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Jul 10, 2008 - 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada ... Nipigon Bay, Lake Superior, were surgically implanted with radio transmitters and ...
Transactions of the American Fisheries Society 137:1203–1212, 2008 Ó Copyright by the American Fisheries Society 2008 DOI: 10.1577/T05-273.1

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Habitat Use and Movement Patterns of Brook Trout in Nipigon Bay, Lake Superior JAMIE M. MUCHA Lakehead University, Centre for Northern Forest Ecosystem Research, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada

ROBERT W. MACKERETH Ontario Ministry of Natural Resources, Centre for Northern Forest Ecosystem Research, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada Abstract.—Brook trout Salvelinus fontinalis are one of two salmonine species native to Lake Superior. Abundant and widely distributed a century ago, Lake Superior’s brook trout (coasters) have been reduced to a few remnant stocks, probably as a result of exploitation and habitat loss. Twenty brook trout captured in Nipigon Bay, Lake Superior, were surgically implanted with radio transmitters and were located from June 1999 to October 2000. Brook trout locations were used to determine the characteristics of utilized lake habitat and to identify streams used for spawning and the spawning areas within them. These locations were also used to establish daily and seasonal movement patterns. A total of 638 locations were obtained during the tracking period, 483 occurring within Nipigon Bay and the remaining 155 within tributaries. Brook trout were located almost exclusively within the shallow nearshore areas of Nipigon Bay; 92% of the locations were less than 7 m deep and 94% were less than 400 m from shore. Brook trout occupied deeper areas with steeper shoreline slopes during July and August, when the water temperatures of shallow nearshore areas were at their highest. Following selected individuals for 24-h periods revealed that brook trout often used deeper areas during daylight hours and moved to extremely shallow nearshore areas during the night. Radio-tagged brook trout began ascending tributaries during late summer in both years of this study. The mean residency time for brook trout in tributaries was 46 d. Spawning occurred in early October, and most radio-tagged brook trout returned to Lake Superior by mid-October. Four different streams were used during the spawning period. Brook trout entering streams exhibited strong spawning site fidelity between years.

The brook trout Salvelinus fontinalis is one of only two salmonine species native to Lake Superior (Cudmore-Vokey and Crossman 2000). Brook trout inhabiting Lake Superior were given the name ‘‘coasters’’ because of their predilection for nearshore areas (Newman and DuBois 1997). The term ‘‘coaster’’ is now used when referencing any brook trout that uses Lake Superior at some point in its life cycle (Becker 1983). Coaster brook trout once provided a worldrenowned sport fishery and were ubiquitous throughout the nearshore waters of Lake Superior; at least 118 documented streams have been identified at some period (Newman et al. 2003). At present, few stocks of coaster brook trout remain in Lake Superior, and information regarding their abundance and distribution is limited (J. W. Slade, unpublished report on the status of coaster brook trout in the waters of Isle Royale National Park, U.S. Fish and Wildlife Service, Ashland, Wisconsin, 1994). Commercial and sport fishing, habitat loss due primarily to the damming of rivers, * Corresponding author: [email protected] Received October 31, 2005; accepted February 22, 2007 Published online July 10, 2008

land use practices such as forestry and mining, and the introduction of nonnative salmonids may all have contributed to the decline of the coaster brook trout (Newman and DuBois 1997). Efforts to rehabilitate Lake Superior’s brook trout have been hampered by a lack of understanding of their life history characteristics, habitat use, behavior, and population status. To date, the published literature concerning coaster brook trout is limited. The basis for our understanding of their ecology stems from studies of inland freshwater brook trout and of anadromous brook trout from the Atlantic Ocean and Hudson Bay. The movement patterns, habitat use, spawning behavior, and spatial distribution of the habitat of Lake Superior’s brook trout are largely unknown. This limited information does not meet the needs of fisheries managers trying to either protect current coaster brook trout populations from further decline or to restore those that have been extirpated. Well-informed decisions regarding land use and harvest regulations cannot be made until empirical knowledge regarding coaster brook trout movement and habitat use both within the lake and tributaries are obtained. This information is essential to protect critical habitat areas from develop-

