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Bits & Pieces (June-July 2005)
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Muskies See, Stalk, Strike
Select Science -- In a laboratory setting, researchers examined when and to what extent muskellunge use their sight and lateral-line senses during feeding.* Different muskie groups were tested, including those with eyes and lateral lines that functioned normally, and others that had their sight, lateral line, or both senses suppressed.
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When presented with a minnow, normal muskies began feeding in distinct phases. First, they oriented toward the prey and then slowly stalked it. Once the fish were in position, they concluded with a fast and furious lunge. Normal muskies had successful death-grips 73 percent of the time.
Muskies with suppressed eyesight didn't orient to or stalk prey and lunged only if an unsuspecting minnow trespassed too closely. Muskies with suppressed lateral lines stalked prey longer, closing a narrow gap before striking. Muskies that lacked both senses didn't feed. These findings indicate that muskies rely more on sight during the first phase of feeding (the stalk), and more on the lateral line system during the final phase (the lunge).
The see-stalk-strike behavior may explain why the figure-8 fishing technique is so effective. The lunge might be cued by the concentrated vibrations of the lure sensed by the muskie's built-in seismograph, more than by the sight. The fish stalked the lure to the boat but for some reason didn't commit -- perhaps not enough commotion was presented along the way. This research shows that lure appearance, presentation, and action can be critical for triggering muskie strikes in different water conditions.
*New, J. G., et al. 2001. Strike feeding behavior in the muskellunge, Esox masquinongy: contributions of the lateral line and visual sensory systems. J. Exp. Biol. 204:1207-1221.
Incredible Shrinking Walleyes?
Curious Science -- After an angler in South Dakota, cited by a conservation officer for possessing walleyes under the minimum length limit, contested the violation by claiming his fish had shrunk, biologists with the South Dakota Department of Game, Fish and Parks investigated the possibility.*
 Walleyes caught in open water were either placed in a cooler with ice or into a livewell. Ice-caught fish were placed in a cooler with ice. Biologists measured all fish at capture every hour for the next 10 hours, and some after 20, 29, 36, and 44 hours. Finally, fish were frozen and checked 30 days later. In poaching cases, fish often are held frozen as evidence for at least 30 days.
The study found that walleyes do indeed shrink after capture, with the extent of shrinkage dependent on water temperature, holding method, and freezing. Fish caught in open water and placed on ice averaged just 1/10 inch shorter after 5 hours, but some shrank by up to 1/3 inch and slightly more after an additional 5 hours. Walleyes caught in cooler water (50F) shrank more than those in warmer conditions.
Shrinkage was attributed to rigor mortis. Walleyes caught and placed in a livewell died quickly and generally increased in length after many hours, though the change was not statistically significant. Expansion was apparently due to a process called "gaping" that takes place after rigor mortis has passed and the flesh is less able to contract due to tissue damage, resulting in increased length. Walleyes caught by ice fishing and placed on ice in a cooler took longer to shrink as they remained alive longer, but 24 hours later showed similar amounts of shrinkage, averaging about 1/5 inch. Greatest shrinkage occurred when walleyes were frozen, with some fish shrinking almost 1/2 inch.
This amount of shrinkage presents a problem for enforcement of length limits when dead fish are checked by officers, and particularly when frozen fish are used as evidence. As a result, the investigators recommended that officers photograph confiscated fish on a measuring board and use the photos as evidence instead of frozen carcasses. We suggest that agencies conduct similar tests on other species to further evaluate this phenomenon, then coordinate law enforcement proceedings accordingly.
*Blackwell, B. G., M. J. Hubers, and R. G. Losco. 2003. Postharvest length changes of angler-caught walleye. N. Am. J. Fish. Mngt. 23:770-778.
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