Management and ecology of Alpine lakes

The 1960s stocking of non-native Brook Trout (Salvelinus fontinalis ) into fishless alpine Bighorn Lake led to the loss of the large crustaceans Hesperodiaptomus and Daphnia, leaving the lake dominated by small cyclopoid copepods and rotifers. Following gillnet removal of Brook Trout in the 1990s, Daphnia returned to pelagic waters. Hesperodiaptomus did not return, due to exhaustion of its resting egg bank. Diacyclops and rotifer abundance remained high in the absence of Hesperodiaptomus. Beginning three years after fish removal began, phytoplankton biomass fell 70-fold relative to reference Pipit Lake. All phytoplankton taxa except for cyanophytes exhibited decreased biomass. Chlorophyll-a concentration was unresponsive to fish manipulation. Secchi disc depth increased from 3.5 m to maximum lake depth (9.2 m), probably as a result of zooplankton filtering of inorganic sediments from lake-water. No changes in nutrient concentrations in lake water were attributable to fish manipulation.; Reduction of angling mortality of Bull Trout (S. confluentus ) on two small lakes had variable effect on Bull Trout abundance. Bull Trout abundance increased ∼5-fold for Harrison Lake and was associated with reduced individual growth rates. Size-based competitive asymmetries between Bull Trout born before and after reduction in angling mortality led to the loss of at least 15 year-classes of large, old Bull Trout. For Osprey Lake, closure of the lake to angling did not increase Bull Trout abundance. One or more of illegal angling, trophic restructuring of the population and high immigration rate probably limited the scope for population abundance responses at this site.; A Common Loon caused strong predator avoidance responses in pelagic adult Bull Trout in Harrison Lake. Size segregation of adult and juvenile Bull Trout was lost when adults occupied littoral predation refuges and cannibalism of juveniles by adults increased. Food consumption of adult Bull Trout increased in the presence of a loon, as a result of increased cannibalism, but decreased for juveniles. Low juvenile food consumption was correlated with low growth rates. Daily juvenile biomass losses due to loon-induced cannibalism were similar to probable losses due to direct predation by loons.