Impact on natural populations

Injuries among natural fish populations have not been frequently reported, possibly because affected fish succumb to opportunistic infections shortly after acquiring this injury and are not readily observed. Nonetheless, solar UV was found to affect the survival of bluegill (Lepomis macrochirus) embryos in natural lakes and the selection of spawning habitat and hatching success of yellow perch (Perca flavescens) [25,26]. Coho salmon (Oncorhynchus kisutch) stocks that develop in relatively sunny rain-free areas of British Columbia appear to be declining compared to more northern coastal stocks that develop under less sunny conditions [27]. The declines are thought to result from solar UVR injuries to early life stages that develop in shallow freshwater streams. UVR has also been implicated in the decline of marine fish such as cod (Gadus morhua) in the North Atlantic where water column UV levels were found to be at irradiance levels harmful to developing embryos [28].

Brown trout (Salmo trutta) endemic to certain clear water streams in Europe have been declining in recent years (http://s.o.w.tripod.com/dyingbrowns.htm). Abnormally dark colored and blind brown trout have frequently been observed in these streams while other fish species in these streams such as rainbow trout and grayling appear unaffected. Many of these streams were channelized and are shallow with little or no canopy cover or protective in-water substrate. Conversely, unchannelized streams that appear to support healthy brown trout populations have rocky bottoms substrates as well as shade from tree canopy that can provide places for fish to avoid excessive solar radiation. These unchan-nellized streams also had deep pools and were often tea-colored from tannic substances. Such observations provide circumstantial evidence that solar UV may be affecting brown trout in certain streams in Europe.

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