Annual production and population dynamics of a relatively unexploited walleye, Stizostedion vitreum vitreum (Mitchill 1818), population in Savanne Lake, Ontario

Abstract

Most studies of the walleye have been conducted on highly exploited populations. Savanne Lake in northwestern Ontario, and having an area of 364.29 ha, was exploited until 1969, whereupon it was designated a fish sanctuary. Study was conducted during the spring months of 1973, 1974 and 1976. Walleyes were marked and recaptured in trapnets. Marking was accomplished by attaching a Floy tag FTF69 under the second dorsal fin with monofilament line. The 6th, 12th and second dorsal spines were clipped In the springs of 1973, 1974 and 1976, respectively, for Schumacher- Eschmeyer estimates. Schumacher-Eschmeyer spring population estimates of 5211, 5184 and 5463 for the three study years represent the spawning population most vulnerable to the gear. The annual mortality rates were a high 8 to 85% for the component of the population sampled and characteristically increased with age over the three years of study. Growth retardation was observed among tagged walleyes; some of the walleyes neither grew in length nor laid down a new annulus in each year of their life. These observations were attributed to limited food supply and extensive handling, tagging and finclipping of the fish over the study period. No relationship was noted between year-class strength and year-class growth. Walleyes showed similar annual percentage increments in their growth during the three sampling years. Relative growth of walleyes increased substantially by the end of the first year, decreased sharply the second year and continued to decrease at a lesser rate until, on the average, the fifth or sixth year. After this, growth was very slow. Annual biomass of 6.58 kg.ha[superscript 1] for 1973-74, 6.06 kg.ha[superscript 1] for 1974-75 and 2.64 kg.ha[superscript 1] for 1975-76 for age groups 7 and older was low. Production estimates of 1.0 kg.ha[superscript 1].yr[superscript 1] for 1973-74, 1.31 kg.ha[superscript 1].yr[superscript 1] for 1974-75 and 0.52 kg.ha[superscript 1].yr for 1975-76 for age groups 7 and older are likely values for a theoretical yield of 1.9 kg.ha[superscript 1].yr[superscript 1] based on the morphoedaphic index (Ryder 1965) and support the 30 percent rule. The brood stock production estimate of 1.85 kg.ha[superscript 1].yr[superscript 1] for fish of age 5.5 (weighted mean age to first maturity) and older, and representing about 20% of the total walleye production in the lake, relates closely to the theoretical yield of 1.9 kg.ha[superscript 1]..yr[superscript 1] based on the 30 percent rule. The slow growth of the walleyes was attributable to chemical characteristics typical of lakes in this region of the Canadian Shield, as well as to their high density relative to the available food.