Canadian Journal of Fisheries and Aquatic Sciences
Otolith microchemistry and acoustic telemetry reveal anadromy in non-native rainbow trout (Oncorhynchus mykiss) in Prince Edward Island, Canada
Journal: Canadian Journal of Fisheries and Aquatic Sciences
Manuscript ID cjfas-2019-0229.R1
Manuscript Type: Article
Date Submitted by the 09-Feb-2020 Author:
Complete List of Authors: Roloson, Scott; University of Prince Edward Island, Biology Landsman, Sean; University of Prince Edward Island, Faculty of Science Tana, Raymond; University of Waikato Faculty of Science and EngineeringDraft Hicks, Brendan; University of Waikato Faculty of Science and Engineering, Biological Sciences Carr, Jonathan; ATLANTIC SALMON FEDERATION, RESEARCH Whoriskey, Frederick G.; Ocean Tracking Network, Dalhousie University van den Heuvel, Michael; University of Prince Edward Island
ESTUARIES < Environment/Habitat, ENVIRONMENTAL CONDITIONS < Keyword: General, EUTROPHICATION < General, DIADROMOUS SPECIES < Organisms, MIGRATION < General
Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :
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1
2 Otolith microchemistry and acoustic telemetry reveal anadromy in non-native rainbow trout
3 (Oncorhynchus mykiss) in Prince Edward Island, Canada
4 Scott D Roloson1*, Sean J Landsman1, Raymond Tana2, Brendan J Hicks2, Jon W Carr3, Fred 5 Whoriskey4, Michael R van den Heuvel1 6
7 1 Canadian Rivers Institute, Department of Biology, University of Prince Edward Island 8 2 University of Waikato, Hamilton, New Zealand 9 3 Atlantic Salmon Federation, St. Stephen, New Brunswick 10 4 Ocean Tracking Network, Halifax, Nova Scotia 11 12 13 Short Title: Anadromy in non-native rainbow trout 14 Draft 15 Author to whom correspondence should be addressed: 16 Scott Roloson 17 Canadian Rivers Institute 18 Department of Biology 19 University of Prince Edward Island 20 550 University Avenue 21 Charlottetown 22 Prince Edward Island 23 Canada 24 C1A 4P3 25 P: 1-902-393-1061 26 F: 1-902-566-0740 27 [email protected] 28
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32 ABSTRACT
33 This study examined the migratory patterns of introduced rainbow trout in three rivers in Prince
34 Edward Island, Canada, using acoustic telemetry and otolith microchemistry. Only 6% of acoustically
35 tagged fish in three river systems left coastal embayments. A cohort of rainbow trout in all three rivers
36 entered saline waters. Habitat use differed among migrants in the three rivers as Montague River fish
37 occupied estuary habitat (mean 20.79 PSU) more often than West and Dunk River fish that tended to
38 occupy both riverine tidal (mean 1.27 and 4.29, respectively) and freshwater habitats (< 0.5 PSU),
39 particularly during summer months (July and August). A second cohort of rainbow trout remained
40 exclusively in freshwater. Migratory individuals were more likely to arise from anadromous mothers, but
41 freshwater mothers produced migratory offspring in all sites. Migratory individuals were significantly
42 larger than non-migratory freshwater residents. This study suggests that partial residency was the primary
43 strategy with prominent tidal occupation. WhileDraft secondary marine and freshwater contingents were
44 included in the full range of successful migratory strategies.
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45 Introduction
46 Rainbow trout (Oncorhynchus mykiss) have been introduced to over 90 countries (MacCrimmon
47 1971; Crawford and Muir 2008) as the species is a valued recreational angling target. However, their
48 impacts on native species include depleting the prey depletion, competing for habitat, and even affecting
49 food web composition in adjacent riparian habitats (Baxter et al. 2007; Blanchet et al. 2008; Jackson et al.
50 2016). Once the species establishes itself, subsequent invasions occur through dispersal between
51 interconnected freshwater watersheds, and potentially to neighboring coastal watersheds where the
52 population has access to the sea. Despite the widespread introductions, only a few instances of anadromy
53 in rainbow trout have been reported (Fasuch 2007) in South America (Pauscal et al. 2001) and several
54 locations in eastern Canada including Quebec (Thibault et al. 2009), Bras D’Or Lakes in Nova Scotia 55 (Madden et al. 2010) and Prince Edward IslandDraft (PEI) (Roloson et al 2018). 56 While anadromy may impart dispersal and fitness advantages (Gross 1987), many factors can
57 restrict the establishment of anadromy in salmonids in new environments. The development of anadromy
58 may be favored or constrained by factors including energetic status of individuals, physiological and
59 osmoregulatory demands of switching from freshwater to salt water, unfavorable conditions in the marine
60 environment and physical barriers that may impede fish passage (Jonsson and Jonsson 2005, Pauscal and
61 Ciancio 2007; Curry et al. 2010, Januchowski-Hartley et al. 2013, Kendall et al 2015, Lecomte et al.
62 2013). Anadromy may also have a genetic basis and has been observed to be retained in landlocked
63 populations for many generations (Hecht et al. 2013).
64 Recent technological advances have given an unprecedented opportunity to study fish migration
65 and life history. Chronological life history reconstruction by otolith microchemistry can distinguish the
66 movement ecology in marine and freshwater species (Walther and Limburg 2012). Delineating movement
67 between fresh water and salt water has been inferred from Sr:Ca and Ba:Ca ratios, where increases in both
68 ratios indicate movement to high and low salinity environments, respectively (Walther and Limburg
69 2012). However, changes in otolith chemistry are not immediate and temporal lags between changes in
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70 ambient chemistry and otolith chemistry may occur over weeks or months (Eldson and Gillanders 2005a).
71 However, otolith chemistry has other strengths, maternal life history can be interpreted from chemical
72 signatures that are incorporated into the otolith core (Courter et al. 2013). Individuals with anadromous
73 mothers can be distinguished by elevated strontium signatures, thereby quantifying the relative
74 contribution of resident (non-anadromous) and anadromous mothers. In contrast to otolith
75 microchemistry, acoustic telemetry involves implanting individual fish with acoustic tags and detecting
76 movements with prepositioned receivers. This technique can definitively movements at a finer spatial
77 scale, and across ecological transitions such as salinity gradients. Acoustic telemetry has been used to
78 determine fish migratory corridors and habitat use (Hussey et al. 2015, Binder et al. 2017; Chaput et al.
79 2018). However, challenges with the deployments of receiver arrays and the cost of acoustic transmitters
80 can inhibit broad application of the technique (Brenkman et al. 2001).
81 On Prince Edward Island (PEI), Canada,Draft over one million rainbow trout were introduced to a
82 variety of watersheds via stocking and aquaculture escapes, and established populations were reported as
83 early as 1971 (MacCrimmon 1971; Guignion et al. 2010). As of 2018, rainbow trout had established
84 populations in 32 rivers (Gormley et al. 2005, Roloson et al. 2018). Watersheds with recently established
85 rainbow trout had source populations nearby, suggesting that anadromous movements between adjacent
86 rivers were perpetuating invasion (Roloson et al. 2018). In eastern Quebec, where some populations of
87 introduced rainbow trout have become anadromous, a genetic study reported several individuals had
88 similar genetic signatures to rainbow trout on PEI, raising questions about the geographic scale of
89 anadromous rainbow trout originating on PEI (Thibault et al. 2009).
90 As anadromy in introduced rainbow trout is rare, their introduction into multiple watersheds on
91 PEI, presents a unique chance to study the development of anadromy in novel environments (Dodson et
92 al. 2013; Pearse et al. 2014). This study investigated the prevalence of anadromy in established
93 populations in three watersheds on PEI, Canada. Movement and residency patterns among these
94 established populations were compared to determine the similarity of habitat use and migratory patterns.
95 To accomplish this, a combination of acoustic telemetry, Floy tagging, and otolith microchemistry was
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96 used. Otolith microchemistry was used to assess size differences and the relationship between
97 anadromous and non-anadromous mothers and the migratory phenotype of subsequent generations.
