Diel Resource Partitioning among Juvenile Atlantic Salmon, Brown Trout, and Rainbow Trout during Summer
Abstract
Interspecific partitioning of food and habitat resources has been widely studied in stream salmonids. Most studies have examined resource partitioning between two native species or between a native species and one that has been introduced. In this study we examine the diel feeding ecology and habitat use of three species of juvenile salmonids (i.e., Atlantic Salmon Salmo salar, Brown Trout Salmo trutta, and Rainbow Trout Oncorhynchus mykiss) in a tributary of Skaneateles Lake, New York. Subyearling Brown Trout and Rainbow Trout fed more heavily from the drift than the benthos, whereas subyearling Atlantic Salmon fed more from the benthos than either species of trout. Feeding activity of Atlantic Salmon and Rainbow Trout was similar, with both species increasing feeding at dusk, whereas Brown Trout had no discernable feeding peak or trough. Habitat availability was important in determining site-specific habitat use by juvenile salmonids. Habitat selection was greater during the day than at night. The intrastream, diel, intraspecific, and interspecific variation we observed in salmonid habitat use in Grout Brook illustrates the difficulty of acquiring habitat use information for widespread management applications.
Received June 13, 2014; accepted January 28, 2015
Understanding the life-stage-specific requirements of a fish species is an important tenet of fisheries management. The environmental conditions during early life history can greatly influence recruitment and, ultimately, population size (Elliott 1989; Zabel and Achord 2004). The larval and juvenile stages of most fishes are perhaps the most vulnerable to environmental influences because of the smaller size and restricted mobility (Litwak and Leggett 1992; Laegdsgaard and Johnson 2001). Consequently, because of the importance of the juvenile stage, a considerable amount of attention has been devoted to examining the habitat and dietary needs of many highly valued and managed fish species as understanding these requirements is a necessary step for proper species management (Rosenfeld 2003). However, the preponderance of these studies have used only daytime observations to describe the resource use. Studies that have examined the nocturnal resource use of juvenile fish have generally documented differences from observations made during the day (Bremset 2000; Bradford and Higgins 2001).
It is generally accepted that social interaction among closely related fish species may lead to interactive segregation, with each species taking a smaller but more specific portion of the available resources (e.g., Nilsson 1967). Atlantic Salmon Salmo salar and Brown Trout S. trutta are closely related salmonid species that occur sympatrically in streams throughout Europe. Consequently, many studies have examined how these species partition resources in streams in their native ranges (Egglishaw and Shackley 1977; Hesthagen 1988, 1990). In North America, Atlantic Salmon occur sympatrically with another native salmonid, Brook Trout Salvelinus fontinalis, and several studies have examined how these species coexist in streams (Gibson 1978; Mookerji et al. 2004; Johnson 2008). Brown Trout and Rainbow Trout Oncorhynchus mykiss have been introduced throughout the native range of Atlantic Salmon in North America (MacCrimmon and Marshall 1968; MacCrimmon 1971). Although some studies have examined aspects of the juvenile stream ecology of two of the species and others have looked at interspecific interactions of all three species in experimental channels (Van Zwol et al. 2012a, 2012b) when occurring sympatrically (Heggenes et al. 1999, 2002; Bremset 2000, Coghlan et al. 2007), there is no information available on diel resource use during summer for these three species while occurring sympatrically in natural streams.
Diel resource partitioning among juvenile Atlantic Salmon, Brown Trout, and Rainbow Trout during summer was examined in a Finger Lakes tributary in central New York. The purpose of the study was to (1) examine interspecific and intraspecific variation in the diurnal and nocturnal habitat use of juvenile salmonids and (2) examine diel variation in the diet composition, feeding periodicity, diet overlap, and principle foraging resource (i.e., benthos or drift) of subyearling salmonids.
METHODS
Diel variation in the habitat use of juvenile Atlantic Salmon, Brown Trout, and Rainbow Trout was examined in Grout Brook, a second-order tributary of Skaneateles Lake in central New York. Habitat observations were made during summer in two representative 2.0-km reaches of the stream, approximately 0.7 km (lower Grout Brook) and 5.0 km (Grout Mill) from the confluence with the lake. Grout Brook is a nursery stream for migratory Rainbow Trout and Brown Trout from Skaneateles Lake. Juvenile Rainbow Trout are abundant throughout the Grout Brook system, whereas natural reproduction of Brown Trout appears to be restricted to the lower 3 km of the stream. Brown Trout and Rainbow Trout spend less than 2 years in the stream before descending to the lake. Although juvenile Atlantic Salmon are annually stocked into the lake, very few adults have been observed in Grout Brook and successful natural reproduction has not been documented. The juvenile Atlantic Salmon that were encountered in this study had been stocked into Grout Brook 2.5 months prior to the habitat and diet observations. Although historically Atlantic Salmon resided in many Finger Lakes, a natural barrier prevented them from gaining access to Skaneateles Lake. Consequently, although Atlantic Salmon are native to the immediate area, they, as well as Brown Trout and Rainbow Trout, are not native to Grout Brook. The only other common fish species in Grout Brook is Slimy Sculpin Cottus cognatus.
