Forage fish distributions in a seasonally hypoxic fjord

Hypoxia (≤ 2 mg L^-1 dissolved oxygen) influences distributions of forage fish and invertebrates in many aquatic ecosystems.  Bottom hypoxia is common, but midwater oxygen minimum layers (OMLs) can occur in freshwater lakes, fjords, and the ocean.  Acoustic and net surveys were conducted from 2006-2008 in the temperate fjord, Hood Canal, WA to examine effects of seasonal oxygen concentrations on fish species composition and density distributions.  Reflected acoustic energy (i.e. backscatter) from fish, was higher in July than in September in all years.  The highest September backscatter occurred during 2006, the year with the lowest dissolved oxygen.  Vertical distributions and migratory responses to OMLs differed between years.  In 2006, invertebrates migrated into waters <20 m at night, but the upper limit of fish vertical distribution stopped at the OML base (35 m).  In 2007, the OML did not affect zooplankton or fish vertical distributions but did slow the ascent of zooplankton into the OML.  Regression analyses examining backscatter and physical factors (bottom depth, tide, distance from shore, location), found that no factor was consistently related to backscatter, but that high backscatter was often associated with shallow bottom depths.  Forage fish may be valuable indicators of ecosystem health, but knowledge of species residency, distributions, and response to hypoxia is needed before appropriate indices can be derived.