Fishes as Consumers
MARE 444
Dr. Jason Turner
Fish as Consumers
Fish are important consumers as they represent
multiple trophic levels in aquatic food webs
Fishes as Consumers
Fish can be classified on the basis of their
feeding habits:
Detritivores - detritus
Herbivores – plants (phytoplankton,
macro algae
Carnivores – fish, zooplankton; animals
Omnivores – mixed diet; multiple sources
Fish as Consumers
Fish must have energy source – metabolism
Food demand – direct function of metabolic
rate
Dietary requirements – protein, lipid,
carbohydrates for growth (anabolism) and
energy to run body machinery (catabolism)
Require essential nutrients – amino & fatty
acids, vitamins, minerals
Fishes as Consumers
Within these categories fish can be
characterized further as:
Euryphagous – having a mixed diet
Stenophagous – eating a limited assortment
of food types
Monophagous – consuming only one sort of
food
Majority of fish are euryphagous carnivores
Feeding Mode
“Oh, no way - where? Holy crap, he's with a girl! But he's the guy
from Depeche Mode! That's impossible! Come on, he's in Depeche
Mode!”
- The Monarch
Feeding mode and food types are
associated with the body form and
digestive system
Herbivores & detritivores – longer gut
length with greater surface area
often take in large amount of
indigestible material
Gut Lengths in Carnivores
Carnivores have shorter gut lengths
gut length greater in those that prey on
smaller organisms
Digestive and absorptive area can also be
increased via spiral valve
Rainbow trout
(carnivore)
Catfish (omnivore 1º animal sources)
Carp (omnivore - 1º
plant sources)
Milkfish (microphagous
planktovore)
Gut Lengths in Carnivores
Wall of the intestine is folded creating a helical spiral
Spiral - slows the passage of food
- increases surface area for absorption
combination increases the digestive performance of the
intestine
Prey-Capture Methods
Three major capture methods among
fishes:
Ram Feeding
Suction Feeding
Manipulation
Ram Feeding
Fish overtakes its prey by rapid
swimming, thereby ramming water
through its open mouth and opercule
Suction Feeding
Fish creates, while stationary, a strong,
inward directed water current by rapid
expansion of the buccal cavity
Manipulation Feeding
Fish using manipulation (e.g., biting,
scraping, clipping, gripping, grasping) to
feed use their true or dermal teeth on
their upper and lower jaws
Marine Fishes
• Superclass Agnatha (jawless fishes)
• Superclass Gnathostoma (cartilagenous
fishes)
• Superclass Osteichthyes (bony fishes)
Superclass Agnatha
Scavengers - hagfish
predators on other fish - lamprey
hagfishes and abyssopelagic
Superclass Gnathostoma
• planktivores (whale shark, basking
shark, manta rays)
• scavengers (opportunistic)
• carnivores
– nektonic hunters (sharks & sawfishes)
• Great White - top predator
– demersal (most rays and sharks)
Planktivores
Scavengers
Nektonic Carnivores
Benthic Carnivores
Superclass Osteichthyes
teleosts - ray-finned bony fishes - most
common
planktivores (anchoveta, herring, flying
fish, lantern fish)
tend to be size-selective feeders
herbivores (damselfish, mullet, etc.)
carnivores
Carnivorous Teleosts
• nektonic hunters (tuna, marlin,
barracuda, ulua, mahi mahi, etc.)
– skipjack tunas are known to consume over
180 different kinds of food items
– small tuna tend to feed on epipelagic
organisms; large tuna feed on mesopelagic
organisms (as do marlin and swordfish)
• demersal (flounder, goatfishes, catfish)
• most fish eat other fish
Fish Ecology
• most plentiful fish occupy lower trophic
levels (plantivores); fewer higher trophic
level fish (WHY?)
• fish may feed on different organisms/at
different trophic levels through life cycle
• more prey = more fish
– tuna migrations - tuna show up when
pelagic crabs are seasonally available
Coral Reef Fish
• unique associations; specific niches in
some cases
• colorful (WHY?)
