The influence of midwater hypoxia on nekton vertical migration

Hypoxia affects pelagic nekton, fish and large zooplankton, distributions in marine and fresh-water ecosystems. Bottom hypoxia is common, but midwater oxygen minimum layers (OMLs) may also affect nekton that undergo diel vertical migration (DVM). This study examined the response of pelagic nekton to an OML in a temperate fjord (Hood Canal, WA, USA). A 2006 study suggested that the OML created a prey refuge for zooplankton. Using acoustics (38 and 120 kHz), the 2007 night DVM patterns of nekton were quantified before (June, August) and during (September) an OML. All months had similar precrepuscular distributions (>50-m depth) of fish and invertebrates. During the September evening crepuscular period, a zooplankton layer migrated upwards (>1.5 m min(-1)), but the layer's rate of ascent slowed to <0.5 m min(-1) when it reached the lower edge of the OML. The bottom edge of the layer then moved below the OML and remained there for 13 minutes before moving through the OML at >1.0 m min(-1). As in June and August, fish in September followed the upward migration of the zooplankton layer to the surface, crossing through the OML. Our results suggest that the 2007 OML did not affect zooplankton or fish vertical distributions.