Northeastern Naturalist 424 L.A. Warner, .F. Purchase, and G. Veinott 22001155 NORTHEASTERN NATURALIST 2V2(o2l). :2422,4 N–4o3. 62 Seasonal Variation in Estuarine Habitat Use by Native Atlantic Salmon (Salmo salar) and Invasive Brown Trout (Salmo trutta) in Southeast Newfoundland Lucas A. Warner1,2, Craig F. Purchase1,*, and Geoff Veinott3 Abstract - In North America, Salmo salar (Atlantic Salmon) populations evolved in the absence of Salmo trutta (Brown Trout) and use estuaries more extensively than in Europe. European Brown Trout were introduced to Newfoundland in the 1880s and are spreading along its coast. Most of the colonized watersheds include distinct estuaries. Unlike for river habitats, knowledge of estuary use by Brown Trout outside of their native range is very limited. We investigated seasonal estuary use by Atlantic Salmon and Brown Trout of different sizes in eastern Newfoundland. We observed parr-sized Brown Trout in June and July, whereas Atlantic Salmon parr were present from April to August. Smolts of both species were most prevalent in spring, but we found them throughout the year. Large Brown Trout were present in the estuary over much of the year. Brown Trout are a freshwater invasive species in much of their introduced range and their year-round estuarine presence in our system raises concerns for native salmonids, particularly Atlantic Salmon on Newfoundland’s south coast, which have been identified as at-risk and evolved in the absence of these invaders. Introduction Migration is return movement between different habitats and occurs at varied temporal/spatial scales. In temperate latitudes, the seas are generally more productive than freshwater systems (Gross 1987), and many species of freshwaterspawning fish migrate to salt water (i.e., become anadromous) to feed. These migrations are often facultative—many anadromous species do not have to go sea to complete their life cycles, which is an example of partial migration (Secor and Kerr 2009). Anadromous migration often includes movement through estuaries, highly productive feeding habitats that also function as physiological-transition zones for some diadromous fishes (Etheridge et al. 2008). Salmo trutta (Brown Trout ) are native to Eurasia, but are commonly transplanted and have been successfully introduced nearly worldwide (Jonsson and Jonsson 2011). As a result of their impacts to native fishes (e.g., New Zealand [McDowall 2006], South America [Pascual 2007], North America [van Zyll de Jong et al. 2004]), they are listed as one of the 100 worst invasive alien species (Lowe et al. 2000). Salmonids are indigenous to the northern hemisphere, 1Fish Evolutionary Ecology Research Group, Department of Biology, Memorial University, St. John’s, NL, A1B 3X9, Canada. 2Current address - Stantec, 200–325 25th Street SE, Calgary, AB, T2A 7H8, Canada. 3Northwest Atlantic Fisheries Center, Fisheries and Oceans Canada, Box 5667, St. John’s, NL, A1E 2H8, Canada. *Corresponding author - craig.purchase@mun.ca. Manuscript Editor: John Waldman Northeastern Naturalist Vol. 22, No. 2 L.A. Warner, .F. Purchase, and G. Veinott 2015 425 which is the setting for interactions between invasive Brown Trout and their close relatives. In North America, competition between introduced Brown Trout and populations of Salmo salar (Atlantic Salmon, native to eastern North America, where they evolved in the absence of Brown Trout, and also western Europe, where they co-evolved with Brown Trout), and Salvelinus fontinalis (Brook Charr, endemic to eastern North America, where they evolved in the absence of Brown Trout) has long been a concern in rivers (Fenderson 1954, Waters 1983). However, interactions in estuaries are undocumented. Furthermore, how facultative anadromous fishes use different habitats when introduced to novel areas, in general, is poorly understood. Little is known of Brown Trout estuary use outside of their native range (Hustins 2007, Scott and Crossman 1964). In Europe, adults and juveniles use estuaries extensively (Knutsen et al. 2001, 2004; Rikardsen et al. 2006), whereas Atlantic Salmon generally do not (Jonsson and Jonsson 2009, Klemetsen et al. 2003). Interestingly, our review yielded only one limited study, conducted in Denmark, which investigated estuary use by both species within the same watershed (Koed et al. 