Nekton

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Nekton
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Nekton
Organisms capable of swimming against a current
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General characteristics of nekton

• Larger body size
• Greater swimming power
• Most nekton animals are vertebrates, and most
  vertebrates are fish
• Only the squid and a few species of shrimps are
  truly nektonic invertebrates
• Few reptiles (turtles and sea snakes), birds
  (penguin) and mammals

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Importance of Nekton

• Large nekton can profoundly influence marine
  communities

• Important in current or historical harvests
• Fishes of critical importance to world food supply

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Vertical distribution

• Epipelagic countershading
• Countershading a nektonic organism is bicolored,
  dark above and light below
• Holoepipelagic shark, tuna, ocean sunfish
• Meroepipelagic herring, salmon

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Morphological features of nekton at different
vertical zones

• Mesopelagic
• Seldom exceed 10 cm
• Equipped with well developed teeth and large
  mouth
• Large light-sensitive eyes, uniformly black
• Photophores light-producing organs
• Abyssalpelagic
• Species-specific pattern of photophores
• Small with flabby, soft, nearly transparent flesh
  supported by weak bones
• Oversized mouth

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Major zones of life in a marine ecosystem
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Nektonic Crustacea

• Pelagic crabs and shrimp
• Larger euphausiids
• Antarctic Krill (Euphausia superba)
• 5-6 cm long
• Dominant food of baleen whales
• Increased fishery for livestock and poultry feeds

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Euphausia superba
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Who eats Krill?
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Krill the Antarctic Food Web
Critical components of Antarctic food webs
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• Krill Fishery
• Annual consumption by natural predators 470
  million MT
• 1972 Japan and Russia began harvesting krill

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Krill Fishery

• Potential harvest 25-30 million MT/yr
• Economic cost of fishery high
• Patchy distribution complicates location
• Depths may be 150-200m
• Single net haul may collect 10 MT
• Ecological consequences of removal poorly
  understood

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• Squids
• Large size range cm gt 20 m
• Giant squid (Architeuthis) largest invertebrate
• Water jet propulsion
• Highly maneuverable and agile
• Up to 10 m/s

• Predators consuming 15-20 body mass per day

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Giant Squid (Architeuthis dux)

• One of the largest marine predators
• Little is known about their ecology
• Diet deep-sea fishes, orange roughy, hokie
• Rapid growth full size in 3-5 years with a life
  span of 7 years

• Predators fishes when squid are young, then
  sperm whales

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• Squid Fisheries
• 70 of present catch of cephalopods
• Major source of human food
• Driftnet fishery began in N. Pacific in 1981
• Driftnets monofilament panels 8-10 m tall and up
  to 50 km long
• Set at night and allowed to drift while
  entangling prey

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Driftnets

• 1989 Japan, Korea, Taiwan were deploying 800
  driftnet vessels in N. Pacific

• Harvested 300,000 T squid annually
• Salmon and tuna also captured as by-catch
• 750,000-1,000,000 seabirds killed annually
• 20,000-40,000 marine mammal deaths
• Destruction to zooplankton not quantified

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• Drift-nets
• 1993 UN General Assembly accepted a resolution
  calling for a moratorium on all high-seas
  drift-netting
• Some illegal drift-netting continues

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Marine Reptiles
Saltwater crocodile
Marine iguana
Sea snake
Marine turtle
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Marine Birds
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Marine Mammals
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• What is the difference between nekton and
  plankton?
• Describe the krill fishery.
• Why did the krill catch drop in 1984 and 1993?
• What is a drift net?