Nekton distribution and midwater hypoxia: A seasonal, diel prey refuge?

Hypoxia affects the distribution of pelagic nekton (i.e., fish and large invertebrates) in both marine and freshwater systems. Bottom hypoxia is common, but midwater oxygen minimum layers (OMLs) also develop in marine offshore regions, fjords, and freshwater lakes. Studies of nekton responses to OML in marine ecosystems have primarily occurred in deep, offshore regions with thick, persistent OMLs. Our study examined the response of pelagic nekton to an OML in a shallow temperate fjord, Hood Canal, WA, U.S.A. Using acoustics, we quantified vertical distribution of nekton at two sites (Hoodsport and Duckabush) before (July) and after (September) OML development. Both Hoodsport and Duckabush had strong OML between 10 and 35 m in September, with lower (minimum 0.63 mg L −1) oxygen levels at Hoodsport compared to Duckabush (1.58 mg L −1). The OML did not affect daytime distribution of fish or invertebrates, with both occupying depths >60 m. At night in July, with no OML, invertebrates migrated into waters