Springtime abundance of chaetognaths in the shelf region of the northern Gulf of Alaska, with observations on the vertical distribution and feeding of Sagitta elegans

Chaetognatha were among the most dominant macrozooplankton taxa collected i n 6 years of springtime collections, both throughout the water column and n ear-bottom on the continental shelf in the northern Gulf of Alaska. Three s pecies (Sagitta elegans, S. scrippsae, and Eukrohnia hamata) were collected in 248 collections, although S. elegans was the numerically dominant speci es overall during most cruises. Collections taken in epibenthic sleds gener ally contained about two orders of magnitude more chaetognaths than those t hat sampled throughout the water column. Two size modes were apparent in th e S. elegans size-frequency distributions which were believed to belong to different cohorts. Growth rate was on the order of 2-3 mm per month. The tw o cohorts showed asynchronous diel vertical distribution patterns, with the smaller individuals found near the surface during the day whereas the larg er individuals were near the surface at night. About 7.5% of the S. elegans examined contained food. Copepods made up the majority (similar to 89% by number) of the diet of both large and small individuals. Euphausiid juvenil es and cirripede larvae were also observed, as well as several incidences o f cannibalism by large chaetognaths on smaller individuals. Based on the re sults of two diel series, this species was observed to feed mainly during t he night-time.