| Summary: | Atlantic saithe (Pollachius virens) and Norwegian spring-spawning herring (Clupea harengus) represent two of the most abundant and ecologically important fish species in the Northeast Atlantic. The two species co-occur in the Vesterålen/Lofoten area during the wintering of herring, but the temporal and spatial overlap between species in different scales as well as their trophic interactions is otherwise largely unknown. Saithe is a potentially important predator on herring. In the present study we found the commercial catches of saithe off western Norway to be closely related to the migration of herring into the coastal spawning grounds, indicating that saithe follow the high concentrations of migrating herring. Acoustic surveys within a defined study area demonstrated that the species overlapped spatially and temporarily. Degree of overlap varied both diurnally and with the maturity status of the herring (pre-spawning, spawning or spent). Concurrent catches of saithe and herring from trawling and gillnets supported an overlap also on the meso-scale. Observations on more fine-scaled behaviour obtained from an underwater rig with highsensity cameras positioned in high-density areas of herring demonstrated that the distribution and behaviour of herring and saithe was closely linked to the diel light cycle and spawning of herring. When looking at the actual outcome of the interactions between the species on a kind of temporally, integrated spatial microscale, it was observed that stomachs of large saithe (>50 cm) on average contained 2–3 herring, while the stomachs of smaller saithe were filled with herring eggs. This suggests a considerable predation pressure by saithe on different life stages of herring.
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