November 01, 2019
By Dr. Rob Neumann
Field Science—Studies suggest that the largest fish within a group can out-compete smaller individuals of the same species for food, resulting in faster growth. It’s believed that this “shoal dominance” also can lead to an order of fishing vulnerability among individuals of different sizes in the group.
Researchers tested this theory by ice-fishing for Eurasion perch in Finland.* Eurasion perch are close cousins of the yellow perch found in North America. The researchers hypothesized that perch size should decrease with catch order. They fished for a maximum of 10 perch for 5-minute periods through 4.5-inch holes. Fishing areas were chosen to ensure different groups, or shoals, of perch were targeted in each trial. Perch were caught from 10 lakes ranging in size from 4 acres to as large as the Baltic Sea.
Perch were caught on a dropper rig consisting of a 5-gram copper-silver colored metal lure with a dropper line attached to a fluorescent orange hook (#14). Hooks were baited with one to three blowfly maggots, which look like standard maggots sold for bait in North America. Length and weight were measured for each perch caught, and perch weren’t returned to the water and thus couldn’t be caught more than once.
With data pooled, up to 25 percent of the variability in perch size was explained by capture order, but the pattern was significant only in two of the lakes studied. The estimated decrease in fish size along capture order was about 1 mm and 1 gram per fish. Among the two lakes where this phenomenon was observed, perch populations had the largest variability in sizes.
Perch formed shoals according to fish size. In fact, perch size among locations within a lake was more variable than perch size among lakes. If anglers are fishing lakes with a wide assortment of sizes and are catching smaller perch, switching to a new location may increase the size of perch caught. The researchers suggest that highly mobile anglers that specifically target the first few large perch in each location maybe more selective for larger perch than less mobile anglers.
Dr. Rob Neumann
*Vainikka, A., J. Koskimaki, P. T. Niemela, and R. Kortet. 2012. Composition of the Eurasian perch (Perca fluviatilis) catches in ice fishing: Does capture order predict body size? Fish. Res.: 115-116:24-30.