Habitat.
Diurnal (1000–1400 hours) and nocturnal (2400–0400 hours) observations of juvenile salmonid habitat were made at both sites 10 d apart. Because in-stream habitat did not change over this period, available habitat within each stream reach was quantified only once, 5 days after the initial observation of juvenile salmonid habitat. Stream temperatures during this period, recorded with a continuous reading thermograph, ranged from 11.7°C to 16.8°C. Juvenile salmonid habitat observations were made using the spot-sample electrofishing technique (Bovee 1986). This is a preferred technique in shallow, first- and second-order streams (Johnson and Kucera 1985; Johnson et al. 1992; Johnson and Dropkin 1996), where snorkeling is not effective (Heggenes et al. 1990). This technique avoids the continuous application of the electric field, which tends to drive fish. When using spot-sample electrofishing to examine fish habitat, care must be taken to ensure that all available habitats are sampled and the “spots” chosen for the application of the electric field are sufficiently dispersed. In instances in which fish were seen to flee prior to the application of the electric field, the affected area was not sampled. Nocturnal habitat observations were made using illuminated headlamps. During day and night periods, fish habitat observations were made over the entire 2.0-km stream reach, proceeding from the lower most boundary and finishing at the upper boundary. Sampling time was about 3 h during the day and 3.5 h at night.
At each site where fish were observed, numbered buoys were placed as the crew proceeded upstream. At this time, the buoy number was recorded on the data sheet along with the number, species, and age (either subyearling or overyearling) of salmonids observed at the spot. Upon retrieval of each buoy, the habitat variables were recorded at each location, including the water velocity, water depth, amount and type of cover, and substrate size. Water depth was measured with a wading rod, and mean water velocity was measured at a depth of 0.6 m from the surface with a Marsh-McBirney model 201d flowmeter. Cover is a difficult variable to quantify, even more difficult to define, and consequently is often omitted from many microhabitat studies (Heggenes et al. 1999). The type of cover was specified as overhead (e.g., overhanging bank), surface turbulence, or bottom substrate (e.g., cobbles or boulder), and the amount was visually estimated at 5% increments as the percentage of cover available within a radius of about four fish lengths surrounding the marker buoy (Johnson and Dropkin 1996). The estimation of cover based on individual fish length allows for more area to be considered for larger fish, which are usually more mobile than smaller fish and use cover over a broader area (Johnson et al. 1992). Substrate materials were classified visually using a modified Wentworth particle-size scale (e.g., sand = 4, gravel = 5, cobble = 6, boulder = 7, bedrock = 8; Orth et al. 1981). Available habitat was estimated from 50 transects located at 40-m intervals in each of the 2.0-km stream reaches. Stations were spaced at 0.5-m intervals along each transect where measurements of water velocity, water depth, percent cover, and substrate size were made. Depending on stream width, the number of stations established along the transects ranged from 7 to 12.
Bootstrapping cluster analysis (McKenna 2003) and canonical correspondence analysis (CCA; ter Braak and Smilauer 2002) were used to evaluate the differences in habitat use by each age-class of each species in each site day or night relative to each other and to the available habitat in each sampling area. Standardized habitat values (Z-scores) allow for the comparison of variables with different measurement units and value ranges and were used as the attributes of conditions associated with locations where age-classes of each salmonid species were collected. The cluster analysis objectively identifies groups of biotic entities (i.e., species age-classes within sites and diel period). We used the Bray–Curtis similarity index (unweighted pair group method using arithmetic averages linkage method, 1,000 bootstrap samples to test each linkage), and replicate samples were arranged by combinations of sample location, species, age, and diel period; available habitat samples within each sample area were considered separate groups. The CCA is a constrained ordination that directly relates species abundances to environmental data. In our case, the biotic entities were age-classes of salmonid species collected day or night at each site and the six microhabitat descriptors described above were available. A permutation test (499 permutations) was used to test the significance of each habitat variable's influence on the distribution of biotic entities. Cluster analysis groups were superimposed on the CCA ordination space to help evaluate the separation of groups from each other and from available habitat conditions. Multiway analysis of variance (ANOVA) was then used to test for differences in habitat conditions among those habitats occupied by the various species and age-classes of juvenile salmonids and the available habitat conditions.
