2008 NORTHEASTERN NATURALIST 15(3):363–374 Seasonal Habitat Use of Brook Trout and Juvenile Atlantic Salmon in a Tributary of Lake Ontario James H. Johnson* Abstract - The seasonal habitat use of Salvelinus fontinalis (Brook Trout) and subyearling Salmo salar (Atlantic Salmon) was examined in Hart Brook, a tributary of Lake Ontario. Fish habitat use and available habitat were examined during summer and autumn. Interspecific differences in habitat use occurred as well as intraspecific seasonal differences. Overyearling Brook Trout were more selective in their habitat preferences than subyearling Brook Trout or juvenile Atlantic Salmon. Depth and the amount of cover were significantly different among the three fish groups. Salmon occupied faster and shallower water than either age group of trout. Atlantic Salmon were also associated with larger-sized substrate materials than either trout age group, and salmon occurred in habitats with less cover than trout. Overyearling Brook Trout occupied deeper water with more cover than subyearling trout. All three salmonid groups occupied areas with more cover in autumn compared to summer. In autumn, subyearling Brook Trout used deeper areas than they had in the summer. In Hart Brook, the habitat of subyearling Atlantic Salmon can be generally characterized as riffles, the habitat of overyearling Brook Trout was deep pools with extensive cover (35%), and that of subyearling trout was any area with moderate flow and at least 20% cover. As efforts proceed to reintroduce Atlantic Salmon in Lake Ontario, further research is needed to ensure the conservation of Brook Trout populations. Introduction Competition for food and space is thought to govern salmonid populations in streams (Chapman 1966, Elliott 1994, Heggenes et al. 2002). For salmonids in lotic environments, this competition usually results in fish establishing and defending territories (Gibson 1988). Territorial behavior may facilitate intraand interspecific coexistence by maximizing the ability to partition available resources among individuals (Hearn 1987). Although competition clearly plays an important role governing the habitat use of salmonids in streams, ontogenetic factors are also important because certain habitats may be critical for completing specific life-history functions (Chapman and Bjornn 1969, Johnson and Kucera 1985, Shirvell and Dungey 1983). Several investigators have documented intraspecific differences in the habitat used by a species attributed to fish size (Bohlin 1977, Heggenes et al. 2002, Moyle and Baltz 1985). These intraspecific size-related differences in habitat use are often examined by age group (Hearn and Kynard 1986, Symons and Heland 1978). However, since growth is most rapid during the first year in stream salmonids, summer-to-autumn habitat differences of subyearlings have been documented for many species (Johnson and Dropkin 1996, Johnson and Kucera 1985, Rimmer et al. 1984). *Tunison Laboratory of Aquatic Science, US Geological Survey, 3075 Gracie Road, Cortland, NY 13045; jhjohnson@usgs.gov. 364 Northeastern Naturalist Vol. 15, No. 3 Salmo salar (Linnaeus) (Atlantic Salmon) was a major component of the Lake Ontario fish community, and Webster (1982) concluded that this population historically “represented the most striking worldwide example of freshwater colonization by Atlantic Salmon.” However, Atlantic Salmon was extirpated from the Lake Ontario watershed and has been absent from the system since 1896 (Parsons 1973). Although efforts to restore anadromous populations of Atlantic Salmon throughout New England have been underway for over 30 years, efforts examining the feasibility of restoring the species in the Lake Ontario watershed are just beginning. A potential impediment to Atlantic Salmon restoration in some southern New England watersheds (Hearn and Kynard 1986), as well as in Lake Ontario tributaries (Jones and Stanfield 1993, Johnson and Wedge 1999, McKenna and Johnson 2005), is interspecific competition with non-native Oncorhynchus mykiss (Walbaum) (Rainbow Trout). However, Raffenberg and Parrish (2003) found that subyearling Atlantic Salmon survival was highest at sites where trout abundance was greatest in Connecticut River tributaries. Coghlan and Ringler (2005) and Coghlan et al. (2007) considered warmer streams more