The body temperatures of 15 hatchery‐origin and two wild brook trout Salvelinus fontinalis and 11 hatchery‐origin rainbow trout Oncorhynchus mykiss were monitored with internally implanted, temperature‐sensitive radio transmitters from June through September 1997 in a fifth‐order Adirondack river (New York). The fish were released into a 12‐km reach that during summer had maximum temperatures near those that are lethal for salmonids. Body temperatures were compared between species and with river temperatures. In 1997, the maximum river temperature was 26.4°C, and the highest average daily temperature was 25.0°C. The brook trout were usually cooler than the main river flow because they used two of five tributary confluences or groundwater discharge areas in two pools within the main river. The temperatures of the brook trout from June through September were an average of 2.3°C cooler than the main flow of the river and differed significantly (P = 0.002) from those of the rainbow trout, which were 1.5°C cooler than the river. When the river temperatures were 20°C or higher, the mean temperatures of the brook trout were 4.0°C cooler than the river and differed significantly (P = 0.002) from those of the rainbow trout, which were 2.3°C cooler than the river. Brook trout, and to a lesser extent rainbow trout, used localized coolwater areas to lower their body temperatures below that of the main river. Groundwater discharge areas within pools and some tributary confluences were critical habitats for behavioral thermoregulation because they provided refuge areas from warm river water during the summer.
Poststocking growth, movement, and catch were compared among hatchery brook trout Salvelinus fontinalis, rainbow trout Oncorhynchus mykiss, and brown trout Salmo trutta in a fifth‐order river. Associations of species, size, and stocking date with angler catch were also examined. The river is episodically acidified, and during summer it approaches lethal maximum temperatures for trout. Catchable‐sized brook and rainbow trout (168–458 mm total length) were stocked in the late spring of 1996 and 1997. Brown trout were stocked only in 1997. Fish were marked with visible implant tags and were recovered through October of each year. All three species had negative daily growth rates in weight over the summer and early fall. Rainbow trout stocked in 1997 tended to move downstream after stocking, whereas the other groups showed no strong movement trend. Recovery rates significantly differed between brook and brown trouts stocked in early June and those fish stocked in late May. Large (>300‐mm) rainbow trout were caught at higher rates than small (<260‐mm) fish were. Anglers were estimated to have caught 72% of the stocked brook trout, 51% of the rainbow trout, and 18% of the brown trout. High summer water temperatures (>20°C) did not affect angler catch rates because cool refuges within the river concentrated and made the stocked fish—especially brook trout—vulnerable to angling. By stocking more than one species, we were able to create diversified angling opportunities and sustain a fishery in this thermally marginal river over the entire summer season.
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