J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 540 2013 NORTHEASTERN NATURALIST 20(3):540–551 Short-term Effects of Beaver Dam Removal on Brook Trout in an Appalachian Headwater Stream Jonathan M. Niles1,3,*, Kyle J. Hartman1, and Patrick Keyser2 Abstract - In North America, Castor canadensis (Beaver) impoundments of low-order streams greatly modify ecological processes, influence stream biota, and impact fish movement. We evaluated the short-term effects of removing a beaver dam from an Appalachian headwater stream on a Salvelinus fontinalis (Brook Trout) population. Prior to dam removal, we found only one marked trout that had navigated the dam moving upstream and no marked trout from above the dam moving downstream. Immediately following dam removal, trout abundance above and below the dam increased 67.1% and 46.0%, respectively. During later samples, however, declines in both trout abundance and relative weight suggest the initial large increases in our study sections after dam removal may have led to increased competition among trout, causing large numbers to move several hundred meters further upstream, beyond our study site, in order to find acceptable habitat. These results demonstrate that the presence and subsequent removal of a beaver pond on a Brook Trout stream can be considered both beneficial and harmful; thus, site-specific evaluation is necessary to determine best whether to retain or remove ponds. Introduction Impoundments by Castor canadensis Kuhl (Beaver) of low-order streams in North America greatly modify ecological processes and influence stream biota. Such impoundments alter nutrient and carbon cycles (Naiman et al. 1991), nutrient availability (Johnston and Naiman 1990), nutrient and material standing stocks and transport (McDowell and Naiman 1986), and water characteristics (Gard 1961). The creation of beaver ponds along streams results in increased nitrogen fixation (Naiman and Melillo 1984), increased supply and retention of coarse particulate organic matter, and increased downstream export of fine particulate matter (Naiman et al. 1986). Beaver dams reduce water velocities, which can lead to sediment deposition and restructuring of the benthic invertebrate community (McDowell and Naiman 1986), resulting in the availability of prey items less desirable to fish (Hale 1966). In changing lotic habitats to lentic habitats, stream temperatures are increased and dissolved oxygen concentrations are reduced (McRae and Edwards 1994). Alteration of stream temperature regimes brought about by Beaver has often been considered detrimental for salmonids in eastern North America (Alexander 1998). Beaver ponds, however, can provide important winter habitat for many stream fishes, and the importance of impoundments increases in streams lacking large deep pools (Cunjak 1996). In general, it appears that Beaver are more beneficial 1Department of Wildlife and Fisheries Resources, West Virginia University, Morgantown, WV 26506-6125. 2Department of Forestry, Wildlife and Fisheries, University of Tennessee, Knoxville, TN 37996. 3Current address - Department of Biology, Susquehanna University, Selinsgrove, PA 17870. *Corresponding author - niles@susqu.edu. 541 J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 to salmonids in coldwater streams of mountainous or semi-arid areas than they are in warmer streams of lower altitudes. Rasmussen (1941) reported they beneficially created deep pools with shade and cover for fish in coldwater mountain streams in Utah, and Alexander (1998) found that growth indices were greater for individual fish that occupy such impoundments. Salyer (1935) and Reid (1952), however, found Beaver to be generally harmful (in hindering passage) to fish in lowland streams. Beaver dams often present barriers to anadromous fish migration, especially during periods of low stream flow (Collen and Gibson 2001), and dams may degrade salmonid spawning areas by reducing stream flow and causing excessive siltation of the spawning gravel (Knudsen 1962). Management agencies and landowners remove dams for a variety of reasons, including those listed above; however, the removal of beaver dams can present problems, especially when dams are removed with explosives, as they often are (WI DNR 2005). This method results in a quick draw down of the pond, but may result in physical damage and death to fish. Alternative removal methods include tearing dams out using hand tools or a backhoe, thereby allowing fish to experience less rapid displacement and mortality; however, such methods are difficult and time consuming, and may not result in as complete a removal as blasting (WI DNR 2005). While some studies have looked at the effects of beaver dam removal on trout populations, these studies utilized explosives as the removal technique with a long time between samples (Avery 1992). We sought to determine the effect of beaver dam removal on Salvelinus fontinalis Mitchell (Brook Trout) without potential detrimental factors like rapid dewatering or explosive concussion, by evaluating the short-term effects of gradual, manual dam removal on trout abundance and movement in a central Appalachian headwater stream. Methods Study area This study was conducted in South Branch Panther Run (SBPR), a tributary of the Middlefork River, located in Randolph County, WV (Fig. 1). A commercial paper company actively managed the surrounding watershed for wood/paper production throughout the study. The surrounding forest