suitable for Atlantic Salmon restoration than for Rainbow Trout because juvenile salmon have a higher thermal optima than juvenile trout. Little attention has been devoted to the impacts of Atlantic Salmon restoration on Salvelinus fontinalis (Mitchill) (Brook Trout) populations because salmon and Brook Trout have coevolved in North America. In New York tributaries of Lake Ontario, Brook Trout populations are greatly reduced due to habitat degradation (primarily loss of riparian cover that results in increased stream temperatures) and competition with non-native salmonids including Salmo trutta (Linnaeus) (Brown Trout), Oncorhynchus tshawytscha (Walbaum) (Chinook Salmon), Oncorhynchus kisutch (Walbaum) (Coho Salmon), and Rainbow Trout. In these streams, Brook Trout generally occur in headwaters or smaller tributaries that maintain suitable summer water temperatures (Johnson 1978). The purpose of this study was to examine the summer and autumn habitat use of Brook Trout and subyearling Atlantic Salmon in a small New York tributary of Lake Ontario to facilitate in the restoration and conservation of these species. Methods Summer and fall habitat use of subyearling (0+) Brook Trout, overyearling (≥1+) Brook Trout, and subyearling Atlantic Salmon was examined in Hart Brook (43°49´N, 76°01´W). Hart Brook is a second-order stream that discharges into Sandy Creek, which then discharges into eastern Lake Ontario. Although resident Brook Trout occur throughout the entire stream, small numbers of resident Brown Trout are present as well as moderate numbers of juvenile Rainbow Trout in the lower reaches. Hart Brook has extensive riparian cover, dominated by Tsuga canadenis (L. Carr) (Eastern Hemlock), that helps maintain summer water temperatures below 17 °C. 2008 J.H. Johnson 365 Approximately 2000 (about 42 mm, total length [TL]) subyearling Atlantic Salmon were stocked in Hart Brook in early May 2002. Salmon were stocked along a 0.5-km stream section that averaged about 2 m in width and had a gradient averaging 2.2%. Summer habitat evaluations began in mid-July, allowing about 2.5 months of acclimation for the stocked Atlantic Salmon. Habitat observations were also made in early October. The spot-electrofishing method was used to capture salmonids to determine their habitat use. This method, used while working upstream, is effective in small shallow streams (<12 cm mean depth) where water depth is insufficient for snorkeling (Heggenes et al. 1990, Johnson and Dropkin 1996). Sample sites were at least 3 m apart to minimize fish disturbance. At the site of each fish collection, a numbered buoy was placed, and the number, species, and age group of salmonid was recorded. Water depth, water velocity (0.6% from the surface), percent cover, and substrate size were recorded at the site of each buoy. Depth was measured with a calibrated wading rod, and water velocity recorded with a Marsh-McBirney model 201d digital flow meter. The amount of cover and substrate size were both estimated visually. Cover was quantified at 5% increments as total available cover within four fish lengths of the location of the buoy. Estimation of cover in this manner allowed more area to be considered for larger fish (i.e., overyearling trout) that are more mobile than smaller fish and use cover over a broader area (Johnson et al. 1992). Substrate size was classified using a modified Wentworth particle-size scale with values of 1 (detritus), 2 (mud), 3 (silt), 4 (sand), 5 (gravel), 6 (rubble), 7 (boulder), and 8 (bedrock) (Orth et al. 1981) and represented the same area that was used to estimate cover. Available habitat within the study reach was quantified from 25 transects across the stream about 20 m apart. Water depth, water velocity, amount of cover, and substrate size were recorded at stations spaced 0.25 m apart along each transect. The distribution of salmonid habitat and available habitat variables were compared using the Kolmogorov-Smirnov test (Slauson 1988). Ordination of salmonid habitat and available habitat was examined using principal component analysis (PCA) (ter Braak 1995, ter Braak and Smilaurer 2002). A significance level of P < 0.05 was used for all comparisons. Results A total of 360 habitat observations were made on subyearling Atlantic Salmon (141), subyearling Brook Trout (124), and