The Vertical Distribution of Marine Macroplankton. V. The Distribution of Animals Caught in the Ring-trawl in the Daytime in the Plymouth Area

Abstract

1. The vertical distribution of many species in daylight in the Plymouth area is shown, as a result of the examination of a number of serial hauls taken with the stramin ring-trawl between the dates April 2nd and August 6th, 1926.2. It is shown that probably the errors introduced by the employment of a non-closing net are for the present purpose negligible.3. The indications are that most species have an optimum level at which they live, in the daytime, the actual depth varying from day to day according to weather and other conditions.4. A few species appear to show no preference, being present equally abundantly at all depths or showing marked changes in the region of greatest abundance from day to day.5. A provisional list is given on page 567, showing the order in which many of the different species appear in greatest abundance from the surface downwards.6. The vertical distribution appears to differ for the two sexes of Calanus finmarchicus, the males tending to avoid the surface layers more than the females.7. The presence of the medusa Cosmetira pilosella in fair numbers in a catch in the daytime in the Plymouth region would seem to be a reliable indication that the net has been fishing below 20 metres.8. A catch of Calanus finmarchicus made by the stramin ring-trawl consists nearly always of over 80 per cent adults.9. No attempt has been made definitely to correlate behaviour with external factors: more observations are to be desired.10. Brief notes are given on the seasonal distribution of various species. The danger of drawing conclusions about seasonal abundance from daylight hauls only is shown by the results for Tomopteris helgolandica (p. 575).