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USING PHABSIM TO ESTIMATE ADULT TROUT CARRYING CAPACITY RICHARD M. ANDERSON COLORADO DIVISION OF WILDLIFE COLORADO SPRINGS, COLORADO 80907 ABSTRACT PHABSIM is a method that will quantify fish microhabitat in a stream. Correlations were run between trout standing stock and the habitat units (W.U.A.). Brown trout biomass was highly correlated with brown trout W.U.A. (r = O.A41). Total trout biomass was correlated with total trout W.U.A. (r = 0.847). Based on the strength of the correlations, a carrying capacity equation was developed: C.C. = TH x B/H , where TH is the total microhabitat of a stream reach an~ B/H ms the biomass one unit supports when fully utilized. Maximum~d.U.A. was used to repre- sent TH The B/H value was obtained from streams felt to be at carrying capaci~y, e.g., the South Platte River (Cheesman Canyon) where biomass was 8.6 g/ft . INTRODUCTION Carrying capacity is usually defined as the adult trout standing stock that a stream can support when its habitat is fully utilized. Carrying capacity can be difficult to determine. Many factors operate that can depress a population below the carrying capacity of the habitat. These include angling mortality, spawning mortality, disease, predation, competition and stream fertility. Also, if the adult population is subjected to winter mortality because of low flow, the available habitat typical of summer and fall cannot be filled. When winter habitat is sufficient, harvest is probably the most critical of these factors that suppress standing stock. If the carrying capacity of the habitat is known, then response of the trout population to a program that ameliorates the most limiting factor can be estimated. METHODS When forage is not limiting, the formula for carrying capacity could be expressed as: 27