Seasonal variations in mean water column temperatures in the northern GOA (obtained from IMS GLOBEC website http://www.ims.uaf.edu/GLOBEC/ ). The temperature is around 5oC throughout the water column from March through May. The upper layer (0-25 m) starts to warm up in summer reaching about 12oC in July-August. By October the surface layer has cooled to about 10oC.
MethodsEuphausiids were collected along the Seward Line and within Prince William Sound in the northern Gulf of Alaska (GOA) during GLOBEC LTOP cruises in 2001-2003. 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 (5oC in March-May, 12oC in July-August and 10oC in October) in the dark and were checked every 12-24 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).
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 growth and molting rates experiments
207GAK12HX279195
173
205
GAK2
GAK4
GAK1
HX267--HX253October
136
156
173
PWS1
GAK4
GAK3
HX276202
200
GAK11
GAK5
HX26350
50
50
GAK12
GAK9
GAK6
HX248August
189GAK8WC307186
174
GAK7
GAK5HX26245
45
GAK9
GAK7HX246July
179GAK2HX273129
198
GAK4
GAK7HX25860
60
GAK13
GAK9HX243May
175
206
192
GAK4
GAK6
GAK1
HX271207
190
GAK11
GAK3
HX25790GAK13HX241April
155
180
GAK12
GAK2
HX269147
88
180
GAK6
GAK3
GAK1
HX25452PWS1HX239March
# of animals in experiment
StationCruise ID# of animals in experiment
StationCruise ID# of animals in experiment
StationCruise ID
200320022001Month
AbstractGrowth 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 2003. 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 moult) 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 5oC to 6 days at 12oC.
15
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.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-0105236.
Thysanoessa spinifera
0
5
10
15
20
25
Total Length, mm
Freq
uenc
y
Thysanoessa inermis
0
5
10
15
20
25
Total Length, mm
Freq
uenc
y
Euphausia pacifica
0
2
4
6
8
10
12
14
16
18
20
Total Length, mm
Fre
quen
cy
Size Group 1 Size Group 2
Size Group 2
Size Group 2
Size Group 1
Size Group 1
3712
52
86
2042
5
23
15
23
22
8
8
4
10
2
5
37
74
14
18
3
13
2
14 5
260
18
34
Conclusions1. 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.
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 5oC and 12oC for Thysanoessa species (Table 2).
11.4 ± 6.7
n=3
18.2 ± 5.0
n=6
11.5 ± 3.4
n=12
8.3 ± 5.0
n=3
6.02 ± 1.18
n=5
6.4 ± 0.4
n=4
June-August
12oC
E. pacificaT. spiniferaT. inermis
8.3 ± 1.1
n=4
9.35 ± 7.3
n=3
8
n=2
October
10oC
March-May
5oC
Table 2. Intermolt periods (IP) calculated from daily molting rates for different incubation temperatures
Growth Rates of Euphausiids in the Northern Gulf of Alaska in 2001-2003.A.I. Pinchuk*, R.R. Hopcroft, K.O. CoyleInstitute of Marine Science, University of Alaska Fairbanks, AK 99775-7220*e-mail: [email protected]
