Summary Euphausiids (krill) are important food items of fish, seabirds and whales: consequently, it is important to understand their seasonal cycles. The interannual, seasonal and spatial abundance, distribution and population dynamics of the euphausiids Thysanoessa inermis, Thysanoessa spinifera, Thysanoessa longipes and Euphausia pacifica were studied in the Northern Gulf of Alaska during the production season from 1997 to The greatest abundance of juveniles, males and females of T. inermis and T. spinifera were observed in March-April and in August on the inner shelf, especially when a strong shelf break front was developed. In contrast, Euphausia pacifica tended to be more abundant on the outer shelf in August-October. Dense aggregations of T. longipes were observed in Prince William Sound in March. The spawning of T. inermis and T. longipes occurred in April-May, while E. pacifica spawned from July through October. The spawning of T. spinifera was extended from April through October. The spawning of T. inermis, T. longipes and T. spinifera appeared to be closely related to the phytoplankton spring bloom on inner shelf, while the spawning of E. pacifica occurred later in season. The euphausiid growth rates were maximal between April and August coinciding with the spring and summer phytoplankton blooms. T. inermis, T. spinifera and T. longipes showed a significant increase in abundance from 1998 to 2000, indicating progressively favorable conditions on the inner shelf. Methods Euphausiids were collected at 13 stations on the Seward Line and 5 stations in Prince William Sound using a 1-m 2 MOCNESS with 0.5 mm mesh nets during GLOBEC cruises in The net were fished from 100 m to the surface in 20 m increments at night to minimize net avoidance. Specimens were preserved in 10% formalin. Euphausiids in the samples were identified, staged, enumerated and the wet weight was measured. The data were uploaded into an MS ACCESS data base, and analysis was done with VBASIC calls to STATISTICA calculating routines. Due to uneven spatial distrubution of euphausiids, the analysis of their abundance was done on power transformed data. Shipboard experiments were conducted in 2001 to determine individual growth rates. To collect live animals for experiments, location and depth of euphausiid aggregations were identified with an HTI acoustic system during night time acoustic survey along the Seward Line. The detected aggregations were fished using the MOCNESS with 100 μm mesh nets. Euphausiids were gently removed from the catch and placed in individual 750 ml tissue flasks filled with seawater collected simultaneously at the site. The animals were maintained at the ambient mixed layer water temperature in the dark and were checked every hours for molts and egg production. If an animal molted, the exuvia was removed and preserved in 5% formalin. At the end of each experiment, all animals were preserved individually, and the length of uropods were measured on all molts and preserved animals using a computer-driven measuring system. Meters Below Mean Sea Level Map showing the location of the study site, the Seward Line stations (labeled GAK), and the Prince William Sound stations. Table 1. Summary of GLOBEC cruises in Distribution and growth of Euphausiids in the Northern Gulf of Alaska. A.I. Pinchuk*, R.R. Hopcroft, K.O. Coyle Institute of Marine Science, University of Alaska Fairbanks, AK * Figure 1. Cross shelf distribution of euphausiids along the Seward line and in Prince William Sound (PWS) (stations MS2, HB2, KIP1, PWS1, PWS2 combined) based on data collected from 1997 to 2000 shows that euphausiid species preferred different environments. Three water masses can be distinguished in the study area: brackish Alaska Coastal Current nearshore, oceanic Alaska Stream offshore and intermediate shelf regime in between. A distinct front often develops separating oceanic outer shelf and intermediate inner shelf waters. Significantly higher abundance of T. spinifera and T. inermis occurred on the inner shelf, where T. spinifera dominated euphausiid populations. In contrast, E. pacifica were most abundant on the outer shelf. Thysanoessa longipes formed dense aggregations in PWS and were virtually absent from other stations. Figure 2. Interannual variations in abundance of euphausiids in the study area show significant increase in abundance of T. spinifera, T. inermis and T. longipes from 1998 through 2000, while abundance of E. pacifica did not changed. This may indicate long-term changes in conditions on the inner shelf, which were favorable for euphausiid reproduction and growth. Figure 3. Seasonal variations in abundance of euphausiids in the study area show two peaks for T. inermis, T.longipes and T. spinifera in early spring and late summer. While the first peak was associated with spawning and constituted predominantly adult animals, the second peak indicated the arrival of a new year-class consisted of juveniles as well as recently maturated individuals hatched out the previous year. Figure 4. Seasonal variations in abundance of euphausiid females with spermatophores indicated considerable differences in their spawning time. While T. inermis and T. longipes had one relatively short spawning event in April- May, T. spinifera continued to spawn from April through July, and E. pacifica spawned in May through August. The observed differences in spawning may be related to the timing of phytoplankton bloom on the inner and outer shelf. Figure 5. Seasonal variations of relative size change of E. pacifica shown as the percentage change in uropod length per day during the first 4 days of shipboard experiments conducted in The maximal growth in length appeared to occur from April through July and coincided with the development of phytoplankton blooms on the outer shelf. The large amount of data collected in 2002 will document seasonal growth in length for the other euphausiid species.