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FIGURE 1.—Map of the study area showing the locations of Nipigon Bay and relevant tributaries: (A) the Jackpine River, (B) Dublin Creek, (C) the Cypress River, and (D) the Little Cypress River.

ment, identify potential angling sanctuaries, and provide adequate open–closed angling season dates. The overall goal of this study was to evaluate the habitat use and movement patterns of Lake Superior’s brook trout by means of biotelemetry. The first objective of this study was to identify the type of lake habitat used by brook trout and to determine whether their use of habitat changes either diurnally or seasonally. The second objective was to examine the movement patterns of coaster brook trout at various timescales (seasonal, between days, and within days). The third objective was to identify which streams spawning coaster brook trout used and to describe the attributes of these systems. The final objective was to examine the movement patterns of coaster brook trout within their spawning streams, including entry and exit dates, instream movement, and residence time. Study Area This study was conducted primarily within Nipigon Bay and four of its tributaries: the Jackpine River, Cypress River, Little Cypress River, and Dublin Creek (Figure 1). Nipigon Bay lies entirely within the Canadian waters of Lake Superior and forms the northernmost portion of the lake. The bay is enclosed by the Black Bay Peninsula to the west and St. Ignace, Simpson, and a number of smaller islands to the south. Water is exchanged between the open waters of Lake Superior and Nipigon Bay through the Nipigon and Moffat straits and the Simpson and Wilson channels. Within Nipigon Bay, there are four conspicuous islands (Vert, La Grange, Burnt, and Outan) and a number of smaller islands. Numerous tributaries drain into Nipigon Bay, including the Nipigon River, Lake Superior’s single largest inflow. Nipigon Bay reaches

a maximum depth of approximately 140 m, but a large proportion of the western end is less than 10 m deep (Canadian Hydrographic Service nautical chart). The four tributaries investigated in this study are all small- to medium-sized high-gradient streams that derive most of their flow from runoff or snowmelt. The watersheds of these tributaries are dominated by bedrock, morainal deposits, and glacial outwash (Gartner 1979). The watershed areas for the Jackpine, Cypress, and Little Cypress rivers and Dublin Creek are 288.1, 166.2, 8.4, and 22.5 km2, respectively. All tributaries, except for the Jackpine River, whose barrier to migration is some 12 km upstream, have an insurmountable barrier falls less than 5 km from Lake Superior. Methods Movement patterns and habitat use by coaster brook trout were studied from May 1999 through October 2000 by means of radiotelemetry. Twenty coaster brook trout were captured by angling between May 20 and June 2, 1999, in numerous locations within Nipigon Bay. All brook trout met the minimum recommended weight such that transmitter weight did not exceed 2% of their body weight in air (Winter 1983). Those implanted with radio transmitters ranged from 35.9 to 52.9 cm fork length and 634 to 2,223 g in weight (Table 1). Each angled brook trout was held in a holding tank equipped with circulating pumps until tagging. Before surgical implantation of the transmitter, each fish was anesthetized with 40 mg/L of clove oil (Anachemia Science, Lachine, Quebec) in 18 L of water. Anesthetized individuals were placed belly-up on an operating table (Courtois 1981) and bathed with anesthetic solution across the gills. A transmitter was surgically implanted into each fish’s body cavity using the shielded-needle technique (Ross and Kleiner 1982). After surgery, each fish was placed back into the holding tank, tagged with two t-bar anchor tags (Floy Tag, Inc., Seattle, Washington) below the dorsal fin, measured for fork length and maximum body width and depth, and weighed. After 45 min, each fish was released at its point of capture. The radio transmitters used in this study were of two different sizes and programming. Thirteen brook trout were implanted with Lotek MCFT coded transmitters (Lotek, Inc., Newmarket, Ontario) that ranged in frequency from 149.400 to 149.700 MHz. These had an operational life of 575 d, were operational 24 h/d, measured 16 3 51 mm, and weighed 18 g. The remaining fish were implanted with Lotek MBFT coded transmitters that ranged in frequency from 151.510 to 151.650 MHz, had an operational life of

