The Vertical Distribution of Marine Macroplankton. III. Diurnal Observations on the Pelagic Young of Teleostean Fishes in the Plymouth Area.

Extract

Results are given of collections made with the stramin ring-trawl to study the vertical distribution of post-larval teleostean fishes throughoutthe period from daylight, through the hours of darkness, to daylight on July 15th-16th, 1924, and June 17th, 18th, and 19th, 1925. At the time of collecting in July, 1924, it was full moon ; in June, 1925, there was no moon.

In no case, except perhaps that of Sardina pilcliardus post-larvse, was there evidence of marked upward migration at night ; the general impression for most species was that while the type of distribution shown in daylight was somewhat modified at night, only very few individuals moved up to the surface layers. There was, however, a notable increase in numbers of certainspecies at all depths at night, whether this was an indication of a movement up from layers very close to the bottom is not known ; it will need many more observations before the exact significance of this can be fully understood.

The numbers of post-larvae taken were in the case of most species unfortunately very small on all occasions; it is not possible, therefore, to come to very definite conclusions.

A possible indication of the effect of moonlight was shown by the post-larval Callionymus lyra, for while in July, 1924, at the time of full moon they showed no upward movement at night, in June, 1925, with no moon a certain proportion reached the surface layers.