1. Natural History of Sharks, Skates, and Rays Food & Feeding MARE 380 Dr. Turner

2. Diet Widely recognized that elasmobranchs play a role in energy transfer b/w upper trophic levels; understanding of how prey is consumed & processed is rudimentary

3. Quantifying Diet Early descriptions – lists of prey items; counts, weight, or volume of stomach w/specific prey types Later index of relative importance (IRI) = %F(%W + %N) Energy budget of prey; convert items into calories or joules

4. Dietary Groups 1° carnivores – limited array of prey compared to teleosts; Consume: plankton, teleosts, elasmobranchs, cephalopods, large fishes, reptiles, birds, marine mammals

5. Diet Shifts That’s right! You fat cats didn’t finish your plankton; now it’s mine! – Chuck Gerebedian Fishing Down Marine Food Webs

6. Feeding Relationships Relatively few investigations comparing diets of elasmobranchs Typically studies involving niche overlap among elsmo, competition w/ teleosts, among size classes of individual species

7. Feeding Patterns Understanding feeding patterns requires knowledge of diet and dynamics of the feeding process, including ecological interactions b/w predator & prey Difficult; ↑ # of empty stomachs – no information

8. Trophic Levels Assumed to be top predators Estimated with models – Ecopath Sharks- tertiary consumers (4 th trophic level); similar to marine mammals, > birds Also estimates using 13 C & 15 N isotopes

9. Stable Isotopes Carbon and nitrogen generally used (sulfur recently) - Both abundant throughout nature Values expressed as ratios of two isotopes - 13 C/ 12 C or 15 N/ 14 N = δ Can determine: primary producer (C&S) - Organic material has “isotopic signature” trophic level of feeding (N) Based upon principle of “Fractionation” Each time stable isotope is metabolized there is “Fractionation” (do not participate equally – bias toward lighter)

10. 15 N Fractionation -20 ‰ 12 C 13 C 12 C +1 ‰ +7 ‰ +10 ‰ +13 ‰+16 ‰ +3 ‰ 15 N 14 N C N -19 ‰-18 ‰-17 ‰ Est. Value

11. Seagrass -10  13 C…origin of organic matter -22 -9-21 -8-20 Fractionation of Carbon  1 ppt per Trophic level Phytoplankton

12.  15 N…Trophic position of consumer Fractionation of Nitrogen  3-4 ppt Per Trophic level 6 18 15 12 9  15 N 14 12 10 8 6 diatoms  13 C -22 -20-18-16 Crustaceans Juvenile fishes tuna & dolphin = unknown

13. Food Consumption Feeding ecology – important aspect of life- history – expressed as food consumption rates Consumption dep. upon gastric evacuation rates

14. Daily Ration Mean amount of food consumed on a daily basis by individuals of a population – expressed as a proportion of mean body weight; measured: 1) in situ – field method; requires amt of food in stomach, gastric evacuation dynamics 2) bioenergetic models – estimated based upon bioenergetic equation: growth + metabolism + excretion + egestion

15. Energy Budget Equation Consumption = growth + metabolism + excretion (urine, U) + egestion (feces, F) C = G + M + U + F Daily energy required for growth (J day -1 ) = growth (g day -1 ) X energy equiv of shark tissues (J g -1 ) Metabolism – MR X oxycalorific value Non-assimilated loss ≈ 27%

16. Evacuation, Excretion, Egestion Limited information regarding gastric evacuation Excretion loses in gill effluent & urine not measured Egestion – spiral value; ↑ SA for digestion, ↓ volume to accommodate large liver

17. Production Growth in body mass – measured via laboratory exper, field mark-recapture, size- at-age Expressed as % body weight ↑ in teleosts compared with elasmo Gross conversion efficiency (K1)

18. Gross Conversion Efficiency Gross conversion efficiency (K1) – efficiency of food conversion to somatic growth; measures proportion of digested food available to next trophic level Typically from 3-40%; decrease with age Based upon Assimilation Efficiency- AE

19. Summary – Food & Feeding Gastric Evacuation & Daily Ration SpeciesStageTGET (hrs)Daily Ration % BW/day Carcharhinus sp. (requim sharks) Juvenile81 - 1040.3 – 2.9 Negaprion sp. (lemon sharks) Juvenile28 - 411.5 – 2.1 Sphyrna sp. (hammerheads) Juvenile5 – 502.9 – 9.4 Trikas semifasciata (Leopard shark) All28 – 320.85 – 2.2 Isurus oxyrinchus (shortfin mako) Adult36 – 482.2 – 3.0

20. Summary – Food & Feeding Elasmobranchs: Typically slow to process (digest) Slow growth – even in juveniles Might change with heterothermic pelagics - difference not seen in mako