98
99 Methods
100 Study area
101 Prince Edward Island, Canada’s smallest province (5,620 km2) has many, short (10-20 km)
102 watersheds fed by groundwater sources (Jiang and Somers 2009; Knysh et al. 2016). Pleistocene glacial
103 melt over sedimentary glacial till carved many short drainage basins into the landscape. Holocene sea
104 level rise has created relatively large estuaries for rivers on the island, commonly considered drowned
105 river valleys (Shaw 2005; van der Poll 1983). Present day PEI is heavily farmed, with approximately 50% 106 of the land area used for agriculture. This resultsDraft in elevated inorganic nitrogen loading to river systems 107 from fertilizer runoff (Jiang et al. 2015; PEI Department of Agriculture 2015). Due to the loading, the
108 river estuaries in this study often experience midsummer anoxia and hypoxia (Coffin et al. 2018).
109 Rainbow trout were studied in the Dunk River (watershed area 146.9 km2), West River (99.7
110 km2), and the Montague, Valleyfield and Brudenell Rivers complex (187.5 km2) (Fig. 1). The Montague
111 and Valleyfield Rivers share a common estuary. The Brudenell River (34.7 km2) also has an estuary
112 within the same embayment as the Montague-Valleyfield Rivers. Here we refer to this three-river
113 complex as the Montague River. Agriculture composes 67%, 39% and 42% respectively of the land use in
114 the Dunk, West and Montague watersheds. Salmonids dominate the fish community in PEI river systems
115 which are comprised of brook trout (Salvelinus fontinalis), Atlantic salmon (Salmo salar), and rainbow
116 trout. Brook trout are found in all rivers and have sympatric anadromous and freshwater resident life
117 histories. Atlantic salmon have been extirpated from much of their original distribution and are now found
118 in about two dozen watersheds (Cairns and MacFarlane 2015). Rainbow trout have expanded their
119 distribution and are now found in at least 32 watersheds (Roloson et al. 2018).
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120 A combination of flow regime and stream morphology were used to delineate four habitat
121 categories: freshwater, tidal, estuarine and outer bay. Freshwater was defined by the presence of
122 unidirectional flow. The boundary between freshwater and tidal (head of tide-freshwater to brackish) was
123 based on changes in water level associated with tidal fluctuations and bidirectional flow. This was
124 determined by placing fixed markers along the shoreline at 50 m intervals and recording water levels at
125 high tide and three hours later. Water level monitoring was conducted three times during spring tides. A
126 water level change greater than 2 cm was defined as tidal. The boundary between tidal and estuary was
127 defined by the morphological characteristics of the system where a tidal stream channel opened into an
128 estuarine bay. This was typically characterized by the appearance of marine-estuarine plants such as Ulva
129 spp. The transition between the estuary and the outer bay is the geographical boundary between the
130 estuary and coastal waters or larger embayments where salt and freshwater are fully mixed, these
131 boundaries were described in Coffin et al. (2018).Draft
132 Loggers were used to measure salinity at the chosen habitat boundaries (Onset Hobo®, U24-002-
133 C, set to 5 min logging interval). Loggers were deployed 0.5 m from bottom for 30 d to include the full
134 monthly range of tidal conditions (two spring tide and two neap tide cycles). Deployment dates ranged
135 from 11 October – 10 November 2014 (Dunk River), 12 August – 11 September 2015 (Montague) and 16
136 September – 16 October 2015 (West River). Loggers could not be deployed long-term as biofouling
137 caused sensor error, particularly in warmer temperatures and additional limitations were caused by the
138 limited storage capacity of the loggers,
139 Fish tagging
140 Floy tagging
141 Rainbow trout were captured either with a two-way fish trap that was positioned in a pool-and-
142 weir fishway at a dam at the outlet of Knox’s Pond, Montague River (at the head-of-tide) or by fly
143 angling in all tributaries entering the Montague Estuary. Volunteer anglers captured the fish and retained
144 them in a live-well until tagging (< 2 h). Rainbow trout >25 cm fork length (FL cm) were tagged with a
145 red external Floy tag (FD-94 3/4" mono Long T). Anglers also recaptured some of the tagged animals.
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146 When a tagged animal was recaptured, distances between tagging and capture locations were measured
147 using the path function in Google Earth Pro. Based upon the habitat designations described above, capture
148 locations were designated as freshwater, tidal, or estuary. Observations (both original and recaptures),
149 were tabulated by month and used as an indication of occupancy of specific habitats. An awareness
150 campaign to promote angler participation included information in provincial angling summaries and signs
151 posted at prominent angling locations. A toll-free hotline was setup for anglers to report their recaptures
152 and a reward ($10 certificate to local fishing store) was offered for the reporting of recaptures.
153 Acoustic tagging
154 Rainbow trout in the acoustic telemetry portion of the study were captured via fly angling,
155 electrofishing, or the previously described fish trap. Electrofishing enabled capture during the spawning
156 period (March – April) when the fish could be targeted in shallow spawning riffles. Angling was the most
157 successful capture method, as fish were mostDraft often found in deeper pools, and at various locations. Fish
158 were captured as they egressed from freshwater in late spring and as they resided in tidal waters over
159 summer months. A total of 61 trout were acoustically tagged between 2011 and 2014 (Dunk n = 25,
160 Montague n = 16, West n = 20).
161 Acoustic transmitters were models V13-1X (13 mm diameter, 36 mm total length, 6 g in water,
162 539-day tag life) and V16-4X (16 mm diameter, 68 mm total length, 10.3 g in water, 924-day tag life)
163 with a 69 kHz frequency (Innovasea Marine Systems Canada, Inc., Halifax, NS) programmed to have a
164 random ping interval between 140-220 s (V13) or 45-85 s (V16). Fish were selected for tagging where the
165 tags would weigh < 2% of body mass as suggested by Lucas and Baras (2000) and Thorstad et al. (2009).
166 Prior to surgery fish were anesthetized in clove oil (40 mg/L, Hilltech Canada Inc.) for 3-5 min, until the
167 opercular beat interval slowed to 1-2 seconds. To insert the tag an approximately 25mm incision was
168 made along the linea alba anterior to the pelvic girdle, (Panther et al. 2011). Following tag implantation,
169 the incision was closed with 2 monofilament sutures with a Surgeon’s knot (Deters et al. 2012). Fish were
170 measured (fork length) to the nearest 0.1 cm, weighed (nearest 10 g), and returned to a recovery live-well
171 for 1 h (Table 1). Fish implanted with acoustic transmitters were also fitted with a yellow Floy tag, which
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172 allowed anglers to discern between those with acoustic transmitters and those without (red Floy tags). A
173 hotline contact number was also provided on both the red and yellow Floy tag. Signs posted at popular
174 angling locations indicated the difference between tag colours and provided a contact number for anyone
175 who captured a tagged fish.
176 Receiver arrays
177 Tagged rainbow trout were tracked by a network of acoustic receivers (VR2 and VR2W,
178 Innovasea Marine Systems Canada, Inc., Halifax, NS). In 2011, 22 receivers were deployed (Montague
179 18, West 4), 30 in 2012, (Montague 18, West 7, Dunk 5), 24 in 2013 (Montague 7, West 8, Dunk 9), and
180 20 in 2014 (1 Montague, 6 West, 13 Dunk). As a priority, receivers were placed at the freshwater-tidal
181 boundary, the tidal-estuary interface (hereafter referred to as the TEI), and at the estuarine-outer bay
182 boundary. They were also placed above the freshwater-tidal interface and at least one receiver was placed
183 centrally within each habitat type. ReceiversDraft in estuary habitat were deployed at chokepoints (<400 m),
184 and in locations that minimized interaction with submerged shellfish aquaculture and boat traffic (Fig. 1).
