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Nekton

2. Nekton Organisms capable of swimming against a current Nekton
When we think of nektonic organisms, fishes are the most likely group to come to mind. Fish are certainly important and prominent groups of nekton, however, there are other nekton including marine mammals, marine reptiles, cephalopods, some crustaceans and sea birds. In the series of lectures, we’ll take a look at nektonic organisms. We’ll begin with the less common groups and finish with fishes.

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
Importance of Nekton
Nekton are the most visible components of marine food webs. Further, their energy requirements are such that they can profoundly influence the marine communities that they inhabit. Recall the influence of just a few orcas on the sea otter populations off Alaska.
Some are of current commercial importance, others such as whales were of historical commercial importance.
A large component of the protein supply of the world comes from the fishes of the oceans. Indirectly, many fishes are captured to manufacture high-protein animal feeds. Thus even when we consume chickens or hogs, we are often deriving energy from the oceans.

5. Vertical distribution
Epipelagic countershading
Countershading: a nektonic organism is bicolored, dark above and light below
Holoepipelagic: shark, tuna, ocean sunfish
Meroepipelagic: herring, salmon
Fish that spend their lives in the epipelagic are termed holoepipelagic. These include white tip sharks, spiny marine bream, yellow fin tuna, striped marlin, oarfish, and ocean sunfish.
The second group of oceanic fish is called meroepipelagic. These fish spend only part of their life in the epipelagic zone. This is a more diverse group and includes fish that spend their adult lives in the epipelagic but spawn in inshore waters (ribbon halfbeak, herring, whale shark, dolphin, ) or in fresh water (salmon).
Another component of the oceanic nekton is the marine mammals. These include whales (order Cetacea), and the seals and sea lions (order Carnivora). Other marine mammals include manatees and dugongs (Order Sirenia), and sea otters (order Carnivora) but these occupy inshore waters.
Nektonic reptiles of the oceanic are almost exclusively turtles and snakes. There are also marine iguanas in the Galapagos Islands and marine crocodiles around some Australian and Indonesian Islands, but these are littoral and inshore animals.

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
Nektonic Crustacea
Nektonic crustacea consist of pelagic swimming crabs, shrimp and larger euphausiids. The most famous of the euphausiids is the antarctic krill (Euphausia superba).
Krill are large animals that may be 5-6 cm in length. They occur in massive swarms that may be kilometers in length. These swarms are a rich food supply for baleen whales.
A commercial fishery for krill has also developed. Krill decompose rapidly and after dying they release powerful enzymes that rapidly break their tissues down. For this reason, it has been difficult to provide a fresh enough product for human consumption. The majority of krill are harvested as animal feeds. They are used in fish farming to provide farmed fish such as salmon with the desirable pink flesh color.

9. Euphausia superba Euphausia superba
Note the large swimming legs on the euphausiid that give them their mobility and classification as nekton.

10. Who eats Krill?

11. Krill & the Antarctic Food Web
Critical components of Antarctic food webs
Krill & the Antarctic Food Web
Krill are essential components of antarctic food webs. They are consumed by baleen whales, seals, squid, penguins and winged birds, carnivorous zooplankton and pelagic fishes. How many trophic levels are their above krill?

12. Krill Fishery Annual consumption by natural predators = 470 million MT
1972: Japan and Russia began harvesting krill
Krill Fishery
This graph illustrates the development of the krill fishery in antarctic. Exploratory fishing took place in 1972 and since ‘73, catches have been recorded. The landings are in thousands of tonnes. Note how the fishery increased through the late 70’s. The dip in the early 80Õs was due to a drop in demand for krill rather than a change in their abundance. The fishery remained high through the rest of the 80’s. In the early 1990’s the Russian component of the fishery diminished leaving the Japanese and Chileans.

