Daytime surface swarming by Meganyctiphanes norvegica (M. Sars) (Crustacea, Euphausiacea) off Brier Island, Bay of Fundy

Abstract

In July–September the euphausiid Meganyctiphanes norvegica (M. Sars) swarms at the surface in daytime off Brier Island, Nova Scotia (ca. 44°15′ N 66°23′ W). Systematic observations of swarming were made, mainly in late August 1974, 1975, 1976, and 1978, supplemented in 1974 and 1975 by the frequency of occurrence of the euphatisiids in the proventriculi of collected greater shearwaters, Puffinus gravis (O'Reilly). Swarms, consisting mostly of immature animals, occurred near an area of turbulence where strong tidal streams ran up against steep underwater ledges, bringing cool subsurface water and also copepods to the surface. Swarming was apparently not an attempt to avoid underwater predators or unfavourable oceanographic conditions, nor was it a means of advancing gonad maturation through exposure to solar irradiation. Swarming was most common in spring tide periods when tidal streams were strongest, and did not occur at all during a neap tide period in late August 1975. It is suggested that M. norvegica swarms were transported passively to the surface by strong vertical turbulence. However, the possibility that they swam actively to the surface in search of copepods and other food passively transported in that way could not be ruled out. It is suggested that the surface swarming of Euphausia pacifica Hansen, the other well-documented survey of the phenomenon, by Y. Komaki in 1967, be reexamined in the light of these conclusions. The significance of surface swarms as food sources for a variety of marine predators is discussed.