overyearling Brook Trout (95) in Hart Brook in addition to 267 observations of available habitat. Subyearling Brook Trout and Atlantic Salmon were approximately the same size during each season (i.e., 60 mm TL in summer and 80 mm TL in autumn). The average size of overyearling Brook Trout was about 160 mm TL during both seasons. During each season, both age groups of Brook Trout occupied deeper areas than salmon, with overyearling trout (mean = 36 cm ± 1.17 SE) using significantly deeper areas than subyearling Brook Trout (mean = 27 cm 366 Northeastern Naturalist Vol. 15, No. 3 ± 1.0 SE) (Fig. 1A, Table 1). The depths where trout occurred were always significantly deeper than the average depth in the study reach (mean = 16 cm ± 1.0 SE). There was no difference between the depths where subyearling Atlantic Salmon occurred in autumn and available depths, but salmon were found at significantly greater depths than were present in the study reach during summer (Table 1). Figure 1A and B. Distribution of subyearling Atlantic Salmon (ATS), subyearling Brook Trout (BT 0+), overyearling Brook Trout (BT 1+ ) and available habitat (AH) during summer and autumn in Hart Brook. (A) = depth and (B) = velocity. 2008 J.H. Johnson 367 Table 1. Results of Kolmogorov-Smirnov tests comparing differences in habitat variables of Brook Trout and subyearling Atlantic Salmon and habitat used by salmonids with available habitat in Hart Brook. Values presented are means with standard error in parenthesis. ATS 0+ = subyearling Atlantic Salmon, BT 0+ = subyearling Brook Trout, BT 1+ = overyearling Brook Trout, AH = available habitat, Asterisk (*) denotes significant difference. Comparison Depth (cm) Velocity (cm/s) Cover (%) Substrate Summer ATS 0+ - AH 17.9 (0.8)–15.3 (1.0)* 10.4 (1.0)–7.0 (0.8) 13.7 (1.4)–8.8 (0.9)* 5.5 (0.08)–5.0 (0.07)* BT 0+ - AH 25.5 (1.6)–15.3 (1.0)* 4.3 (0.6)–7.0 (0.8) 18.8 (1.5)–8.8 (0.9)* 4.9 (0.1)–5.0 (0.07) BT 1+ - AH 35.2 (2.2)–15.3 (1.0)* 3.4 (0.5)–7.0 (0.8)* 33.9 (2.5)–8.8 (0.9)* 4.3 (0.1)–5.0 (0.07)* ATS 0+ - BT O+ 17.9 (0.8)–25.5 (1.0)* 10.4 (1.0)–4.3 (0.6)* 13.7 (1.4)–18.8 (1.5)* 5.5 (0.08)–4.9 (0.1)* ATS 0+ - BT 1+ 17.9 (0.8)–35.2 (2.2)* 10.4 (1.0)–3.4 (0.5)* 13.7 (1.4)–33.9 (2.5)* 5.5 (0.08)–4.3 (0.1)* BT 0+ - BT 1+ 25.5 (1.6)–35.2 (2.2)* 4.3 (0.6)–3.4 (0.5) 18.8 (1.5)–33.9 (2.5)* 4.9 (0.1)–4.3 (0.1)* Autumn ATS 0+ - AH 17.0 (1.7)–16.0 (0.9) 26.7 (3.3)–13.2 (1.5)* 20.2 (1.3)–8.6 (1.0)* 5.8 (0.08)–5.4 (0.07)* BT 0+ - AH 28.1 (1.4)–16.0 (0.9)* 6.6 (0.8)–13.2 (1.5)* 30.4 (2.0)–8.6 (1.0)* 5.4 (0.07)–5.4 (0.07)* BT 1+ - AH 37.6 (2.7)–16.0 (0.9)* 6.1 (1.6)–13.2 (1.5)* 42.5 (3.0)–8.6 (1.0)* 5.2 (0.1)–5.4 (0.07)* ATS 0+ - BTO+ 17.0 (1.7)–28.1 (1.4)* 26.7 (3.3)–6.6 (0.8)* 20.2 (1.3)–30.4 (2.0)* 5.8 (0.08)–5.4 (0.07)* ATS 0+ - BT 1+ 17.0 (1.7)–37.6 (2.7)* 26.7 (3.3)–6.1 (1.6)* 20.2 (1.3)–42.5 (3.0)* 5.8 (0.08)–5.2 (0.1)* BT 0+ - BT 1+ 28.1 (1.4)–37.6 (2.7)* 6.6 (0.8)–6.1 (1.6) 30.4 (2.0)–42.5 (3.0)* 5.4 (0.07)–5.2 (0.1)* Seasonal Summer-Autumn ATS 0+ 17.9 (0.8)–17.0 (1.7) 10.4 (1.0)–26.7 (3.3)* 13.7 (1.4)–20.2 (1.3)* 5.5 (0.08)–5.8 (0.08)* BT 0+ 25.5 (1.6)–28.1 (1.4)* 4.3 (0.6)–6.6 (0.8) 18.8 (1.5)–30.4 (2.0)* 4.9 (0.1)–5.4 (0.07)* BT 1+ 35.2 (2.2)–37.6 (2.7) 3.4 (0.5)–6.1 (1.6) 33.9 (2.5)–42.5 (3.0)* 4.3 (0.1)–5.2 (0.07)* AH 15.3 (1.0)–16.0 (0.9) 7.0 (0.8)–13.2 (1.5)* 8.8 (0.9)–8.6 (1.0) 5.0 (0.07)–5.4 (0.07) 368 Northeastern Naturalist Vol. 15, No. 3 Subyearling Atlantic Salmon occupied significantly faster areas (mean = 19.6 cm/s ± 2.4 SE) than either subyearling Brook Trout (mean = 5.5 cm/s ± 0.7 SE) or overyearling Brook Trout (mean = 4.8 cm/s ± 1.3 SE) during both seasons (Fig. 1B, Table 1). The water velocities where salmon were found were also significantly faster, on average, than velocities throughout the study reach (mean = 10.1 cm/s ± 1.3 SE) of Hart Brook. Figure 2A and B. Distribution of subyearling Atlantic Salmon (ATS ), subyearling Brook Trout (BT 0+ ), overyearling Brook Trout (BT 1+ ) and available habitat (AH ) during summer and autumn in Hart Brook. (A) = cover and (B) = substrate. 