|Young-of-year (YOY), or recently spawned black bass are surveyed annually by Division of Fisheries Management staff at major rivers statewide as an index of reproductive success. Since 2005, this index has also been used to determine the prevalence of a recently emerging disease that has been affecting YOY smallmouth bass in the Susquehanna River system.
Division of Fisheries Management staff from Areas 3, 4, 6, and 7 surveyed YOY black bass (primarily smallmouth bass) in the West Branch Susquehanna River, upper Susquehanna (North Branch), lower Susquehanna, and middle Susquehanna River, respectively. Surveys are conducted using backpack electrofishing gear targeting near-shore, gravel and cobble habitats favored by YOY smallmouth bass at early stages of their development.
Survey results varied by reach, generally decreasing in density in a downstream direction. The upper Susquehanna River (North Branch) proved to have very strong smallmouth bass reproduction in 2010: the third highest on record (Figure 1). The West Branch Susquehanna River and middle and lower Susquehanna River all had catch rates below the historic average, but better than 2009 (Figures 2-4).
Figure 1: Historic catch rate of YOY smallmouth bass at the upper Susquehanna River (North Branch).
Figure 2: Historic catch rate of YOY smallmouth bass at the West Branch Susquehanna River. Note difference in scale among figures.
Figure 3: Historic catch rate of YOY smallmouth bass at the middle Susquehanna River (Sunbury to York Haven Dam). Note difference in scale among figures.
Figure 4: Historic catch rate of YOY smallmouth bass at the lower Susquehanna River (York Haven Dam to Holtwood Dam). Note differences in scale among figures.
Prevalence of disease among YOY smallmouth bass sampled in 2010 was low to moderate in the mainstem Susquehanna River and West Branch Susquehanna River at the time of sampling (Figure 5). Surveys on the mainstem Susquehanna River were conducted two weeks earlier than normal due to the rapid growth of the YOY smallmouth bass and may have not allowed for the disease to fully develop. Since the initial surveys, diseased fish have been more prevalent at all portions of the Susquehanna River but does not appear to be as severe in the middle Susquehanna River as in past years. Follow up surveys in the next few days to weeks will give a better appreciation as to the scope of the disease impacts on YOY smallmouth bass in summer 2010.
Low YOY smallmouth bass catch rates in the middle and lower Susquehanna River this year may be due to a multitude of factors: including; sample timing and efficiency, predation, disease, and river flows. Survival can fluctuate widely from year-to-year, even in the absence of disease. Future boat electrofishing surveys targeting adult smallmouth bass in the late summer and early fall, although not an accurate measure of YOY smallmouth bass numbers, will provide further insight into the contribution level of the 2010 year class to the catchable bass population in the West Branch Susquehanna and Susquehanna rivers.
Figure 5: Prevalence of diseased YOY smallmouth bass (as a percentage) found in directed surveys of the Susquehanna River since the discovery of the condition in 2005.
|– Susquehanna River Biologist Geoff Smith and Area Fisheries Managers Jason Detar, Rob Wnuk, Mike Kaufmann, and Kris Kuhn|
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