The Vertical Distribution of Marine Macroplankton. VII Observations on the Behaviour of Calanus finmarchicus

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Abstract
SUMMARY: The vertical distribution of Calanus finmarchicus adults in the daylight in the Plymouth region as shown by stramin ring-trawl catches in described for the period April to September in 1926. The general results confirmed the indications given in 1925 that there is a, gradual descent of the region of maximum abundance from about 10 metres in April to-metres in June, with a definite rise towards the surface ia July, August, and September.2. The rise towards the surface was evident on sunny days as well as dull, indicating that the suggestion that dull weather and low light intensity was its cause in 1925 may possibly have been incorrect.Measurements of the total lengths of Calanus were made which showed a seasonal change, a brood of small adults occurring in July, August, and September, as opposed to a brood of large adults whichdominated in the spring.It is suggested that possibly these two broods are physiologically different, and that the small type of adult prefers a higher light intensity and lives nearer the surface than the large type.The males and females differed in their behaviour, the males being always slightly deeper in the water than the females.Some indications are given of the course of the life-cycles of Calanus in the Plymouth area.While from April to the end of June the abundance of the last copepodid stage (Stage V) rarely exceeded more than 20% of the total Calanus in any one catch, they became very much more abundant in July, August, and September, reaching even 60% and 70%.