Effects of spatial variability on the exposure of fish to hypoxia: a modeling analysis for the Gulf of Mexico

Abstract

Abstract. The hypoxic zone in the northern Gulf of Mexico varies spatially (area, location) and temporally (onset, duration) on multiple scales. Exposure of fish to hypoxic dissolved oxygen (DO) concentrations (< 2 mg L−1) is often lethal and avoided, while exposure to 2 to 4 mg L−1 occurs readily and often causes the sublethal effects of decreased growth and fecundity for individuals of many species. We simulated the movement of individual fish within a high-resolution 3-D coupled hydrodynamic water quality model (FVCOM-WASP) configured for the northern Gulf of Mexico to examine how spatial variability in DO concentrations would affect fish exposure to hypoxic and sublethal DO concentrations. Eight static snapshots (spatial maps) of DO were selected from a 10 d FVCOM-WASP simulation that showed a range of spatial variation (degree of clumpiness) in sublethal DO for when total sublethal area was moderate (four maps) and for when total sublethal area was high (four maps). An additional case of allowing DO to vary in time (dynamic DO) was also included. All simulations were for 10 d and were performed for 2-D (bottom layer only) and 3-D (allows for vertical movement of fish) sets of maps. Fish movement was simulated every 15 min with each individual switching among three algorithms: tactical avoidance when exposure to hypoxic DO was imminent, strategic avoidance when exposure had occurred in the recent past, and default (independent of DO) when avoidance was not invoked. Cumulative exposure of individuals to hypoxia was higher under the high sublethal area snapshots compared to the moderate sublethal area snapshots but spatial variability in sublethal concentrations had little effect on hypoxia exposure. In contrast, relatively high exposures to sublethal DO concentrations occurred in all simulations. Spatial variability in sublethal DO had opposite effects on sublethal exposure between moderate and high sublethal area maps: the percentage of fish exposed to 2–3 mg L−1 decreased with increasing variability for high sublethal area but increased for moderate sublethal area. There was also a wide range of exposures among individuals within each simulation. These results suggest that averaging DO concentrations over spatial cells and time steps can result in underestimation of sublethal effects. Our methods and results can inform how movement is simulated in larger models that are critical for assessing how management actions to reduce nutrient loadings will affect fish populations.

Effects of habitat usage on hypoxia avoidance behavior and exposure in reef-dependent marine coastal species

“…While hypoxic conditions are a major consideration, sublethal DO concentrations are also important. Many laboratory studies document that DO concentrations between 2 and 5 often result in less avoidance (i.e., significant exposure), and individuals show reduced growth and fecundity (Uphoff et al, 2011;LaBone et al, 2021). Our study showed that the presence of ARs would likely increase the exposure of the two rockfish species to 3 mg/ L. Therefore, both hypoxia and higher (moderate) DO concentrations should be part of the design and assessment of performance for ARs.…”

“…In large-scale sea areas, hypoxic conditions may change gradually and can be distributed in patches (LaBone et al, 2021). A relatively stationary and stable low-oxygen location allows most organisms, especially fish, to avoid this area.…”

“…Low DO can affect the prey and predators of the species of interest, which then, even without direct exposure, affect growth and mortality (David and David, 2000). Major indirect effects occur from the avoidance movement in response to hypoxia exhibited by many mobile species to reduce their exposure to low DO (Craig, 2012;Lagos et al, 2015;Jorissen and Nugues, 2021;LaBone et al, 2021). Avoidance results in indirect effects because it is not the exposure to DO that causes physiological effects but rather the consequences of individuals being forced to move to less preferred locations and habitats where mortality, growth, and reproduction would be different than if they were able to inhabit the avoided areas (Eby and Crowder, 2002;LaBone et al, 2021).…”

Metabolic prioritization of fish in hypoxic waters: an integrative modeling approach

“…Such simplifying assumptions are common in ecosystem models. Examples include ignoring changes in all but one or two activities in response to hypoxia and the aggregation of several activities into a higher order process such as growth (Karim et al, 2003;de Mutsert et al, 2016;LaBone et al, 2021). This is a valid approach which provides valuable insight into fish population and community response to hypoxia.…”