185 Receivers were relocated as the tagging effort shifted between years. VR2 receivers were used in all years
186 except 2014, when 15 VR2W receivers were deployed on the Dunk and West. Receivers in estuarine
187 waters were deployed in April and May and retrieved in November and December just prior to winter ice
188 formation. Estuary receivers were fixed to a rope riser 1 m off bottom. The riser had two floats, one that
189 remained submerged just below the water surface at low tide (to ensure the receiver remained suspended
190 vertically in the water column) and a second which broke the water surface at high tide allowing retrieval
191 of the units at any tide level. In tidal and freshwater, receivers were attached to concrete cinder blocks
192 positioned on the bottom and deployed year-round. At each site an attempt was made to maintain two
193 receivers throughout the year: one at the uppermost extent of tidal waters and another in freshwater in the
194 lower section of the river, enabling directional evaluation of movements throughout the year. Additional
195 receiver coverage was obtained by submitting detection data to the Ocean Tracking Network (OTN) to
196 determine if any tagged fish were detected on their regional receiver arrays operated by the Atlantic
197 Salmon Federation or the OTN (www.oceantrackingnetwork.org).
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198 Preliminary acoustic receiver range tests were conducted during receiver deployment and more
199 rigorous range testing occurred at representative receivers from each habitat type (fresh, tidal, estuary and
200 outer). Results indicated that all receiver deployments used in this study would have reliable, high
201 probabilities of detecting tagged animals. Range tests used a V13 test tag (fixed delay, signaling every 10
202 seconds). Freshwater receivers were deployed in holding pools and range testing indicated that they
203 provided coverage across the entire stream width. An upstream-downstream range test showed freshwater
204 receivers recorded 100% of transmissions at 80 meters and 68% at 130 meters. The estuary range test
205 showed efficient detection across entire channel width (93% of tag signals detected at 204 m). At a
206 representative outer receiver detection efficiency was 87% at 167 meters and 36% at 443 meters. An
207 extended range test was conducted at a tidal receiver in August 2015. Detection efficiency (binned per
208 hour) was tested during each of the four tidal phases (falling, low, high, rising) as defined by Spares et al
209 (2015). Detection efficiency was not statisticallyDraft different among tidal phases and ranged between 88%
210 (low tide) and 92% (high tide). Additional information on range testing is available in the supplemental
211 information (Supplemental Material Fig. S1).
212
213 Occupancy and event metrics
214 Prior to analysis, the database was filtered to remove false detections (records that did not stem
215 from any of this study’s tagged fish). We also removed from consideration detections of tags that were
216 stationary within range of a given receiver for months, or tags that were only detected at a single receiver
217 over the course of the study. These animals were assumed to have died or lost their tags. In total,
218 detections from a total of 12 tags were removed from the analysis. Following protocols used in previously
219 published telemetry studies, single detections at a receiver were also not included in analysis of
220 movement (Miles et al 2018, Clements et al. 2005). Residence was defined as time spent within the
221 detection radius of a specific receiver. A residence event commenced following two consecutive
222 detections (360 s) of a tag and ended if a fish was not detected for 12 consecutive hours, which is the
223 interval of one tidal cycle. Non-residence events recorded the time spent at different places within the
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224 same habitat, outside of the detection radius, but between receivers. Non-residence events between
225 receivers of different habitat types were not included as it was not always possible to determine the
226 relative proportion of time spent in either habitat. A non-residence began when an individual was not
227 detected for 360 s (> 2 detections) and concluded when a fish was detected on a different receiver, or the
228 same receiver after >12 hours. Thus, if a fish left a receiver and came back within 12 h it registered as a
229 residence and if it came back after more than 12 h it was a non-residence. Non-residence events were only
230 registered for movements between receivers of the same habitat type. In combining residence and non-
231 residence time, a proportion of time spent in each habitat could be generated, which was defined this as
232 occupancy. Occupancy for each fish was separated into months and each fish had monthly occupancy
233 between 0-1 for each habitat. Fish that were not detected in a specific habitat had an occupancy
234 proportion of 0, while fish that spent the entire month in a habitat had an occupancy value of 1. An
235 “unknown” location was recorded when a fish’sDraft location could not be confirmed, such as when a fish was
236 between habitat types or if a fish was in Tidal or Estuary habitats during winter, when arrays were
237 restricted to freshwater and upper tidal zones.
238 In addition to the occupancy-based metrics, movements were summarized into events that
239 indicated movements among habitats. Events were summarized by month to investigate patterns
240 throughout the year. Events were categorized as either freshwater exit, freshwater entry, or estuarine
241 movements. Movements into freshwater were compared among sites to provide insight on spawning
242 related movement timing. A freshwater exit was registered when a fish was detected moving downstream
243 from freshwater into tidal or estuary habitat and vice-versa for a freshwater entry. Estuarine movements
244 were registered each time a fish moved between the various adjacent habitats to the estuary. Each time a
245 fish moved from an estuary receiver to an outer receiver an E-Out event was registered, this enabled the
246 examination of individuals that may have dispersed beyond the estuary receiver array. The TEI (Tidal
247 Estuary Interface) receiver was identified as an important location as it represented a movement from low
248 salinity (tidal-brackish) to higher estuarine salinity. Each time a fish moved to the TEI an event was
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249 registered, these included: D-TEI (down to TEI), TEI-Up (TEI up to Tidal), TEI-Out (TEI out to estuary)
250 and E-Up (estuary up to Tidal).
251
252 Otolith microchemistry
253 Extraction and polishing
254 Rainbow trout that were selected for otolith sampling were sacrificed and brought back to the lab
255 for immediate otolith removal and to have mass and length recorded. In some cases where anglers
256 submitted specimens from the study sites, the heads were frozen until delivery to the laboratory where
257 otolith removal occurred. Otolith pairs were washed with deionized water and left to air dry for 24 h.
258 Otoliths were kept in acid washed 50 ml glass vials and handled with acid washed plastic utensils. To
259 remove any trace metal contamination, samples were sonicated for 5 minutes in milli-Q water, then rinsed
260 with milli-Q water, soaked for 3 minutes inDraft 3% hydrogen peroxide, then triple rinsed in milli-Q water and
261 left to air dry for 24 h. All decontamination procedures (excluding sonication) were conducted in a
262 laminar flow fume hood. Otoliths were mounted sulcus side upward, with thermosetting glue
263 (Crystalbond, Aremco Products, Inc, USA) on small sections of pre-cut glass slides, enabling several
264 sections to be combined onto a single slide for chemical analysis. Exposure of each otolith’s nucleus was
265 achieved by sequentially polishing the structure using 1200, 2000, and then 4000 grain wetted silicon
266 carbide waterproof sandpaper. After polishing, samples were combined (15-20) on a single glass slide to
267 allow for more efficient laser operation. After polishing, samples were sonicated again in milli-Q water,
268 rinsed in 3% nitric acid for 15 seconds to remove any residual metals that might have been introduced
269 during polishing, and washed with milli-Q water.
270
271 Otolith processing
272 Chemical analysis of fish otoliths was conducted at the University of Waikato’s Mass Spectroscopy
273 Suite using a Perkin Elmer Elan SCIEX DRCII inductively coupled mass spectrometer (ICP-MS) with a
274 New Wave Research Nd: YAG 213 nm wavelength laser. National Institute of Standards and Technology
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275 (NIST) 612 standard reference material was used to calibrate the laser, prior to, during and after samples
276 were ablated. To account for any instrument drift the NIST standard was ablated for approximately one
277 minute after every fifteen-minute period of otolith ablation. Prior to ablation the otolith core was identified
278 with transmitted light and lines were subsequently pre-set from the core to the outer edge. The laser spot
279 size was set to 20 μm and the laser fired at a 20 Hz repetition rate and travelled at 10 μm/s. Ablated material
280 was carried from the laser chamber to the ICM-MS via a helium-argon carrier gas. Prior to the analysis of
281 each sample, a background gas blank was run for 20s to allow the detection of baseline isotope levels
282 present in the system. This analysis measured a broad range of isotopes but for this study, only 88Sr, 43Ca,
283 137Ba were of interest. Elemental concentrations are presented as mean counts per second (ratioed to
284 calcium) to account for the lack of matrix-matched standards (Morales-Nin et al. 2005).
