Marine Zoology & Ecology: Pelagic fishes

Pelagic meaning

relating to/living/occurring in the open sea

Characteristics of Open Ocean:

less productive

lower biomass

lower diversity

Adaptations: Body plan

streamlined, laterally compressed body

stiff fins

small pectoral fins

Adaptations: Colouration

countershading

Marlin- use iridophores to rapidly change colour

Adaptations: Body composition

scales oriented to disrupt boundary layer- reduced or absent

specialised denticles- sharks

oils/mucus coating to reduce drag

firm and muscular

powerful tails

Adaptations: Biolumniescence

camouflage: some can match ventral light intensity to downwelling light

attracting prey

courting/sexual dimorphism

schooling behaviour

Adaptations: Buoyancy

swim bladder

increased lipid storage

reduced density- body can be up to 95% water

Adaptation: Senses

large eyes

lateral line

Small pelagic fish: keystone role

in forage fish (schooling)

high oil content

tropical or subtropical

Small pelagic fish: food

consume zooplankton- prey for higher trophic levels

Small pelagic fish: Ecological role

energy transfer

Small pelagic fish: commercial role

food, oil or bait

Large pelagic fish: keystone role

apex predator

migratory fish

Large pelagic fish: food

smaller pelagic fishes

control species abundance at different trophic levels

Large pelagic fish: ecological role

population control of lower trophic levels

Large pelagic fish: commercial role

food, recreational fishing

Deep sea pelagic fish: distribution

patchy distribution

highly diverse

Deep sea pelagic fish: morphological traits

significantly different than epipelagic

Deep sea pelagic fish: survival strategies

Body morphology & colour

eye, mouth and teeth shape

lure appendages

prey illumination

counterillumination

Deep sea pelagic fish: body morphology & colour

great diversity of body shapes and colours

Deep sea pelagic fish: eye, mouth & teeth shape

several adaptations: tubular and enlarged eyes

large expandable mouth

long teeth

Deep sea pelagic fish: lure appendages

structures used to glow light & attract prey

symbiosis with bioluminescent bacteria

Deep sea pelagic fish: counterillunimation

camouflage involving use of ventral photophores to match dim light from surface

Ecological types of pelagic fish

oceanic & neritic

Ecological types of pelagic fish: Oceanic

spend most life in open ocean

true residents: tuna, swordfish, marlins, sunfish

partial: juveniles of benthic fish, whale sharks

Ecological types of pelagic fish: Neritic

spend most life in water above continental shelf & open ocean

most abundant fish in world: herring, sardines, anchovy

feed in highly productive inshore waters

Migration

follow food or move in search of spawning grounds

pelagic movements driven by changing conditions, often seasonal

Oceanic hotspots:

fixed structures

eddies

seasonally variable

Diel Vertical Migration

daily cycles of movement leads to formation of deep scattering layer

creates a nutrient cycle- leading to increased productivity and enhancing biodiversity

leads to pelagic carbon cycle

Pelagic carbon cycle: Trophic cascade carbon

food web dynamics- help maintain carbon storage and function of coastal marine ecosystem

Pelagic carbon cycle: biomixing carbon

turbulence & drag- movement of marine vertebrates

enhanced mixing of nutrient rich water from deeper in water column

enhance primary production

Pelagic carbon cycle: Bony fish carbonate

excrete metabolised carbon as calcium carbonate

enhance alkalinity- provide buffer against acidification

Pelagic carbon cycle: Whale pump

nutrients from faecal material of whale

enhance primary production by phytoplankton & uptake of dissolved CO2

Pelagic carbon cycle: twilight zone carbon

Mesopelagic fish feed in upper ocean layer at night

transport consumed organic C to deeper waters at day

Major challenges

overfishing

bycatch

climate change