Abstract
Acoustic studies and sampling during 4 winters in a Norwegian fjord showed that the clupeid fish sprat Sprattus sprattus inhabited severely hypoxic waters. Their lower tolerable oxygen content was at similar to 7% O-2 saturation (similar to 0.5 ml O-2 l(-1) at 7 degrees C), and they occurred as deep as this limit allowed in the 150 m water column. Sprat in this hypoxic habitat were foraging on dormant copepods Calanus spp. in the daytime, or did not feed at all. Use of hull-mounted, submerged and bottom-mounted echosounders allowed observations of in situ swimming behavior of individual sprat, revealing that they were continuously moving up and down. This behavior can likely be ascribed to negative buoyancy, compensated for by repetitive upward swimming. A large portion of the sprat population occurred in deep water during both day and night, yet the sprat did undertake both synchronous and asynchronous nocturnal vertical migrations. Some individuals swam all the way to the surface, apparently to refill their swimbladder, before immediately diving. Gadoid predators seemed to avoid waters with oxygen contents below similar to 15 to 20% O-2 saturation so that few predators occurred in the deep, low-oxygen sprat habitat, However, gadoids were foraging on the vertically migrating sprat at night. We hypothesize that overwintering sprat may take advantage of low oxygen waters due to higher tolerance for hypoxia than their predators.