285 286 Statistics Draft 287 Acoustic occupancy model
288 The occupancy model used percent occupancy (in a given habitat) as the response variable and habitat,
289 year, site and month as predictor variables. Data were extracted using the VTrack software package in R
290 (Campbell et al. 2012). Fish ID was held as a random variable. Predictor variables were first checked for
291 collinearity by calculating their variance inflation factors (VIFs; Zuur et al. 2010). Any variables with
292 VIFs >3.0 were considered collinear and the variable with the highest VIF was removed. This procedure
293 led us to drop “Year” from our models, but all other variables were retained. Prior to analysis, we built a
294 frequency histogram of the response variable; results indicated that 65.0% of the proportions were 0, and
295 8.5% were 1, thus zero- and one-inflated. The beta distribution with zero-one inflation is most appropriate
296 for this type of a continuous proportion response variable. The R ‘gamlss’ package (Rigby and
297 Stasinopolous 2005), which contains a distribution family ‘BEINF’ (beta inflated) that allows proportions
298 to include both 0s and 1s was used to model occupancy. Furthermore, given that individual fish were
299 repeatedly sampled, a zero-one inflated mixed modeling approach where Fish ID was held as a random
300 effect was used. Therefore, we chose a zero-one inflated beta mixed model. We applied an a priori model
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301 building procedure to test our null hypothesis that occupancy would not be affected by site, month, or
302 habitat. Each fixed factor was assessed for significance using likelihood ratio tests to compare models
303 without each term against the full model (Zuur and Ieno 2016). Predicted values were generated from the
304 mu component (proportions 0 < y < 1) of the model to assist in understanding movement patterns. Model
305 validation and goodness-of-fit was assessed using the plot.gamlss and Rsq functions (Cragg and Uhler
306 pseudo R2 values) in the gamlss package, respectively. Temporal autocorrelation was minimized by
307 examining data on monthly time scales. The absence of temporal autocorrelation was confirmed by
308 plotting the ACF and PACF plots (Zuur et al. 2010) using the 'acfResid' function in the gamlss package.
309
310 Otolith analysis
311 Otolith Sr:Ca and Ba:Ca line scans were measured in order determine if chemical profiles could
312 differentiate habitat use. Only fish over 19.8Draft cm were included in the migratory analysis as this was the
313 smallest fish to exhibit elevated Sr:Ca values, which are associated with exposure to elevated salinity. The
314 analysis of Sr:Ca line scans for migratory-sized fish was comprised of 26, 22, and 23 individuals from the
315 Dunk, Montague, and West Rivers, respectively (total 71). To avoid possible interference from
316 anadromous mothers the first 300 µm was removed for statistical analysis. We excluded Ba:Ca ratios
317 from further statistical analysis due to the high variability in this ratio among and within sites (Magath et
318 al. 2013, Walther and Thorrold 2008). Following Magath et al. (2013) the Sr:Ca values were binned into
319 three migratory contingents where all measured values fell within predetermined ranges. Sr:Ca < 1
320 mmol/mol throughout the whole line scan were assigned to the freshwater contingent (Austin et al. 2019);
321 Sr:Ca between 1 – 3.5 mmol/mol was the Tidal contingent; and Sr:Ca values > 3.5 mmol/mol were the
322 Estuary contingent. The tidal cut-off of 3.5 mmol/mol has been previously used to establish anadromy in
323 invasive rainbow trout in Gulf of St. Lawrence watersheds (Thibault et al. 2010). The hypotheses that all
324 sites had similar otolith chemistry was tested using analysis of similarity (ANOSIM) in PRIMER
325 (Anderson 2008). This test is a non-parametric one-way permutational ANOVA that is designed to
326 identify differences between predetermined groups or sites. Data were square root transformed and a
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327 Bray-Curtis resemblance matrix was generated. Data were plotted with a multidimensional scaling plot
328 (MDS) and the ANOSIM was conducted with the capture watershed as a fixed factor.
329 Additional analysis was conducted to determine if capture location or migratory history
330 influenced the size at capture (fork length). Length data were visually examined to assess normal
331 distribution and analyzed in a two-way ANOVA. The Dunk River was the only site where all factorial
332 combinations were present. The interaction between capture location and migratory strategy was
333 evaluated only at this site to determine the effect on fish size. In contrast, the relationship between
334 migratory strategy and length was examined at all sites, with length as the dependent variable and
335 migratory strategy and site as the categorical factors. Acoustic tracking data was available for one
336 individual that was recaptured after spending two growing seasons at large within the acoustic array. This
337 individual was euthanized, its otolith was aged using a compound microscope and the position of annuli
338 were superimposed onto the Sr/Ca line scanDraft to illustrate the period of overlapping chemistry and acoustic
339 tracking.
340 Primordial otolith core values were evaluated in order to examine maternal origins and the
341 contribution of anadromous mothers. Otolith core values were assessed by taking the average of
342 measurements from the first 200 µm, which represented the entire otolith core (Courter et al. 2013).
343 Rather than choosing a predetermined cut-off for each life history, this value was determined by
344 univariate K-means clustering where individuals with core values above the threshold were determined to
345 have mothers with anadromous origin (Courter et al. 2013).
346
347 Results
348 Salinity delineation of habitat boundaries
349 Salinity profiles in the study systems documented clear differences among the habitats available
350 to the migratory trout (Table 2) and provided a framework for the interpretation of otolith microchemistry
351 and movements of acoustic tagged fish. The tidal habitat in the three systems had a mean salinity of < 5
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352 PSU and a median value > 1 PSU, but levels varied with both lower and higher salinities recorded (Table
353 2). Mean salinity at all Tidal-Estuary Interface (TEI) sites was below 12 PSU (Dunk, 11.34, Montague,
354 10.99 and West, 9.55). Estuary habitats all had mean salinities > 18 PSU and the three study sites had
355 median values of 20.4 (Dunk), 19.03 (Montague), 20.71 (West).
356 Floy tagging
357 In total, 39 of the 173 (23%) Floy tagged fish were recaptured including two that were recaptured
358 twice. Fifteen recaptures (38%) occurred at the original tagging location, 13 of which occurred within 11
359 days. A total of 32 (82%) recaptures occurred less than 2 km from the original tagging location. Overall,
360 20 recaptures occurred within 30 days (51%), 8 between 30-100 days (21%), and 11 (28%) occurred more
361 than 100 days after tagging and release. The farthest straight-line distance between release and recapture
362 sites for a Flog tagged rainbow trout was 28 km. This individual was initially captured in freshwater in
363 Montague River. It was later recaptured 266Draft days after tagging in the Mitchell River, a tributary of
364 Cardigan Bay. Given that the recapture occurred during spawning season, it may be that this individual
365 was attempting to spawn in the Mitchell River. The second farthest distance recorded between release and
366 recapture for a Floy tagged rainbow trout was 3.6 km. The fish was recaptured in the Montague estuary,
367 downstream of its freshwater tagging location.
368 Acoustic tracking
369 Of the 62 tagged fish 50 were included in the analysis of movement and occupancy (Dunk n = 21,
370 Montague n = 14, West n = 15; Table 1). Acoustic tracking resulted in a database with 1,283,409
371 detections (650,706 on Dunk, 182,802 on West, and 449,901 on Montague). Across the three systems,
372 tagged fish stayed local, residing primarily in tidal and estuary habitats (Fig. 2). No fish were detected on
373 any distant receivers away from PEI within the Gulf of St. Lawrence operated by the Ocean Tracking
374 Network. Out of the 50 tagged fish, eight were detected on receivers at the outer bay receivers, three of
375 were not detected again and the remaining five returning to their inner estuary. The three that were not
376 detected again may have dispersed to new sites or could have died. However, the dependence on estuary
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377 and tidal habitats for most acoustically tagged fish suggests that long distance migration and straying
378 among systems is the exception, rather than the norm.
379
380 Freshwater entry - exit
381 In total there were 78 freshwater entry or exit movements documented, with 28 recorded entries
382 into freshwater (Dunk 11, Montague, 10, West 7) and 50 freshwater exits (Dunk 23, Montague 12, West
383 15). There was a general pattern of downstream movement following the spring spawning period.
384 Freshwater exit from the Dunk River was highest in May (10 exits) with other fish departing in April (7)
385 and June (6). In contrast, freshwater exit from the Montague was earlier and peaked in the month of April
386 (4 exits), with 2 departures each in May and December and 1 each in February, March, July and
387 September. Individuals from the West River moved downstream latest in spring with exits peaking in July
388 (5 exits). However, some fish from this systemDraft moved to sea both earlier and later than July (3 exits each
389 in May and June, 2 in April, and 1 in August).