Diet.
The diel feeding ecology of subyearling Atlantic Salmon, Brown Trout, and Rainbow Trout was examined over a 24-h period in lower Grout Brook in late July, 1 week after completion of the habitat analysis. Salmonids were collected at 4-h intervals using a backpack electrofisher. The subyearling salmonids used for diet analysis were collected within a 0.25-km reach of the 2.0-km stream section where habitat observations were made. The collections of fish at each successive 4-h time interval were made immediately up stream of where collections ended for the previous 4-h interval. Fish collections at each 4-h interval generally took 20 min. Upon collection, fish were immediately placed in 10% buffered formalin. Food availability in each area was assessed concurrently with the diet examination using a Surber sampler (bottom) and drift nets. Fifteen (five per interval) bottom samples (0.09 m2; mesh size = 0.75 mm) were taken at 1200, 2000, and 400 hours to reflect benthic composition during diurnal, crepuscular, and nocturnal periods, respectively. Eighteen (three per interval) drift nets (aperture = 30.5 cm × 30.5 cm; mesh size = 0.60 mm) were set and emptied at 4-h intervals to measure invertebrate drift. Surber samples and drift net samples were taken at the top of the 250-m stream section used for diet examination. Bottom and drift samples were preserved in 70% ethanol.
Laboratory analysis.
In the laboratory, subyearling salmonids were measured (total length; mm) and weighed (nearest 0.001 g). Stomachs were removed and weighed (full and empty) prior to diet examination. Aquatic taxa were generally identified to family and terrestrial taxa to order for diet and bottom and drift samples. Diet composition, bottom samples, and drift net samples were quantified based on dry weight (dried at 105°C for 24 h) for each taxon.
Data analysis.
The weight of the stomach contents divided by the weight of the fish for each 4-h interval provides an index of stomach fullness that was used to estimate feeding periodicity over the 24-h period. Feeding periodicity estimates for each 4-h interval were used to construct a 24-h diet for subyearling salmonids in each stream. Diet composition at each 4-h interval was weighted by the feeding periodicity value for that period and this value was then summed for all intervals for each taxon and divided by the number of feeding intervals (six).
where Ct is the amount of food consumed in t hours, St is the mean stomach contents at the end of the interval, So is the mean stomach contents at the beginning of the interval, and R is the exponential rate of gastric evacuation. These food ingestion rates were then summed to obtain an estimate of the daily food consumption. Stream salmonids feed more or less continuously, but So of the first interval was unknown. We used the last observed stomach content (ST) value for the initial So. This is more reasonable than assuming stomachs began empty but assumes the consumption is roughly similar from one day to the next at this time of year. The values for R were derived from Hayward and Weiland (1998). Daily food consumption (based on wet weights in milligrams) of subyearling salmonids was estimated by summing interval values for the 24-h period. Daily ration was expressed as total daily food consumption divided by fish weight.
Bootstrapping cluster analysis was also used to evaluate differences in the day and night subyearling diets of each salmonid species that used lower Grout Brook. This cluster analysis used the same method as that described above, but the weights of each taxon in the diet or the available food resources were used as attributes and replicate samples were by species–diel period combinations or available forage samples, which were separated into drift or benthic resources; this allowed the analysis to determine the similarity of any diet to each of the separate food resources.
Because the Shapiro–Wilks test showed that diet data were not normally distributed, we assessed the significance of diet composition differences between each time period within each stream using Kruskal–Wallis one-way ANOVA (Statistix 8.0 Analytical Software, Tallahassee, Florida).
RESULTS
A total of 3,695 observations were made on the diel habitat use of juvenile salmonids, with similar sample sizes between diurnal (51%) and nocturnal periods (49%) (Table 1). An additional 523 (207 Atlantic Salmon, 195 Rainbow Trout, 121 Brown Trout) subyearling salmonids collected from the lower Grout Brook site were examined for dietary analysis (Table 2). Collections occurred over six, 4-h intervals beginning at 1200 hours and ending 24 h later at 1200 hours. A minimum of 28 Atlantic Salmon, 23 Rainbow Trout, and 14 Brown Trout were collected and examined at each 4-h interval.
