Climate-related variability in abundance and reproduction of euphausiids in the northern Gulf of Alaska in 1998–2003

Interannual variability in abundance of the dominant euphausiids Thysanoessa inermis, Thysanoessa spinifera and Euphausia pacifica was studied in the northern Gulf of Alaska during the production season from 1998 to 2003. T. inermis abundance increased significantly over the shelf from 1998 to 2002, the abundance of T. spinifera (which also inhabits the shelf) did not change, while the abundance of E. pacifica (usually common on the outer shelf) declined in 2001. Based on the incidence of spermatophores on the females, reproduction of T. inermis occurred during April in all years, and was extended through May in 2000–2002. The major spawning of T. spinifera and E. pacifica occurred from April through July, and from late May through August, respectively. The spawning of T. inermis and T. spinifera appeared to be closely related to the spring diatom bloom on the inner shelf, while the spawning of E. pacifica occurred later in season, when the temperature of the mixed layer increased. A strong increase in abundance of T. inermis, associated with the extended colder phase in the North Pacific, indicates that progressive cooling in 1999–2002 may have resulted in greater reproductive success of the early spawning T. inermis on the inner shelf.