|On May 26 and 27, 2009, Trindle Spring Run (locally known as Silver Spring Run), located in a highly urbanized portion of Cumberland County in the vicinity of the town of Mechanicsburg, was reexamined to evaluate the status of the naturally reproducing rainbow trout population.
Trindle Spring Run, not unlike Letort Spring Run or Big Spring Creek also located in Cumberland Valley, is a low gradient, fertile, limestone spring stream. Trindle Spring Run begins its path to the Conodoguinet Creek south-southwest of Mechanicsburg arising from small spring sources in the limestone geology of the Cumberland Valley, and flows approximately 5.6 miles to its mouth. Portions of the stream flow underground between Mechanicsburg and Silver Spring. Aquatic plants common to limestone spring streams such as watercress and water-starwort provide cover for trout. The downstream portion of the stream from a spring source near the Silver Spring Meeting House downstream to its mouth (approximately 0.9 miles) supports a naturally reproducing population of rainbow trout, uncommon in Pennsylvania. A low-head dam was removed from this stream reach during 2006.
This section of Trindle Spring Run is designated as High Quality-Cold Water Fishes (HQ-CWF) under Chapter 93 by the Pennsylvania Department of Environmental Protection (DEP). Historically, the PFBC stocked Trindle Spring Run sporadically with adult and fingerling brown, rainbow, and brook trout from 1932 to 1950. Additionally, the Mechanicsburg Sportsmen’s Club has historically stocked the stream with trout; however, they discontinued this practice at the request of the Area Fisheries Manager following a 1992 survey that revealed a thriving wild rainbow trout fishery. The stream is currently managed as a Class A wild rainbow trout fishery by the Pennsylvania Fish and Boat Commission (PFBC) and, as such, is not stocked by the PFBC. Commonwealth Inland Waters regulations apply. Trindle Spring Run has a fish consumption advisory of one meal per month due to the presence of PCBs originating from a variety of sources within the watershed.
During May 2009, PFBC biologists from the Division of Fisheries Management conducted a routine electrofishing survey at an historic survey site representative of the stream section to evaluate the current status of the wild rainbow trout population. The site, approximately 340 yards in length, is located just upstream from the mouth in the vicinity of the former low-head dam. Species captured during the survey include rainbow trout, brown trout, rock bass, green sunfish, white sucker, and slimy sculpin. A total of 62 wild rainbow trout between three and 13 inches in length were captured during the first pass of the two-pass mark-recapture electrofishing survey, of which 35 fish were between seven and 13 inches (Table 1). Based on these results, the estimated abundance of legal size (> 7 inches) wild rainbow trout residing in the 0.9 mile section of stream from Silver Spring Meeting House downstream to the mouth was 201 fish. Biomass or pounds per acre of fish residing in the stream, was estimated at 26.37 pounds per acre. Additionally, a small number of wild brown trout ranging from two to 19 inches were captured during the survey (Table 1).
The wild rainbow trout population of Trindle Spring Run has fluctuated throughout the period of record from 1982 to 2009 (Table 2). Rainbow trout biomass was estimated at its highest level during a 1992 survey (95.84 pounds per acre) and lowest during a 1982 survey (25.82 pounds per acre; Figure 1). Fluctuations in abundance are not uncommon in wild trout populations. Commonly these fluctuations may be attributed to environmental factors leading to natural variation in reproductive success to which many wild trout populations have demonstrated great resiliency. Additionally, increased commercialization in the surrounding watershed may cause more dramatic population fluctuations than can be explained by environmental factors alone.
Figure 1. Estimated biomass of rainbow trout at the historic survey site from 1982 to 2009.
The recent decline in the Trindle Spring Run wild rainbow trout population is likely not attributable to overharvest by anglers. This portion of the stream was randomly selected as part of the 2004 Angler Use, Harvest and Economic Assessment on Wild Trout Streams in Pennsylvania. This statewide assessment revealed that anglers fishing wild trout streams released the majority of the wild trout caught on large streams and all of the rainbow trout caught on small streams similar to Trindle Spring Run. Based on this assessment there does not appear to be a need for the use of special regulations or more conservative regulations to adequately protect wild rainbow trout from overexploitation in small streams similar to Trindle Spring Run. Given the urban location of the watershed, public awareness and appreciation of this valuable resource and its self-sustaining rainbow trout population will be extremely valuable to the preservation of Trindle Spring Run for future generations.
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