Michigan Department of Natural Resources
Fisheries Research Report No.1895, 1981
Analysis of "Quality Fishing'' Regulations Through Mathematical Simulation of a Brown Trout Fishery
Richard D. Clark, Jr.
Abstract.-The population simulator, TROUT.DYNAMICS, was calibrated with mortality, growth, and reproduction statistics for the brown trout (Salmo trutta) fishery in a section of the Au Sable River, Michigan. It was then treated as a "test-tube" fishery for experimenting with different "quality fishing" regulations. A range of inverted and slot size limits was tested under a flies-only gear restriction for conditional fishing rates (m) of 0.20 to 0.60. The performance of these regulations was compared to typical minimum size limits, an unrestricted fishery, and a catch-and-release fishery. The potential impact of hooking mortality on the fishery was examined by simulating a wide range of hooking mortality rates for a catch-and-release regulation. Also, the effects of gear restrictions, such as fly-fishing-only or artificial-lures-only, were examined under a hypothetical scenario in which it was assumed that they would reduce fishing pressure, as well as hooking mortality. Two statistics were used as the major indices of fishery performance under the various regulations--the number of trophy-size trout (over 406 mm long) caught annually and the total annual harvest (or kill) in numbers of legal-size trout. Results showed that the catch of trophy-size fish was inversely related to the total harvest in numbers. The greatest number of trophy fish was caught under a catch-and-release fishery in which no harvest was permitted. The greatest total harvest in numbers of fish was obtained, but the lowest number of trophy fish was caught in an unrestricted fishery. Hooking mortality did not have a serious impact on total catch of trout until the portion of fish dying after catch and release (h) exceeded 40% and the fishing rate (m) exceeded 0.30. In contrast, the catch of trophy fish was reduced considerably by relatively small increases in hooking mortality. With respect to gear restrictions, it was found that use of fly-fishing only regulations maximized the number of trophy fish in the population, but artificials-only regulations maximized the annual catch of trophy fish from the population. The main reason for this was the assumption that the average angler had a higher catch rate with spinner type lures than with flies. Any-lure regulations maximized the total catch for the fishery. The exact numerical results of this analysis applied only to the study fishery, but the general trends in fishery statistics should apply to most stream trout fisheries and to any other fishery which conforms to major model assumptions.