Foraging and predator effects of sharks. Foraging ecology. What do they eat? How much do they eat? How often do they eat? What time do they eat?. How do they catch prey?. Chase (e.g. Blue shark). Stalk and ambush (e.g. white shark). Greenland sharks (stalk and ambush?).
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Foraging ecology • What do they eat? • How much do they eat? • How often do they eat? • What time do they eat?
How do they catch prey? • Chase (e.g. Blue shark)
Co-operative hunting (sevengill sharks)? Ebert 1991
Diet • Stomach content analysis: • Index of Relative Importance (IRI) • Dietary breadth (Levin index, Shannon-Weiner diversity index) • “Snap shot” in time, high resolution
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
Generalists vs. specialist • Generalist: consume many prey item, relative to their abundance • Specialist: consume a few prey items, independent of their abundance
Not so simple….. • Stable isotopes reveal hidden INDIVIDUAL specialization within populations • Bull sharks in the Everglades are individual specialists (Matich et al., 2010).
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 ?!)
But diet changes…. • With age (ontogenetic shift) • With season (movements of prey and movement of sharks) • With geographic location
What time of day? • Mostly considered nocturnal based on tracking data • In many cases not validated • Determined using stomach content analysis, gastric evacuation experiments
Juvenile scalloped hammerhead sharks Bush 2003
Some sharks asynchronous Juvenile lemon sharks feed all times of the day Cortes and Gruber 1993
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
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
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
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….
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
Stevens et al., 2000 Remove tigers Remove reef sharks Remove both
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)
Tiger sharks • Tiger sharks very broad diets (generalists) • One of the largest sharks • How do tiger sharks regulate ecosystems?
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
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
Effects of removal of sleeper sharks Sleeper sharks consume seals in Alaska What would happen if they were removed?
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
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?
Line Islands Pacific Ocean Palmyra and Kingman uninhabited and US wildlife refuges
How does community structure differ between islands? Kingman Palmyra
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?
Some problems • Visual surveys for shark abundance often biased and exagerrated • Surveys don’t take into account changes in shark behavior
Results change our understanding of how ecosystem works! Top predators Primary producers Normal biomass pyramid Inverted biomass pyramid