DAYTIME SURFACE SWARMS OF THYSANOESSA SPINIFERA (EUPHAUSIACEA) IN THE GULF OF THE FARALLONES, CALIFORNIA

Abstract

Daytime surface swarms of the euphausiid Thysunoessa spinifera were observed and sampled in spring I985 in the Gulf of the Farallones off San Francisco, California. Samples were dipnetted at the surface from nine discrete swarm patches: and depth-stratified plankton tows were made before, during, and after swarm activity. Analyses of sex and maturity stage of surface .patch samples showed that 94% of all euphausiids were in breeding condition. Sex ratios differed significantly in six of the nine patches, with males dominating four and females dominating two patches. This sexual segregation is thought to be related to differences in size between sexually mature males and females. Biological and oceanographic observations at the swarm site, and examinations of food baskets of swarm individuals, gave little indication that the animals were passively transported to the surface, or that they were there to feed or avoid predators. T. spinferu eggs and Stage I nauplii were numerous in plankton collections taken the night following, and day after surface swarming was observed. It is concluded that swarming in this species is related to reproduction. Its adaptive value in regard to different breeding-related activities is still unclear, but probably involves a combination of factors that relate to the timing of sexual maturation, egg release, and larval dispersal in the changeable upwelling environment off central California. The neritic euphausiid Thysanoessa spinifera aggregates in conspicuous daytime swarms at the ocean surface along the California coast during spring and summer. Little is known about these surface swarms or what triggers their formation, probably because of the unpredictable nature of their onset and duration. The behavior occurs during daylight when most vertically migrating euphausiids, including T. spinifera (Alton and Blackburn, 1972), are usually found at greater depths. In general, upwelling centers have been associated with highest densities of T. spinifera (Brinton, 1962); and in the Gulf of the Farallones, daytime swarms occur sporadically from about April to July, when upwelling events are the most frequent and intense along the central California coast (Bakun, 1975). During this time, northwest winds intensify, triggering the offshore transport ofwarm surface waters, which are then replaced by cool, nutrient-rich water from the depths (Bolin and Abbot, 1962). Periodically during the upwelling season, northwesterly winds slacken, the process temporarily subsides, and surface temperatures may rise due to solar radiation or intrusion of warmer offshore water (Bolin and Abbot, 1962; Hatfield, 1983). When the rust-colored patches appear, many of the commercial sport fishing boats, guided by flocks of feeding seabirds, seek them out in search of salmon, which we have found to feed heavily on euphausiids at this time of year. Although this daytime swarming is often highly visible and appears to be a relatively common event off central California, very little information is available on swarm population structure. The only published reports consist of brief accounts of swarms that washed ashore at La Jolla, California (Boden et al., 1955), and Bandon, Oregon (Pearcy and Hosie, 1985), which were composed entirely of spent females, and those observed in Monterey Bay by Barham (1956) for which sex and maturity composition are not known.