Population Abundance and Stock Characteristics of Flathead Catfish in the Lower St. Joseph River, Michigan

Abstract

Little information exists regarding the biological attributes and stock dynamics of flathead catfish Pylodictis olivaris in lotic systems throughout the northern United States. We examined the population abundance, annual survival, growth, condition, size structure, and age structure of flathead catfish in the lower St. Joseph River, Michigan, to direct future management efforts in this system and increase our knowledge of northern flathead catfish stocks. Fish were collected by means of electrofishing during June through September 2002 and 2003. Population abundance was estimated at 5,452 individuals (range, 3,985–7,277 fish), and annual survival was estimated at 67%. Analysis of pectoral spine cross sections revealed that growth was greatest for fish younger than age 6 (range, 83–98 mm/year) and decreased among older age-classes (range, 18–70 mm/year). Relative weight was greatest for individuals less than 300 mm total length (TL; 115%) and declined with increasing fish length. The size structure and age structure of flathead catfish were dominated by fish less than 400 mm TL and younger than age 4 (89% and 79%, respectively), although individuals greater than 1,100 mm TL and up to age 17 were present in the population. Relative stock density estimates indicated that 58% of fish greater than the minimum stock size were of quality length, while flathead catfish of preferred, memorable, and trophy sizes represented 28, 7, and 1% of fish collected, respectively. Despite their presence on the northern fringe of the species' geographic distribution, the flathead catfish in the lower St. Joseph River exhibited biological characteristics and stock dynamics that were similar to those reported for other lightly exploited stocks throughout the midwestern United States.