1. Natural History of Sharks, Skates, and Rays
Food & Feeding
MARE 394
Dr. Turner
Summer 2008

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. That’s right! You fat cats didn’t finish your plankton; now it’s mine!
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 (4th trophic level); similar to marine mammals, > birds
Also estimates using 13C & 15N isotopes

9. Stable Isotopes
Carbon and nitrogen generally used (sulfur recently)
- Both abundant throughout nature
Values expressed as ratios of two isotopes
- 13C/12C or 15N/14N = 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. C N Fractionation -20 ‰ -19 ‰ -18 ‰ -17 ‰ +7 ‰ +10 ‰ +13 ‰ +16 ‰ 12C
Est. Value
+1 ‰
+1 ‰
+1 ‰
14N
14N
14N N
15N
15N
15N
+7 ‰
+10 ‰
+13 ‰
+16 ‰
Est. Value
+3 ‰
+3 ‰
+3 ‰

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

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

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
Species
Stage
TGET (hrs)
Daily Ration
% BW/day
Carcharhinus sp.
(requim sharks)
Juvenile
0.3 – 2.9
Negaprion sp.
(lemon sharks)
1.5 – 2.1
Sphyrna sp.
(hammerheads)
5 – 50
2.9 – 9.4
Trikas semifasciata
(Leopard shark)
All
28 – 32
0.85 – 2.2
Isurus oxyrinchus
(shortfin mako)
Adult
36 – 48
2.2 – 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