25 March, 2000

Fish for Bioassays

Question 35: What is "spy hopping"?

Since it is Saturday, part of my day is dedicated to station tasks--house mouse at 2 pm, all hands meeting at 3:30 pm, and gashing tonight. Gash is the nightly clean-up and sanitizing of the kitchen, food preparation and dining area. Four to five people do it nightly after dinner. It is voluntary but if someone is shirking the once-a-week responsibility, he may find himself signed up every night by other people. In a small community like this it is impossible to go unnoticed.

In the morning I helped get our fish settled in their new home. They will be part of another bioassay. For the bioassay to work properly, they need to be comfortable enough to behave naturally and feed. We have approximately twenty fish (Notothenia coriiceps) that Jim and Andy have caught at the shoreline. The end of the pier by the seawater intake line is actually a hollow tower with a room down inside and a hatch that opens to the ocean. We can fish there in all weather.

Katrin and I transferred the fish from a glass holding tank to one of the large tanks on the aquarium deck. Although the single fish (lighter colored than the rest) we could see was acting more naturally in the less exposed area, the rest of the dark gray/black fish were totally invisible on the bottom of the black tank. We decided to sew a white plastic mesh covering for the bottom of the tank. It floated, so we weighted it down with lead weights, being careful not to trap or squish the fish! Before we carry out the bioassay, we need to construct dividers for several tanks so the fish can be isolated.

The fish species we are using is 18 to 42 cm long and a mottled greenish black with green bands on their fan-like fins. N. coriiceps are opportunistic generalists, which means they will eat whatever turns up. This includes limpets, isopods, amphipods (look like tiny shrimp), gastropods (snails) and macroalgae. The adults are benthic (live by the sea floor). They are found as deep as 90 meters and all the way up into shallow coastal waters where they hide among the seaweed and are eaten by cormorants.

N. coriiceps, like all Antarctic fish, are not very active. In the tanks, as in the ocean, they remain quietly on the bottom, moving only in response to some stimuli (e.g. food, being chased by a net). Their body shape is not as streamlined as that of pelagic species (ones that live in the water column). A cross-section of N. coriiceps body is more teardrop shaped, while a cross-section of a pelagic fish is shaped more like a cat's pupil. They are negatively buoyant (they sink) and have heavier skeletons than species that spend more time in the water column. When they rest on the substrate (rock, sand, etc.) their body is supported by the pelvic and anal fins. In benthic Notothenioids (the large group most Antarctic fish belong to) the fins are modified for their contact with the substrate and body support. The rays of the fins are thicker and shorter while the skin that makes up the fin is much thicker.

While diving we have seen several other species of fish, all hiding quietly in the macroalgae among the rocks. On one of our first dives we found a dragonfish, a member of the only group of fish here that does not include species that live north of the Antarctic Convergence.

Answer 34: The Orca or Killer whale. While in some other oceans groups may eat mainly fish, in the Southern Ocean they mostly eat penguins, seals, and other whales (especially Minkes).

Andy going fishing.

Fish fin diagram.

Katrin transferring a fish.

Fishing inside the pier tower.

Putting netting in bottom of outdoor fish tank.