13. Krill Fishery… Potential harvest = 25-30 million MT/yr
Economic cost of fishery high
Patchy distribution complicates location
Depths may be m
Single net haul may collect 10 MT
Ecological consequences of removal poorly understood
Krill Fishery…
Why are we so interested in harvesting krill? This is potentially one of the largest single species fisheries in the world. The potential krill harvest is estimated at million metric tons per year.
Economically, it can be a difficult species to catch. The location of the fishery is remote and conditions in Antarctic waters can be difficult. The patchy distribution of krill means that swarms can be hard to locate. The depths of the swarms may also complicate collection. Once located, a single net tow through a large patch may yield 10 MT.
We still don’t know what the consequences of the removal of krill are to the Antarctic ecosystem. Clearly this is an important species in that food web. We must ensure that sufficient krill are left to sustain the other users of the resource. The fishery has had a very positive impact on research in the Antarctic. The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) has conducted the most extensive research on the Antarctic ecosystem. Funding for CCAMLR came from interest in managing the krill fishery. Without a fishery for krill, it is unlikely that we would have learned nearly as much as we have about that ecosystem.

14. Nektonic Cephalopods
Nektonic cephalopods consist of squids, cuttlefish and octopods. Squid make up the largest proportion of commercially-fished cephalopods and the potential world catch is estimated at 10 million tons annually. (Compare that to the potential million tons of krill that could be harvested each year).
We will concentrate on squids. Cuttlefish and octopods are less common and more often are associated with demersal habitats.

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
Squids
Squids come in a wide of sizes. Small species may only be a few centimeters in length while the giant squid (Architeuthis) is the largest invertebrate on the planet with a length of over 20 m.
All utilize waterjet propulsion. Water is taken in via the mantle and expelled through a nozzle called a siphon. This nozzle can be rotated and provides the squid with remarkable agility and speed. Squid can move at up to 10 m/s.
They are voracious predators that may consume % of their body mass per day.
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
Giant Squid
These enormous animals are poorly understood. The problem is that their habitat is the deep-sea and no live specimen has ever been collected. All we know of them comes from dead specimens and the stomach contents of their predators (sperm whales).
What do they eat? As one scientist who studies them put it Òanything they want to!Ó Their diet includes deep-sea fishes and other squids.
Most cephalopods grow very rapidly. For example, an octopus can reach full size in less than a year and may only live one year. Giant squids are similar. Their large size doesnÕt imply longevity and slow growth. They can reach full size in 3-5 years and probably only live for up to 7 years.
When young, they are prey for a variety of fishes including the species that they later consume as adults. When they mature, their only natural predators are sperm whales.
Predators: fishes when squid are young, then sperm whales

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
Squid Fisheries
As mentioned earlier, squids make up 70% of the world cephalopod catch and they are a major source of food for humans. Large number of squid inhabit the pelagic waters of the North Pacific.
In 1981, a driftnet fishery for squid began in that region. Driftnets are vertical panels much like gillnets. They may be 8-10 m tall and hang vertically in the water-column. What makes them different is their length. A single net may be as much as 50 km in length. The nets are set at night and allowed to drift while entangling squid that swim into them.

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
Drift Nets
The problem with driftnets is that they are indiscriminate in their catch and many organisms other than squid are captured. The capture of non-target organisms is called by-catch. Bycatch in the N. Pacific driftnet fishery became a serious problem.
By 1989, three countries: Japan, Korea and Taiwan were deploying 800 drift-net vessels in the region. This fleet harvested 300,000 tons of squid annually. The bycatch from this fishery was enormous. Salmon and tuna were intercepted and landings from the driftnet fishery constituted a large source of mortality for species that were already fished heavily.
Diving birds also encountered the nets and 750,000-1 million birds of a variety of species were thought to have died each year.
Marine mammals had difficulty detecting the thin nets and 20,000-40,000 cetaceans died each year.
The nets probably resulted in the destruction of salp chains and other gelatinous zooplankton and that impact wasn’t 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
Drift Nets…
In 1993 the United Nations passed a moratorium on high seas drift netting and the fishery ended. Illegal drift-netting operations continue in the Pacific and drift-nets are still used in the Mediterranean by countries such as Italy.

20. Marine Reptiles Saltwater crocodile Marine iguana
Marine Reptiles (Sea Snakes)
Marine Reptiles include sea snakes, saltwater crocodiles, marine iguanas and sea turtles. Sea snakes are a group of true snakes that have completely adapted their life-histories to live in the sea.
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?