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TABLE 1.—Summary of brook trout capture dates, transmitter frequencies, size attributes, and number of locations, Nipigon Bay, Lake Superior, June 1999 through October 2000. Brook trout number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Date tagged 20 21 21 26 26 27 28 26 28 27 21 27 28 1 21 28 21 27 28 20

May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999 Jun 1999 May 1999 May 1999 May 1999 May 1999 May 1999 May 1999

Transmitter frequency/code

Fork length (mm)

Weight (g)

151.510 151.540 151.550 151.570 151.610 151.640 151.650 149.400/51 149.400/55 149.420/51 149.420/53 149.420/54 149.420/55 149.420/56 149.420/58 149.700/70 149.700/71 149.700/73 149.700/74 149.700/75

501 412 385 400 376 359 385 470 446 522 419 529 410 516 456 479 506 480 486 426

1,515 820 690 810 710 634 845 1,319 1,180 1,543 945 2,223 935 1,685 1,085 1,535 1,701 1,363 1,620 1,238

375 d, were operational from 0900 to 2100 hours, measured 11 3 49 mm, and weighed 9 g. Brook trout were located throughout the study period; the majority of the effort concentrated on the open-water months of May through October. Tracking intensity averaged 40–50 h/week May through October and 10–15 h/month November through April. For each day of lake tracking, the tracking route, start–stop times, surface water and air temperatures, wind speed and direction, and lake conditions were recorded. Tagged fish were located with a SRX 400A receiver (Lotek, Inc.) and three-element Yagi antennae on board a boat or walked along the tributaries. A few locations were made in the winter (during ice cover) with allterrain vehicles. Most tracking was done between 0900 and 2100 hours, when all transmitters were active. Individuals carrying 24-h transmitters were selected at random and tracked at 4-h intervals for 24-h periods to investigate within-day movements. Brook trout could generally be detected at a distance of 500 m, the distance decreasing with the depth of the fish or the presence of ice cover. The accuracy of the location of a radio-tagged brook trout could most often be determined to within 5 m, as occasional visual observation confirmed. At every location site, a depth reading (60.1 m) was taken with a Humminbird 200 depth sounder (Humminbird, Eufaula, Alabama). It was not possible to ascertain the actual depth in the water column at which the fish resided (unless visually observed); therefore, only depth of the location was recorded. In addition, a distance-to-shore measure (m) was taken with a Bushnell 400 rangefinder (Bushnell

First location date

Final location date

16 18 18 17 16 10 12 19 1 17 17 28 17 15 18 16 18 28 8 1

28 26 8 31 6 21 4 1 7 8 14 7 9 9 27 9 21 19 14 28

Jun 1999 Jun 1999 Jun 1999 Jun 1999 Jun 1999 Jun 1999 Aug 1999 Jul 1999 Jul 1999 Jun 1999 Jun 1999 Jun 1999 Jun 1999 Jul 1999 Jun 1999 Jun 1999 Jun 1999 Jun 1999 Jul 1999 Jul 1999

Oct 1999 Jul 2000 Nov 1999 Jul 2000 Jul 2000 Jul 1999 Jul 2000 Oct 2000 Dec 1999 Nov 1999 Sep 2000 Jul 2000 Oct 2000 Sep 1999 Jun 2000 Aug 1999 Jul 1999 Oct 2000 Sep 2000 Aug 2000

