Nekton: The free swimmers.

The large organisms that can freely swim against ocean currents are called the nekton. The nekton includes some invertebrates like the squid, a wide variety of fishes, sea turtles and the whales and their kin. It is these organisms that we interact with in world wide fisheries.

Here the life histories of two members of the nekton are explored. These are the bluefin tuna (Tunus tunus) and the swordfish (Xiphas gladius, or sword sword in Greek and Latin respectively). Both of these are the target of major fisheries in the North Atlantic and elsewhere around the world. In fact bluefin is the most expensive fish in the world with record prices for a single fish at the Tokyo fish market exceeding $50,000! These fish are caught, flash frozen
in liquid nitrogen and shipped to Tokyo. Swordfish are targeted by long-line fishing boats throughout the world. For a good acount of the swordfish and what it is like to live (and die) on a long-liner read the bestseller, "The Perfect Storm" by Sebastian Junger.

Both of these fish are warm-blooded or endotherms in that they heat there blood in large muscle masses in their tails. The bluefin has better control of its temperature and can forage in waters colder than 8 C. The swords seem to be restricted to waters warmer that 10 C. Both fish use the warm blood flow to heat their large eyes. This improves vision in these visual predators.

Swordfish and bluefin have very different social behaviors. The bluefin forms large schools. The complicated interactions between individual fish include the formation of linear "soldier schools" that breakout of larger schools. It is thought that these might be related to foraging by the fish. Swordfish are solitary animals and are extremely aggressive. The sword or rostrum is used as a weapon. They will spear other swordfish, whales, boats, etc. One attacked the submerssible Alvin and stuck its sword into the insulation around Alvin's pressure hull. Swords are also fairly active acoustically.

When they lunge after prey they form a bubble around their head that pops creating a distinctive sound. It possible that they use these noises to detect each other and maintain a safe range. These fish only come together to mate.


 


Migration:

Both bluefin and swordfish migrate across large distances on an annual basis. Bluefin spawn in the Gulf of Mexico in April and May. This places the planktonic larval fish in the Gulf Stream where they are carried northwards to the waters off New England where they feed.

(Click here for a map of bluefin distribution)

The adults also swim down the Gulf Stream and then into the coastal waters of the Gulf of Maine or off Nova Scotia. Some of the older adults (>500 lb giants) may travel as far as the cold waters off Norway to feed. Swordfish follow a similar route to forage in the Stream edge or along the canyons at the shelfbreak instead of up on the colder banks.

One problem of understanding nekton is deciding how they navigate during their migrations. It is very difficult to understand the behavior of these fish because their world is so different from the the terrestrial environment that we live in. One way of studying these fish is to track them acoustically by first placing an acoustic tag on the fish and then following them with a ship. A nice description of a tagging experiment off Cape Hatteras, North Carolina can be found in Carey and Robison (1981, Fish. Bull.). They tracked one swordfish for three days as it swam eastward along the Gulf Stream. The fish
spent its days in the deep waters around 10C and its nights feeding in the near surface waters. The conclusion of the study was that the fish used a combination of thermal and optical (light) clues to determine where it went. In fact one can make a chart or map of the fish's preferred light and temperture levels and produce a "highway" of sorts up the Gulf Stream where there is a narrow band of optimal light and temperature levels.

This region in the Gulf Stream is called the Gulf Stream front. It is similar to the cold and warm fronts that form in the atmosphere. The hypothesis is that the migrating fish follow the Gulf Stream front by shifting back and forth to find the narrow band of temperature and light that they prefer. This may have other advantages since the currents that produce the sharp changes in temperature and light absorbtion also concentrates prey for the swordfish and the organisms such as squid that they eat.