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Aim Shoaling of large oxygen minimum zones (OMZs) that form along eastern margins of the world's oceans can reduce habitat availability for some pelagic fishes. Our aim was to test the hypothesis that habitat compression caused by shoaling of the Pacific OMZ in tropical regions creates a boundary to the southern distribution of shortfin mako sharks (Isurus oxyrinchus) in the Eastern North Pacific Ocean. Location Eastern North Pacific and Western North Atlantic oceans. Methods We compared environmental conditions between areas used by satellite‐tagged mako sharks in the Eastern North Pacific, encompassing the world's largest OMZ, to those used in the Western North Atlantic where no OMZ is present. In the Pacific we quantified the effects of temperature and dissolved oxygen (DO) on depth use and tested if sharks spent less time in areas with strong habitat compression over the OMZ than expected by chance. Results The southern distribution of sharks in the Pacific corresponded with the apex of OMZ shoaling in the North Equatorial Current. Sharks in the Atlantic occupied areas with warm surface temperatures (≥ 26°C) more often than the Pacific, and waters with these temperatures in the Atlantic had greater DO at depth. Sharks in the Pacific reduced time near the surface in warm temperatures and consistently avoided depths with low DO and spent less time in areas with strong habitat compression than expected by chance. Main Conclusions The combination of warm surface temperatures and shoaling of the OMZ creates a soft boundary to mako shark movements in the Eastern North Pacific Ocean. The expected expansion of OMZs due to climate change could have considerable impact on future distribution of mako sharks and other pelagic fish. As such, development of species distribution models to predict the effects of climate change on pelagic fish distributions should incorporate oxygen availability.

Recent evidence suggests predators may change colonization rates of prey in nearby predator-free patches as an example of context-dependent habitat selection. Such remote predator effects can be positive when colonizers are redirected to nearby patches (habitat compression), or negative when nearby patches are avoided (risk contagion). However, it is unknown to what extent such responses are predator-and prey-specific and change with increasing distance from predator patches. We evaluated how cues of fish, backswimmers and dragonfly larvae affect habitat selection in replicated pond landscapes with predator-free patches located at increasing distances from a predator patch. We found evidence for risk contagion and compression, but spatial colonization patterns were both predator-and prey-specific. The mosquito Culex pipiens and water beetle Hydraena testacea avoided patches next to patches with dragonfly larvae (i.e. risk contagion). Predator-free patches next to patches with backswimmers were avoided only by mosquitoes. Mosquitoes preferentially colonized patches at some distance from a fish or backswimmer patch (i.e. habitat compression). Colonization patterns of beetles also suggested habitat compression, although reward contagion could not be fully excluded as an alternative explanation. Water beetles preferred the most isolated patches regardless of whether predators were present in the landscape, showing that patch position in a landscape alone affects colonization. We conclude that habitat selection can be a complex product of patch isolation and the combined effects of different local and remote cues complicate current attempts to predict the distribution of mobile organisms in landscapes.

Oxygen minimum zones in the open ocean are predicted to significantly increase in volume over the coming decades as a result of anthropogenic climatic warming. The resulting reduction in dissolved oxygen (DO) in the pelagic realm is likely to have detrimental impacts on water-breathing organisms, particularly those with higher metabolic rates, such as billfish, tunas, and sharks. However, little is known about how free-living fish respond to low DO environments, and therefore, the effect increasing OMZs will have cannot be predicted reliably. Here, we compare the responses of two active predators (bigeye tuna Thunnus obesus and yellowfin tuna Thunnus albacares ) to DO at depth throughout the eastern Pacific Ocean. Using time-series data from 267 tagged tunas (59,910 days) and 3D maps of modelled DO, we find that yellowfin tuna respond to low DO at depth by spending more time in shallower, more oxygenated waters. By contrast, bigeye tuna, which forage at deeper depths well below the thermocline, show fewer changes in their use of the water column. However, we find that bigeye tuna increased the frequency of brief upward vertical excursions they performed by four times when DO at depth was lower, but with no concomitant significant difference in temperature, suggesting that this behaviour is driven in part by the need to re-oxygenate following time spent in hypoxic waters. These findings suggest that increasing OMZs will impact the behaviour of these commercially important species, and it is therefore likely that other water-breathing predators with higher metabolic rates will face similar pressures. A more comprehensive understanding of the effect of shoaling OMZs on pelagic fish vertical habitat use, which may increase their vulnerability to surface fisheries, will be important to obtain if these effects are to be mitigated by future management actions.