Number of locations 52 23 28 58 7 7 6 42 36 27 73 35 53 22 18 9 10 29 55 48

Corporation, Overland Park, Kansas). Universal Transverse Mercator coordinates were also taken at each fish location site with a Garmin 45 global positioning system unit (Garmin International, Inc., Olathe, Kansas); the horizontal accuracy of these units is generally within 15 m. To examine lake habitat use seasonally, the mean depth, distance-to-shore, and slope-to-shore values for each tagged individual were grouped by month and evaluated with a one-way analysis of variance (ANOVA). A Tukey’s honestly significant difference (HSD) test was used to assess individual differences. Differences in range sizes were evaluated against both fork length and number of locations with a standard linear regression. Results A total of 638 telemetry locations were made between June 10, 1999, and October 19, 2000. Of these, 483 locations were made in Lake Superior and 155 were made in tributaries. The minimum and maximum number of locations for individual brook trout was 6 and 73, respectively (Table 1). Fourteen of 20 radio-tagged brook trout were located more than 20 times. Locating certain individuals was extremely difficult because of the large study area, and a number of radio-tagged brook trout were out of contact for extended periods. Three active transmitters were collected from fish that had either perished or expelled their transmitter. Table 1 summarizes the attributes of the brook trout radio-tagged for this study.

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FIGURE 2.—Depth distribution of individual brook trout locations, May 1999 through October 2000.

Lake Habitat Brook trout were located almost exclusively in the shallow-water areas of Nipigon Bay during their lake residency. Of 483 lake location sites, 444 (92%) were in areas less than 7 m deep. The mean and median depths of the pooled location sites were 3.4 and 2.5 m, respectively, the shallowest being 0.6 m and the deepest 26.4 m. Individual brook trout were located in similar depth ranges when compared with each other (Figure 2). Brook trout were most frequently located in areas close to shore in Nipigon Bay. Of 483 distance-toshore readings, 454 (94%) were less than 400 m from shore. The mean and median pooled distances to shore were 116.1 and 76 m, respectively. Brook trout were located in areas ranging from 5 to 1,390 m from shore. Differences between individual brook trout were observed; some individuals remained relatively close to shore while others were often found at great distances from shore (Figure 3). During the open-water months (May–December), the mean depth at which brook trout were located differed by month (ANOVA: F ¼ 16.146; P , 0.001; Figure 4); the brook trout were located in significantly deeper areas during July and August than during the other months (Tukey’s HSD: P , 0.05). The mean depths at which brook trout were located during July

and August were 4.2 and 3.6 m, respectively. The mean depth for all brook trout locations in each of the remaining months was less than 3 m. Brook trout located at 4-h intervals throughout a 24h period were found in deeper areas during the day than at night (ANOVA: F ¼ 3.187; P , 0.05; Figure 5). The mean depths at which brook trout were located were significantly lower at 0400 hours (1.2 m) than at 1200 hours (2.6 m; Tukey’s HSD: P , 0.05). Individual brook trout location depths and distanceto-shore values were used to calculate the slope from the shoreline to the location point. The slope of shoreline where brook trout were located differed by month (ANOVA: F ¼ 5.62; P ¼ 0.013). Brook trout occupied areas of greater slope in July, August, and September than in all other months (Tukey’s HSD: P , 0.05). Lake Movement The maximum distance traveled by individual brook trout in this study, calculated as the straight-line distance between the furthest two location points, ranged from 2.9 to 46.0 km. The mean and median ranges for brook trout were 17.9 and 16.4 km, respectively. Twelve of the 20 tagged brook trout exhibited a range that exceeded 10 km. The range of individual brook trout was not related to its number of

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FIGURE 3.—Distance-to-shore distribution of individual brook trout locations, May 1999 through October 2000.

telemetry locations (r2 ¼ 0.046; P ¼ 0.363) or fork length (r2 ¼ 0.009; P ¼ 0.691). The distance traveled for brook trout located on consecutive days was calculated for the months of

June, July, and August. Daily distance values were calculated as the straight-line distance between locations obtained on consecutive days. Daily movements were relatively short, 66% (140/209) of movement

FIGURE 4.—Distribution of brook trout location depths by month, 1999–2000.

