This paper synthesizes the trophic dynamics of a Canadian arctic marine ecosystem in so far as it is known, using new data on primary production, zooFlankton, the bivalve Mya truncata, and arctic cod (Boreogadus saida), as well as literature values for marine mammals and seabirds. The 98 000 km reglon has a high rate of primary production relative to other parts of arctic Canada. About 60 g Cm-' are fixed annually, of which approximately 90% is contributed by phytoplankton, 10% by ice algae, and 1% by kelp. Phytoplankton production is twofold higher along the south coast of Comwallis Island than elsewhere in Barrow Strait. Four copepod species, of which Pseudocalanus acuspes is the most important energetically, graze about one-third of the phytoplankton production. Bivalves maintain high biomass but low energy flow, acting as sedimenting agents. Arctic cod is a major component, with 125 000 tonnes bein consumed by marine mammals and 23 000 tonnes by seabirds annually. Our hydroacoustic estimate for mean arctic cod density, 0.0022 fish.m', is probably too low, partly because we have been unable to quantify dense aggregations of schooling fish. The ecological efficiency of ringed seal is near maximum, with 5% of ringed seal ingestion going to bears and man as seal flesh. The data on total kill and prey consumption in whales and birds is incomplete because they migrate out of the Lancaster Sound region in winter. The food chain is very long, with bears occupying the fifth trophic level; this is reflected by high biomagnification factors for persistent lipophilic pollutants such as PCBs. There are major data gaps for some zooplankton and most of the benthos, as well as for winter populations and energetics. This trophic analysis is therefore incomplete and efficiencies for entire trophic levels cannot be calculated.RgSUMfi. Cet article rBsume la dynamique trophique de 1'6cosysttme marin dans le Canada arctique dans la mesure oh il est connu, en utilisant de nouvelles donntes sur la production primaire, le zooplancton, le bivalve Mya fruncata, et la m o m polaire (Boreogadus saida), ainsi que les valeurs trouvees dans la documentation sur les mammieres et les oiseaux marins. Cette rigion d'une superficie de 98 000 k m ' posstde un taux BlevC de production primaire par rapport B d'autres parties du Canada arctique. Environ 60 g C.m-' sont fix& annuellement, dont environ 90 p. cent par le phytoplancton, 10 p. cent par les algues glaciaires et l p. cent par les laminaires. La production de phytoplancton est deux fois plus Blevte le long de la c6te meridionale de l'ile Comwallis qu'ailleurs dans le dCtroit de Barrow. Quatre esp&ces de cop6podes, dont la Pseudoculunus acuspes est la plus importante du point de vue tnergitique, utilisent environ un tiers de la production de phytoplancton pour se noumr. Les bivalves contribuent de faGon importante A la biomasse mais peu au flux BnergBtique, Ctant des agents de s6dimentation. La morue arctique est une composante importante, itant consommBe au taux annuel de 125 000 tonnes par...

We document the occurrence of large schools of Arctic cod (Boreogadus saida) in the Barrow Strait region of the eastern Canadian Arctic during the open water season. Schools were most frequently observed near shore, often in depressions inside bays. Schools ranged up to 130 O00 mz surface area and contained on the order of 4 X 108 fish, weighing 12 O00 tonnes. Evidence indicates that schools form before the arrival of predators, but when they occur in shallow water, they are often subjected to intense predation by thousands of seabirds and marine mammals, primarily black-legged kittiwakes (Rissa tridacfyla), northern fulmars (Fulmam glacialis), harp seals (Phoca groenlandicus), beluga (Delphinaptem Zeucas) and narwhal (Monodon monoceros). The reasons for such schooling behavior are unknown.

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Relatively little is known about the distribution of Wsh in deep water (>200 m) in the Beaufort Sea. Data collected by an Acoustic Doppler Current ProWler operated in the Chukchi and Beaufort seas in summer were examined for evidence of Wsh biomass detections between 18 and 400 m. The presence of Wsh in waters between 1 and 30 m was explored opportunistically with a non-scientiWc echo sounder. Evaluation of Wndings was enhanced by measurements of water column properties (temperature, salinity, Xuorescence and transmissivity). Relatively small shoals of Wsh were detected on the Chukchi shelf and eastern Chukchi shelf break, and also on the Alaskan and Canadian Beaufort shelves in the upper 20 m (T = 2-5°C). Much larger shoals (putative polar cod) were detected within Atlantic Water along the Beaufort continental slope (250-350 m) and near the bottom of Barrow and Mackenzie canyons, where temperatures were above 0°C. A warm-water plume of Alaska Coastal Current water with high concentrations of phytoplankton, zooplankton, and Wsh was found extending along the shelf 300 km eastward of Barrow Canyon. In contrast to the warm surface and Atlantic Water layers, very few Wsh were found in colder, intermediate depth PaciWc-origin water between them. The large biomass of Wsh in the Atlantic Water along the continental slope of the Chukchi and Beaufort seas represents previously undescribed polar cod habitat. It has important implications with regard to considerations of resource development in this area as well as understanding impacts of climate change.

Aggregations of Arctic cod (Boreogadus saida) in Allen Bay, on the southern coast of Cornwallis Island, Lancaster Sound were studied with fishery acoustics techniques in 1989 and 1990. Nonschooling fish were also examined in several Cornwallis and Devon Island bays. Although acoustic signal shading within the schools minimized abundance estimates (error may exceed 50%), measurements in 1989 indicated that two aggregations of adult Arctic cod together comprised at least 900 million individuals. Combined surface area of these two schools was 59 hectares, and total biomass was 30 715 tonnes (520 t•ha-1). Mean density in one school was 91 fish•m-3. In the other it was 307 fish•m-3 , each fish occupying a space equivalent to about one body length cubed. A smaller school studied in the same location a year later had a mean density of 72 fish•m-3. Mean length of net-captured fish from that school was 18.7 cm (TL), and mean weight was 44.5 g. School surface area was about 4.6 hectares, and it contained an estimated 2835 t of biomass (616 t•ha-1). Given that significant concentrations of biomass occurred in this small portion of the Lancaster Sound region, we concluded that, in theory, sufficient biomass was sequestered in fish schools within the region to support energy flows through the food web. Acoustic measurements indicated that fish at a school edge were larger (target strength [TS] =-44.7 dB, equivalent size = 19.3 cm) than nonschooling fish elsewhere (TS =-53.4 dB, equivalent size = 8.4 cm). Zooplankton abundance near a school was markedly reduced. Near Cornwallis Island, where water temperatures throughout the water column were >-1.3˚C, fish were scattered from surface to bottom. But near Devon Island, fish were common only in warmer surface waters (> 2.0˚C) above a marked thermocline at about 35 m, but not deeper, where water temperature was-1.5˚C.