2006). Furthermore, there is a dearth of information for either species on seasonal estuary use by individuals at different life stages. Contrary to habitat-use patterns where the 2 species co-evolved in Europe, juvenile Atlantic Salmon have been repeatedly shown to exploit estuaries in eastern Canada (Thorpe 1994), which may or may not be related to the absence of Brown Trout—a known predator (Larsson 1985). The extensive work on estuarine use by Atlantic Salmon parr conducted by Cunjak (1992) and colleagues (Cunjak et al. 1989, 1990) in northern Newfoundland was in an area not yet colonized by Brown Trout. Here we provide the first comparison of estuary use between Brown Trout and Atlantic Salmon in a North American watershed, by season, for different size classes. One of the earliest introduction sites of Brown Trout in North America was in the 1880s near the city of St. John’s on the island of Newfoundland (Hustins 2007, Scott and Crossman 1964), which is now part of Canada. Introduced individuals were initially descendants of non-migratory fish, and stocking activities ceased in the early 1900s (Hustins 2007). However, Brown Trout are spreading and are now established in numerous watersheds on the Avalon, Burin, and Bonavista peninsulas (Westley and Fleming 2011). It has been suggested that straying anadromous individuals established these new populations (van Zyll de Jong et al. 2004, Westley and Fleming 2011). Methods Study area We sampled the Renews River estuary (46º55'N, 52º56'W), NL, Canada ~90 km south of St. John’s (Fig. 1). The estuary is connected to a narrow bay, ~0.5 km x 3 km, before becoming open ocean. The estuary is relatively small (18.6 ha) and shallow with a tidal amplitude of 0.7–1.5 m. Many shallow areas become dry during the lowest tides; however, deeper trenches remain filled with water at all Northeastern Naturalist 426 L.A. Warner, .F. Purchase, and G. Veinott 2015 Vol. 22, No. 2 times. A human-made breakwater largely encloses the estuary and protects it from easterly winds and wave action. Veinott (2009) determined mean salinities to be 3.2 and 7.9 psu at the surface and bottom, respectively. The substrate consists of a gently sloping bottom of small rocks and gravel, which extends 5–10 m offshore. Silt and mud accumulate with increasing distance from shore and aquatic vegetation, including Zostera marina L. (Eelgrass) and Ascophyllum nodosum (L.) Le Jolis (Rockweed), is abundant throughout. Phoca vitulina L. (Harbor Seal) are regularly present, and a small island supports a seasonal Sterna paradisaea Pontoppidan (Common Tern ) nesting colony (Veinott 2009). The amount of Atlantic Salmon and Brown Trout eaten by these animals has not been quantified. Anglers regularly target anadromous Brown Trout in the estuary (Veinott 2011). Atlantic Salmon reproduce (1 sea-winter grilse) throughout the watershed (as evidenced by presence of young of year), Brown Trout reproduce in the lower main river, and large numbers of Brook Charr reproduce in the upper tributaries (Warner 2013). Fish sampling We specifically chose 7 sampling sites within the confined estuary (Fig. 1) because they could be consistently sampled throughout the duration of this study. We undertook preliminary exploration, testing, and careful planning to ensure that we would be able to make unbiased seasonal comparisons in catch rates. The Figure 1. Map showing location of the Renews River estuary, NL, Canada. Northeastern Naturalist Vol. 22, No. 2 L.A. Warner, .F. Purchase, and G. Veinott 2015 427 closest site to the river was 320 m from where it enters the estuary. We conducted standardized sampling of the estuary over an entire year, commencing in May 2009. Sampling was bi-monthly and timed to occur when peak-high tide was between the daylight hours of 10:00 and 16:00. We set nets on 2 days within each bi-monthly high-tide cycle, and on each day all sampling occurred within a 4-h time window (2 h ± high tide) to maintain consistency. Temporary ice-cover prevented sampling during 1 tide cycle in each of December 2009, January 2010, and February 2010; however, we successfully sampled the other tide cycle within each of these 3 months. We used 2 gear types (beach seine and gill net) because they could effectively sample the estuary and capture different-sized fish. We sought to examine temporal changes in the