. The estimation of daily rates of food consumption for fish. Journal of Animal Ecology 47: 977-991.
Ferry, L.A., S.L. Clark, G.M. Cailliet 1997. Food habits of spotted sand bass (Paralabrax maculatofasciatus, Serranidae) from Bahia De Los Angeles, Baja California. Bulletin of the Southern California Academy of Sciences 96(1): 1-21.
Kohler, N.E. 1987. Aspects of the feeding ecology of the blue shark, Prionace glauca in the western North Atlantic. PhD Dissertation, University of Rhode Island, Kingston, 163pp.
Liao, H., C.L. Pierce, J.G. Larscheid 2001. Empirical assessment of indices of prey importance in the diets of predacious fish. Transactions of the American Fisheries Society 130: 583-591.
Medved, R.J. 1985. Gastric evacuation in the sandbar shark, Carcharhinus plumbeus. Journal of Fish Biology 26: 239-253.
Pinkas, L., M.S. Oliphant, I.L.K. Iverson 1971. Food habits of albacore, bluefin tuna, and bonito in California waters. California Fish and Game 152: 1-105.
Stillwell, C.E. and N.E. Kohler 1982. Food, feeding habits, and estimates of daily ration of the shortfin mako (Isurus oxyrinchus) in the northwest Atlantic. Canadian Journal of Fisheries and Aquatic Sciences 39: 407-414.