2008 J.H. Johnson 369 During both seasons, less than 20% of the subyearling Atlantic Salmon were found at water velocities less than 0.5 cm/s, whereas about 55% of the subyearling and 70% of the overyearling Brook Trout occurred at these velocities. Except for subyearling Brook Trout during summer, the velocities where both subyearling and overyearling Brook Trout occurred were significantly less than velocities available within the study reach (Table 1). During both seasons, all three salmonid groups were associated with more cover than was, on average, present in the study reach. Overyearling Brook Trout used significantly more cover (mean = 38% ± 2.1 SE) than the other two salmonid groups, and subyearling trout (mean = 25% ± 1.4 SE) used significantly more cover than subyearling Atlantic Salmon (mean = 17% ± 1.1 SE) (Fig. 2A, Table 1). Subyearling Atlantic Salmon used significantly larger-sized substrate than either age group of Brook Trout (Fig. 2B, Table 1). The substrate where salmon occurred was also significantly larger than available substrate, whereas the size of substrate used by overyearling Brook Trout was significantly smaller than what was generally available in the study reach of Hart Brook. There was no significant difference in the size of substrate used by subyearling Brook Trout and the size of available substrate materials (Table 1). The only available habitat variable that changed from summer to autumn was velocity, which was significantly higher in autumn (Fig. 1B, Table 1). Consequently, any discussion of intraspecific seasonal differences in velocities where salmonids were found must take this into consideration. Although mean water velocities occupied by all three salmonid groups were greater in autumn, the difference was only significant for subyearling Atlantic Salmon (Fig. 1B, Table 1). Intraspecific seasonal differences among the three salmonid groups were most pronounced for cover and substrate. Subyearling Atlantic Salmon and subyearling Brook Trout were found in areas that had significantly more cover and larger-sized substrate materials in fall (Figs. 2A and 2B, Table 1). Using principal component analysis (PCA), Axis 1 explained 94.8% of the variation and axis 2 explained 5.1% (Table 2, Fig. 3). PCA indicated that subyearling Atlantic Salmon habitat during both summer and fall was characterized by higher velocity and larger substrate size (Fig. 3). Summer habitat of subyearling Brook Trout and summer and fall habitat of overyearling trout was largely associated with higher cover and depth, whereas fall habitat use of subyearling trout was influenced by increased depth and substrate size. PCA also revealed that habitat selection in Hart Brook was most pronounced for overyearling Brook Trout (Fig. 3). Discussion The three groups of salmonids (i.e., subyearling Atlantic Salmon, subyearling Brook Trout, overyearling Brook Trout) generally occupied 370 Northeastern Naturalist Vol. 15, No. 3 different habitats in Hart Brook. Subyearling Atlantic Salmon used riffles and subyearling trout occupied areas with moderate flow (i.e., <10 cm/s) and at least 20% cover. Overyearling Brook Trout were found in deep pools with substantial cover (>35%). These findings are consistent with previous research on the habitat use of these species (Cunjak and Green 1983; DeGraaf and Bain 1986; Johnson and Dropkin 1996; Morantz et al. 1987; Rimmer et al. 1983, 1984; Sotiropoulos et al. 2006). However, Hedger et al. (2005) reported that depth and substrate size were more important in determining habitat use of juvenile Atlantic Salmon in a Quebec river. In both allopatry and sympatry with Brook Trout, subyearling Atlantic Salmon occupy riffles, and their use of these fast-water habitats is thought to be facilitated by their large pectoral fin which allows them to maintain position when placed in contact with the substrate (Kalleberg 1958). Saunders (1965) speculated that greater buoyancy of Brook Trout as compared to juvenile Atlantic Salmon may provide an advantage for trout at slower water velocities. Juvenile Atlantic Salmon move to deeper areas as they grow (Symons and Heland 1978), and in Lake Ontario tributaries, most Atlantic Salmon will smolt at two years (McKenna and Johnson 2005). Consequently, these pre-smolt overyearling Atlantic Salmon will likely occupy deeper stream areas with Brook Trout, thus increasing the possibility of inter-specific competition. Mookerji et al. (2004) found significant differences in the diet of overyearling Atlantic Salmon and Brook Trout in sympatry. However, Figure 3. Ordinal