combined catch from all standard daily fishing effort. We chose 5 sites to sample habitats that could not be effectively seined and used sinking gill-nets in these locations. Each gill net had 2 panels (2.54-cm and 5.08-cm stretched mesh), that measured 30 m x 1.8 m; we chose this size mesh to focus our effort on sizes of fish likely to be captured with reasonable frequency. No multi sea-winter salmon are present in this part of Newfoundland (thus virtually all spawners found here are <60 cm) and anglers rarely catch Brown Trout >50 cm in this estuary (Veinott 2011). We standardized the location and fishing time of each gill-net set. We set out gill nets for 1 hour to avoid sampling mortality. We released all fish and sampled each gill-net site once each day, except one site, which was sampled twice each day. We sampled 2 sites using a large beach seine that we deployed from a boat. We specifically chose these locations because they were free of large rocks and could be effectively seined; no other suitable sites existed. The net measured 22 m by 2 m, with 35-m ropes attached to spreader bars on the end of each wing. The wings of the seine consisted of 19-mm stretched mesh, and the codend consisted of 13-mm stretched mesh, with a 9-mm liner. We placed standardized landmarks on shore above the monthly peak high-tide water level, deployed the seine 35 m from shore between these landmarks, and then pulled it onshore (1 pull = 1 sample). The area sampled during each pull was approximately 550 m2. We sampled each seine site twice (2 pulls) each day. For both sampling methods described above and all fish captured, we counted, identified to species, and recorded fork length before releasing them. It can be difficult to identify an individual’s life-cycle stage because the expression of phenotypic traits, including color and body shape, can vary between populations of salmonids. Because we did not record clear distinctions between parr, smolt, and adult life-cycle stages, we grouped fish into 3 size classes (Fig. 2). We classified individuals that measured ≤100 mm as small, and were typically parr; fish that measured >100 mm and less than 250 mm as medium, and typically represented individuals that were undergoing or had completed smoltification (typically called smolts), which is the physiological transition for life in the marine environment; and individuals that measured ≥250 mm as large—these were post-smolts or adults. Northeastern Naturalist 428 L.A. Warner, .F. Purchase, and G. Veinott 2015 Vol. 22, No. 2 Temperature data We deployed 4 temperature loggers in the study area: 2 in the estuary and 2 in the harbor. Each logger recorded temperature every hour, was installed next to a large boulder, and always remained under water (~2 m). We initially calibrated loggers by immersion in buckets of water of known temperature. We averaged by month the temperature data retrieved from each logger, and chose the average of the 2 loggers to represent the monthly water temperature of the estuary and harbor. We also recovered temperature data from the Department of Fisheries and Oceans Canada (DFO) Station 27 (47º32'N, 52º35'W; located approximately 8 km outside of St. John’s harbor) from May 2009 to April 2010. Station 27 is the first hydrographic monitoring station in the DFO standard St. John’s to Flemish Cap transect, which was established in 1946. The station is located within the Avalon Channel and is used to represent typical water temperature on the continental shelf. We selected the average monthly surface temperature (less than 10 m depth) to represent water temperature of the open ocean, except for January and February 2010 when data were not collected. Figure 2. Summary of the number of Atlantic Salmon and Brown Trout captured, by fork length. Individuals were grouped into 3 size classes: small (≤100 mm; n = 35 Atlantic Salmon, 29 Brown Trout), medium (>100 mm and less than 250 mm; n = 734 Atlantic Salmon, 193 Brown Trout), and large (≥250 mm; n = 1 Atlantic Salmon, 28 Brown Trout). Northeastern Naturalist Vol. 22, No. 2 L.A. Warner, .F. Purchase, and G. Veinott 2015 429 Statistical analyses We employed the R statistical package version 2.10.0 to conduct fish-catch precision statistics, set significance at α = 0.05, and tested assumptions of parametric statistics by examining model residuals. To