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FIGURE 5.—Depth distribution of brook trout located at 4-h intervals in a 24-h cycle, May 1999 through October 2000.

distances being less than 500 m and many being less than 200 m. The mean and median daily distances traveled were 757 m and 359 m, respectively. Some brook trout periodically moved longer distances within 24-h periods, the maximum being 16.5 km. Stream Movements Brook trout were located in four different tributaries of Nipigon Bay in the fall of 1999. Twelve of the 20 tagged brook trout were detected in streams during the spawning period. Of these 12, 8 were found in the Cypress River, 2 in the Jackpine River, and 1 each in Dublin Creek and the Little Cypress River. The first detection of a brook trout in a stream in 1999 was on July 27 and the last was on October 11. Eleven of the 12 had first been detected in streams between August 15 and September 9. Seven of the 12 brook trout entering streams in the late summer and fall were observed to enter for brief durations before returning to Lake Superior, only to reenter at a later date. Two brook trout entered and exited multiple times; all of the other brook trout exhibiting this behavior did so only once. Some brook trout were also located in more than one tributary stream in the fall of 1999. Of the 7 tagged brook trout that exited a stream shortly after entering it, 4 entered a different stream soon afterwards. The brook trout entering the Cypress and Jackpine rivers in 1999 tarried in the estuary and lower stream reaches before moving upstream. Sixty-six percent

moved to upstream reaches between September 7 and 14. By September 27, 10 of 12 brook trout that entered streams had migrated to upstream areas. Brook trout generally exited streams and returned to Lake Superior in mid-October 1999. By October 25, 1999, 9 of 12 had done so. By November 9, 1999, all tagged brook trout that had entered streams that fall were again located back in Nipigon Bay. The brook trout that resided in tributary streams of Lake Superior for extended periods in the fall of 1999 did so for an average of 46 d. The minimum and maximum stream residencies of radio-tagged brook trout were 2 and 72 d, respectively. No tagged brook trout were located in tributaries outside of the latesummer–fall period, even though sporadic tracking was done in these areas. Although no tagged individuals were ever located, small numbers of large (.35-cm) brook trout were observed in tributaries in early May 2000, holding behind spawning rainbow trout Oncorhynchus mykiss. Stream Habitat Within upstream reaches, brook trout were usually first found occupying deep pools. The use of these deep pools decreased in the latter part of the river residency period of 1999. Ninety-eight percent of the stream locations made in September were in pools with maximum depths exceeding 1.5 m. In October, the number of locations in such pools decreased to 57%.

HABITAT AND MOVEMENT OF BROOK TROUT

Stream Fidelity Brook trout exhibited strong fidelity to the streams they had entered in the fall of 2000. Although the number of brook trout with operating transmitters had decreased by the fall of 2000, six brook trout were detected in streams that year. Each brook trout also showed strong fidelity to their individual spawning area by being located in the same 2- to 3-m2 spawning area as in the previous year. Tracking intensity was reduced during the fall of 2000, so that stream residence times could not be calculated for this year. However, from the limited number of days spent locating fish, it can be surmised that the timing and habitat selection were very similar to 1999. Similar to the previous year, brook trout in 2000 were located in shallow (,50 cm) instream spawning areas in early to mid-October and exited by the third week of October. Discussion The brook trout occupying Nipigon Bay used areas with definable habitat characteristics. Given the diversity of habitats available within Nipigon Bay, brook trout primarily utilized the shallow nearshore areas, comprising only a small percentage of the total habitat that is potentially available. This tendency was also found by Newman et al. (1999) and Slade (1994). Anadromous brook trout also occupied the 2–4-m nearshore depth contour (White 1940). The habitat most highly utilized by Nipigon Bay brook trout can be described as a band of nearshore waters adjacent to the shorelines of both the mainland and adjacent islands. The width of this habitat band is dictated by both depth and distance to shore. If nearshore waters have a shallow slope, then a broader area is utilized. Conversely, a steep nearshore slope yields only a narrow band of such habitat. These findings are consistent with those of studies of habitat use by lake-dwelling brook trout, which indicate that although this species may inhabit a range of depths, it is generally located in nearshore areas within 1 m of the bottom (Baldwin 1948; Flick and Webster 1962; Lackey 1970). Areas further than 1 km from shore, regardless of depth, were rarely used by Nipigon Bay brook trout. The isolated telemetry locations recorded from these offshore areas were probably the result of individuals being located in transition between mainshore and offshore island habitats, as no brook trout were detected in these areas on consecutive days. After ice-out, brook trout were located most frequently in extremely shallow areas close to shore. These shallow nearshore areas are the first to warm up after ice-out, and brook trout typically seek out the warmest possible water after ice-out, as these areas are