representation of habitat data using principal component analysis (PCA). ATS 0+ = subyearling Atlantic Salmon, BT 0+ = subyearling Brook Trout, BT 1+ = overyearling Brook Trout, AH = available habitat, s = summer, f Table 2. Weighted scores for habitat variables and eigenvalues for the first two PCA axes. Habitat variable Axis 1 Axis 2 Depth 0.948 0.319 Velocity -0.576 -0.054 Substrate -0.697 0.264 Cover 0.987 -0.159 Eigenvalue 0.948 0.051 2008 J.H. Johnson 371 they also reported that allopatric Brook Trout fed nearly twice as much as sympatric trout during the day and concluded that this was due to interspecific interactions with Atlantic Salmon. The amount of cover used was the only habitat variable that was always significantly greater for all three groups of salmonids than what was available within the study reach of Hart Brook. The amount of cover used by all three groups of salmonids was also significantly greater in autumn than in summer. These findings are consistent with previous work on subyearling Atlantic Salmon (Rimmer et al. 1984) and Brook Trout (Johnson and Dropkin 1996). Cunjak and Green (1983) reported that Brook Trout were found at slower water velocities and used more cover than Rainbow Trout. They further concluded that the presence of Rainbow Trout did not influence Brook Trout habitat since there was no difference in habitat use of Brook Trout in allopatry or sympatry. An increase in the amount of cover used by subyearling stream salmonids in autumn is thought to be due to an increase in fish size (i.e., 20–30 mm) and is sometimes associated with an increase in the size of substrate materials as well (i.e., they occur over larger substrate materials). This association is especially true for subyearling salmonids that occupy riffles such as Atlantic Salmon (Rimmer et al. 1984) and Rainbow Trout (Johnson and Kucera 1985). In Hart Brook, subyearling Atlantic Salmon were always associated with significantly larger-sized substrate material than either age group of Brook Trout. Overyearling Brook Trout used smaller-sized substrate materials than subyearling trout, suggesting that subyearling trout were utilizing substrate for cover and substrate size was not an important factor governing habitat use of overyearling Brook Trout. Decreasing stream temperatures also influence the habitat use of Brook Trout and juvenile Atlantic Salmon. In autumn as stream temperature approaches 10 °C, Atlantic Salmon begin sheltering in rubble as do Brook Trout to a lesser extent (Gibson 1978, Rimmer et al. 1983), although there is also some evidence of sheltering at much higher temperatures (Gries and Juanes 1998). Stream temperature in Hart Brook in early October was 11.5 °C, so sheltering by the majority of stream salmonids had probably not begun. When considering all available habitat in Hart Brook, habitat selection was most pronounced for overyearling Brook Trout (i.e., their habitat centroids diverged most from available habitat centroids in the PCA). PCA was useful in determining which habitat variables were most important to each of the three salmonid groups on a seasonal basis. Relatively fast current velocity and large substrate size were important to subyearling Atlantic Salmon, whereas greater depth and cover were important to Brook Trout. However, in autumn, increased substrate size was also important to subyearling trout. In Hart Brook, these three groups of salmonids used different habitat during summer and autumn. Further research is needed to examine ecological associations between juvenile Atlantic Salmon and Brook Trout (especially overyearlings) in Lake Ontario tributaries to better understand the effects of reintroduced Atlantic Salmon on native Brook Trout populations. 372 Northeastern Naturalist Vol. 15, No. 3 Acknowledgments I would like to thank Tim Wallbridge and Kevin Douglass for assistance in the field and Marc Chalupnicki and Jim McKenna for assistance with data analysis. This article is Contribution 1452 of the USGS Great Lakes Science Center. Literature Cited Bohlin, T. 1977. Habitat selection and intercohort competition of juvenile Sea-trout, Salmo trutta. Oikos 29:112–117. Chapman, D.W. 1966. Food and space as regulators of salmoid populations in streams. American Naturalist 100:345–357. 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