390 With regards to freshwater entry, the patterns varied among the river systems and movements
391 occurred over a longer period than for the freshwater departures. For the Dunk River, four freshwater
392 entries were recorded in May, with an additional two in April and one each in January, March, June,
393 November and October. On the Montague River, three entries were recorded in October, two each in
394 August and November, and one in February, September and December. Finally, for the West River 5
395 freshwater entries occurred in October with single events recorded in April and May.
396
397 Estuary entrance and exit events
398 Estuary events were analyzed to indicate movement between habitats in order to separate
399 instances when an individual moved between fresh (< 0.5 PSU), tidal (1-10 PSU) water and the estuary,
400 (>10 PSU). There were 595 estuarine movement events (278, 215 and 102 on the Dunk, Montague, and
401 West Rivers, respectively). Almost all movements were between estuary and either TEI or tidal habitats
402 (> 95% of all events), but rarely to the outer estuary. The number of events in a given month, by habitat
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403 movement type, was divided by the total number of events at that habitat in the entire study, giving a
404 monthly relative proportion of events (Fig. 3b). Fish from the Dunk River moved from tidal habitat down
405 to the TEI, but rarely further (132 movements down to TEI but only 5 movements beyond TEI into the
406 Estuary). The two movements beyond the TEI and into the estuary occurred in the autumn (September
407 and October). In 2012 on the Dunk three fish were recorded the estuary in May. Event counts varied by
408 month as fish routinely moved down to TEI (D-TEI) in June (20), September (12), October (18) and
409 November (67). However, fish rarely moved down to TEI in August (4) and July (0). By comparison, fish
410 on the Montague River spent more time in the Estuary and movements upward (E-Up) to the TEI were
411 highest during summer months (July, 15 events; August, 40 events; and September, 13 events). The West
412 River had the fewest estuary movement events (102), attributable in part to the overall length of the
413 system (> 40 km) and the difficulty in covering the entire length with the available receivers. Here
414 movements down to the estuary peaked in SeptemberDraft (13) and October (7). Only one movement to the
415 estuary was recorded in July or August in all years of tracking. At all sites, movements to the outer
416 estuary were infrequent, with only 14 movement events recorded (2 Dunk, 3 West, 9 Montague).
417
418 Occupancy
419 Likelihood ratio tests comparing models with and without each fixed effect indicated that site,
420 habitat, month and their interactions all significantly affected occupancy of particular habitats (Likelihood
421 ratio tests, p’s < 0.05). Overall, the inclusion of all fixed factors and their interactions accounted for
422 81.1% of variation in the data. The predicted values of the mu component (proportions 0 < y < 1)
423 confirmed occupancy differences among sites (Fig. 3a) consistent with the movement event observations.
424 On the Montague River, fish occupied the estuary habitat throughout summer months while the Dunk
425 River fish occupied tidal areas throughout summer months. West River trout tended to stay in freshwater
426 longer but were observed in tidal or estuary habitat during summer and fall months. Tidal occupancy was
427 highest by Dunk and West trout (60% and 44%, respectively) and lowest for Montague (6%) fish. Estuary
428 occupancy was higher for Montague (39%), than fish from Dunk (0%) or West (4%).
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429 Otolith chemistry
430 Otolith inferences from acoustic tracked fish
431 One fish on the Dunk River was tracked with acoustic telemetry from 7 May 2013 - 9 December
432 2014 and serendipitously recaptured during otolith collection surveys. In its time at liberty it grew from
433 45.2 cm (1.0 kg) to 54.7 (2.2 kg). This individual’s acoustic telemetry movements indicated it remained in
434 tidal habitat during the study period. In 2013, it moved over the period 18 May – 28 June to between the
435 first and second receivers of tidal habitat, indicating that it was exposed to low salinity during this period.
436 In 2014, during the same dates (15 May to 29 June), the fish made several trips from tidal waters down to
437 the TEI but did not enter estuary habitat. Otolith mass spectrometer scans suggest that Ba:Ca otolith
438 levels may have responded to these movements, but Sr:Ca levels did not elevate at any time during the
439 period when the fish carried the acoustic tag.
440 Ba:Ca Draft
441 For the other fish whose otoliths were analyzed for elemental ratios (N = 70), in some cases,
442 spikes in Ba:Ca were proceeded by, and inversely proportional to, elevations in Sr:Ca. These movements
443 provide evidence of low salinity and tidal habitat occupancy, which was common on the West and Dunk
444 Rivers. However, problematically some individuals exhibited episodic Ba:Ca spikes during the presumed
445 obligate freshwater juvenile rearing period which raised concerns on the reliability of Ba:Ca as an
446 indicator of tidal occupation (examples given in Fig. 4).
447 Migratory life history and maternal contribution
448 Pooling data from all sites, otolith microchemistry showed the freshwater resident, tidal, and
449 estuary contingents comprised 48% (n=34), 33% (n=24), and 18% (n=13), respectively of the sample
450 (Fig. 4). Fish at the Dunk, Montague and West rivers exhibited an exclusively freshwater life history
451 signature (58%, 32% and 52% respectively). Tidal migratory contingents were found on all three systems
452 and comprised 38%, 18% and 43% of the sample from the Dunk, Montague and West rivers. Fish with an
453 estuary/marine profile were most common on the Montague (50%), while only one individual displayed
454 this signature in the West and Dunk River samples (4% of the fish examined in each river). The ANOSIM
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455 tests for between-site differences in chemical signatures showed significant differences (p = 0.0004).
456 Pairwise tests showed significant differences between the Montague and Dunk (p = 0.0001), and
457 Montague and West (p = 0.0012) fish. However, their Sr:Ca contingents on the West and Dunk Rivers
458 were similar (p = 0.0676).
459 Maternal origin analysis showed that all individuals sampled from the Dunk River had freshwater
460 resident mothers. By contrast, the percentage of freshwater mothers was 68% and 57%, for the Montague
461 and West rivers, with 32% and 43% respectively of the individuals in the rest of these samples having
462 anadromous mothers (Table 3). Anadromous mothers tended to produce migratory offspring (either tidal
463 or estuarine). For the Montague and West Rivers, 86% (6 out of 7) and 80% (8 out of 10) of migratory
464 individuals that entered the tidal or estuary habitats had anadromous mothers, indicating anadromous
465 mothers tended to produce anadromous offspring. By contrast offspring from freshwater mothers
466 produced 42%, 41%, and 13% of anadromousDraft migratory offspring on the Dunk, Montague and West
467 Rivers, respectively. A two-way ANOVA with life history and river system showed a significant
468 difference in length between life history types (p=0.007), but not between sites, and there was no
469 significant interaction between the two variables. Migratory individuals were 11.2 cm larger than non-
470 migratory individuals. When the effect of migratory strategy on length was assessed separately by site,
471 only Dunk River showed that migratory strategy was a significant factor influencing size (p=0.019).
472
473 Discussion
474 This study confirmed that short distance movements were the primary migratory strategy of
475 rainbow trout in all the study rivers. Only 6% of our acoustically tagged fish left the local river systems or
476 their estuaries. Similarly, a low number of recaptures were made outside of the systems in which fish
477 were marked with Floy tags. Both acoustic telemetry and otolith microchemistry verified that habitat use
478 differed among the study sites. Montague River fish were more prone to occupy saline estuary habitat,
479 while West and Dunk River fish spent more time in lower salinity tidal and freshwater habitats. All
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480 populations had higher occupation of tidal or estuarine habitat use during summer months (July and
481 August). Each site also had fish that showed periods of repeated movements across the salinity gradient.
482 While both anadromous and freshwater mothers tended to produce offspring with similar life histories to
483 the maternal life history, both maternal types produced offspring that exhibited the opposite life history
484 strategy. The results of this study suggest that partial migration to brackish and saline waters reflect the
485 life history patterns of rainbow trout on PEI, rather than the typical pattern of long duration steelhead
486 migration observed in the native range (Quinn and Myers 2004; Kendall et al. 2015). However, the
487 benefits of anadromy were that migratory individuals were significantly larger than non-migratory
488 individuals, and they were more likely to produce migratory offspring than freshwater residents.
