Stocked Trout Study Gives Glimpse Into Fishing Residency

HARRISBURG – Anglers counting down the days to their first crack at the bigger trout being stocked this year by the Pennsylvania Fish and Boat Commission can count on finding fish. A study conducted last spring on trout residency in the days following stocking indicates that for the vast majority of waters, hatchery trout distribute themselves within proximity of the stocking site.

The 2006 study involved 135 stream sections that were evaluated using electrofishing at randomly selected stocking sites 10-20 days after stocking and the start of the open season. (Electrofishing is a common fisheries survey process which passes a mild electric current through water, drawing in fish so that they can be evaluated.) The intent of the study was to determine the percentage of the stocked trout remaining within 300 meters of the stocking points. In all, 259 samples were taken and scored as “excellent” (>90 percent recaptured), “good” (75 percent – 89.9percent), “fair” (40 percent – 74.9 percent), “poor” (19percent – 39.9 percent) or “very poor (<9.9 percent). The results: 72 percent of the sites studied held numbers of trout that were excellent, good or fair.

That should be reassuring news for trout anglers, says Tom Greene, Coldwater Unit Leader for the PFBC. “We don’t want all trout staying right at the stocking point, nor do we want them traveling too far. Seeing the number of fish we collected within 300 meters of stocking sites is a good indication that we’re getting an appropriate amount of dispersal on most stocked waters,” said Greene.

Not surprisingly, streams that had a great number of fallen logs, undercuts banks and boulders with few erosion problems tended to provide higher recapture rates than waters without those habitat features. However, there was enough variability in the recapture rates in these streams that fisheries biologists say habitat structure factors alone were not the only things affecting how many trout stayed within the sampling sites. In fact, the study suggests that no single variable appears to determine how many trout stay near the point of release.

While seven sub-basins sample showed no poor or very poor sites, Greene said that excellent to very poor sites occurred throughout the state. “There was no discernable pattern of trout re-captures rates on a statewide basis,” he said.

For example: In sub-basin 9, a drainage that is part of the West Branch of the Susquehanna River basin, four of nine sites showed poor recapture rates. However three sites there had recapture rates in excess of 90 percent.
In attempting to look for patterns, the Commission cross referenced its finding on the percentage of fish recaptured with variables in stocking such as water temperatures, pH and total alkalinity between hatcheries, truck tanks and the receiving streams. No significant influences were found. Factors such as transportation time from hatcheries to the water the numbers of stops made along the way or the density of trout in each tank showed no significant differences.

Likewise, the Commission found no single factor that would explain the relative absence of stocked trout in those waters that returned poor or very poor recapture rates. Streams in sub-basin 8 of the West Branch of the Susquehanna River had relatively low recapture rates. Of the eight sample sites in sub-basin 8, six were below the 40% recapture rate. A portion of sub-basin 4 (the upper North Branch of the Susquehanna River basin) also showed localized poor trout recaptures. Those sub-basins are known to be vulnerable to acid precipitation and pH has been observed to be a factor affecting trout residency. However, that relationship was weak based on the 2006 data.

The complete 2006 study report is available on the Commission’s web site at

In 2007 Commission biologists will continue to study trout residency, but will concentrate on revisiting the waters sampled in 2006 that had low recapture rates. If results from the 2007 data suggest that changes in the stocking program on some streams are needed, options could include stocking trout just prior to opening day or stocking only during the open season.

Daryl Pierce, a Commission biologist who was lead author of the 2006 study report, said the collection of additional data will give fisheries managers more information to consider. “Even if a sample site had poor or very poor retention of stocked trout in 2006, this does not imply that the entire stream or even the section is poor water for trout or trout fishing. Our sample sites encompass a relatively small distance, when given some of the sizes of the sections that we stock; the planted trout might just have simply moved to the nearest pool that was outside of our sampling area.”

The ongoing study of stocked trout residency is part of an agency strategic initiative to re-engineer trout fishing opportunities. Improvements such as consolidating special trout regulation programs, stocking of larger hatchery trout, providing specific details on stockings during the open season and an earlier opening day in those areas of the Commonwealth that warm sooner in the spring are all results of this process.

“Trout stocking is one of the most valued services the Commission provides to anglers. So making sure we get the best possible recreational use for the millions of adult trout we are producing is essential. Evaluating when, why and where trout move after stocking will have a direct impact on how we manage stocked trout going into the future,” said Doug Austen, PFBC Executive Director.

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