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generally closest to their temperature optimum at this time of year (Biro 1998). As summer progressed, brook trout were found further from shore in deeper areas. This probably reflects their thermal preferences as well since deepwater habitat is often used by brook trout in lakes during July and August so they can avoid temperatures above their tolerance limits (Baldwin 1948; Lackey 1970; Olson et al. 1988). In addition to using deepwater habitat as a temperature refuge, brook trout in July and August were located in localized shallow shoreline areas that appeared to be springs with groundwater discharge. Biro (1998) also found that brook trout used groundwater upwelling areas in lakes as midsummer thermal refugia. In areas with very shallow littoral zone slopes that lacked deepwater habitat, we observed that, as water temperatures warmed, brook trout moved to other shoreline areas with deepwater habitat. This took only a few days, although for fish residing adjacent to offshore islands it typically occurred a week later. This delay is probably the result of cooler water temperatures in these areas because of their proximity to Lake Superior’s colder offshore waters. After spawning in late October, brook trout returned to Lake Superior from tributary streams. At this time, they once again used the same shallow nearshore areas they inhabited after ice-out and before nearshore water temperatures increased to their summer highs. Brook trout remained in these shallow nearshore areas throughout the fall and winter. Lackey (1970) also noted that brook trout use increasingly deeper waters throughout summer and shallower waters in fall. Closer scrutiny of the nearshore habitat utilized by brook trout throughout the year revealed some physical differences between the habitat types. Nearshore areas used by coaster brook trout in the summer had steeper littoral slopes than did the nearshore areas that saw use in spring–early summer and in fall after spawning was completed. Rather than move further from shore to deeper areas of preferable temperature, brook trout moved laterally, selecting adjacent nearshore areas with steeper slopes that provided deep, cool waters closer to shore. Offshore areas may not be attractive to brook trout because these locations are too far from preferred foraging areas. Shallow nearshore habitat also had a much higher occurrence of cover, including large boulders, shoal edge, and aquatic vegetation. Given the shallow depths that brook trout inhabit during certain months of the year, they may be attracted to areas that provide escape and concealment from predators. Based on visual observations, deep nearshore habitat was relatively devoid of the aforementioned cover types. Cover may not be critical to brook trout when occupying deep