determine the variability at each sampling level, we examined variance components for the number of fish captured using each sampling gear (seine and gill nets) for random factors with a fully nested analysis of variance (ANOVA). Response variables were catch-numbers of large, medium, and small size-classes for Atlantic Salmon and Brown Trout, and a pool of all other species. Random factors for the seine-net catches were season, month, tide cycle, day, site, and set, while gill-net catches were season, month, tide cycle, day, and site (the second set was removed from the one duplicated gill-net site for the purposes of this analysis). Large Atlantic Salmon were not captured using seine nets, and small Atlantic Salmon and Brown Trout were not captured using gill nets. In response to the high incidence of zero-catch, we transformed data using the equation Catch = √(Catch + 0.5) This transformation, recommended for data with a high incidence of zeros, produced the model that best fit our data (Sokal and Rohlf 1995, Za r 1998). Fine-scale spatial and temporal variability was much more pronounced than large temporal trends. Results from the variance-component analysis (Fig. 3) show that the majority of the variability in catch was among sets and sites when sampling with seine nets (74% average across fish species and size classes), and among sites when sampling with gill nets (92% average across fish species and size classes). These results illustrate that catch results can vary greatly among samples and sites, and demonstrate the importance of repeated sampling. Results We captured a total of 19,848 fish representing 16 species during the 12-month sampling period (Table 1). Gasterosteus aculeatus (Threespine Stickleback; 62%), Apeltes quadracus (Fourspine Stickleback; 26%), and Myoxocephalus scorpioides (Arctic Sculpin; 5%) accounted for 93% of the total catch. These species of small fishes were also the most consistently captured. We captured a total of 1024 salmonids representing 3 species: Atlantic Salmon (n = 770), Brown Trout (n = 250), and Brook Charr (n = 4). Salmonids accounted for 5% of the total number of fish collected. Atlantic Salmon accounted for 75% of salmonids collected (most of which were caught on 1 day, see below). Although only 24% of salmonids collected were Brown Trout, it was the dominant large fish among all species captured. Atlantic Salmon and Brown Trout catch-rates varied by season and body size (Fig. 4). We captured small (parr size) Atlantic Salmon from May to August 2009, and again in April 2010 (none from September 2009 to March 2010); they were most abundant during spring. Similar-sized Brown Trout were only present in June and July (Fig. 4). Northeastern Naturalist 430 L.A. Warner, .F. Purchase, and G. Veinott 2015 Vol. 22, No. 2 Medium salmonids (smolt size) were the most frequently captured size class and were present in every month except January and March 2010. We captured a single large pulse on 13 May 2009 that consisted of 673 Atlantic Salmon and 54 Brown Trout—fish collected in this sample accounted for 92% and 27% of medium Atlantic Salmon and Brown Trout, respectively, captured during the study. This aggregation was only present that one day, and the large pulse greatly affected relative abundance comparisons between Atlantic Salmon and Brown Trout on an annual Figure 3. Results of 11 fully nested analyses of variance (ANOVAs) separating variance components for fish-catch results for (a) gill-net sites and (b) seine-net sites, among seasons (n = 4), months within seasons (n = 3), tide cycles within months (n = 2), days within tide cycles (n = 2), sites within days (n = 2 for seines, 5 for gill nets) and, for seinenet sites only, sets within sites (n = 2). Fish species included Atlantic Salmon, Brown Trout, and all other species. Salmonids were grouped into 3 size classes: (≤100 mm; n = 35 Atlantic Salmon, 29 Brown Trout), medium (>100 mm and less than 250 mm; n = 734 Atlantic Salmon, 193 Brown Trout), and large (≥250 mm; n = 1 Atlantic Salmon, 28 Brown Trout). Northeastern Naturalist Vol. 22, No. 2 L.A. Warner, .F. Purchase, and G. Veinott 2015 431 basis. Including all sampling events (42 days), we captured more medium-size Atlantic Salmon than Brown Trout; however, excluding the 13 May sample, more Brown Trout were present in the other 41 samples. On 