Natural History of Sharks, Skates, and Rays

Food & Feeding

MARE 394Dr. Turner

Summer 2008

DietWidely recognized that elasmobranchs play a role in energy transfer b/w upper trophic levels;

understanding of how prey is consumed & processed is rudimentary

Quantifying DietEarly 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

Dietary Groups1° carnivores – limited array of prey compared to teleosts;

Consume: plankton, teleosts, elasmobranchs, cephalopods, large fishes, reptiles, birds, marine mammals

Diet ShiftsThat’s right! You fat cats didn’t finish your plankton; now it’s mine!

– Chuck Gerebedian

Fishing Down Marine Food Webs

Feeding RelationshipsRelatively few investigations comparing diets of elasmobranchs

Typically studies involving niche overlap among elsmo, competition w/ teleosts, among size classes of individual species

Feeding PatternsUnderstanding 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

Trophic LevelsAssumed to be top predators

Estimated with models – EcopathSharks- tertiary consumers (4th trophic level); similar to marine mammals, > birds

Also estimates using 13C & 15N isotopes

Stable IsotopesCarbon 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)

15N15N

Fractionation

-20 ‰

12C

13C 13C 13C

12C 12C

+1 ‰ +1 ‰ +1 ‰

+7 ‰ +10 ‰ +13 ‰ +16 ‰

+3 ‰ +3 ‰ +3 ‰

15N

14N 14N14N

C

N

-19 ‰ -18 ‰ -17 ‰Est. Value

Est. Value

Seagrass

-10

13C…origin of organic matter

-22

-9 -21

-8 -20Fractionation of

Carbon 1 ppt per

Trophic level

Phytoplankton

15N…Trophic position of consumer Fractionation of

Nitrogen 3-4 ppt

Per Trophic level

6

18

15

12

9

15N

14

12

10

8

6

diatoms

13C

-22 -20 -18 -16

Crustaceans

Juvenile

fishes

tuna & dolphin = unknown

Food ConsumptionFeeding ecology – important aspect of life-history – expressed as food consumption rates

Consumption dep. upon gastric evacuation rates

Daily RationMean 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 dynamics2) bioenergetic models – estimated based upon bioenergetic equation:growth + metabolism + excretion + egestion

Energy Budget EquationConsumption = 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%

Evacuation, Excretion, EgestionLimited information regarding gastric evacuation

Excretion loses in gill effluent & urine not measured

Egestion – spiral value; ↑ SA for digestion, ↓ volume to accommodate large liver

ProductionGrowth 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)

Gross Conversion EfficiencyGross 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

Summary – Food & FeedingGastric Evacuation & Daily Ration

Species Stage TGET (hrs) Daily Ration

% BW/dayCarcharhinus sp.

(requim sharks)

Juvenile 81 - 104 0.3 – 2.9

Negaprion sp.

(lemon sharks)

Juvenile 28 - 41 1.5 – 2.1

Sphyrna sp.

(hammerheads)

Juvenile 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

Summary – Food & FeedingElasmobranchs:

Typically slow to process (digest)

Slow growth – even in juveniles

Might change with heterothermic pelagics - difference not seen in mako