Nekton

1. 015d Nekton

2. Nekton Organisms capable of swimming against a current

3. 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

4. Importance of Nekton • Large nekton can profoundly influence marine communities • Important in current or historical harvests • Fishes of critical importance to world food supply

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

6. 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

7. Major zones of life in a marine ecosystem

8. 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

9. Euphausia superba

10. Who eats Krill?

11. Krill & the Antarctic Food Web Critical components of Antarctic food webs

12. Krill Fishery • Annual consumption by natural predators = 470 million MT • 1972: Japan and Russia began harvesting krill

13. 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

15. Squids • Large size range: cm … > 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

16. 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 http://evomech7.blogspot.com/2006/12/japan-researchers-film-live-giant.html

17. 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

18. 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

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

20. Marine Reptiles Saltwater crocodile Marine iguana Sea snake Marine turtle

21. Marine Birds

22. Marine Mammals

23. Inquiry • 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?