13 August 2009, we captured the only large Atlantic Salmon observed during the standardized sampling period; it had a fork-length of 562 mm (2 kelts were captured during preliminary work in April 2009). We captured large Brown Trout from May to October 2009, December 2009, and April 2010, with largest catches in spring (May–June 2009, and April 2010). Average water temperature in the Renews River estuary was consistently higher than in the harbor and the ocean during May–November 2009 and reached a maximum of 16 ºC in August 2009 (Fig. 4). Based on the available temperature data from our loggers and from DFO Station 27, the average water temperature in the estuary, harbor, and the ocean remained above 0 ºC throughout the winter months. Water temperatures measured at DFO Station 27 during the winter of 2009–2010 (December 2009, March and April 2010) were the warmest since recording started in 1946 (Colbourne et al. 2011). Discussion This study represents the first reported investigation of seasonal estuary use by Atlantic Salmon and Brown Trout outside of the native range of the latter. Atlantic Salmon use estuaries more extensively in North America, which we suggest may be Table 1. Summary of fish caught in the Renews River estuary with 2 gears: gill net and seine net (May 2009–April 2010) % of Gill Seine total Scientific name Common name net net Total catch Gasterosteus aculeatus L. Threespine Stickleback 3 12,276 12,279 61.865 Apeltes quadracus (Mitchill) Fourspine Stickleback 1 5222 5223 26.315 Myoxocephalus scorpioides Arctic Sculpin 2 959 961 4.842 (Fabricius) Salmo salar L. Atlantic Salmon 6 764 770 3.879 Salmo trutta L. Brown Trout 28 222 250 1.260 Gadus ogac Richardson Greenland Cod 3 170 173 0.872 Gasterosteus wheatlandi Putnam Blackspotted Stickleback 0 96 96 0.484 Pseudopleuronectes americanus Winter Flounder 1 44 45 0.227 Walbaum Anguilla rostrata (Lesueur) American Eel 0 19 19 0.096 Osmerus mordax (Mitchill) Rainbow Smelt 3 8 11 0.055 Clupea harengus L. Atlantic Herring 3 4 7 0.035 Urophycis tenuis (Mitchill) White Hake 0 6 6 0.030 Salvelinus fontinalis (Mitchill) Brook Charr 0 4 4 0.020 Ammodytes americanus Kendall American Sandlance 0 2 2 0.010 Cyclopterus lumpus L. Lump Fish 1 0 1 0.005 Pholis gunnellus L. Rock Gunnel 0 1 1 0.005 Total 19,848 100.000 Northeastern Naturalist 432 L.A. Warner, .F. Purchase, and G. Veinott 2015 Vol. 22, No. 2 Figure 4. Monthly catch of (a) small, (b) medium, and (c) large fish and average temperature in the estuary, harbor, and DFO station 27. Catch was summed to tide cycle; data points represent average catch-per-month and error bars (only shown below the mean) represent standard deviation between 2 high-tide cycles. Error bars are absent from December 2009–February 2010 because only 1 tide cycle was sampled; all other data points where error bars are absent is because catches of both tide cycles were identical. Temperatures were unavailable from the harbor for April–October 2009, and from DFO station 27 for January–February 2010. Y-axis is at different scales for each size class. Northeastern Naturalist Vol. 22, No. 2 L.A. Warner, .F. Purchase, and G. Veinott 2015 433 due to the absence of Brown Trout. Our findings highlight increased inter-specific overlap between these species in North America compared to Europe, and the potential for negative effects on recovering Atlantic Salmon populations. Brown Trout estuary use in this invaded system was similar to use patterns reported in their native Europe, where extensive documentation indicates they feed heavily (Knutsen et al. 2001, 2004; Rikardsen et al. 2006). Recent work by Westley and Fleming (2011) found that the majority of systems in Newfoundland colonized by Brown Trout include distinct estuaries that are important conduits of source– sink population dynamics related to recreational fisheries (Veinott et al. 2012). We observed salmonids of all life stages in the estuary through much of the year. This finding supports the belief that Brown Trout tend to remain in coastal areas and do not migrate very far from their home rivers (Jonsson and Jonsson 2006, Klemetsen et al. 2003, O’Connell 1982). Brown Trout have poorer osmoregulatory capacity to tolerate salt water than Atlantic Salmon, which is further compromised at low temperatures (see Jonsson and Jonsson 2011). Coastal Newfoundland