Demersal Fish Biomass in Relation to Expansion and Shoaling of the Oxygen Minimum Zone off the U.S. West Coast

 Understanding the relationship between environmental variables and fish distribution and abundance has long been a goal of fisheries biologists. Since 2002, hypoxic conditions have been observed on the continental shelf off the coast of the Pacific Northwest in a region not previously characterized by hypoxic conditions. In addition, major declines in dissolved oxygen have been observed in the oxygen minimum zone (OMZ) within the southern California Current system as well as a shoaling of the OMZ. Despite these recent increases in frequency, duration, and spatial extent of hypoxia and the recognition of hypoxia as a threat to worldwide fish production, little is known about its effects on upper trophic levels. In 2007, the Northwest Fisheries Science Center (NWFSC) initiated studies on the extent of hypoxic conditions on the continental shelf and slope along the west coast and the influence of hypoxia on demersal fishes and invertebrates, including commercially important groundfish. This project was developed as an extension of the NWFSC West Coast Groundfish Bottom Trawl Survey.  In 2009 and 2010, working with oceanographers at Oregon State University, the NWFSC expanded its hypoxia research coast-wide by deploying an oceanographic sensor package for monitoring oxygen on ~1,100 tows of the NMFS bottom trawl survey.  Total catch per unit effort (ln CPUE, kg hectare^-1) and species diversity (number of species, n) were significantly (P<0.05) and positively related to oxygen concentration within hypoxic zones offshore Oregon (2007 – 2009) and southern California (2008). Coast wide the relationship between CPUE and near-bottom oxygen was highly significant (P<0.0001) within hypoxic waters (DO<1.43 ml l^-1) although the relationship with species richness was significant only at depths <100 m.