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areas because the increased depth reduces the risk of predation. Throughout the day, the brook trout in Nipigon Bay are usually associated with drop-off structure, large boulders, or aquatic vegetation. During crepuscular time periods, brook trout were observed leaving cover or deeper waters and moving to extremely shallow areas close to shore with limited cover. These individuals were probably leaving their daytime cover locations to feed during these low-light periods. Similarly, in shallow areas along Minnesota’s Lake Superior shoreline, brook trout were observed leaving cover areas in the late evening to feed on insects (Newman et al. 1999). Although not known as lowlight predators, brook trout are capable of feeding in complete darkness and are aided by an acute olfactory sense (Hoar 1942). When the range of maximum distance traveled is used as a measure of how much total area is used, the brook trout of Nipigon Bay appear to be quite variable with respect to the amount of total area they use. By plotting individual brook trout, our locations suggest that individuals either inhabit a small number of core areas, resulting in a small range, or move frequently using numerous areas for only a brief duration and yielding a much larger range. Our results are somewhat consistent with those for anadromous saltwater brook trout, which rarely stray more than a few kilometers from their natal stream (Naiman et al. 1987). When residing in small streams, brook trout often occupy areas of less than 100 m2 (Power 1980). The brook trout is not physically adapted to long-distance traveling in a lentic environment: its square tail and low-aspect ratio are not conducive to high-speed swimming and pelagic cruising (Naiman et al. 1987). Brook trout appeared to follow fairly consistent movement patterns and habitat selection during their occupation of Nipigon Bay tributaries, each selecting the same discrete areas during its stream residency. Brook trout began entering tributaries sporadically beginning in late July. Their residency times during these early upstream movements were generally brief, lasting no more than a few days before the individuals returned to Lake Superior. These early upstream movements could be the result of the first spawning urges or an attempt to find more preferable temperature conditions. During periods of unfavorably high lake temperatures, brook trout often move into tributaries seeking temperature refugia (Power 1980). Movement back to Lake Superior may also have been an act of predator avoidance, as numerous northern river otters Lontra canadensis were observed in these areas during this time. The possibility that brook trout entered streams to forage cannot be overlooked. However, after

observing numerous aggregations of brook trout within these lower reaches, they did not appear to be involved in any foraging activity and were often quite reluctant to strike any offerings made by anglers. Ephemeral upstream movements continued until mid-August, when brook trout ceased to return to Lake Superior after a few days within their tributaries. This probing pattern differs from that exhibited by mature anadromous brook trout that return to their natal streams in a nonpulsed manner throughout the months of August and September (MacGregor 1973; Montgomery et al. 1990). Upon first entering Nipigon Bay tributaries in late summer and early autumn, brook trout inhabited deep pools in the lower river or remained within the estuarine portion of the river. Brook trout in the lower estuaries were observed aggregated and moved as a group. Similar aggregations of anadromous brook trout were observed within still-water sections of Nova Scotia’s Moser River in August and September (Wilder 1952). The lower river and estuary portions of most of the Nipigon Bay tributaries utilized by brook trout were generally deep and unaffected by periods of low streamflow. Movements of brook trout from the lower estuary waters to upstream holding pools occurred directly after significant rain events in both 1999 and 2000. White (1940) observed a similar correlation between an increase in streamflow and an increase in the upstream movement of anadromous brook trout within their spawning streams. Movements from the estuaries and lower river portions to upstream holding areas by tagged brook trout were generally accomplished in a single day. Due to the presence of natural barriers to migration, three of the four streams used by tagged coaster brook trout have less than 5 km of river accessible from Lake Superior. Upstream movements of this magnitude by coaster brook trout are similar to single-day upstream movements by anadromous brook trout that ascend several kilometers daily to reach their spawning areas (Naiman et al. 1987). Unlike in other tributaries, no tagged brook trout entered the Little Cypress River until late September. The individual using this stream during the spawning period was located in the lower estuarine waters of an adjacent river for the three previous weeks. This departure from the norm may reflect the fact that the Little Cypress River is much shallower in both its estuary and lower reaches and is probably not suited for holding large brook trout for extended periods. Once ascending tributaries to the upstream reaches after significant rain events, brook trout once again sought out deep pools and remained there until the last week of September. Throughout the last week of

