1. AB Egg release of T. inermis and E. pacifica in the northern GOA. Most of T. inermis females started to release eggs within first 2 days of incubation with the average number of released eggs per female being similar in Day 1 and 2, but significantly smaller on Day 3 and 4 (A). About 25% of females were continuously releasing eggs over 3 days rather than producing a single distinctive brood, while the number of eggs in each consecutive clutch significantly decreased (B). In contrast, most of E. pacifica females released eggs on the first day of observation (A) and only 2 females produced eggs repeatedly (B). Sample size and 95% confidence intervals are noted. Relationship between brood size and female total body length (um) of T. inermis and E. pacifica. The brood size significantly increased with female size, however a large scatter of the data occurred, indicating that some other factors determined the brood size as well. Developmental time (D, days) from hatching to calyptopis 1 and temperature (T, o C) for E. pacifica based on our results and on data available in literature with 95% confidence intervals and sample size noted. The reasonable fit of our data to those from other regions suggests that non-feeding larvae of E. pacifica develop similarly in various regions of the North Pacific. That, together with apparently low mortality rates, observed in our and other studies, indicates their ability to tolerate wide temperature ranges they may experience inhabiting the mixed layer. Our data for T. inermis (superimposed on the graph and not included in the regression equation) appears to fit the trendline, suggesting, that the broad temperature tolerance of larvae may be a common trait of other euphausiids occurring in temperate waters. The Intermolt Periods (IP) in days for early stages of T. inermis and E. pacifica, measured at different temperatures. All stages of both species showed shorter IP at higher temperatures and appeared to have similar growth patterns. The IP of non-feeding nauplii (N1 & N2) and metanauplii (MN) were significantly shorter that those of feeding calyptopises (C1-C3). Development times from egg to the first furcilia (F1) stage as estimated from IP would range between 20 d and 33 d for T. inermis and 15 d and 45 d for E. pacifica at 12 and 5 o C respectively. Conclusions 1. The spawning of Thysanoessa inermis occurred in spring, while that of Euphausia pacifica started in summer and extended through fall. 2. Thysanoessa inermis may repeatedly produce eggs over 4 days with the number of released eggs declining with each consecutive clutch. Euphausia pacifica appear to release most of their eggs in a single distinctive brood over 24 hours. It is not clear how many broods can both species release over the spawning season in the northern GOA. The brood size significantly increased with female size for both species. 3. The development times from egg to the first furcilia stage ranged between 20 d and 33 d for T. inermis and 15 d and 45 d for E. pacifica at 12 and 5 o C respectively. 4. The non-feeding larvae of E. pacifica develop similarly in various regions of the North Pacific, which, together with low mortality rates, indicates their ability to tolerate wide temperature ranges. Reproduction and early development of Thysanoessa inermis and Euphausia pacifica (Crustacea: Euphausiacea) in the northern Gulf of Alaska. A.I. Pinchuk *, R.R. Hopcroft Institute of Marine Science, University of Alaska Fairbanks, P.O.Box 730, Seward, AK 99664 *e-mail: ftaip1@uaf.edu Abstract Early life history patterns were studied in the dominant euphausiids Thysanoessa inermis and Euphausia pacifica from the northern Gulf of Alaska (GOA) in 2001-2004. Gravid females of T. inermis were observed in April and May. Most of females started to release eggs within first 2 days of incubation. The average number of released eggs per female XXX was similar in Day 1 and 2, but significantly smaller on Day 3 and 4. About 25% of females were continuously releasing eggs over 3 days rather than producing a single distinctive brood. In contrast, gravid females of E. pacifica were observed from early July through October. Most of females released eggs on the first day of observation and only 2 females produced eggs repeatedly. Average brood size appeared to increase with female size. Hatching and early development (from egg to furcilia stage) was studied at 5, 8 and 12 o C. Hatching lasted over several hours depending on the incubation temperature. Development times from egg to the first furcilia stage ranged between 20 d and 33 d for T. inermis and 15 d and 45 d for E. pacifica at 12 and 5 o C respectively. Methods Euphausiids were collected along the Seward Line and within Prince William Sound in the northern 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 black 100 μm mesh nets. Gravid females 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 12 hours for eggs. When eggs were released, the female was removed from the flask to prevent cannibalism. The female was transferred to a new container filled with fresh seawater and incubated for 4 days to estimate the length of an individual spawning event. At the end of each experiment, all animals are preserved individually. The eggs were counted, then either preserved or incubated. Incubated eggs were placed in multiwell trays (15 ml well volume), 25-50 eggs per well, and were kept at 5  0.5 o C, 8  0.5 o C and 12  0.5 o C until they hatched or became moribund. The nauplii, which emerged from the eggs, were transferred into multiwell trays with fresh seawater. Since the nauplii and metanauplii do not feed, no food was introduced until they molted into calyptopis. The trays were inspected every 3–12 hours depending on temperature. After the larvae reached calyptopis stage, they were individually transferred into multiwell trays and were inspected daily. Water and food were changed every 3-5 days. The larvae were fed ad libitum mix of Thalassiosira spp., Isochrisis galbana, Pavlova lutheri and Chromonas spp. Feeding was augmented with in situ food (10-150 µm) collected in Resurrection Bay to ensure a more nutritionally balanced diet. Cyanobacteria, ciliates, flagellates, diatoms, eggs, small nauplii, and small larvaceans were observed in varying concentrations in these in situ samples. Spawning of T. inermis and E. pacifica in the northern GOA as reflected by abundance of adult females carrying attached spermatophores. The spawning of T. inermis occurred in April-May, while that E. pacifica extended from July through October. The spawning of T. inermis 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 (data obtained from GLOBEC LTOP data set for the northern GOA from 1998 through 2002). Vertical bars – 95% confidence intervals. 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. Stage Thysanoessa inermisEuphausia pacifica 5oC5oC8oC8oC12 o C5oC5oC8oC8oC10 o C12 o C IP±95%CIn n n n n n n Egg2.5 ± 0.21121.5 ± 0.425--2.6 ± 0.3691.8 ± 0.21601.2 ± 0.3701.1 ± 0.532 N11.8 ± 0.2891.5 ± 0.4250.7 ± 1.331.6 ± 0.3691.2 ± 0.21591.1 ± 0.3700.9 ± 0.528 N24.1 ± 0.284--1.6 ± 1.333.6 ± 0.3691.5 ± 1.531.0 ± 0.3701.1 ± 0.528 MN4.3 ± 0.79--2.6 ± 0.4256.4 ± 0.2144--2.9 ± 0.3702.3 ± 1.07 C16.9 ± 0.88--4.8 ± 0.34413.4 ± 0.2144--6.9 ± 0.3705.1 ± 1.34 C26.9 ± 0.87--5.8 ± 0.4319.2 ± 0.2118--7.5 ± 0.4534.3 ± 1.53 C36.3 ± 0.87--5.9 ± 0.4339.1 ± 0.386--6.9 ± 0.7162.52 F120.3 ± 1.33--5.3 ± 0.799.4 ± 1.07--3.02 2 nabr2r2 Fp T. inermis188-577.938.820.22453.8<.000 E. pacifica58-224.8 0.0170.31225.35<.000 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