Freshwater fish as aquatic moles

Posted on July 2, 2014

The surface waters of streams often connect with the underlying groundwater by way of the saturated zone below the stream bed. Although this zone offers a potential refuge habitat for fish and other surface water species, very little is known about the capacities or willingness of such animals to penetrate stream bed substrates. In field experiments carried out in the Isielvar River of south-east Norway, biologists laid one-metre long, 14 cm diameter PVC tubes horizontally along the stream bed parallel to the current. The middle section of each tube was filled with coarse gravel to simulate the stream bed environment, while the end sections of the tubes were empty of gravel. Young Atlantic salmon were introduced into the end segments and fine mesh was fitted over the tube openings to prevent their escape. Each tube was translucent, which allowed snorkeling observers to record movements of fish into the gravel. With a gravel particle diameter of 24-60 mm, over 80% of salmon averaging 28 mm in length entered the substrate in the first 15 minutes of a trial. Observations showed that fish could travel through 60 cm of gravel in a few seconds. When experimental fish were larger (52-85 mm) or when the gravel was finer (16-22 mm), rates of entry were lower but still significant. Other experiments were carried out with vertically-oriented tubes: in these, a hose directed water upward through a tube to replicate upwelling groundwater and fish were introduced into the top of the tube. Rates of entry were similar to those when tubes were horizontal. These results suggest that the active use of within-bed habitats by salmonid fish, even when the average particle size is smaller than their own length, is more common than was previously assumed. Presumably, fish gain benefits in the form of increased protection from extreme temperatures, high flows and large predators.

Reference: Heggenes, J., Bremset, G. & Brabrand, A. 2013. Visiting the hyporheic zone: young Atlantic salmon move through the substratum. Freshwater Biology 58, 1720–1728.