River discharge and seasonal, eutrophication-driven hypoxia within coastal waters of the northern Gulf of Mexico create a wide range of environmental conditions for fishes. Environmental conditions are most dynamic on the Louisiana shelf, a region in which oil and gas platforms (hereafter platforms) are abundant and serve as artificial reefs. Platforms provide a unique, vertically oriented substrate for fouling organisms and nekton throughout the water column and often span substantial gradients of salinity, temperature, dissolved oxygen (DO), and water clarity. During the summers of 2013 and 2014, we used paired video and hydrographic samples taken at 150 small platforms sited in nearshore waters (3.7−18.0 m water depth) to document the responses of platform-associated fishes to coastal water quality. Responses of the 26 analyzed fish species included substantial changes in habitat selection coupled with vertical habitat compression. The vertical extent of the water column suitable for fishes was compressed due to avoidance of hypoxic bottom water (DO <50% saturation) and apparent avoidance of eutrophic surface strata with supersaturated oxygen conditions (140.7 ± 7.0% saturation [95% CI]). Generalized linear mixed models, habitat suitability indices, and factor analysis suggested that fish habitat-selection patterns varied significantly in the presence of hypoxia by up to 10 salinity units, 10°C, 30% DO saturation, 6 m water depth, and 4.5 m Secchi depth. The differences in intraspecific distributions occurring in the presence and absence of hypoxia were often greater than interspecies separations within comparisons. These findings demonstrate an important indirect effect of eutrophication- driven hypoxia on fishes.

Climate change has induced ocean deoxygenation and exacerbated eutrophication-driven hypoxia in recent decades, affecting the physiology, behaviour and ecology of marine organisms. The high oxygen demand of visual tissues and the known inhibitory effects of hypoxia on human vision raise the questions if and how ocean deoxygenation alters vision in marine organisms. This is particularly important given the rapid loss of oxygen and strong vertical gradients in oxygen concentration in many areas of the ocean. This review evaluates the potential effects of low oxygen (hypoxia) on visual function in marine animals and their implications for marine biota under current and future ocean deoxygenation based on evidence from terrestrial and a few marine organisms. Evolutionary history shows radiation of eye designs during a period of increasing ocean oxygenation. Physiological effects of hypoxia on photoreceptor function and light sensitivity, in combination with morphological changes that may occur throughout ontogeny, have the potential to alter visual behaviour and, subsequently, the ecology of marine organisms, particularly for fish, cephalopods and arthropods with ‘fast’ vision. Visual responses to hypoxia, including greater light requirements, offer an alternative hypothesis for observed habitat compression and shoaling vertical distributions in visual marine species subject to ocean deoxygenation, which merits further investigation. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.

Community scientists have illustrated rapid declines of several aphidophagous lady beetle (Coccinellidae) species. These declines coincide with the establishment of alien coccinellids. We established the Buckeye Lady Beetle Blitz program to measure the seasonal occupancy of coccinellids within gardens across a wide range of landscape contexts. Following the Habitat Compression Hypothesis, we predicted that gardens within agricultural landscapes would be alien‐dominated, whereas captures of natives would be higher within landscapes encompassing a high concentration of natural habitat. Within the state of Ohio, USA, community scientists collected lady beetles for a 7‐day period across 4 years in June and August using yellow sticky card traps. All identifications were verified by professional scientists and beetles were classified by three traits: status (alien or native), mean body length, and primary diet. We compared the relative abundance and diversity of coccinellids seasonally and determined if the distribution of beetles by size, status, and diet was related to landscape features. Alien species dominated the aphidophagous fauna. Native aphidophagous coccinellid abundance was positively correlated with forest habitat while alien species were more common when gardens were embedded within agricultural landscapes. Urbanization was negatively associated with both aphidophagous alien and native coccinellids. Synthesis and Applications: Our census of native coccinellid species within residential gardens—a widespread and understudied habitat—was enabled by volunteers. These data will serve as an important baseline to track future changes within coccinellid communities within this region. We found that native coccinellid species richness and native aphidophagous coccinellid abundance in gardens were positively associated with forest habitat at a landscape scale of 2 km. However, our understanding of when and why (overwintering, summer foraging, or both) forest habitats are important remains unclear. Our findings highlight the need to understand how declining aphidophagous native species utilize forest habitats as a conservation priority. We established the Buckeye Lady Beetle Blitz community program to track native lady beetle communities in residential gardens. We found a strong positive relationship between the abundance of forested habitat in the surrounding landscape and native aphidophagous coccinellid abundance.

