Species-specific responses of demersal fishes to near-bottom oxygen levels within the California Current large marine ecosystem
Long-term environmental sampling provided information on catch and near-bottom oxygen levels across a range of depths and conditions from the upper to the lower limit of the oxygen minimum zone and shoreward across the continental shelf of the US west coast (US-Canada to US-Mexico). During 2008 to 2014, near-bottom dissolved oxygen (DO) concentrations ranged from 0.02 to 5.5 ml l-1 with 63.2% of sites experiencing hypoxia (DO < 1.43 ml l-1). The relationship between catch per unit effort (CPUE) and DO was estimated for 34 demersal fish species in 5 subgroups by life history category (roundfishes, flatfishes, shelf rockfishes, slope rockfishes and thornyheads) using generalized additive models (GAMs). Models included terms for position, time, near-bottom environmental measurements (salinity, temperature, oxygen) and bottom depth. Significant positive relationships between CPUE and DO occurred for 19 of 34 groundfish species within hypoxic bottom waters. Community effects (total CPUE and species richness for demersal fishes) also exhibited significant and positive relationships with low near-bottom oxygen levels. GAM analysis revealed an apparent threshold effect at lower oxygen levels, where small changes in oxygen produced large changes in catch for several species, as well as total catch and species richness. An additional 7 species displayed negative trends. Based on Akaike’s information criterion values, near-bottom oxygen played a major role in the distribution of flatfishes, roundfishes and thornyheads. By examining similarities and differences in the response of various subgroups of commercially important groundfish species to low DO levels, we uncovered ecological inferences of potential value to future ecosystem-based management.