Growth Rates of Euphausiids in the Northern Gulf of Alaska in A.I. Pinchuk *, R.R. Hopcroft, K.O. Coyle Institute of Marine Science, University of Alaska Fairbanks, AK * Abstract Growth rates of the three most abundant euphausiid species (Thysanoessa inermis, T. spinifera and Euphausia pacifica) in the northern Gulf of Alaska were measured from March through October in 2001, 2002 and Shipboard experiments were conducted to obtain in situ growth rates using the instantaneous growth rate technique, which involves incubating individual animals at ambient temperature and food, and measuring the change in length of the uropods after molting. The highest mean growth rates (over 5% of length change 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 more oceanic T. spinifera and E. pacifica, suggesting tight coupling with food availability. The molting rate appeared to be strongly influenced by temperature ranging from 11 days at 5 o C to 6 days at 12 o C. Methods Euphausiids were collected along the Seward Line and within Prince William Sound in the northern Gulf of Alaska (GOA) during GLOBEC LTOP cruises in To collect live animals for experiments, location and depth of euphausiid aggregations were identified with an HTI acoustic system operating at 42, 120, 240 and 420 kHz during night-time acoustic survey along the Seward Line. The detected aggregations were fished using 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 (5 o C in March-May, 12 o C in July-August and 10 o C in October) in the dark and were checked every hours for molts. If an animal molted the exuviae and the animal were preserved in 5% formalin. At the end of each experiment all animals were also preserved. The experiment duration was 48 hours. The length of uropods were measured on all molts and preserved animals using a digitized measuring system (Roff & Hopcroft, 1986). Map showing the location of the study site, the Seward Line stations (labeled GAK), and the Prince William Sound stations. Meters Below Mean Sea Level Table 1. Summary of growth and molting rates experiments 207GAK12HX GAK2 GAK4 GAK1 HX267--HX253October PWS1 GAK4 GAK3 HX GAK11 GAK5 HX26350 GAK12 GAK9 GAK6 HX248August 189GAK8WC GAK7 GAK5 HX26245 GAK9 GAK7 HX246July 179GAK2HX GAK4 GAK7 HX25860 GAK13 GAK9 HX243May GAK4 GAK6 GAK1 HX GAK11 GAK3 HX25790GAK13HX241April GAK12 GAK2 HX GAK6 GAK3 GAK1 HX25452PWS1HX239March # of animals in experiment StationCruise ID# of animals in experiment StationCruise ID# of animals in experiment StationCruise ID Month Seasonal variations in the molting rates of Thysanoessa inermis, T. spinifera and Euphausia pacifica in the northern GOA. The molting rate expressed as mean percentage of population per day, with 95% confidence intervals noted. The seasonal molting pattern was similar for all species with lowest rates in spring and highest in late summer, apparently being strongly influenced by temperature. The intermolt periods (IP) calculated for different temperatures were significantly different for 5 o C and 12 o C for Thysanoessa species (Table 2) ± 6.7 n= ± 5.0 n= ± 3.4 n= ± 5.0 n= ± 1.18 n=5 6.4 ± 0.4 n=4 June-August 12 o C E. pacificaT. spiniferaT. inermis 8.3 ± 1.1 n= ± 7.3 n=3 8 n=2 October 10 o C March-May 5 o C Table 2. Intermolt periods (IP) calculated from daily molting rates for different incubation temperatures Size Group 1Size Group Size Group 2Size Group Size Group 2Size Group 1 Seasonal variations in the growth rates of Thysanoessa inermis, T. spinifera and Euphausia pacifica in the northern GOA. The growth rates expressed as mean percentage change in uropod length at molt, with 95% confidence intervals and sample size noted. Two size groups were found in experimental populations for all three species presumably representing the one- and two-year old generations. The growth rates were calculated separately for each size group to account for the effect of body size. The seasonal patterns of growth were different for species as well as for size groups. The minimal growth rates for both groups were observed in March indicating close to 0 growth or shrinkage for the Thysanoessa species. The growth intensified through the spring, peaking in May. By the mid summer, the growth rates significantly decreased for T. inermis Size Group 2, while remaining high for other euphausiids. In late summer and fall, growth rates of T. inermis Size Group 2 remained low while those of the Size Group 1 significantly decreased. In contrast, both size groups of T. spinifera had similar growth rates from May through the end of production season. Euphausia pacifica appeared to follow the same pattern. It appears that seasonal changes in growth rates were not induced by temperature, but rather other factors such as food availability. Conclusions 1. Growth of euphausiids in the GOA was close to negative in March, but reaching maximum values by May. 2. Seasonal growth pattern of coastal T. inermis was different from those of T. spinifera and E. pacifica, and was characterized by a sharp decline in growth rates in late summer and fall, possibly resulted from lack of large diatoms in the coastal area. 3. The molting rate appeared to be affected largely by temperature, rather then other environmental conditions such as food limitation. 4. In contrast, growth expressed as % of uropod length change, did not appeared to be affected by temperature, but controlled by other conditions such as food availability. This research is supported by the U.S. GLOBEC Northeast Pacific Program, jointly funded by the National Science Foundation and the National Oceanic and Atmospheric Administration under NSF Grant OCE Seasonal variations in mean water column temperatures in the northern GOA (obtained from IMS GLOBEC website ). The temperature is around 5 o C throughout the water column from March through May. The upper layer (0-25 m) starts to warm up in summer reaching about 12 o C in July-August. By October the surface layer has cooled to about 10 o C. Seasonal variations in water column chlorophyll-a content (μg l -1 ) in 2002 in the northern GOA (obtained from T. Whitledge). The dots denote sampling points through upper 50 m layer along the Seward Line. The spring bloom starts nearshore in April and reaches maximum in May extending over the shelf. In summer and fall chlorophyll-a values are lower. Depth, m Distance, km Depth, m Distance, km Depth, m Distance, km Depth, m Distance, km Depth, m Distance, km Depth, m Distance, km April AugustJulyOctober MarchMay