Food Consumption and
Feeding Habitats
Introduction
• Knowledge of prey consumption and food
processing is basic
• Rates of consumption, feeding pattern,
and digestion studied in few species
• Distinctive morphology (spiral valve
intestine) might influence digestion,
absorption, etc.
Diet
•
•
•
•
•
•
Early studies = lists of prey
Frequency of Occurrence (O)
Total number of specific prey (N)
Total weight of specific prey (W)
Volume of specific prey (V)
Problems associated with each one
Cortes 1997 & 1999…
• Combination of several factors
• Index of Relative Importance
–IRI: %F (%W + %N)
Still controversial but, useful in
comparisons
Limitations of stomach contents
• Rate of digestion vary with size and type
of prey
• Problems with capture techniques
– Food regurgitated when captured at depth or
captured using gill nets
Perhaps: determine diet in energetic terms
(again, limitations)
Dietary Groups of Elasmobranchs
• All are carnivorous
• Wide variety of prey
plankton
whales
Filter Feeders
Mollusks
Worms
Parasitic
Sea lions
Whales
Teleosts
Sea birds
Sea turtles
Diet Shifts
• Some ontogenetic changes occur
– from a diet of inverts to a diet with teleosts
and other elasmos, mammals, and birds
• Other times no change
Dietary Differences
•
•
•
•
•
Geographic differences
Habitat type
Water depth
Seasonal variations
Sex
Feeding relationships
• Overlap among sympatric species (low to
high)
• Competition between consecutive size
classes
• Competition between same size classes
• High overlap in adjacent areas
Generalized vs. Specialized
• Top predators = generalists
• Others (skates/rays) = specialists
• Variation tends to increase with size and
age or decreases in size and age
Feeding patterns
• Must know more than just diet
• Frequency, weight, stage of digestion,
gastric evacuation dynamics of each item
• Analysis of stomach contents interferes
with feeding patterns
• High frequency of empty stomachs =
sharks are intermittent feeders
– Exceptions: demersal carnivores & filter
feeders
Trophic level
• Accepted sharks are top predators
• No quantitative estimate
– Cortes, 1999
• Tertiary consumers
• Stable isotopes provide further information
Processing of Food
Food Consumption Rate
• Determination requires regular
examination of stomach contents
• Difficult to determine
• Influenced by many factors including age,
feeding history, reproductive factors,
location, habitat, water temperature, prey
availability and gastric evacuation rate
• Influences growth, survival, reproduction
and rate of predation
Daily Ration
• Amount of food consumed in a day
• Expressed as a proportion of mean body
weight
• Wetherbee et al., 1990 determined daily
ration to be a good measure for
comparative studies
Estimating Daily Ration
• Can be done using field methods but this
requires more than examining stomach
contents, you must have knowledge of
gastric evacuation rates
• Can be done using bioenergetic models
Bioenergetic Models
• Estimate daily ration using growth rate,
metabolic rate, and energetic losses
• For example:
Consumption = Growth + Metabolism + Urine +
Feces
Gastric Evacuation Rate Is
Influenced By:
• Temperature (increases as temp
increases)
• Meal Size
• Food Type
• Determination by surface area models
may be the most accurate for multi-prey
item meals
Excretion
• Losses from gill effluent and urine have
not been measured in elasmobranchs they
are assumed to be similar to teleost fishes
at 7% of the energy budget
• Difficult to measure due to the large
volume of water in the habitat and the fact
that some nitrogenous wastes are retained
for osmoregulatory purposes
Egestion
• The process of removing undigested food
from the body as feces
• Prolonged in elasmobranchs possibly due
to the spiral valve intestine
• Extended time may be related to low food
consumption rate
• Slow rate limits growth and reproductive
rates
Spiral Valve Intestine
• Primitive design
• Increases digestive surface area
• Conserves abdominal space for the large
liver and large embryos, in an average
adult human the combined length of the
small and large intestine is about 8
meters, in a comparably sized shark the
spiral valve is about 30 centimeters long
Spiral Valve Intestine Governs
Food Consumption Dynamics
Through:
•
•
•
•
Time for passage of food, slower emptying
Efficiency of absorption
Rate of consumption, decreased
Available energy for growth and
homeostasis
• Wetherbee and Gruber (1993) measured
the average efficiency at close to 80%
(similar to carnivorous teleosts)
Obstacles To Digestive Efficiency
Studies
• Prolonged time to evacuate a meal in
elasmobranchs, from 1 to several days
• Difficulty of maintaining sharks in captivity
• Fecal collection is very labor intensive
Production
• Increase in body mass
• Methods of measurement
– Lab Experiments
– Field Mark and Recapture
– Indirectly through size at age relationships
Relative Rates of Production
• Are expressed as a percentage of body
weight
• Are lower in elasmobranchs than in teleost
fishes
• Higher in juveniles than adults
• Small coastal and pelagic species have
faster rates possibly due to increased risk
of predation
Gross Conversion Efficiency
• Efficiency of food conversion to somatic
growth
• Allows determination of the proportion of
ingested food that will be available to the
next trophic level
• Decreases with age
• Is believed to be comparable to teleosts
Conclusion
We study what they eat, the rate
of ingestion, energy processing
and nutrients available in the prey
to better understand energy flow
through elasmobranchs to marine
communities