Researchers placed an omnidirectional camera on a robotic arm to collect data about zebrafish movements in their native streams in India, then used the information to understand how the brain helps fish orient in current.
September 18, 2024
By Dr. Rob Neumann, Steve Quinn, Dr. Hal Schramm & Ralph Manns
Fish Brain Breakthroughs: How Fish Orient in Moving Water Anglers and biologists often take for granted the ability of stream and river fish to orient themselves in water that’s continually moving past and pushing them about. For the brain, it represents a complex problem and solving it allows stream fish to move about their environment and thrive.
Recently, a large team of scientists from Northwestern University led by Dr. Emma Alexander combined field observations of stream fish with their brain activity to understand their ability to operate in these environments.* As water moves, fish are constantly trying to self-stabilize to stay in place or swim directionally. Their brain needs visual information to accomplish this. Alexander compares the situation to sitting in a train that isn’t moving. If a train next to yours starts moving, your brain gets tricked into thinking that you’re moving instead. The visual cue from the other train is so strong that it overrides the input from your other senses that indicate you’re still sitting still. Elevators can provide similar experiences.
They conducted the field study on the well-known aquarium fish, the zebra danio, in its natural environment of streams in India, using an omnidirectional underwater camera mounted on a robotic arm to record fish swimming behavior. Analyzing these observations with computer modeling indicated that this species, and likely other stream fishes, regularly scans the bottom to pick out features to orient by, since they remain static while the rest of their environment moves with the current. This allows their brain to instinctively calculate swimming speed as well.
The authors note that the concept of “optic flow” relates to the way the vertebrate brain interprets movement. For example, many bird species fly through narrow corridors and among obstacles by balancing the lateral visual flow on either side of their head to maintain course and avoid collisions. Humans, too, use optic flow in our speed and distance paradigm that tells us when and how hard to break while moving toward a moving object. This ability weakens with age, causing some elderly drivers to lose their driver licenses.
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Different species of fish, inhabiting vastly different habitats, use some form of this brain function as they swim, whether it’s a trout feeding in a small brook, or a walleye migrating toward a spawning shoal. Offshore pelagic species sense currents and sun position in a similar way, allowing them to operate in seemingly wide open spaces. The marvels of fish biology can, at times, help us to understand our quarry better and catch them. In other instances, we can simply marvel at their intricate nature.
–Steve Quinn
*Alexander, E., L. T. Cai, S. Fuchs, T. C. Hladnik, Y. Zhang, V. Subramanian, N. C. Guildeau, C. Vijayakumar, M. Aronchem, S. A. Juntti, T. R. Thiele, A. B. Arrenberg, and E. A. Cooper. 2022. Optic flow in the natural habitats of zebrafish support spatial biases in visual self-motion estimation. Current Biol. 32:5008-5021.
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Research Notes: Preservation Methods and Fish Weights Several studies have examined how post-catch holding treatments can affect the length of fish —for instance, freezing, or holding fish on ice or in a livewell. Changes in length post-catch, particularly shrinkage, can have important implications for enforcement of length limit regulations .
Little information exists, however, regarding how common holding methods can affect fish weight. If the conditions in which fish are held cause changes in fish weight, this can have important implications for certifying record fish, if for instance the fish is caught at a time when natural resources department agents aren’t available and/or a certified scale isn’t accessible at the time of the catch and the fish needs to be held for an expended period prior to weighing.
Florida researchers set out to tackle the question of how fish holding methods and holding times can affect weight of four common gamefish species: black crappie, bluegill, flathead catfish, and largemouth bass. Fish were collected by electrofishing, and four holding (i.e., preservation) methods were evaluated: held live in an aerated and temperature-controlled cattle tank; on ice in a 380-liter cooler allowed to drain excess water; in a 50:50 ice/water slurry in a 380-liter cooler, and frozen in plastic bags at 0°F. For the first three treatments, fish were weighed at 0, 6, 12, 24, and 48 hours. Freezer fish were weighed at 24 and 48 hours, after being thawed in water in the plastic bag in which they were stored.
The effect of holding method on weight varied by species. Changes in weight of black crappies was insignificant at 24 hours for those held in the freezer, on ice, or live, but fish held in the ice/water slurry had a 6 percent gain in body weight, on average. At 48 hours, changes in weight were nearly identical to those seen at 24 hours. For bluegills, those held in the ice/water slurry had an average gain in weight of approximately 10 percent in 24 hours; those held in the freezer lost about three percent body weight, while those held on ice and alive had minimal changes in weight. For flathead catfish, those held alive experienced average weight losses of 4.8 percent and 5.6 percent at 24 and 48 hours, respectively, and weight gains of about 4 to 5 percent over those same time intervals for those held in the ice/water bath. For largemouth bass, freezer storage and being held on ice had minimal effect on weight, while those in the ice bath gained about 5 percent of body weight on average, and those held alive had about a 3 percent loss in body weight.
Average proportional change in weight at 24 and 48 hours for four fish-holding treatments. (Example: A proportional change in weight of 0.06 is a weight gain of 6 percent. Negative numbers are weight losses.) Results showed that holding method can significantly affect the weight of fish, which varied by species and holding method. Holding fish in the ice/water bath resulted in gains in weight for all species, and when held alive, largemouth bass and flathead catfish had significant weight losses. For anglers needing to hold a potential record fish before it can be weighed, the researchers suggest either holding fish on ice in a cooler with the drain plug open, or to freeze it in a plastic bag and thaw it in the same bag in water before weighing.
–Dr. Rob Neumann
*Hatcher, H. R., P. A. Strickland, S. M. Bisping, and R. D. Henry. 2023. Evaluation of methods to minimize weight change of potential record fish during the certification process. J. Southeast. Assn. Fish and Wild. Agencies. 10:45-50.
