Fishes as Consumers

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Fishes as Consumers

• MARE 444
• Dr. Jason Turner

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Fish as Consumers
Fish are important consumers as they represent
multiple trophic levels in aquatic food webs
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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

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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

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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

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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

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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

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Rainbow trout (carnivore)
Catfish (omnivore - 1º animal sources)
Carp (omnivore - 1º plant sources)
Milkfish (microphagous planktovore)
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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
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Prey-Capture Methods

• Three major capture methods among fishes
• Ram Feeding
• Suction Feeding
• Manipulation

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Ram Feeding

• Fish overtakes its prey by rapid swimming,
  thereby ramming water through its open mouth and
  opercule

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Suction Feeding

• Fish creates, while stationary, a strong, inward
  directed water current by rapid expansion of the
  buccal cavity

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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

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Marine Fishes

• Superclass Agnatha (jawless fishes)
• Superclass Gnathostoma (cartilagenous fishes)
• Superclass Osteichthyes (bony fishes)

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Superclass Agnatha

• Scavengers - hagfish
• predators on other fish - lamprey
• hagfishes and abyssopelagic

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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)

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Planktivores
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Scavengers
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Nektonic Carnivores
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Benthic Carnivores
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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

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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

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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

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Coral Reef Fish

• unique associations specific niches in some
  cases
• colorful (WHY?)
• abundant (WHY?)
• impacts

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Fish Ecology

• More fish in temperate waters (WHY?)
• higher diversity in (sub)tropics (WHY?)
• fewer fish in deeper waters (gt300 m)
• nektonic fishes in general are non-specialized,
  non-selective feeders
• feeding is size-dependent

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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

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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

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Recruitment and Growth Hypotheses

• Growth hypothesis - size and numbers of fish
  indicate growth and survivability respectively
• dependent on temperature
• ?temperature ?growth
• ?adult size ?fecundity

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Growth vs. Predation
Quantity Quality of food bigger larvae
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plankton vs growth
Bigger larvae Decreased predation Bigger is
Better Hypothesis
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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
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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
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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
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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
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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
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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
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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
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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