MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser Vol 436 207 218 2011 doi 10 3354 meps09270 Published August 31 Impact of declining intermediate water oxygen on deepwater fishes in the California Current J Anthony Koslow1 Ralf Goericke1 Ana Lara Lopez1 William Watson2 1 Scripps Institution of Oceanography University of California San Diego La Jolla California 92093 0218 USA NOAA National Marine Fisheries Service Southwest Fisheries Science Center La Jolla California 92037 USA 2 ABSTRACT Although declining oxygen concentration has been reported for the oxygen minimum zones OMZs of the tropical oceans and the North Pacific Ocean consistent with model predictions of the effects of global warming its ecological impacts are poorly understood We report the apparent impact of declining oxygen on midwater fishes within the OMZ of the southern California Current CC Principal component analysis of the California Cooperative Oceanic Fisheries Investigations CalCOFI ichthyoplankton time series from 1951 to 2008 indicates that the dominant temporal pattern principal component 1 PC1 represents the marked decline of the region s mesopelagic fishes during periods of reduced oxygen Of the 27 taxa with loadings 0 5 on PC1 24 were mesopelagic PC1 was strongly correlated with intermediate water oxygen concentrations r 0 75 p 0 05 which were about 20 lower in the past decade and the 1950s than in the period from 1970 to 1995 The abundance of mesopelagic fishes represented by PC1 was reduced on average by 63 between periods of high and low oxygen concentrations We hypothesize that the underlying mechanism is the shoaling of the hypoxic boundary layer during periods of reduced oxygen which renders the mesopelagic fauna more vulnerable to visually orienting predators The mesopelagic fish fauna provides a vital trophodynamic link between the marine plankton and many higher predators The decline of deepwater fish populations has profound implications for commercial fisheries marine food webs and marine conservation climate models predict a 20 to 40 decline in global deepwater oxygen concentrations over the coming century KEY WORDS Oxygen Mesopelagic fishes Oxygen minimum zone Climate change California Current Resale or republication not permitted without written consent of the publisher Global ocean climate models predict that oxygen levels in the deep ocean will decline by approximately 20 to 40 over the coming century This is a result of projected increases in surface warming which leads to enhanced vertical stratification of the upper mixed layer which reduces ventilation of deeper waters Sarmiento et al 1998 Matear Hirst 2003 Shaffer et al 2009 Oxygen levels have already declined since the mid twentieth century in the tropical oxygen minimum zones OMZs of the Indian Pacific and Atlantic oceans and across the North Pacific basin Keeling Garcia 2002 Whitney et al 2007 Bograd et al 2008 Stramma et al 2008 although the trend has not been monotonic in the southern California Current CC McClatchie et al 2010 Northeast Pacific waters are naturally old which has led to the formation of a distinct OMZ at mid depths 200 to 1200 m in the CC Mean oxygen concentrations at 200 to 300 m are 2 0 ml l 1 an approximate upper limit for hypoxia Diaz Rosenberg 2008 and decline to a minimum of 0 5 ml O2 l 1 a level of severe hypoxia at about 500 m see Fig 4 and Discussion Oxygen levels in the OMZ of the CC have declined substantially approximately 20 Email jkoslow ucsd edu Inter Research 2011 www int res com INTRODUCTION 208 Mar Ecol Prog Ser 436 207 218 2011 since 1984 Bograd et al 2008 leading us to examine the impacts on deepwater fishes Although there are reports of the effect of hypoxia and of hypoxic intrusions on various metazoan groups particularly in the benthos Levin 2003 Vaquer Sunyer Duarte 2008 the potential effect of long term declining oxygen levels in the water column anticipated under most climate change scenarios is poorly known The greater motility of pelagic organisms has no doubt contributed to a belief that ecological effects in the water column will be mitigated although habitat compression due to a shallow OMZ can significantly influence the foraging dynamics of pelagic fishes Prince Goodyear 2006 The mesopelagic and continental slope demersal fishes which are most vulnerable to reduced oxygen in the OMZ represent a diverse fauna Of the 30 000 extant fish species more than 10 live in the world s oceans below 200 m depth and most of these are mesopelagic or inhabit the seafloor of the continental slope Cohen 1970 Nelson 2006 In the ichthyoplankton data set from the California Cooperative Oceanic Fisheries Investigations CalCOFI program mesopelagic fishes are the most diverse group representing 38 of 586 ichthyoplankton taxa recorded Moser Watson 2006 Their abundance is second only to the coastal pelagic species and they comprise 20 of the total larvae Globally the biomass of mesopelagic fishes is estimated to be on the order of 109 t cf annual global marine fish landings of about 108 t Gj saeter Kawaguchi 1980 As a major prey of various commercially exploited fishes Bertrand et al 2002 Potier et al 2007 marine mammals Pauly et al 1998 squids Kozlov 1995 and seabirds Thompson et al 1998 mesopelagic fishes represent a vital link in oceanic food webs worldwide The fishes inhabiting the OMZ have evolved various adaptations to living in a low oxygen environment including reduced metabolism and enhanced gas exchange mechanisms but they rely on aerobic metabolism and the physiological burden of reduced oxygen levels is unclear Childress Seibel 1998 Of potentially greater importance given the ability of fish to avoid hypoxic water may be the contraction of their habitat as OMZs expand This fauna is thus at potential risk owing to reduced oxygen levels and expanding hypoxic regions globally Paleo oceanographic episodes of expanded OMZs particularly in the late Cretaceous Eocene and Miocene are believed to have contributed significantly to present patterns of biodiversity and biogeography of fishes at mid ocean depths White 1987 The ichthyoplankton data set from the CalCOFI program allows us to investigate the potential effects of climate variability and climate change on the diverse fish communities both commercially exploited and unexploited of the southern CC The time series which extends back to 1951 provides data on the quantitative abundance no fish m 2 of several hundred taxa from a range of habitats