Factors Affecting Abundance of Lake Herring (Coregonus artedii Lesueur) in Western Lake Superior

Abstract
A field and laboratory study was conducted in 1965–1968 to investigate factors affecting lake herring (Coregonus artedii Lesueur) abundance in western Lake Superior. Commercial landings in November-December in the Duluth-Superior area dropped from 1,117,000 pounds in 1954 to 2000 pounds in 1966. Wisconsin catches also declined markedly. Abundance and fishing intensity indices decreased with landings. Commercial catches and abundance of chubs (Coregonus spp.) and smelt (Osmerus mordax) increased simultaneously with the herring reduction. Biological characteristics of Duluth-Superior and Apostle Islands (Wisconsin) herring stocks were examined and compared. Herring eggs were more abundant and viable in the latter area, and laboratory hatching success was significantly greater for Apostle Islands eggs. Herring larvae were generally more abundant and smaller in the Apostle Islands; distribution and growth did not differ between areas. Abundance of larvae in both areas declined sharply in June-July. Average age of adult herring in commercial catches increased from 3.9 years in 1950–1959 to 5.1 in 1966–1968 but showed no significant difference between areas. Percentage of females did not show a trend from 1960 to 1968; however, the 1968 percentage of 85.0 was the greatest since 1962. Percentage of females increased with age. Average length and weight increased from 11.3 inches and 0.33 pounds in 1950–1959 to 13.4 inches and 0.67 pounds in 1966–1968. Average number of eggs per ounce of fish increased from 842 in 1950–1954 to 1105 and 1019 in the Duluth-Superior and Apostle Islands areas, respectively, in 1966–1967. Smelt was the most abundant species in both areas and more abundant at Duluth-Superior. Zooplankton were more abundant at Duluth-Superior. Sediment soil in the Duluth-Superior area contained a higher percentage of silt and organic matter than soil in the Apostle Islands and was possibly related to lower egg viability in the former area. Correlations between fishing intensity and herring abundance suggest that overfishing was not the major factor in the decline of the species. Predation was not found to be a serious influence on herring. Food of herring, bloaters (Coregonus hoyi), and smelt was quite similar during early stages. Herring larvae consumed exclusively copepods. Food similarities and highly significant negative correlations between herring and bloater abundance and herring and smelt abundance suggest that competition for food at the critical larval stage was likely the major influence in the lake herring decline.

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