No abstract

Apart from a few scattered observations (Hensen, 1887; Gran, 1902) the study of the food of Calanus began with Dakin (1908). He examined the remains present in the gut, and this was the method used also by Esterly (1916), Lebour (1922) and Marshall (1924). They all found that these remains consisted of a greenish mush containing the skeletons of numerous planktonic organisms, chiefly diatoms and dinoflagellates. Naked flagellates were, how-ever, occasionally seen, and it was realized that the food might in reality consist largely of organisms which had no skeleton and could leave no recognizable remains.

1. The seasonal changes in the population of Calanus in Loch Striven in 1933 have been investigated.2. There were three main breeding periods. The first was in February and March, an unsuccessful beginning of the second in April, the true second brood in May to June, the third in July, and subsidiary broods in August and September.3. Numbers were very low until April when the copepodites of the first brood caused a large increase; there was a second increase in May consisting mainly of the eggs of the second brood, and the maximum for the year came in July. The last and subsidiary broods provided the stock for the autumn and winter of 1933.

In a recent study (Butler, Corner & Marshall, 1969) it was found that the excretion of nitrogen and phosphorus in soluble form by Calanus finmarchicus caught at Garroch Head in the Clyde sea-area was significantly higher in spring when plant food was plentiful, than in autumn when plant food was in relatively short supply. The present survey has extended this earlier study to include more detailed data at all times of the year, particular attention being paid to the spring diatom increase of 1969 when plant food in the sea near Garroch Head rose above the level which Beklemishev (1962) regards as inducing superfluous feeding, a wasteful process partially involving the inefficient assimilation of foodstuffs (see ‘Discussion’, p. 549).

The seven species of small copepod common in Loch Striven have been studied from vertical tow-net hauls taken there throughout the year 1933. These copepods were Pseudocalanus minutus, Paracalanus parvus, Microcalanus pygmaeus, Centropages hamatus, Temora longicornis, Acartia clausi and Oithona similis.In general, the copepods began to reproduce about the time of the spring diatom increase in March or April, and produced a succession of broods throughout the summer; apart from the first these broods were not as a rule so clearly marked as in Calanus. Microcalanus begins to breed before the spring increase and has clearly marked broods like Calanus.