Natural History of Sharks, Skates, and Rays Early Chondrichthyes MARE 380 Dr. Turner
Natural History of Sharks, Skates, and Rays Food & Feeding MARE 394 Dr. Turner Summer 2008
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Transcript of Natural History of Sharks, Skates, and Rays Food & Feeding MARE 394 Dr. Turner Summer 2008
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