Bits & Pieces: Erie Smallies and Aquatic Oxygen

From the Archives: Aquatic Oxygen

Sunlight drives photosynthesis, and oxygen is a product of that process. So is more oxygen found around and under floating vegetation than in nearby open water?

Researchers at the University of Washington measured dissolved oxygen concentrations under and near thick mats of vegetation in two ponds.* In Bull Lake, mats of mixed coontail, milfoil, and Elodea produced high concentrations of dissolved oxy­gen at the surface (15 to 21 parts per milllion) during the day. The surface of nearby open water had 11 parts per million (ppm). Oxygen levels at the bottom were reversed. Open water contained 5 ppm, while mats held 1 to 3 ppm.

At Keevies Lake, watershield (a plant with leaves like small lily pads) covered most of the surface. The sur­face and bottom often held less than 1 ppm dissolved oxygen.

Evidently, much of the oxygen produced by floating leaves escapes into the air rather than into the water. Moreover, thick mats of floating weeds block light penetration to the bottom, so little photosynthesis and oxygen production results.

Vegetation mats also limit wind-generated currents, preventing warmer surface water from mixing down­ward. Below the surface, water can become oxygen poor and stagnant. Mini thermoclines can occur under large, heavily matted weedbeds. The upper layer of water holds ample oxygen and fish, while the layer near bottom may be barren.

Currents move water through weedbeds, mixing oxygen levels and reducing layering. Water under weedmats becomes similar to open areas in temperature and oxygen content. But stagnant bottom water under matted vegetation often holds few fish because water lilies and other floating plants prevent mixing and encourage stagnation without add­ing oxygen to surface water.

In-Fisherman

*Frodge, J. D., G. L. Thomas, and G. B. Pauley. 1991. Sediment phosphorus loading beneath dense canopies of aquatic macro­phytes. Lake Reserv. Mgmt. 7(1):61-71.

Research Results: Lake Erie’s Nomadic Smallmouths

Stream-dwelling smallmouth bass are noted for making long-distance movements, particularly in northern climates to seek winter refuges; but smallmouths in lakes are typically regarded as often locally mobile but with little long-distance movement. Past movement studies, which were based on tagging and recapture, found most fish were recaptured within several miles of their original capture and tagging site.

Development and application of biotelemetry to fishery studies—surgically implanting small radio or acoustic transmitters into a fish and monitoring the fish’s position by radio or hydroacoustic receivers—allows continual, remote detection of a fish’s location and has greatly expanded fishery scientists’ knowledge of fish movement. Biotelemetric tracking of smallmouth bass in lakes Michigan and Ontario found smallmouths moved up to 90 miles, but 87 to 97 percent of the tracked smallmouths moved much shorter distances.

In a recently published study, Ohio DNR fishery scientists Zak Slagle and Matt Faust implanted acoustic transmitters into 23 tournament-captured smallmouth bass (18 inches average length) returned to a Sandusky Bay, Ohio, weigh-in site on the southern side of the western basin of Lake Erie.* Acoustic-tagged fish were detected by an array of fixed acoustic receivers deployed throughout Lake Erie. Eight of the 19 tagged fish were detected outside of Sandusky Bay in the first year after release, and six during the second year after release. These fish moved an average of 68 miles per year and an average of 119 miles over the two-year tracking period.

One individual swam 314 miles during the two-year tracking period. Forty-two percent of the fish moved at least 19 miles beyond Sandusky Bay. All these distances are minimal estimates of distances traveled because they’re calculated as straight-line distances between the receivers they were detected at. Six of the eight fish crossed jurisdictional boundaries, five into Ontario waters and one into Michigan waters; three of the five fish that swam to Ontario waters returned to Ohio waters. The fates and locations of the fish after two years cannot be known.

Some conditions of the conduct of the research may have contributed to the observed high movement. For example, the angler-caught fish, which have been found in other studies to be more aggressive and have higher metabolism, may be more prone to making longer movements. Further, these fish were displaced from what may have been a home range (15-mile average displacement distance after capture), possibly stimulating greater movement searching for a new home area. But it is also possible that the lake-wide array of receivers may have detected movement of long-distance swimmers overlooked in other studies.

To bolster their findings and remove potential shortcomings of this initial study, Slagle and Faust will be implanting acoustic tags into 180 additional smallmouths collected by electrofishing and angling and released at the sites of capture, then monitoring movement for two more years. While the researchers await the results of the greatly expanded study, the fish monitored so far demonstrate a far greater mobility than expected from previous studies. The high mobility is also compatible with the low genetic differences among smallmouths throughout the large lake, which would be a consequence of lake-wide interbreeding.

Dr. Hal Schramm

*Slagle, Z. J., and M. D. Faust. 2023. Are smallmouth bass more mobile in large lakes than once thought?  J. Great Lakes Res. 49:554-560

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