| Summary: | and Euphausia pacifica (Hansen 1911) are key pelagic grazers and also important prey for many commercial fish species in the Gulf of Alaska (GOA). To under-stand the role of the euphausiids in material flows in this ecosystem their growth rates were examined using the instantaneous growth rate (IGR) technique on the northern GOA shelf from March through October in 2001–2004. The highest mean molting increments (over 5 % of uropod length increase per molt) were observed during the phytoplankton bloom on the inner shelf in late spring for coastal T. inermis, and on the outer shelf in summer for T. spinifera and more oceanic E. pacif-ica, suggesting tight coupling with food availability. The molting rates were higher in summer and lower in spring, for all species and were strongly influenced by temperature. Mean inter-molt periods calculated from the molting rates, ranged from 11 days at 5C to 6 days at 8C, and were in agreement with those measured directly during long-term laboratory incubations. Growth rate estimates depended on euphausiid size, and were close to 0 in early spring, reaching maximum values in May (0.123 mm day–1 or 0.023 day–1 for T. inermis) and July (0.091 mm day–1 or 0.031 day–1 for T. spinifera). The growth rates for E. pacifica remained below 0.07 mm day–1 (0.016 day–1) throughout the season. The relationship between T. inermis weight specific growth rate (adjusted to 5C) and ambient chlorophyll-a concentration fit a Michaelis–Menten curve (r2 = 0.48) with food saturated growth rate of 0.032 day–1 with half saturation occurring at 1.65 mg chl-a m–3, but such relationships were not significant for T. spinifera or E. pacifica.
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