HABITAT AND MOVEMENT OF BROOK TROUT

September and early October, brook trout moved into shallow runs, pool tailouts, and tributaries to the main rivers. Similarly, anadromous brook trout entering a large river system remained within areas of the main river system and entered smaller tributaries to spawn in October (Wilder 1952). The streams where our brook trout were located were shallow, making it possible to observe them during the first 2 weeks of October in acts of courtship, redd excavation, and egg deposition. The use of groundwater inputs for spawning areas by brook trout has been identified in many studies (Witzel and MacCrimmon 1983; Curry and Noakes 1995; Blanchfield and Ridgway 1997). Streambanks adjacent to the areas used for brook trout spawning were observed to either be wet well above the level of flow or to have clusters of marsh marigold Caltha palustris, which are an indicator of focused groundwater discharge (Rosenberry et al. 2000). By the third week in October, most brook trout had returned to Nipigon Bay. The descent from their spawning areas was brief and highly synchronized, and most made the descent within the same 3-d period. The deep pools that held some brook trout for over a month during the prespawning ascent were utilized for no more than 1–2 d during their descent. The mean stream residency time for coaster brook trout is similar to the 60-d mean duration over which brook trout were located on or near their spawning grounds in a small southern Ontario lake (Blanchfield and Ridgway 1997). The vast differences in residency times between brook trout in our study could be related to the sex of the individuals, as female brook trout generally spend less time on spawning grounds than males (Blanchfield and Ridgway 1997). However, the brook trout in this study were tagged in the spring, when they did not exhibit any external sexual characteristics and could not be sexed; therefore, residency times could not be compared between males and females. From observations of redd excavation by brook trout in Nipigon Bay tributaries, it appears that most spawning occurred during the second week of October, when water temperatures declined to 88C. Blanchfield and Ridgway (1997) observed that the peak spawning periods for brook trout were associated with drops in temperature from 11.38C to 10.38C and from 8.88C to 5.98C. Radio-tagged brook trout did not overwinter within spawning streams; all streams were vacated by the first week of November. This behavior differs from that of anadromous brook trout, which overwinter exclusively within their freshwater spawning tributaries (Naiman et al. 1987; Montgomery et al. 1990). This difference is probably attributable to the intolerance of brook trout

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to the low temperatures and high salinities characteristic of seawater during the winter months (Saunders et al. 1975). During the winter months, brook trout probably have more feeding opportunities within Lake Superior proper than in its tributaries and can avoid super-cooled water and frazil ice. Nipigon Bay brook trout appear to have strong homing abilities, both to their home streams and, more precisely, to specific spawning reaches. Very few studies have examined the ability of brook trout to return to a natal area for reproduction. White (1940, 1941) found evidence of homing in anadromous brook trout within the Moser River, but with some error in choosing between tributaries of the same river system. Similarly, displaced brook trout from Matamek Lake, Quebec, returned with great precision to their natal streams (O’Connor and Power 1973). Although our study did not identify any lake-spawning areas within Nipigon Bay proper, it is highly possible that some lake-spawned individuals contribute to the brook trout population of Nipigon Bay, as lake-spawning populations exist at Isle Royale in Lake Superior as well as in Lake Nipigon. Obtaining useful telemetry data on these brook trout was difficult because of the vast amount of potential habitat within both Nipigon Bay and the surrounding areas. Although most radio-tagged brook trout remained within Nipigon Bay for the duration of the study, it is possible that some individuals strayed outside this area and did not return. Numerous individuals could not be located for extended periods and some were never located again. Locating radiotagged individuals during the spawning period was especially challenging, given that numerous tributaries were accessible only by walking and their stream residency time was brief. The data in this study suggest that Nipigon Bay’s brook trout inhabit shallow nearshore areas throughout most of the year, moving to deeper nearshore areas during midsummer and ascending tributaries in late summer. Movements inshore to extreme shallow areas, presumably to forage, occur during the night. Brook trout were located in four different tributaries in the fall and used very few discrete areas within each tributary for spawning. Protection of these vital areas from further degradation will certainly assist in achieving the longterm viability of this native species. Acknowledgments This research was made possible through funding by the Ontario Ministry of Natural Resources. We are especially grateful to the Upper Great Lakes Management Unit–Lake Superior for its assistance throughout this study, as well as the Nipigon District Ontario

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