Moving on up: Vertical distribution shifts in rocky reef fish species during climate‐driven decline in dissolved oxygen from 1995 to 2009

Anthropogenic climate change has resulted in warming temperatures and reduced oxygen concentrations in the global oceans. Much remains unknown on the impacts of reduced oxygen concentrations on the biology and distribution of marine fishes. In the Southern California Channel Islands, visual fish surveys were conducted frequently in a manned submersible at three rocky reefs between 1995 and 2009. This area is characterized by a steep bathymetric gradient, with the surveyed sites Anacapa Passage, Footprint and Piggy Bank corresponding to depths near 50, 150 and 300 m. Poisson models were developed for each fish species observed consistently in this network of rocky reefs to determine the impact of depth and year on fish peak distribution. The interaction of depth and year was significant in 23 fish types, with 19 of the modelled peak distributions shifting to a shallower depth over the surveyed time period. Across the 23 fish types, the peak distribution shoaled at an average rate of 8.7 m of vertical depth per decade. Many of the species included in the study, including California sheephead, copper rockfish and blue rockfish, are targeted by commercial and recreational fisheries. CalCOFI hydrographic samples are used to demonstrate significant declines in dissolved oxygen at stations near the survey sites which are forced by a combination of natural multidecadal oscillations and anthropogenic climate change. This study demonstrates in situ fish depth distribution shifts over a 15‐year period concurrent with oxygen decline. Climate‐driven distribution shifts in response to deoxygenation have important implications for fisheries management, including habitat reduction, habitat compression, novel trophic dynamics and reduced body condition. Continued efforts to predict the formation and severity of hypoxic zones and their impact on fisheries dynamics will be essential to guiding effective placement of protected areas and fisheries regulations.