FISH ASSEMBLAGE STRUCTURE AND GROWTH IN THE LOWER MILK RIVER, MONTANA IN RELATION TO ENVIRONMENTAL CONDITIONS

Abstract

With the major habitat alterations on the Missouri River in the 20th century, native fishes must rely more heavily on the larger, more natural, inflowing tributaries for spawning and rearing habitat. A two-year study was conducted to investigate the occurrence and abundance of fishes in the lower Milk River, Montana, which enters the Missouri River immediately below Fort Peck Dam. In sampling conducted from May to August in successive years (2002, 2003), the fish species assemblage included multiple species of special concern (blue sucker Cycleptus elongatus, paddlefish Polydon spathula, sauger Sander canadense) and multiple watch list species identified by the Montana Natural Heritage Program (burbot Lota lota, brassy minnow Hybognathus hankinsoni, plains minnow Hybognathus placitus). Relationships with environmental conditions and their interactions with temporal variables (month, year) were investigated for occurrence and total catch data. Models were generally similar for individual species with temperature and turbidity being the primary environmental conditions influencing fish occurrence and abundance. Age and growth analysis was conducted on channel catfish (Ictalurus punctatus), sauger, walleye (Sander vitreus), northern pike (Esox lucius) and shovelnose sturgeon (Scaphirhynchus platorynchus). Channel catfish, sauger, walleye and shovelnose sturgeon all grew slower and lived longer in the lower Milk River than populations at lower latitudes. In view of the lower Milk River’s role as spawning and rearing habitat for native fishes and its history of alterations from upriver dams and irrigation withdrawals, more attention should be given to maintaining or improving existing habitat conditions, including adequate instream flows and turbidity.