489 In this study, Montague River fish had extended estuary residence and unequivocal saline
490 environment Sr:Ca signatures, indicating clear exposure to high salinity. In contrast, the Dunk and West
491 River fish occupied low salinity, and their otolithDraft signatures exhibited an intermediate signature. In this
492 study, acoustic telemetry revealed that some tagged individuals moved across the salinity gradient over 20
493 times in a month and were exposed to elevated salinity on timescales of multiple hours to multiple days.
494 Such rapid movements between environments cannot presently be resolved by otolith chemical
495 signatures. In addition to temporal constraints, interpretation of otolith chemistry can difficult between
496 low salinity (~ 5 ppt) and high salinity (~ 38 ppt) (MacDonald and Crook 2010). Australian bass
497 (Macquaria novemaculeata) are a catadromous species that travels from freshwater to the estuary for
498 spawning, although some adults have also been observed in fully marine coastal waters (Trnski et al.
499 2005). In a manipulative laboratory experiment, Macdonald and Crook (2010) observed that Sr:Ca
500 signatures were similar for fish living in salinities ranging from 5 – 38 ppt. Additional research with
501 striped bass (Morone saxatilis) showed rapidly increasing Sr:Ca to near marine signatures at low salinities
502 between 5 and 15 ppt (Phillis et al. 2011). The results of study are in concordance with previous literature
503 highlighting the difficulty interpreting temporal dynamics and concluding salinity exposure from otolith
504 chemistry line scans.
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505 Considering the range of factors that can influence otolith chemistry, there is a need for studies to
506 validate inferences drawn from chemical analyses (Honda et al. 2012; Crook et al. 2016; Miles et al.
507 2018). Otolith chemistry can be affected by complex responses to salinity changes (MacDonald and
508 Crook 2010). For example, several studies involving multiple species have documented delayed changes
509 (> 20 days) in otolith chemistry after entry into saltwater (Elsdon and Gillanders 2005a; Lowe et al. 2009,
510 Miles et al. 2018). Changes in otolith chemistry have been shown to occur more gradually in waters with
511 salinities <5 ppt (Secor et al. 1995).
512 This study also underscores the challenges of using barium as an indicator of low salinity
513 occupation (Coffey et al. 1997; Walther and Limburg 2012). It was anticipated that this ratio would
514 provide a reliable indicator of occupancy in low salinity habitat, but this was not the case. Freshwater
515 resident trout showed approximately 10-fold variability in their otolith Ba:Ca ratios over time, making
516 this ratio of little utility. Macdonald and CrookDraft (2010) also concluded that this ratio was not useful
517 following a controlled laboratory study with Australian bass that found that individuals exposed to the
518 exact same conditions varied widely in response to changes in ambient Ba:Ca ratios. The variability
519 observed in this study is possibly due to seasonal differences in incorporation or availability of the ions,
520 and complex water chemistry (e.g., Martin and Wuenschel 2006; Macdonald and Crook 2010; Miles et al.
521 2018). Dissolved Ba in freshwater ranged by nearly an order of magnitude among the study sites (28 –
522 175.3 µg/l). According to provincial water quality data, the lowest Ba values were on the Montague River
523 followed by the West River and Dunk River at 27.6 µg/l, 172.6 µg/l, and 175.3 µg/l, respectively (Qing
524 Li, PEI Dep. Comm. Land and Env. personal communication) potentially confounding Ba interpretation
525 further. In streams, Ba can be dissolved or adsorbed on seasonally varying particulate matter and the
526 amount bound to particulate matter can constitute a significant proportion of the available Ba (Hanor and
527 Chan 1977). Whether variation in turbidity affects barium availability for otolith incorporation is
528 unknown.
529 Within PEI estuaries, abiotic conditions, including water temperature and oxygen, may influence
530 rainbow trout movements and explain the observed among-watershed behavioural differences. Rainbow
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531 trout prefer temperatures below 20 °C, with an optimum between 12-18 °C (Raleigh et al. 1984; Chen et
532 al. 2015). On the Brudenell, West and Dunk Rivers when estuary temperatures in the summer exceeded
533 20 °C for nearly a month (Coffin et al. 2018), tagged fish were seldom observed in the estuary during
534 these conditions. However, when confronted with this thermal challenge, individual responses were not
535 uniform. For example, in the Brudenell River estuary, two of the three tagged fish responded by moving
536 into coastal waters and ultimately into the neighboring Montague River estuary. Conversely, on the Dunk
537 River, fish remained in tidal habitat and there were no movements into the estuary during July and
538 August. In the Montague River estuary, water temperatures remained suitable, but oxygen levels
539 fluctuated. For instance, in 2013 dissolved oxygen was < 4 mg/l for 33% of July and 63% of August
540 (Coffin et al. 2018). Given that the median lethal concentration of oxygen depletion to fingerling rainbow
541 trout is just under 2 mg/L (Landman et al. 2005), these concentrations would certainly be stressful, if not
542 lethal, to larger trout. During these months Draftthere were temporary interruptions of estuarine residence by
543 trout at this site and the fish made short duration upstream movements to lower salinity followed by
544 subsequent displacements back down to the estuary. During anoxic episodes, vertical stratification
545 resulted in anoxic substrates and the occurrence of a shallow oxygenated layer extending 1-2 meters
546 below the surface (Coffin et al. in prep). This pattern can concentrate prey resources as they seek
547 favorable environmental conditions (Steele and Steele 1991; Breitburg 2002). Thus, the seasonal periods
548 of regular movements across the salinity gradient observed at all sites may result from a combination of
549 avoidance of stressful abiotic conditions and opportunistic foraging on concentrated prey sources
550 (Breitburg 2002). Further research using telemetry in association with detailed water chemistry could
551 reveal information about the interplay between oxygen and temperature conditions and rainbow trout
552 movement in these novel ecosystem conditions (Hobbs et al. 2006).
553 The establishment of anadromy can favor rainbow trout invasion and persistence in coastal
554 watersheds (Fausch 2007), but the geographic extent of movements and underlying dynamics of
555 anadromy in invading and relatively newly established populations are not well understood (Ewel and
556 Putz 2004). In rivers draining to the upper St. Lawrence River, for example, reports of anadromous
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557 rainbow trout date back to 1973 (Whoriskey et al. 1981). The primary source of dispersing individuals is
558 thought to be upstream in Lake Ontario and the downstream spread has been described as a complex
559 stepping-stone dispersal model (Thibault et al. 2009, 2010). In this situation, newly established
560 populations and continued colonists from the original source contribute to extending colonization by the
561 invader to other sites. Within invading populations, long-distance migrants are generally considered rare -
562 though significant contributors to invasion into new areas. Beyond this, our understanding of the life
563 history of anadromy in non-native rainbow trout is limited. The present study confirms that these distant
564 migrants on PEI are indeed rare. However, we did observe short-distance movements between adjacent
565 estuaries (>20 km), suggesting that PEI rainbow trout may act as interconnected metapopulations
566 (Schtickzelle and Quinn 2007). Indeed, proximity to an established population of rainbow trout is a
567 significant factor influencing the probability of rainbow trout establishing in any given system on PEI
568 (Roloson et al. 2018). The species has had populationsDraft present in the area for a century, (MacCrimmon
569 1971, Quinn et al. 2001; Hendry et al. 2000; Phillis et al. 2016), yet a long-term, long-distance migratory
570 strategy has apparently yet to develop. The short length of rivers and large estuaries on PEI are highly
571 conducive to anadromy. The innate flexibility of the species also promotes anadromy, as the traits
572 associated with it can remain latent within freshwater populations for decades (Thrower et al. 2005;
573 Quinn et al. 2017). Furthermore, mothers of either phenotype (anadromous or freshwater) can give rise to
574 a mix of both phenotypes (Christie et al. 2011; Courter et al 2013). This suggest that a relatively rare
575 alignment among adaptive capacity, genetics, behaviour, and environmental cues are needed to favor
576 development of longer term, long distances migrations.
