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Survey of large circular and octagonal tanks operated at Norwegian commercial smolt and post-smolt sites

Summerfelt, Steven T., Mathisen, Frode, Holan, Astrid Buran, Terjesen, Bendik Fyhn
Aquacultural engineering 2016 v.74 pp. 105-110
Salmo salar, business enterprises, fiberglass, fish production, floors, geometry, harvesting, hatcheries, labor, pipes, smolts, surveys, tanks, water quality
A survey was conducted to determine the geometry, operating parameters, and other key features of large circular or octagonal culture tanks used to produce Atlantic salmon smolt and post-smolt at six major Norwegian Atlantic salmon production companies. A total of 55 large tanks were reported at seven land-based hatchery locations, i.e., averaging 7.9 (range of 4–12) large tanks per land-based site. In addition, one 21,000m3 floating fiberglass tank in sea was reported. Culture volume ranged from 500 to 1300m3 for each land-based tank. Most tanks were circular, but one site used octagonal tanks. Land-based tank diameters ranged from 14.5 to 20m diameter, whereas the floating tank was 40m diameter. Maximum tank depths ranged from 3.5 to 4.5m at land-based facilities, which produced diameter-to-average-depth ratios of 3.6:1 to 5.5:1m:m. The floating tank was much deeper at 20m, with a diameter-to-average-depth ratio of only 2.4:1m:m. All land-based tanks had floors sloping at 4.0–6.5% toward the tank center and various pipe configurations that penetrated the culture tank water volume at tank center. These pipes and sloping floors were used to reduce labor when removing dead fish and harvesting fish.Maximum flow ranged from 3 to 19m3/min per land-based tank, with 400m3/min at the floating tank, but tank flow was adjustable at most facilities. Land-based tanks were flushed at a mean hydraulic retention time (HRT) of 35–170min. Maximum feed load on each land-based tank ranged from 525 to 850kg/day, but the floating tank reached 3700kg/day. Almost half of the large tanks reported in this survey were installed or renovated since 2013, including the three tank systems with the highest flow rate per tank (greater than 17.6m3/min). These more recent tanks were operated at more rapid tank HRT’s, i.e., from 34.8 to 52.5min, than the 67–170min HRT typical of the large tanks built before 2013. In addition, flow per unit of feed load in land-based tanks that began operating before 2010 were lower (19–30m3 flow/kg feed) than in tanks that began operating later (33–40m3 flow/kg feed). In comparison, the floating tank operates at a maximum daily tank flow to feed load of 160m3 flow/kg feed, which is the least intensive of all tanks surveyed. Survey results suggest that the recently built tanks have been designed to operate at a reduced metabolic loading per unit of flow, a tendency that would improve water quality throughout the culture tank, all else equal. This trend is possible due to the ever increasing application of water recirculating systems.