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

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

“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

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)

Wall of the intestine is folded creating a helical spiralSpiral - slows the passage of food - increases surface area for absorption combination increases the digestive performance of the intestine

Gut Lengths in Carnivores

Prey-Capture Methods

Three major capture methods among fishes:

Ram Feeding

Suction Feeding

Manipulation

Ram FeedingFish overtakes its prey by rapid

swimming, thereby ramming water through its open mouth and opercule

Suction FeedingFish creates, while stationary, a strong,

inward directed water current by rapid expansion of the buccal cavity

Manipulation FeedingFish 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 AgnathaScavengers - 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

• More fish in temperate waters (WHY?)

• higher diversity in (sub)tropics (WHY?)

• fewer fish in deeper waters (>300 m)

• nektonic fishes in general are non-specialized, 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. PredationQuantity & Quality of food = bigger larvae

plankton vs growthBigger 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 recruitmentOcean stability hypothesis – aggregations of food, rather than total integrated food, were more important to larval survival

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

e.g., - Hjort, Cushing, Lasker, Sinclair

Starvation and its effects upon recruitmentMatch-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 recruitmentMember-vagrant hypothesis – importance of the relationship between spawning time and stable oceanographic features which retain larvae in favorable environments

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

e.g., - Sinclair

Starvation and its effects upon recruitmentBigger is better hypothesis – since mortality rates decline with size during ELH, it might be expected that getting big quickly will minimize mortality events

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

e.g., - Houde

If it were, then natural selection would drive the genetic capacity for growth to the maximum permitted by

physiological and phylogenetic constraints