ranged in age from 80 to 90 years and was dominated by Betula alleghaniensis Britt (Yellow Birch), Liriodendron tulipifera L. (Yellow Poplar), and Acer saccharum Marsh (Sugar Maple). All of the riparian areas were logged, mostly during the early 1900s. SBPR is a small, second-order, relatively high-gradient stream. Since the mid- 1990s, the state of West Virginia added limestone sand to the SBPR headwaters annually to enhance water quality and mitigate the effects of acid mine drainage and acid precipitation. This method of water quality enhancement has been successful in elevating stream pH, increasing macroinvertebrate abundance, and restoring fish communities in streams (Clayton et al. 1998). Stream elevation ranged from 807 to 830 m, and canopy cover ranged from 80 to 90%. Stream temperatures remained adequate for Brook Trout most of the year, but maximum daily summer temperatures (21 °C) did approach the thermal limit for the species (about 23.5 °C; Wehrly et al. 2007). SBPR has low fish J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 542 species diversity, typical of Appalachian headwater streams; Brook Trout was the dominant species, and Cottus bairdi Girard (Mottled Sculpin) was the only other species found. Data collection The study began on 19 March 2005 and concluded on 1 August 2005. Prior to dam removal, two 125-m study sections were established, one immediately above the beaver dam impoundment and one immediately below the dam (Fig. 1). Brook Trout populations were assessed on five occasions before dam removal Figure 1. Beaver pond removal study area at South Branch Panther Run, Randolph County, WV. 543 J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 and five occasions after dam removal with a two-pass electrofishing depletion method (Zippin 1958). On each occasion before sampling, block nets were placed at the upstream and downstream ends of each study section. Brook Trout were captured using a single-pulsed DC backpack electrofishing unit. All trout captured were anesthetized in a 120-mg/L solution of clove oil (Anderson et al. 1997), then measured to the nearest mm (total length), weighed to the nearest 0.5 g, and released back into the section of their capture, following completion of the second electrofishing pass and their selective marking. Brook Trout were separated into young-of-the-year (YOY) and age-1+ age classes according to length. In order to determine movement of trout before and after removal of the beaver dam, all trout over 90 mm were given a visible implant elastomer (VIE) mark (marking with VIE was not physically feasible for Brook Trout less than 90 mm). The VIE marks were implanted into the caudal fin of the trout. Trout captured above the dam were given a red mark, while trout captured below the dam were given a green mark. All Brook Trout over 90 mm were marked with the unique identifying color during each sample date. Movement was considered a change in location (upstream-to-downsteam or downstream-toupstream) from the previous sample date. The beaver dam was 3.5 m wide at the base, 3.2 m in height and 23.5 m in length. The pond behind the dam covered about 1200 m2. On 9–11 June 2005, the dam was removed manually with a pick-ax and shovels. The dam was breached at the center and was drawn down at a rate of approximately 10 cm per hour. This technique was minimally destructive, unlike the common practice of removing beaver dams with dynamite. Statistical analysis Population estimates of Brook Trout were calculated using the two-pass Zippin method in order to determine population change and estimate trout density (Zippin 1958). Estimates of the population of the two study sections were calculated by adding the populations and standard errors of each reach (Mood et al. 1974). If less than 30 fish were captured, then the actual number of fish caught was substituted for an estimate (Riley and Fausch 1992). Relative weight (Wr ), as described by Wege and Anderson (1978), was used as an index of fish condition: Wr = (W / Ws) * 100 with W = the actual weight of a fish and Ws = the standard weight for a fish of the same length. The equation used to relate standard weight (g) to total length (TL; mm) for Brook Trout followed Hyatt and Hubert (2001). The proposed metric (g and mm) standard weight equation for Brook Trout in lotic habitats is log10Ws = -5.186 + 3.103 log10TL. We did not assess condition on trout less than 120 mm, again following Hyatt and Hubert (2001). We used several before-after statistical approaches to identify possible effects of dam removal. We tested for changes immediately after the removal using a two-way analysis of variance (ANOVA) to determine differences among the two sample dates, one just prior to, and one just after dam removal. We also performed a two-way ANOVA to compare the effect of section (upstream, downstream) and treatment (before, after removal) for each of the following factors across sample J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 544 dates: age 1+ population, YOY population, and condition. A t-test was used to compare average total length of trout in the two samples before and after dam removal. To determine whether movement within a treatment was statistically biased, we employed the chi square goodness-of-fit test using the total number of upstream and downstream emigrants before and after removal. An alpha value of 0.05 was used for all statistical procedures. Results In the two weeks immediately following dam