WATER MOVEMENTS AND ANTARCTIC KRILL

My contribution is no more than the ideas of a physical oceanographer not too familiar with all the biological considerations. But I used to go to sea with James Marr, who wrote his large report on krill 20 years ago, and had long discussions with him. He was very cautious in presenting his data, leaving his readers to form their own conclusions, but many of his ideas are convincing and seem to become more useful as time goes on. One of his first generalizations was that adult krill live near the surface. Once the first calytopis stage reaches the near-surface water to feed on phytoplankton, a month or so after the eggs were spawned, the later larval and adult stages remain at shallow depths, and the eggs are spawned there. Our host, Professor George, at this meeting has shown that the later larval stages and the adults are intolerant of high pressures, although the nonfeeding early larval stages tolerate high pressure. It seems that, although gravid and spent females occur over the whole range of distribution, the eggs hatch more successfully in some places than others. The most favorable conditions seem to occur near the continental slope of the Antarctic continent, where at the right time of year, nets always take early larval stages. There is also evidence of successful hatching where the water temperatures indicate strong northward flow, particularly in the Weddell Sea current, but also in other places not far from the continental margin. In contrast, it seems that the eggs do not succeed near the northern limit of krill distribution. In spite of intensive fishing round South Georgia, the first larval stages have not been found there, but only the later stages that are likely to have been transported from more successful breeding areas farther south. Although spawning seems to take place not far from the surface, there is, in spite of laboratory measurements, evidence to show that the eggs hatch at con siderable depths, below 1,000 m and perhaps twice as deep. There seems to be no doubt that the eggs go below 1,000 m. It does not seem likely that pressure is an essential factor for development since eggs in laboratory tanks hatch success fully at atmospheric pressure. Isolation from the adults and possible predators might be important. Although the female krill disperses her eggs posteriorly, away from danger, the krill have very active feeding mechanisms, and in a swarm of males and females eggs could scarcely escape unless they sink rapidly. If they remain shallow and are caught by the feeding mechanisms, our nets would have caught more of them. To succeed, the early larvae have to climb towards the surface while developing through several stages. Though still oval, its first nauplius has three pairs of appendages that must reduce its density and possibly give it some motive power, but the climb towards the phytoplankton is probably made easier in some places than others by the water movements. Near the continental slope there are likely to be upward movements in the warm deep water as well as downward movement of the cold shelf water. Without suggesting that the early larval stages ride thermals like a glider, the overall effect may help them upwards. There will also be upward movements in the divergence zone between the mainly east and west wind regions, and in fronts between cold currents like the Weddell Sea current and neighboring warmer water. The apparent failure of hatching as indicated by the absence of the first larval