Dynamic hypoxic zones in Lake Erie compress fish habitat, altering vulnerability to fishing gears

Abstract

Seasonal degradation of aquatic habitats from hypoxia occurs in numerous freshwater and coastal marine systems and can result in direct mortality or displacement of fish. Yet, fishery landings from these systems are frequently unresponsive to changes in the severity and extent of hypoxia, and population-scale effects have been difficult to measure except in extreme hypoxic conditions with hypoxia-sensitive species. We investigated fine-scale temporal and spatial variability in dissolved oxygen in Lake Erie as it related to fish distribution and catch efficiencies of both active (bottom trawls) and passive (trap nets) fishing gears. Temperature and dissolved oxygen loggers placed near the edge of the hypolimnion exhibited much higher than expected variability. Hypoxic episodes of variable durations were frequently punctuated by periods of normoxia, consistent with high-frequency internal waves. High-resolution interpolations of water quality and hydroacoustic surveys suggest that fish habitat is compressed during hypoxic episodes, resulting in higher fish densities near the edges of hypoxia. At fixed locations with passive commercial fishing gear, catches with the highest values occurred when bottom waters were hypoxic for intermediate proportions of time. Proximity to hypoxia explained significant variation in bottom trawl catches, with higher catch rates near the edge of hypoxia. These results emphasize how hypoxia may elevate catch rates in various types of fishing gears, leading to a lack of association between indices of hypoxia and fishery landings. Increased catch rates of fish at the edges of hypoxia have important implications for stock assessment models that assume catchability is spatially homogeneous.

Coastal Upwelling Influences Hypoxia Spatial Patterns and Nearshore Dynamics in Lake Erie

“…Our observations and simulations documented similar short‐term upwelling events driven by northeast winds and Ekman transport on the Ohio coast. Kraus et al () monitored DO at 14‐ to 19‐m depth along the Ohio coast and found greater than expected variability at the shallower stations, which they attributed to advection of hypolimnetic water by internal waves. In addition to the short‐term events, we documented a persistent downwelling along the Ohio coast that occurred in July of most years, associated with frequent southwest winds.…”

“…Rapid changes in bottom DO occur in coastal areas, caused by upwelling or internal waves that result in shoreward advection of hypoxic bottom water. These dynamics can alter fish distribution, cause fish kills, or cause rapid changes in water quality at drinking water plant intakes (Kraus et al, ; Rao et al, ; Ruberg et al, ).…”

Resilience indicators support valuation of estuarine ecosystem restoration under climate change

“…, Craig , Campbell and Rice , Kraus et al. ). On the other hand, hypoxia can induce high concentrations of prey, resulting in efficient commercial fishing (Craig ) and efficient feeding opportunities for piscivores (fish that eat other fish), which include many sportfish (Pihl et al.…”

“…This tactic risks loss of persistence resilience in stressed and changing environments by ignoring habitats that may respond differently to stress (Kraus et al. ) or by concentrating individuals in protected habitats, thus increasing their risk to catastrophic events (McGilliard et al. ).…”

“…Survey sampling can support metrics and define trends in incidence of occurrence and diversity of habitat use by fishes in nursery habitats and indicate how fishes respond to habitat‐limiting conditions such as hypoxia (Campbell and Rice , Kraus et al. ).…”