Diet & Foraging Ecology

Fishery management plans have begun to stress the need for an ecosystem approach to management. The current Fishery Management Plan for sharks gives little consideration to ecosystem function because there are few quantitative species-specific data on competition, predator-prey interactions, and habitat requirements of sharks.

Cortés (1999) presented standardized diet compositions and calculated trophic levels for a suite of sharks species in an effort to better define the ecological role of these predators in marine communities.

The main results of this study suggest that sharks as a group are tertiary consumers and share the top trophic position in marine communities with carnivorous marine mammals. For detailed results, see: Cortés, E. (1999) Standardized diet compositions and trophic levels of sharks. ICES J. Mar. Sci. 56: 707-717. For a spreadsheet containing the summarized diet compositions from the original studies, see:

Summarized Diet Compositions of Sharks Spreadsheet

Bethea et al. (2004. Bethea, D.M., J.A. Buckel and J.K. Carlson. Foraging ecology of the early life stages of four sympatric shark species. Mar. Ecol. Prog. Ser. 268: 245-264.) recently examined stomach contents and catch data of the early life stages of Atlantic sharpnose Rhizoprionodon terraenovae, blacktip Carcharhinus limbatus, finetooth C. isosdon, and spinner sharks C. brevipinna taken from fishery-independent surveys in Apalachicola Bay, Florida, to test for overlap in resource use. The main results of this study show that diet overlap is high for species-life stage combinations that are similar in size and that most species-life stages do not show significant habitat overlap. Additionally, predator-prey relationships were developed for these species. For detailed results, see:

Bethea et al. (2006. Bethea, D.M., J.K. Carlson, J.A. Buckel, M. Satterwhite. Ontogenetic and site related trends in the diet of Atlantic sharpnose shark Rhizoprionodon terraenovae from the northeast Gulf of Mexico. Bull. Mar. Sci. 78(2): 287-307.) described Atlantic sharpnose shark diet from Crooked Island Sound, an embayment of the northeast Gulf of Mexico, and compared with the above published diet information from St. Vincent Island in Apalachicola Bay, an adjacent estuary. The main results of this study show that young-of-the-year sharks in Crooked Island Sound fed on a mix of teleosts (mostly clupeids) and invertebrates, juveniles on sciaenids and clupeids, and adults on sciaenids. Spearman correlation analysis indicated ontogenetic diet shifts within both sites. In addition, simple correspondence analysis showed that life stage diet differed between sites.

Through collaboration with Mote Marine Laboratory Center for Shark Research, the diet and daily ration of the bonnethead shark, S. tiburo, was described from three areas in the eastern Gulf of Mexico by life stage (Bethea, D.M., L. Hale, J.K. Carlson, E. Cortés, C.A. Manire, and J. Gelsleichter. 2007. Geographic and ontogenetic variation in the diet and daily ration of the bonnethead shark, Sphyrna tiburo, from the eastern Gulf of Mexico. Mar. Biol. 152: 1009-1020). Young-of-the-year stomachs from northwest Florida contained a mix of seagrass and crustaceans while juvenile stomachs contained a mix of crabs and seagrass and adult stomachs contained almost exclusively crabs. Crabs made up the majority of both juvenile and adult diet in Tampa Bay. Juvenile stomachs from Florida Bay contained seagrass and a mix of crustaceans while adult stomachs contained more shrimp and cephalopods.

Plant material was found in large quantities in all stomachs examined from all locations (>15 %IRI in 6 of the 7 life stage-area combinations, >30 %IRI in 4 of the 7 combinations, and 62 %IRI in young-of-the-year diet in northwest Florida). Using species- and area-specific inputs, a bioenergetic model was constructed to estimate daily ration. Models were constructed under two scenarios: assuming plant material was and was not part of the diet. Overall, daily ration was significantly different by sex, life stage, and region. The bioenergetic model predicted increasing daily ration with decreasing latitude and decreasing daily ration with ontogeny regardless of the inclusion or exclusion of plant material. Results of this study are currently in press at the journal of Marine Biology. Future research on bonnethead sharks will be in conjunction with colleagues from the University of Hawaii Zoology Department, using heavy isotope analysis on muscle tissue to determine trophic level.

Skates are an important component of benthic marine ecosystems. Fishery management stresses the need for an ecosystem approach, but skates have often been ignored. To evaluate trophic role, the diet and feeding habits of the roundel skate, Raja texana, is being examined from offshore waters in the northern Gulf of Mexico. Preliminary analysis of stomachs from 31 juveniles (25 non-empty; mean DW=23.5 cm) and 46 mature individuals (39 non-empty; mean DW=32.2 cm) indicate shrimps make up almost all juvenile skate diet. Osteichthyes (Micropogonias undulatus and Ophidium sp.) were also found in the diet of juvenile skates although in much smaller amounts. Mature skate diet was also predominantly shrimp.

Crab and other crustaceans (e.g., Squilla sp.) were also found in the diet. Osteichthyes (all unidentifiable) made up about one-forth of mature skate diets. Preliminary analysis does not indicate ontogenetic diet shifts; however, mature individuals consistently have larger and more than one prey item or type in their stomachs. These results were presented at the skate symposium at the 2006 Joint Meeting of the American Society of Ichthyologists and Herpetologists and the American Elasmobranch Society in New Orleans, LA US, and are in preparation for publication (Bethea, D.M., L. Hale, In prep, Diet of the roundel skate Raja texana from the northern Gulf of Mexico).

The Shark Population Assessment Group is currently examining the diet and foraging ecology of several elasmobranch species, including the Atlantic angel shark Squatina dumerili, blacktip shark C. limbatus, cownose ray Rhinoptera bonasus, finetooth shark C. isodon, night shark C. signatus, scalloped hammerhead shark S. lewini, spinner shark C. brevipinna, and other pelagic species of shark. Results from previous diet studies can be found by clicking the Publications link on the right.