Sustainability of Trout Fishery
The following article has been condensed from the publication “The Fishery of Spring Creek – A Watershed Under Siege” by Robert F. Carline, Rebecca L. Dunlap, Jason E. Detar, and Bruce A. Hollender.
Urban influences in the upper part of the watershed are having negative effects on Spring Creek’s trout fishery. Given the extensive development in and around the Borough of State College and the PSU campus, impacts on Spring Creek are not surprising. The effects of urbanization on stream ecosystems have been well documented in the literature, and most of these impacts are readily apparent in nearly all reaches of Spring Creek.
Urbanization reflects the conversion of forest and agricultural land to impervious surfaces that can be categorized as buildings (rooftops) and the transportation system. An increase in impervious surface coverage has profound effects on the hydrologic cycle, because the amount of rainfall that infiltrates into the groundwater is much reduced, and surface runoff, often referred to as storm flow, is greatly increased. Reduced recharge of groundwater translates to reduced stream base flow, which is particularly important to the maintenance of coldwater stream communities. As imperviousness increases, the frequency and magnitude of storm flow events increase, and the physical components of the receiving streams are altered. Higher stream flows require larger stream channels to transport water; hence, the response is erosion of stream banks (widening) and downcutting of the stream bottom (deepening). These types of alterations degrade habitat for invertebrates and fishes. Surface runoff from impervious surfaces during summer months can deliver warm water to the stream and cause elevated stream temperatures, which can lead to a shift from coldwater to warmwater fish communities. Storm water runoff affects water quality by increasing sediment load, reflected in turbidity, and by increasing the amount of nutrients and pollutants in the stream. The cumulative effects of physical, thermal, and chemical alterations linked to storm water runoff lead to degradation of biological communities.
Imperviousness, expressed as surface area in a watershed serves as a unifying theme to quantify the degree of urbanization, and urban streams could be classified on the basis of percentage of impervious surface area in the watershed. On the basis of available literature, if an urban watershed is less than 10% impervious, stream channels are stable and the diversity of biological communities is protected. Results from several recent studies on trout streams seem to indicate coldwater fish communities are not likely to be protected when watersheds reach 10% imperviousness. A study at 100 sites in Maryland that supported brook trout or did not support brook trout but had their preferred habitat concluded that brook trout were lost when imperviousness reached 6.6%. In southern Ontario they found that salmonid populations were eliminated when imperviousness ranged from 6.6 to 9%. In 39 southwest Wisconsin and southeast Minnesota streams significant declines in trout populations occurred when imperviousness was 6 to 11% and no trout when it exceeded 11%.
Spring Creek seems to be an anomaly compared to previously cited studies, because imperviousness is well beyond the range where trout populations persist. In 1995, it was estimated that 12% of the Spring Creek watershed was impervious, and, given that the population has been growing steadily, imperviousness must be greater than 12%. The Centre Region encompasses about the upper one-half of the watershed, and in 2002, imperviousness was 19.3%. Clearly, if Spring Creek followed the pattern of other watersheds, we would not expect wild trout populations to survive in this environment.
We suggest that the persistence of wild trout in the Spring Creek watershed is attributable to the large number of springs that contribute well-oxygenated coldwater to the stream and serve to moderate high summer temperatures (see the article “Why two boundaries”). Among the watershed studies reviewed, the two most frequently cited factors critical to maintenance of coldwater fish communities were stream base flow and summer water temperatures. Stream base flow reflects the amount of groundwater entering the stream, which directly influences stream temperature; hence, streams with large groundwater inputs have cooler summer temperatures and warmer winter temperatures relative to streams with minimal groundwater inputs. From these observations, we infer that the future of wild trout populations in the Spring Creek watershed is dependent upon maintenance of adequate groundwater inputs and summer water temperatures suitable for trout.
We are not aware of any long-term records of water temperature for Spring Creek; hence, we cannot make any generalizations about long-term trends. Our analysis of stream flow at the Axemann gage suggests that mean annual flow has not changed, nor have annual 7-day low flows. Whether stream flow will remain reasonably stable in the face of a growing population remains uncertain. Clearly, the amount of groundwater pumped from the aquifer has increased with the growing population in the watershed (3.1 mgd in 1980 to 9.1 mgd in 2002), but much of this water is treated and returned to the stream. The difference between the amount pumped and the amount returned to the stream is that lost to evapotranspiration, where homeowners and businesses irrigate lawns and gardens. Evapotranspiration undoubtedly accounts for a large proportion of PSU’s treated wastewater that is spray irrigated during the growing season. As the population continues to increase, we can expect more losses to evapotranspiration with potential consequences to the groundwater reserves.
The temperature of Thompson Run was monitored just downstream of a small impoundment that receives storm water runoff from the Borough of State College and part of the PSU campus. In June 1995, hourly temperature increases of up to 6.6 oC (11.9 F)following thunderstorms were recorded. In 1999, temperature monitors were installed in Thompson Run about 0.7 km downstream of the site monitored above. During the months of June through September 1999 to 2007, an average of 22 storms per year produced hourly increases of >2oC (3.6 F) and the mean increase for these storms was 3.7 oC (6.7 F). The highest hourly increase was 9.6 oC (17.3 0F). Despite these temperature increases, wild brown trout continue to persist in Thompson Run. It is likely that as imperviousness increases, we can expect more inflows of heated storm water runoff. Careful management of land and water resources can forestall likely outcomes of urbanization, and some such management practices are already in place or planned.
There are also some invasive plant and animal species that have the potential to degrade the trout fishery. A recent invasion by rusty crayfish poses a significant threat to Spring Creek (Figure 1). In other Pennsylvania streams, rusty crayfish numbers increased rapidly after introduction, and they displaced native crayfishes. High densities of rusty crayfish can negatively affect macroinvertebrates that are an important food source for trout and other fishes.
Figure 1. Photograph of a Rusty Crayfish. (Source: Wisconsin DNR)
The New Zealand mud snail has recently been found in Spring Creek. This small snail can attain high densities and can outcompete native invertebrates, thus reducing the food supply for trout.
The diatom Didymosphenia geminata is a microscopic plant that can form large mats on the stream bottom and reduce the numbers of invertebrates (Figure 2). Anglers are being advised to thoroughly clean their boots after wading in streams outside of the watershed. Such preventive measures are the best method available to keep Didymosphenia out of the watershed.
Figure 2. Schematic and photograph of a Didymosphenia geminata. (Source: Minnesota Sea Grant)
These three invasive species, either singly or in combination, have the potential to have a measurable effect on the health of Spring Creek. When one adds other stressors, such as storm water runoff and erosion from new construction, the likelihood that the trout fishery will be harmed seems to increase. Protection of groundwater recharge areas is vital to ensuring that spring inflows to Spring Creek are maintained. Strict controls on storm water runoff and innovative ways to treat and dispose of domestic wastewater will help to reduce future urbanization impacts, but the watershed is clearly under stress.
Jason E. Detar and David Kristine, Fisheries Biologists, Pennsylvania Fish and Boat Commission