Internal seiches as drivers of fish depth use in lakes

Abstract

Inland temperate lakes undergo various physical processes, such as thermal stratification, that dictate the spatial availability of suitable temperature and dissolved oxygen conditions. Here, we use intensive limnological monitoring and acoustic telemetry transmitters implanted in wild fish to document the magnitude and frequency of thermocline deflection events (i.e., wind driven internal seiches that lead to upwelling of hypoxic hypolimnetic water) and their influence on freshwater fish depth use in a coastal embayment of Lake Ontario. The embayment experienced around 100 internal seiche events during a 3-month period and tracking of walleye (Stizostedion vitreus) vertical positions in the water column showed clear trends of avoidance of low dissolved oxygen. Quantile regression showed a significant correlation between walleye vertical position and the depth of the 3, 4, and 5 mg L À1 oxyclines, with the 3 mg L À1 oxycline having the largest effect. Upwellings of the hypoxic hypolimnion forced walleye to use the water column above these fluctuating oxyclines (5 th percentile, p < 0.001), with 94.2% of detections occurring at depths above the 3 mg L À1 oxycline. Understanding how fish respond to upwelling events (both temperature and oxygen) is important for fisheries assessment, management, and habitat restoration planning as there is clear avoidance of suboptimal oxygen conditions and sampling in the overcrowded fringes of these low-oxygen zones could artificially inflate population estimates.

The Ecological Consequences of Hypoxia in a Temperate, Freshwater Ecosystem.

“…A digital elevation model (DEM) was obtained from and used to calculate a hypsographic curve to determine the area (and subsequently the volume) for each 1 m depth of the Harbour, based on an average surface value above sea level of 75 m (Table C-2 (Brooks et al, 2022;Flood et al, 2021). The thermocline at the center of the system near Center Station is the most stable (refer to Figure 4 of Flood et al, 2021) and represents, on average, the depth of the thermocline throughout the Harbour.…”

“…However, the system continues to experience low DO issues (Bowlby et al, 2016;Gertzen, 2016). The littoral areas of Hamilton Harbour have been documented to experience wind-driven upwellings of low DO and cooler waters during thermal stratification periods (Brooks et al, 2022;Flood et al, 2021) and, due to the dynamic mosaic of temperature and DO conditions, it provides a suitable model system to better understand abiotic habitat selection.…”

“…Many of these listed studies test for relationships with (relatively) static habitat types such as percent vegetation cover, distance from water, distance to land, percent coral cover, and mesohabitat type etc. The abiotic habitat in the coastal areas of Hamilton Harbour is very dynamic as water temperature at a single location can swing from ~16 °C to ~22 °C over several hours and DO from 0 to 15 mg/L over a similar period (Brooks et al, 2022;Flood et al, 2021). These swings in thermos-oxic conditions are due to frequent wind driven upwellings of cool hypoxic deep waters into the littoral zones (Brooks et al, 2022;Flood et al, 2021).…”

“…The abiotic habitat in the coastal areas of Hamilton Harbour is very dynamic as water temperature at a single location can swing from ~16 °C to ~22 °C over several hours and DO from 0 to 15 mg/L over a similar period (Brooks et al, 2022;Flood et al, 2021). These swings in thermos-oxic conditions are due to frequent wind driven upwellings of cool hypoxic deep waters into the littoral zones (Brooks et al, 2022;Flood et al, 2021). To effectively sample this mosaic of habitat to provide an unbiased representation of 'unused' abiotic habitat variables, we sampled every meter of the water column above and below each fish detection at that same timestamp, and then the same positions in the water column for the same time for every day of the study.…”