removal, mean Brook Trout populations (YOY and age 1+) above and below the dam increased 67.1% and 46.0%, respectively. Specifically, however, age 1+ trout abundance significantly increased in the upstream section (mean estimated population: before = 44, after = 63; F = 5.141, P = 0.038) and significantly decreased in the downstream section (mean estimated population: before = 72, after = 46; F = 0.098, P = 0.045) after dam removal (Fig. 2). Contrastingly, we found a significantly greater mean estimated YOY trout population in the downstream (43) compared to the upstream (31) section after dam removal (F = 10.993, P = 0.011) (Fig. 3). During the final two samples of the study (July 21 and August 1), age 1+ trout populations decreased to below pre-dam removal estimates for both sections (Fig. 2). We found significant differences before and after removal in the condition of age 1+ trout (F=8.248, P= 0.010), as mean relative weight was greater before removal (78.9 g) Figure 2. Estimated number of age 1+ Brook Trout as calculated by the Zippin method (bars with ± standard error) within downstream and upstream study sections. Abundance increased in the upstream section (P = 0.038) and decreased in the downstream section (P = 0.045). Dam and pond removal occurred 9–11 June 2005. 545 J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 than after removal (73.9 g) (Fig. 4); however, relative weight for age 1+ trout after removal was not significantly different between the upstream (74.3 g) and downstream sections (73.4 g) (F = 0.875, P = 0.362). Mean length of age 1+ trout in the upstream section was significantly greater comparing the two sample dates immediately before (129.5 mm) and after (150.4 mm) dam removal (t = 6.314, P = 0.024; Table 1). Figure 3. Estimated number of YOY Brook Trout as calculated by the Zippin method (bars with ± standard error) within downstream and upstream study sections. There were significant differences between the downstream and upstream section after dam removal (P = 0.011). Dam and pond removal occurred 9–11 June 2005. Table 1. Average total lengths (mm) of age 1+ Brook Trout before and after Beaver dam removal (occurring 9–11 June 2005). Mean lengths of trout in the upstream section were greater for the two sample dates after dam removal than before (P = 0.024). Date Downstream Upstream 19 March 125.2 114.6 3 April 140.8 120.4 17 April 140.4 122.6 18 May 141.7 127.7 8 June 143.3 131.2 13 June 139.4 150.2 29 June 138.6 150.5 11 July 130.9 140.5 21 July 125.6 136.4 1 August 130.4 133.3 J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 546 Prior to dam removal, we only detected one Brook Trout (length = 164 mm) that traversed the dam, moving upstream before the 8 June sample. Two weeks after dam removal, movement of all trout between sections significantly increased in both the upstream and downstream direction (upstream: P = 0.006, downstream: P = 0.008; Table 2). Trout movement between sections after removal Figure 4. Mean relative weight (g) of Brook Trout greater than 120 mm. There were significant differences before and after dam removal in the mean relative weight of trout (P = 0.010). Relative weight after removal was not significantly different between the upstream and downstream sections (P = 0.362). Dam and pond removal occurred 9–11 June 2005. Table 2. Percentage and number of age 1+ Brook Trout captured in study section that emigrated from the other study section. The dam was removed 9–11 June 2005. Beginning with 29 June, movement of trout between sections increased significantly (upstream: P= 0.006, downstream: P= 0.008). Downstream Upstream Date Percentage Number Percentage Number 19 March n/a n/a n/a n/a 3 April 0.00 0 0.00 0 17 April 0.00 0 0.00 0 18 May 0.00 0 0.00 0 8 June 0.00 0 2.33 1 13 June 0.00 0 2.31 3 29 June 18.52 10 20.48 17 11 July 16.67 8 12.82 5 21 July 3.23 1 31.58 12 1 August 22.73 5 26.92 7 547 J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 remained similar across dates, with the exception of samples taken on 21 July, in which a relatively large number of tagged trout (31.6%) had moved upstream and few had moved downstream (3.2%) (Table 2). Additional sampling on 5 August 2005, not related to this project, found 22 tagged Brook Trout from this study over 700 m upstream of both study sections. Discussion The immediate rapid increase and subsequent decline in Brook Trout numbers in the weeks following dam removal suggested large numbers of trout were displaced by the draining of the pond habitat created by the beaver dam. While we were not able to electroshock the pond before dam removal, it can be assumed from the increase in trout abundance within study sections after removal (June 13 sample) that the pond was inhabited by robust populations of YOY and age 1+ Brook Trout. In the 2 weeks after dam removal, the overall population of trout in the upstream section was over one and a half times the population during the two weeks prior to removal. Before dam removal, we found a significantly higher population of YOY trout in the upstream section. After dam removal we found YOY trout population decreased in the upstream section but increased in the downstream sections. This change could be attributed to the physical removal of the dam and redistribution of YOY trout into a newly accessible habitat. After removal, we found a large amount (78% of population) of untagged age 1+ fish in the upstream section. The rapid increase in the age 1+ population in