577 Although PEI rainbow trout demonstrated local salt water residency, they did not show evidence
578 of the extensive anadromous migratory behaviour as it is typically defined for highly-migratory
579 salmonids. The classical strategy of anadromy in rainbow trout, as manifest in their native range in the
580 Pacific Northwest and western Pacific (e.g., Kamchatka Peninsula), is characterized by smoltification as
581 juveniles followed by spending at least one year at sea in marine feeding grounds (Quinn and Myers
582 2004; Kendall et al. 2015). In South America, introduced anadromous rainbow trout have been shown to
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583 return to freshwater after one summer at sea with an overwintering period in freshwater (Riva-Rossi et al.
584 2007). Similar patterns of short salt-water residency were noted in steelhead populations in northern
585 California and southern Oregon (Kesner and Barnhart 1972). This is in stark contrast to anadromous
586 steelhead in their native range where the fish are known to embark on multi-sea-winter journeys (Quinn
587 and Myers 2004; Kuzishchin et al. 2007). We are unaware of any records of true steelhead (i.e., those that
588 spend at least 1 year at sea) having been documented outside the species native range.
589 The present study suggests that some of the benefits of anadromy may occur by using a mixed
590 migration strategy. Anadromous individuals must obtain significant benefits of increased growth and
591 fecundity to overcome the additional risk of long migrations to sea (Phillis 2014). The prevalence of tidal
592 occupation suggests that individuals may be obtaining benefits of anadromy without entering fully marine
593 water and thus encountering the dangers therein. Salt water migratory individuals captured in this study
594 were larger (by over 10 cm) than freshwaterDraft counterparts and through larger sizes it is presumed they
595 achieved higher fecundity and higher fitness.
596 Given the evidence established in this study, partial anadromy, defined as a mix of both salt water
597 and freshwater contingents may best describe life history diversity on PEI, as has been reported in other
598 salmonid populations (Chapman et al. 2012, Kendall et al. 2015; Austin et al. 2019). The cohort of
599 rainbow trout on PEI, that used the fully anadromous history achieved sizes (up to 650 mm) similar to
600 anadromous trout in their native range without extended residence in marine waters (Howell et al. 1985).
601 However, eutrophic estuarine conditions on PEI (due to high nitrate loads) may fuel productivity and
602 create a favorable suite of conditions that favors migration to intermediary salinities, as it may favor
603 occupation in highly productive inner estuaries (Coffin et al. 2018). Nutrient enrichment in these systems
604 may also lead to more abundant and diverse invertebrate resources in freshwater and tidal habitats that
605 were occupied more in the West and Dunk rivers (Somers et al. 1999; Purcell 2003). Thus, in these
606 systems, there may be a lower impetus to migrate if food is abundant in fresh and tidal waters.
607 Otolith microchemistry and telemetry revealed that some individuals resided in freshwater
608 throughout an entire summer, while others returned from anadromous movements and took up extended
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609 freshwater residence. This latter result is similar to findings by Bond et al. (2015) who showed that
610 anadromous dolly varden (Salvelinus malma) may retire from anadromous life history and establish
611 freshwater residence. Within the native range, additional diversity in life history forms can exist within
612 limited geographical separation (Kendall et al. 2015). This suggests that the traditional dichotomy
613 between freshwater resident and typical steelhead life history strategies is inadequate in describing the
614 diverse life history forms of migratory rainbow trout (Kendall et al. 2015, Bond et al. 2015), including on
615 PEI. As with many studies of rainbow trout, fish with different life history strategies were sympatric
616 during times of reproductive activity. In fact, otolith microchemistry showed freshwater resident fish from
617 both anadromous and freshwater resident mothers, demonstrating the observed partial migration.
618 The development of anadromy in rainbow trout has been described by the conditional strategy
619 theory (Kendall et al. 2015), which maximizes fitness by responding to proximate cues in the
620 environment and selecting from alternative Drafttactics (Hazel et al. 1990; Sloat et al. 2014). Successful
621 genotypes are favored through phenotypic plasticity, and the offspring have a higher propensity to
622 commit to the tactic (Hutchings et al. 2011; Kendall et al. 2015; Liberoff et al. 2014). In novel
623 environments, anadromous individuals experience unknown and unpredictable conditions and expression
624 of anadromy (i.e. phenotypic plasticity) may only be favored in predictable environments (Reed et al.
625 2010). Thus, unpredictable abiotic conditions in the marine environment offer one plausible explanation
626 for the lack of anadromy in rainbow trout outside their native range (Pauscal and Ciancio 2007). In this
627 study, the lack of long-distance migrants suggests greater uncertainty than short distance migrations with
628 more predictable food availability, stable abiotic conditions and possibly lower mortality risk. Overall,
629 opportunistic use of anthropogenically-modified estuaries in this study suggests that rainbow trout may be
630 passengers to human mediated ecological change in novel ecosystems (Ewel and Putz 2004; Didham et al.
631 2005).
632
633
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634 Acknowledgements
635 The authors would like to thank Michael Coffin, Kelly Aylward, Travis James, Christina Pater, Ashley
636 Alberto, Todd Dupuis, Daryl Guignion and Rosie MacFarlane for their time and expertise during this
637 research. Additional thanks anglers who contributed to the success of this study, including Michael
638 Mailman, Michael McQuillan, Elliot Williams, Emery Crawford, Charlie Trainor, Jamie Landry and
639 many others who reported tagged fish. Funding for this research was provided by the Atlantic Salmon
640 Federation as an industrial partner in the NSERC Industrial Post-Graduate Scholarship to SR, by a
641 Canada Research Chair to MRV, and by the PEI Wildlife Conservation Fund.
642
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Table 1: Acoustic tagged rainbow trout (n = 50), in Prince Edward Island, Canada. N/A = mass not available, MT:M = transmitter mass: fish mass, ratio based on mass measured in air (V13-1x = 11g, V16-4x = 24g).
