

How many smallmouth bass live in the North Branch of the Susquehanna River? That might seem like an impossible problem to solve but biologists can, within limits, come up with a reasonable answer. The technique we use is called a Petersen Population Estimate. To conduct the estimate, we first collect all the smallmouth bass we encounter from a stretch of river, usually by electrofishing. Next, we mark them by removing a small portion of their tail fin before putting them back. The small portion removed will grow back within several months. Next, we come back on a second night, electrofish the exact same stretch of river, and once again collect all of the smallmouths we can. Some of the fish caught on the second night will have a portion of their tails removed, indicating that we had caught them on the first night, and some will not. We can calculate how many fish are in the river using the proportion of marked to unmarked bass on the second night of electrofishing.
We mentioned that there are limits to the technique. One limit is the size of the fish. In general, night electrofishing in a river during May is not an effective way to capture smaller bass. So, we confined our estimate to smallmouths that measured at least 9 inches long. Another limit is the size of the river. The North Branch of the Susquehanna River is far too large to conduct an estimate over its entire length, so we decided to do an estimate that applied only to Section 06. Section 06 runs 36.08 miles from the confluence with Meshoppen Creek downstream to the confluence with the Lackawanna River. While Section 06 is a much more manageable stretch than the entire river, it’s still far too big to electrofish its whole length. To get around this problem, we worked a smaller site to determine how many bass that site supported per mile. Once we knew the number per mile, it was easy to calculate how many fish were in all of Section 06 because we knew how long the section was. The key to making the number per mile calculation work was to choose a site where physical habitat was representative of the section as a whole. That way, the number of bass per mile at the site would be the same as the number of bass in an average mile of the entire section. The site we chose to represent Section 06 was in the vicinity of the Pennsylvania Fish and Boat Commission Access Area at Apple Tree. Our sampling site included both sides of the river and extended the entire length of the pool. We chose this site because, like most of Section 06, one bank of the river was very rocky while the other was muddy. We sampled the site in May 2010. With all of that background out of the way, we can finally answer the question: “How many smallmouth bass over 9 inches long live in the North Branch of the Susquehanna River between Meshoppen Creek and the Lackawanna River?” The answer is 34,280, or 950 in every mile of the river. We can do the same calculations for bass > 12 inches and > 15 inches. There were 361 smallmouths > 12 inches per mile and 61 smallmouths > 15 inches per mile. It’s also interesting to look at the lengthfrequency distribution of the population estimate (Figure 1). From this graph, you’ll notice that there are peaks in the estimated numbers of fish at 10 inches and again at 13 inches. These peaks correspond to fish that are 3 and 5 years old, respectively. The 3 year old fish were born in 2007 and the 5 year old fish were born in 2005. Smallmouth bass reproduction in both of those years was above average (http://fishandboat.com/images/fisheries/afm/2009/4x11_05nbr.htm). As the fish from the large year classes produced in 2007 and 2005 grow, the number of big smallmouths in the river should increase. We normally report black bass density as Catch per Unit Effort (CPUE). For comparison’s sake, the CPUE values we obtained for smallmouth bass along the rocky shoreline at Apple Tree during the population estimate were:
We reported CPUE along the rocky shoreline only because that is the type of habitat we normally electrofish when we do our annual smallmouth bass monitoring sites. All of the CPUE values were extremely high and indicated that the smallmouth population in Section 06 was in excellent condition. The CPUE values were much higher than we would see during our typical July sampling. The river is much warmer and its flow is much lower in July, and many of the smallmouths move offshore as a result. Figure 1. Lengthfrequency distribution of the 2010 population estimate for 9 inch and larger smallmouth bass in Section 06 of the North Branch Susquehanna River.


— Rob Wnuk, Area 4 Fisheries Manager  
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