a relatively small area may have resulted in increased competition for space and food resources. Previous research suggested that slow growth and high mortality of intermediate-size to large Brook Trout at high densities may be due to intense intraspecific competition for food (Rabe 1970, Reimers 1958, Walters and Vincent 1973). Relatively large numbers (n = 22) of trout apparently moved several hundred meters upstream in order to find acceptable forage and habitat presumably due to increased competition in study sections after dam removal. Moreover, high Brook Trout densities reduce prey size and density, thereby reducing the caloric reward and increasing the energetic cost of capturing prey (Rabe 1970, Reimers 1958, Walters and Vincent 1973). The decrease in mean relative weight after dam removal demonstrated that trout began to lose weight after several weeks, possibly due to decreased forage and increased competition. Johnson et al. (1992) concluded that Brook Trout growth rates decreased as density increased, an effect reported for populations in several regions (Carline 1977, Rabe 1970). The removal of the beaver pond not only removed physical habitat from the system, but may have also removed a key food source. The large surface area of the pond may have been an important component in securing food for Brook Trout. Food resource abundance for salmonids typically increases with habitat volume (Keeley and Grant 2001). Gard (1961) concluded that beaver ponds contained greater numbers and greater weights of bottom organisms than did the surrounding stream, providing more potential food items. While we were not able to measure fish found within the pond, there were increases in the size of fish found in the J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 548 upstream section of stream after dam removal, suggesting that there were larger fish inhabiting the pond. Gard (1961) compared trout of similar population densities in a stream and beaver pond and found the pond trout much larger (mean = 147 mm), compared to the stream trout (110 mm). Rutherford (1955) also found significantly larger Brook Trout in streams with beaver ponds than in adjacent streams without them. In 2005, the average pool area of the stream upstream of the dam (1000 m transect) was 11.9 m2 with a total pool area 356.2 m2 (J. Niles, unpubl. data). In this study, the beaver pond was approximately 1200 m2 of pool habitat, which was three times more than the total available pool habitat in a 1000-m reach of the stream, making it a potentially valuable in-stream food and refuge resource. The beaver pond probably functioned as an important habitat and feeding area for portions of the upstream section, too. Our results confirm those of previous studies, in which the beaver pond habitats contained larger individual Brook Trout, and such ponds may act as important habitat for nearby streams. The removal of the beaver dam allowed for increased movement upstream from downstream sections, as evidenced by the increase in tagged fish moving between the downstream and upstream reaches. Beaver dams hindered movements of autumn spawners (Brook Trout) during low flow conditions in New York (Cook 1940), western Montana (Munther 1983), and Maine (Rupp 1955). During drier months (e.g., August and September), beaver dams may present complete barriers to upstream migrating Brook Trout. Removal or collapse of a beaver dam has considerable impacts on in-stream fauna and the surrounding landscape. While most natural collapses of dams occur over several years of decay, the occasionally rapid collapse of dams has a dramatic impact. Stock and Slosser (1991) found that the catastrophic collapse of a beaver dam resulted in dramatic, short-term (≈60 days) depressions in the abundance and diversity of organisms at multiple trophic levels; as such, the increasing practice of beaver dam removal using explosives may be extremely detrimental to the fish overall population. Gard (1961) found that an initial population of 103 trout totaling 8.3 lbs decreased drastically to only 19 trout totaling 2.7 lbs after a beaver dam washed out. Neff (1957) made a similar conclusion that the removal of Beavers and drainage of their ponds resulted in a great reduction in trout numbers and fishing potential. By removing the dam slowly and drawing the water down, we were better able to assess the effect of beaver dam removal on Brook Trout populations without confounding factors like rapid dewatering or explosive concussion. The slow draw down of the pond allowed resident trout to more easily adjust to the changes in water quality, water depth, and habitat availability. Slow removal of the dam allowed for better understanding of what happens to a Brook Trout population when dams and the large habitat associated with these features are removed from a stream. The results of this study support previous research, i.e., that the presence and subsequent removal of a beaver pond on a Brook Trout stream can be considered both beneficial (increased connectivity) and harmful (increased competition for 549 J.M. Niles, K.J. Hartman, and P. Keyser 2013 Northeastern Naturalist Vol. 20, No. 3 space and resources, and subsequent population reduction). Overall, it may be necessary to assess possible trends in Brook Trout populations over several years after beaver pond removal, as the short-term effects presented here may not reflect long-term effects. 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