Age, Growth, and Diet of an Introduced Minnow, Redside Shiner Richardsonius balteatus, in Ross Lake, Washington

Since its introduction to Ross Lake reservoir in North Cascades National Park, the redside shiner (Richardsonius balteatus) population has exploded. Ross Lake is also home to one of Washington’s most robust populations of ESA-Threatened bull trout (Salvelinus confluentus). Although this bull trout habitat is largely protected from development, pollution and overuse, the redside shiner represents an unregulated potential threat to bull trout through competition for food and even predation.  Therefore, we studied aspects of redside shiner life history and dietary habits in Ross Lake on a seasonal basis through the course of one year.  We collected redside shiners from winter of 2009/10 through fall of 2010 using minnow traps and by seining. Based on measurements of size distribution and observations of reproductive maturity, we conclude that this population demonstrates two major spawning events, one in the spring and one in the fall. In early spring both sexes initially show maturity at a length of about 50 mm with adults ranging in size up to 85 mm.  During spring while the reservoir rises 15 m to full pool and during summer and early fall redside shiners are feeding on prey from both the benthic and pelagic zones of nearshore habitat.  Most pelagic prey are Cladocera with a few Copepoda.  Nearly all fish have food present in the gut, and estimated individual cladocerans typically are on the order of hundreds per stomach. Common benthic prey are segmented worms (Oligochaeta), midges (Chironimidae) with a few other macroinvertebrates. In mid-winter planktonic algae were also found in the diet, especially diatoms.  The cestode parasite Ligula intestinalis occurs in fish larger than 45 mm. Historically found in trout in this drainage, this parasite causes direct and indirect mortality and inhibits gonad development. In light of this preliminary research, the widely held perception that redside shiners are not competing with native trout for food in this system needs to be reconsidered. We have established that the potential for resource competition exists between redside shiners and native trout, particularly bull trout, and recommend that further research focus on this interaction. Future lake management decisions may need to consider the effects on the dynamics of the relationship between non-native shiners and native trout, especially considering potential uncontrolled effects that climate change may have.