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Foraging and predator effects of sharks

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  1. Foraging and predator effects of sharks

  2. Foraging ecology • What do they eat? • How much do they eat? • How often do they eat? • What time do they eat?

  3. How do they catch prey? • Chase (e.g. Blue shark)

  4. Stalk and ambush (e.g. white shark)

  5. Greenland sharks (stalk and ambush?)

  6. Stalk and deceive (e.g. Oceanic whitetips)?

  7. Lie in wait ambush (e.g. angel sharks)

  8. Co-operative hunting (sevengill sharks)? Ebert 1991

  9. Filter feeders (e.g. whale and basking sharks)

  10. Diet • Stomach content analysis: • Index of Relative Importance (IRI) • Dietary breadth (Levin index, Shannon-Weiner diversity index) • “Snap shot” in time, high resolution

  11. Diet: stable isotopes • 15N and 13C stable isotopes • 15N is an indication of an animals trophic level • 13C is an indicator of where foraging occurs (e.g. benthic vs. pelagic) • Low dietary resolution but time integrated • Non-lethal sampling

  12. Generalists vs. specialist • Generalist: consume many prey item, relative to their abundance • Specialist: consume a few prey items, independent of their abundance

  13. Not so simple….. • Stable isotopes reveal hidden INDIVIDUAL specialization within populations • Bull sharks in the Everglades are individual specialists (Matich et al., 2010).

  14. What do they eat? • Fishes • Crustaceans (crabs, lobster) • Molluscs (squid, octopus) • Birds (Tiger sharks, Blacktip reef sharks ?!) • Mammals (white sharks, tigers, sevengills, sleeper sharks) • Reptiles (tiger sharks, Blacktip reef sharks ?!)

  15. But diet changes…. • With age (ontogenetic shift) • With season (movements of prey and movement of sharks) • With geographic location

  16. What time of day? • Mostly considered nocturnal based on tracking data • In many cases not validated • Determined using stomach content analysis, gastric evacuation experiments

  17. Juvenile scalloped hammerhead sharks Bush 2003

  18. Some sharks asynchronous Juvenile lemon sharks feed all times of the day Cortes and Gruber 1993

  19. How often do they eat? • Every four days (sandbar sharks) • Every day (lemon sharks) • Once per month (great white sharks?) • Data obtained at population level-may obscure individual variation

  20. How much do they eat? • Population averaged range 1-2% BW/day • Juvenile hammerhead sharks 4% BW/day • Yellowfin tuna 12% BW/d • Some reef fish 20% BW/d! • Sharks don’t consume very much • Slow gastric evacuation rates

  21. What effect do they have on the ecosystem? • Diet data shows that many sharks occupy tertiary trophic levels (Cortes 1999) • However they don’t eat that much…. • What are their direct effects on prey groups (e.g. prey they eat)? • Models used to predict what the changes in the ecosystem will be • Results vary

  22. Juvenile sharks in Florida Bay • Juvenile sharks targeted by fishing • What happens to ecosystem dynamics if fishing stops? • Ecopath/Ecosim model used. Based entirely on who eats what and how much….

  23. Carlson 2007

  24. Sharks at French Frigate Shoals, Hawaii • Federal Wildlife refuge-no fishing • Healthy shark population • What would happen if sharks were removed from ecosystem? • Ecopath model

  25. Stevens et al., 2000 Remove tigers Remove reef sharks Remove both

  26. Behavioral modification • What effect does the presence of predators have on prey behavior? • The ecology of fear • The “fear response” may lead to rapid depletion of prey patches (rather than prey are all consumed) • Behaviorally Mediate Indirect Interactions (BMII’s)

  27. Tiger sharks • Tiger sharks very broad diets (generalists) • One of the largest sharks • How do tiger sharks regulate ecosystems?

  28. Shark’s bay Australia

  29. Heithaus 2001

  30. Heithaus and Dill 2002

  31. Heithaus and Dill 2002 Better food in shallow habitats for dolphins When tiger sharks present dolphins forage in deeper waters There is a trade off between foraging success and safety

  32. Behaviorally Mediated Indirect Interactions on dolphins • Dolphins rarely found in tiger shark stomach • Dugongs regular tiger shark prey • Tiger sharks likely search shallow habitats for dugongs • Dugongs INDIRECTLY regulate dolphin habitat use via tiger shark presence

  33. Not just dolphins….e.g. cormorants

  34. Effects of removal of sleeper sharks Sleeper sharks consume seals in Alaska What would happen if they were removed?

  35. Effects of sleeper shark removal • Model simulated effects of sleeper shark removal on seal behavior • Sleeper sharks use deep water, seals shallow • If sharks removed, seals use deep water • Subsequently, seals are predicted to switch diet • Resulting diet switch will have fisheries implications

  36. Frid et al., 2009

  37. Empirical observations of the effects of sharks removal • Human populated islands and atolls exert heavy fishing pressure on sharks • Compare marine community structure with ‘pristine’ ecosystems • Do difference in structure arise from the removal of sharks?

  38. Line Islands Pacific Ocean Palmyra and Kingman uninhabited and US wildlife refuges

  39. How does community structure differ between islands? Kingman Palmyra

  40. Fanning Christmas

  41. Sandin et al., 2008

  42. Differences? • Human inhabited: less sharks, more small planktiverous fishes, reef algae dominated Uninhabited: lots of sharks, reef coral dominated • Does fishing of shark cause shift in reef sharks?

  43. Some problems • Visual surveys for shark abundance often biased and exagerrated • Surveys don’t take into account changes in shark behavior

  44. Results change our understanding of how ecosystem works! Top predators Primary producers Normal biomass pyramid Inverted biomass pyramid