Background Globally, temperate lakes are experiencing increases in surface water temperatures, extended periods of summer stratification, and decreases of both surface and deep water dissolved oxygen (DO). The distribution of fish is influenced by a variety of factors, but water temperature and dissolved oxygen are known to be particularly constraining such that with climate change, fish will likely feel the “squeeze” from above and below. Methods This study used acoustic telemetry to explore the effects of both thermal stratification and the deoxygenation of the hypolimnion on walleye ( Sander vitreus ) movements in a coastal embayment in Lake Ontario. Using historical water quality monitoring data, we documented seasonal and annual fluctuations in availability of both ‘suitable’ (all temperatures, DO > 3 mg/L) and ‘optimum’ (temperatures 18–23 °C, DO > 5mg/L) abiotic habitat for walleye and determined how these changes influenced walleye movements over a three-year period. Results Hypoxia (< 3 mg/L DO) was present in Hamilton Harbour every summer that data were available (32 of the 42 years between 1976 and 2018), with a maximum of 68.4% of the harbour volume in 1990. We found that thermal stratification and a hypoxic hypolimnion greatly reduced the volume of suitable habitat during our telemetry study. The reduction of suitable habitat significantly reduced walleye movement distances, however as the summer progressed, this remaining suitable habitat warmed into their thermal optimum range which was found to increase walleye movement distances. Despite the seemingly poor conditions, tagged walleye remained in the harbour for most of the year, and were the fastest growing individuals compared to other sampled coastal subpopulations in Lake Ontario. Conclusions Although we documented a reduction in the quantity of non-hypoxic habitat available to walleye, the water temperature of the remaining habitat increased throughout the summer into the physiologically optimum range for walleye and increased in metabolic quality. Many abiotic factors influence how, where, and what habitat fish choose to use, and this study reveals the importance of considering both habitat quality (temperature and dissolved oxygen) and quantity when evaluating fish habitat use and behaviour.

Hypoxia is thought to have negative effects on fish in coastal ecosystems, but quantifying those effects can be difficult. Direct exposure to hypoxia can reduce fish growth or survival, but fish can also rapidly detect and avoid low dissolved oxygen levels. However, avoidance behavior may result in indirect effects that reduce fish growth. For example, when hypoxic conditions expand, fish densities may increase in nearshore oxygenated refuges, potentially causing density-dependent reductions in growth. We evaluated this hypothesis for juvenile demersal fish species (primarily spot Leiostomus xanthurus and Atlantic croaker Micropogonias undulatus). By monitoring water quality and fish density across the Neuse River Estuary, North Carolina, USA, under varying water quality conditions during summer 2007, we showed that fish effectively avoided hypoxia despite rapidly changing conditions (minutes to hours), moving away from incursions of hypoxic water and then rapidly redistributing into affected areas after these events passed. Fish densities in nearshore oxygenated refuges increased nearly 2-fold when habitat was compressed by hypoxic waters. Spot in compressed refuges also had significantly less food in their stomachs during June. Based on published estimates of density-dependent spot growth, we estimated that average spot growth rate was reduced 17% during habitat compression events, which occurred 21.5% of the time, translating into an average reduction in growth rate of 4% over the summer. This likely is a conservative estimate of indirect hypoxia effects on growth, as hypoxia was relatively mild in 2007, and density dependence is only one indirect mechanism by which hypoxia may potentially reduce growth.

Oil and gas platforms (platforms) provide high-relief habitat in the northern Gulf of Mexico's hypoxic zone that are important to associated fishes. Hypoxia develops near the bottom and reef-associated fishes utilize vertical structure in the well-oxygenated waters overlaying hypoxia. A video array was used to profile the water column and to estimate abundances and depth distributions of fishes before, during, and after summer hypoxia at platforms experiencing intense (seaward) and mild hypoxia (shoal). Gray snapper abundance increased at shoal platforms (10× greater after vs. before the hypoxia season), while abundance remained stable at seaward platforms. However, there was no significant relationship between gray snapper abundance and oxygen concentrations. Sheepshead, Atlantic spadefish, blue runner, and Atlantic bumper abundances varied throughout the summer, but there was no significant effect of hypoxia. Occupation of bottom waters by fishes was consistent throughout the study period at shoal platforms, but fishes were rarely observed in the bottom 3 m and congregated in the water immediately above the hypoxic layer when hypoxia was present at seaward platforms. Nevertheless, patterns of fish abundances were not driven by the presence or absence of hypoxia. The vertical dimension of platforms is a unique and key aspect of their ecological value, especially in the hypoxic zone, and should be considered for artificial reef management.

Ocean deoxygenation, warming, and acidification resulting from global change and increasing nutrient inputs are major threats to marine ecosystems. Despite this, spatial and temporal patterns of oxygen availability and their impacts on marine life are understudied compared to warming and acidification, particularly in coastal tropical ecosystems. We measured the abundance of major groups of zooplankton in the context of five covarying environmental parameters [temperature, salinity, dissolved oxygen (DO), dissolved organic matter (DOM), and chlorophyll concentration] in a tropical estuary for one year. Partial least squares demonstrated that environmental conditions explained 20% of the variation in the community and found that temperature, salinity, DO, and DOM were most important (Variable Importance in Projection [VIP] > 0.8). A generalized linear model identified depth, DO, salinity, and chlorophyll as significant main effects, and temperature and DOM were also significant via two-way interactions (p < 0.05). When examined separately, the abundance of each zooplankton group was explained by a slightly different combination of environmental factors, but in all cases DO had large, significant effects, and in most cases DOM or its interactions were also significant. These results demonstrate that the seasonal cycle of hypoxia in this system significantly impacts the abundance of major zooplankton groups and likely also recruitment of benthic fauna through impacts on meroplankton and benthic-pelagic food webs.