Analysis Acoustic Release FL Mass Tag Release Release Days Site M :M ID ID Date (cm) (kg) T Type Latitude Longitude Tracked Detections Dunk D01 3952 2012-03-29 43.8 0.91 1.21 V13 46.31956 -63.46544 407 30 D02 19385 2012-03-29 54.0 2.04 1.18 V16 46.31956 -63.46544 763 422 D03 19383 2012-04-06 49.5 1.36 1.77 V16 46.31956 -63.46544 220 332 D04 19391 2012-04-06 54.6 1.36 1.77 V16 46.31956 -63.46544 65 720 D05 51757 2012-04-06 61.0 2.36 1.02 V16 46.31956 -63.46544 787 4244 D06 51759 2012-04-06 59.7 2.31 1.04 V16 46.31956 -63.46544 47 176 D08 11708 2012-04-19 46.4 1.25 0.88 V13 46.31956 -63.46544 20 137 D09 51755 2012-04-26 57.8 2.27 1.06 V16 46.34106 -63.66800 70 2924 D10 7254 2013-04-23 47.2 1.10 1.00 V13 46.34106 -63.66800 548 29316 D11 51262 2013-05-01 53.9 1.45 1.66 V16 46.34106 -63.66800 2 9595 D12 7256 2013-05-02 45.2 Draft0.87 1.26 V13 46.34628 -63.63246 548 59770 D13 51263 2013-05-02 56.0 1.91 1.26 V16 46.34106 -63.66800 261 52818 D14 7257 2013-05-07 45.2 0.96 1.14 V13 46.34106 -63.66800 549 102586 D16 23521 2013-07-12 47.0 1.32 0.83 V13 46.34106 -63.66800 518 69082 D17 23523 2013-08-22 48.3 1.59 0.69 V14 46.34106 -63.66800 94 18008 D20 23525 2014-04-30 45.0 0.98 1.12 V13 46.34195 -63.61072 7 99 D21 33277 2014-05-01 51.8 1.62 1.48 V16 46.31956 -63.46544 54 10419 D22 51261 2014-05-01 64.0 2.48 0.97 V16 46.31956 -63.46544 144 120810 D23 23526 2014-05-05 45.2 1.09 1.01 V13 46.34195 -63.61072 113 19056 D24 51756 2014-06-04 48.1 1.36 1.77 V16 46.34106 -63.66800 191 127711 D25 19379 2014-06-05 52.9 1.77 1.36 V16 46.34106 -63.66800 193 82852 Montague M01 3957 2011-05-14 39.4 N/A N/A V13 46.20192 -62.65604 362 20771 M02 3961 2011-06-01 43.2 1.36 0.81 V13 46.16410 -62.64796 70 10953 M03 3958 2011-06-03 41.3 0.82 1.35 V13 46.16410 -62.64796 433 32152 M04 3950 2011-06-07 39.4 0.91 1.21 V13 46.16410 -62.64796 563 38921 M05 3956 2011-06-11 41.9 1.13 0.97 V13 46.16410 -62.64796 116 10491 M06 3959 2011-06-11 39.4 0.91 1.21 V13 46.16410 -62.64796 72 23115 M07 3960 2011-06-16 42.5 1.13 0.97 V13 46.16410 -62.64796 244 17906 M08 19380 2011-08-08 53.3 1.93 1.25 V16 46.15466 -62.65378 511 27383
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Montague M10 19381 2011-08-14 66.0 3.62 0.66 V16 46.15466 -62.65378 340 29306 M11 19387 2011-08-16 50.8 1.81 1.32 V16 46.15845 -62.67550 550 193999 M12 19384 2011-08-18 64.1 2.94 0.82 V16 46.15466 -62.65378 221 4316 M14 3954 2012-03-25 46.4 N/A N/A V13 46.20192 -62.65604 208 18270 M15 11716 2012-05-03 47.0 N/A N/A V13 46.20192 -62.65604 260 7935 M16 11711 2012-07-02 40.0 N/A N/A V13 46.15856 -63.67526 234 14313 West W01 19389 2011-06-24 47.6 1.36 1.77 V16 46.23098 -63.35085 18 472 W02 3955 2012-03-18 43.2 N/A N/A V13 46.28568 -63.34615 550 3603 W03 3953 2012-03-26 47.0 1.13 0.97 V13 46.20990 -63.34615 31 16227 W04 19379 2012-03-26 59.9 2.72 0.88 V16 46.28568 -63.34615 490 2166 W05 11710 2012-04-01 43.8 0.91 1.21 V13 46.28568 -63.34615 47 198 W11 11709 2012-04-24 50.2 1.47 0.75 V13 46.23098 -63.35085 78 5465 W13 7258 2013-04-16 41.9 0.89 1.23 V13 46.28568 -63.34615 418 122 W14 51756 2013-04-19 54.3 1.52 1.58 V16 46.23098 -63.35085 155 27127 W15 7253 2013-04-20 48.5 Draft1.11 0.99 V13 46.23098 -63.35085 119 16208 W16 51261 2013-04-20 56.7 1.75 1.38 V16 46.23098 -63.35085 40 13766 W17 7255 2013-04-25 46.7 1.08 1.02 V13 46.23098 -63.35085 548 1665 W19 51266 2013-07-21 51.5 1.53 1.57 V16 46.19649 -63.35028 304 31688 W20 33274 2013-08-09 52.8 1.94 1.24 V16 46.19649 -63.35028 47 9597 W21 33275 2013-08-21 62.9 3.93 0.61 V16 46.19649 -63.35028 55 26268 W22 23522 2013-08-29 52.0 1.86 0.59 V13 46.19649 -63.35028 430 17341
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Table 2: Summary of salinity profiles in three study estuaries and the designation for each site. Tidal-Estuary Interface (TEI) defined as the zone of transition between tidal-brackish salinity and estuarine salinities (>12 ppt). Estuary loggers on Dunk and Montague became fouled and therefore, had reduced sample size. The Dunk River had two loggers within the tidal zone, with the first placed upstream of the second which was positioned at the lower boundary between tidal and TEI habitat. River Designation Mean (± SE) Median Range Percent Percent Percent n Latitude Longitude <0.5 PSU > 5 PSU >12 PSU Dunk Tidal 0.61 ± 0.02 0.14 0.01 - 13.28 0.88 0.03 0.01 8616 46.34683 -63.68305 Tidal 4.29 ± 0.06 0.99 0.01 - 18.38 0.45 0.34 0.14 8616 46.35745 -63.68909 TEI 11.34 ± 0.08 13.39 0.14 - 22.35 0.15 0.74 0.56 8616 46.35780 -63.70409 Estuary 18 ± 0.11 20.40 0.5 - 23.64 0.00 0.97 0.87 2856 46.35059 -63.72541 Montague Tidal 0.1 ± 0.03 0.60 0.01 - 19.66 0.93 0.03 0.03 8787 46.15314 -62.65421 TEI 10.99 ± 0.11 10.90 0.01 - 24.49 0.38 0.53 0.49 8787 46.15592 -62.65334 Estuary 20.51 ± 0.05 19.03 1.00 - 24.49 0.00 0.99 0.91 8787 46.37201 -63.77526 Estuary 20.79 ± 0.05 22.02 5.54 - 24.96 0.00 1.00 0.96 5139 46.16461 -62.64721 West Tidal 0.16 ± 0.00 0.09 0.08 - Draft3.86 0.96 0.00 0.00 8731 46.19804 -63.35191 Tidal 1.27 ± 0.02 0.20 0.09 - 10.37 0.62 0.08 0.00 8731 46.19619 -63.34583 TEI 9.55 ± 0.05 9.91 0.34 - 18.32 0.00 0.72 0.42 8731 46.20748 -63.33089 Estuary 20.03 ± 0.05 20.71 7.86 - 23.48 0.00 1.00 0.99 8731 46.19728 -63.29616
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Table 3: Mean (SEM, n) fork length (cm) for individual rainbow trout categorized by individual life history and proportion (n) of rainbow trout distributed by categories combining maternal life and individual life history (e.g. Ana-Fresh indicates an anadromous mother and a freshwater progeny). Asterisks indicate significant differences in size between individual life histories within river systems.
Status Dunk Mont West Individual Life History Freshwater 37.0 (3.3, 15) 33.6 (6.5, 7) 33.0 (2.1, 12) Migratory 48.2 (2.7, 11)* 39.8 (3.1, 15) 37.8 (1.9, 11) Maternal Life Ana-Fresh 0 0.05 (1) 0.09 (2) History Ana-Ana 0 0.27 (6) 0.35 (8) Fresh-Fresh 0.58 (15) 0.27 (6) 0.43 (10) Fresh-AnaDraft0.42 (11) 0.41 (9) 0.13 (3)
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Figure Captions
Fig. 1. Map of study location including the locations of acoustic receivers in the respective watersheds, with scale bar representing 1 km. Receiver
arrays depicted are from 2011 on the Montague River, 2013 on the West River and 2014 on the Dunk River. Map was prepared using Arc GIS (v.
10.2) (ESRI, Redlands, California, USA).
Fig. 2. Representative acoustic telemetry tracks of tagged fish from the three study sites (D = Dunk, M = Mont, W = West). Individuals with
multiple years of tracking are presented in adjacent panels. Habitat zones delineated based upon salinity. Shading indicates the season. Tracks start
on the left of each panel and move to the right. Habitats that the animals are in at the time on a track are given on the Y axis. An “Unknown”
location was assigned when the location of an individual could not be determined.
Fig. 3. Predicted proportion habitat occupancy (a) grouped byDraft site. Panel (b) summarizes the proportion of total movement events that occurred
between Tidal Estuarine Interface (TEI) and adjacent habitats throughout the year. Months on the x axis start with January (1) and end with
December (12)
Fig. 4. Representative otolith plots from each migratory contingent, black indicating Sr:Ca and red indicating Ba:Ca. Individual D31 is the
individual with both telemetry (bottom right) and otolith chemistry (bottom left), with lines representing approximate location of annuli and
asterisk estimating the timing of capture for acoustic tracking.
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Draft
771x543mm (96 x 96 DPI)
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Draft
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338x190mmDraft (96 x 96 DPI)
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338x190mmDraft (96 x 96 DPI)
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