• abundant (WHY?)
• impacts
Fish Ecology
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More fish in temperate waters (WHY?)
higher diversity in (sub)tropics (WHY?)
fewer fish in deeper waters (>300 m)
nektonic fishes in general are nonspecialized, non-selective feeders
• feeding is size-dependent
Recruitment and Growth
• most teleosts produce between 1,000
and 1,000,000 eggs
• mortality rates vary between 99.9 and
99.99%
• slight changes in mortality rates (+/0.01%) can result in 10-fold change in
recruitment
Recruitment and Growth
Hypotheses
• starvation hypothesis - if there is not
enough planktonic food available, larval
fish will starve to death
• predation hypothesis - heavy
predation may result in fewer young
• advection hypothesis - currents may
transport young into unfavorable
conditions
Recruitment and Growth
Hypotheses
• Growth hypothesis - size and numbers
of fish indicate growth and survivability
respectively
– dependent on temperature
– temperature = growth
– adult size = fecundity
Growth vs. Predation
Quantity & Quality of food = bigger larvae
plankton vs growth
Bigger larvae = Decreased predation
Bigger is Better Hypothesis
Factor Controlling Recruitment
Recruitment – the number of individuals that
reach a specified stage in the life cycle (e.g., metamorphosis, settlement, joining the fishery)
Factors influencing recruitment
abundance and distribution of adult population
number and viability of eggs produced
survival of eggs and larvae
Factor Controlling Recruitment
Over 99% mortality occurs between egg
fertilization and settlement or recruitment of
juveniles
Important period – small variations in
mortality rates have profound effects on
subsequent abundance
e.g., - higher fecundity is associated with
greater recruitment variability
Factor Controlling Recruitment
Fisheries based upon one or two year classes are highly
dependent upon successful recruitment
Poor recruitment when fishing effort is very high may
cause collapse
Mortality during early life history (ELH)
Development – behavioral and physiological performance
are key to survival and subsequent recruitment
Growth - leads to changes in size or abundance of existing
features
Ontogeny - leads to the appearance of new features and
reorganization or loss of existing ones
metamorphosis – transformation from one body form
(larval) to another (juvenile)
endogenous – exogenous feeding – transition from yolk sac
to external feeding
“point of no return” – point at which larvae become too
weak to feed and recover (starvation threshold)
- resistance to starvation increases as larvae grow
Starvation and its effects upon recruitment
Ocean stability hypothesis – aggregations of food, rather than
total integrated food, were more important to larval survival
e.g., - Hjort, Cushing, Lasker, Sinclair
Patches of high food concentration 
as ocean stability 
Larvae in patches could feed
effectively
When ocean is rough, prey would
become dispersed and density would
become too low to support larvae
Starvation and its effects upon recruitment
Match-mismatch hypothesis – interannual variation in larval
survival could be explained by the match or mismatch between
the timing of the production cycle and the peak of spawning
time
e.g., - Cushing, Mertz & Myers, Pope et al.
If there is mismatch in space or time between larval food
production and larval hatching time then the larvae may
not encounter sufficient food and reach the “point of no
return”
Starvation and its effects upon recruitment
Member-vagrant hypothesis – importance of the relationship
between spawning time and stable oceanographic features
which retain larvae in favorable environments
e.g., - Sinclair
Emphasizes the role of physical rather than biological
factors in governing spawning or year-class success
Reality: - physical and biological processes will interact
and both will be important
Starvation and its effects upon recruitment
Bigger is better hypothesis – since mortality rates decline
with size during ELH, it might be expected that getting big
quickly will minimize mortality events
e.g., - Houde
High growth rates have costs that can lead to increased
mortality, and actually growth rate evolved to balance the
costs and benefits
Reality: - Bigger may be better but is not necessarily
the best strategy to get big quickly
If it were, then natural selection would drive the genetic
capacity for growth to the maximum permitted by
physiological and phylogenetic constraints