waters are very cold in winter, and we expected to see an influx of Brown Trout into the estuary; however, this movement did not occur which may or may not have been due to winter 2009–2010 being the warmest on record (Colbourne et al. 2011). In contrast, Atlantic Salmon are generally thought to not utilize estuaries for feeding, simply moving through them during spring migration or return movements from the sea (Jonsson and Jonsson 2009, Klemetsen et al. 2003). However, adults may hold in estuaries for several months if river-water levels are insufficient for passage upstream on return migration (Jonsson and Jonsson 2009, Jonsson et al. 2007), and kelts may overwinter in estuaries if suitable habitat is limited in fresh water (Hubley et al. 2008). We found juvenile Atlantic Salmon in the estuary throughout much of the year, and that the smolt run was more temporally compressed in Atlantic Salmon than Brown Trout. Similar to our results, other studies have shown Atlantic Salmon to be present in estuaries through spring and summer (Cunjak et al. 1989, Klemetsen et al. 2003, Pinder et al. 2007). Parr have low salinity tolerances (Parry 1960), and the general consensus is that they do not occur in salt water (Jonsson and Jonsoon 2011). However, we found that small parr-sized Atlantic Salmon used the Renews River estuary extensively, especially in spring. This finding is consistent with observations from a few other studies (Cunjak 1992; Cunjak et al. 1989, 1990; Thorpe 1994), and suggests juvenile Atlantic Salmon exploit estuaries to a greater extent in eastern Canada than Europe. Differences in habitat use by Atlantic Salmon in Europe and North America may be due to the presence or absence of Brown Trout. A substantial portion of parr from many Canadian Atlantic Salmon populations likely rely on estuaries for food. These fish are probably in direct competition with invasive juvenile Brown Trout and are also appropriate-sized prey for large Brown Trout (the dominant large fish present in our estuary). Brown Trout have been shown to be a key predator to juvenile Atlantic Salmon during seaward migrations through estuary and coastal habitats in Europe (Larsson 1985). The Committee on the Status of Endangered Wildlife in Canada recently suggested that hundreds of Northeastern Naturalist 434 L.A. Warner, .F. Purchase, and G. Veinott 2015 Vol. 22, No. 2 Atlantic Salmon populations are at risk; those from southern Newfoundland are considered threatened (COSEWIC 2010). Our finding of nearly year-round estuary use by various-sized Brown Trout and juvenile Atlantic Salmon presents a potentially serious threat to the recovery and sustainability of eastern North America’s native Atlantic Salmon populations. With Brown Trout continuing to disperse to new systems, including those with threatened Atlantic Salmon populations, more research is needed to better understand how they affect salmon. Naturalized Brown Trout are prized game by anglers in eastern Newfoundland, and more information is needed to decide whether the species should be managed as a sport fish or an invasive species. Acknowledgments We would like to thank the many people who helped with fieldwork, including Michael Hurley, Grant Samson, Marcel Field, Michael Vilimek, Peter Westley, Jeremy Mitchell, Olivia Puckrin, and Brendan Wringe. Comments from anonymous reviewers improved an earlier version of the manuscript. Funding and/or equipment were provided by Memorial University, Fisheries and Oceans Canada, the Renews River Conservation Association, the Conservation Corps Newfoundland and Labrador, and via grants to C.F. Purchase from the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Canada Foundation for Innovation, and the Research and Development Corporation of Newfoundland and Labrador. Literature Cited Colbourne, E., J. Craig, C. Fitzpatrick, D. Senciall, P. Stead, and W. Bailey. 2011. An assessment of the physical oceanographic environment on the Newfoundland and Labrador Shelf during 2010. DFO Canadian Science Advisory Secretariat Research Document 2011/089. iv + 31 pp. Available online at http://www.dfo-mpo.gc.ca/csas-sccs/Publications/ ResDocs-DocRech/2011/2011_089-eng.html. Modified 11 February 2014. Committee on the Status of Endangered Wildlife in Canada (COSEWIC). 2010. 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