| Atlantic Salmon | Brown Trout | Rainbow Trout | ||||
|---|---|---|---|---|---|---|
| Time (hours) | Number examined | Mean length | Number examined | Mean length | Number examined | Mean length |
| 1200 | 28 | 65.9 (58–80) | 17 | 64.4 (55–72) | 26 | 56.7 (44–78) |
| 1600 | 29 | 62.2 (51–78) | 18 | 64.2 (51–74) | 30 | 49.2 (40–59) |
| 2000 | 29 | 63.6 (53–73) | 14 | 67.1 (54–84) | 28 | 54.9 (41–65) |
| 2400 | 34 | 65.5 (57–77) | 16 | 62.3 (55–75) | 22 | 53.6 (45–65) |
| 0400 | 28 | 67.8 (59–77) | 20 | 65.3 (48–78) | 33 | 58.2 (47–81) |
| 0800 | 31 | 65.5 (57–77) | 18 | 67.6 (60–79) | 29 | 53.7 (40–67) |
| 1200 | 28 | 62.4 (52–76) | 18 | 63.9 (56–77) | 27 | 55.2 (47–69) |
| Sites | ||||
|---|---|---|---|---|
| Lower Grout Brook | Grout Mill | |||
| Category | Day | Night | Day | Night |
| Atlantic Salmon, subyearling | 264 | 332 | 276 | 321 |
| Brown Trout, subyearling | 99 | 88 | ||
| Brown Trout, yearling | 27 | 26 | ||
| Rainbow Trout, subyearling | 592 | 488 | 324 | 248 |
| Rainbow Trout, yearling | 171 | 153 | 145 | 141 |
Diet
Cluster analysis revealed that the diets of subyearling Brown Trout and Rainbow Trout were most similar, whereas the diet of subyearling Atlantic Salmon was different from the other two species (Figure 1). The diets of both Brown Trout and Rainbow Trout were more closely associated with the composition of the drift than the benthos. The diet of Atlantic Salmon was less similar to the composition of the drift than was the diet of either trout species, which indicates that a larger component of the Atlantic Salmon diet came from the benthos compared with the Brown Trout and Rainbow Trout diets. This suggests that Atlantic Salmon feed more from benthic habitat than do either of Brown Trout or Rainbow Trout species (Figure 1). Casual inspection of Atlantic Salmon diet composition supports this contention (Table 3), especially considering the much lower contribution of terrestrial invertebrates in Atlantic Salmon diets compared with Brown Trout and Rainbow Trout diets. Baetids (20.3–45.6%) and hydropsychids (19.1–34.0%) comprised a large amount of the diet of subyearling Atlantic Salmon at each of the 4-h intervals and contributed 30.7% and 27.2%, respectively, of the total 24-h diet (Table 3). Overall, aquatic invertebrates made up 96.7% of the 24-h diet of Atlantic Salmon and terrestrial invertebrates contributed 3.3%. Baetids (4.7–33.1%) and hydropsychids (4.5–21.9%) were also the major prey consumed by subyearling Rainbow Trout, contributing 21.5% and 14.2% of the 24-h diet, respectively (Table 3). Aquatic invertebrates made up 82.3% and terrestrial invertebrates 17.7% of the 24-h diet of Rainbow Trout. Terrestrial invertebrates (11.1–51.6%), baetids (8.1–15.2%), asellids (1.4–29.2%), elmids (0.0–29.2%), and hydropsychids (1.3–23.1%) were the major prey of subyearling Brown Trout and contributed 26.3%, 10.9%, 10.2%, 9.9%, and 9.4% of the 24-h diet, respectively (Table 3).
| 0400 hours | 0800 hours | 1200 hours | 1600 hours | 2000 hours | 2400 hours | 24-h diet | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Diet item | BRN | RBT | ATS | BRN | RBT | ATS | BRN | RBT | ATS | BRN | RBT | ATS | BRN | RBT | ATS | BRN | RBT | ATS | BRN | RBT | ATS |
| Stonefly, order Plecoptera | 2.9 | 3.8 | 2.3 | 11.0 | 2.9 | 1.9 | 6.9 | 0.3 | 1.2 | 6.3 | 3.4 | 0.9 | 2.7 | 1.5 | 0.0 | 0.0 | 0.0 | 0.7 | 4.9 | 2.1 | 1.2 |
| Mayfly, order Ephemeroptera | 8.8 | 1.7 | 0.4 | 1.1 | 1.5 | 1.2 | 5.1 | 4.0 | 0.9 | 8.1 | 12.3 | 0.0 | 5.9 | 12.1 | 0.3 | 2.8 | 2.2 | 2.0 | 5.3 | 5.3 | 0.8 |
| Family Baetidae | 8.3 | 4.7 | 34.5 | 8.1 | 15.3 | 31.0 | 9.9 | 23.9 | 23.2 | 11.4 | 33.1 | 20.3 | 12.7 | 29.5 | 27.7 | 15.2 | 29.5 | 45.6 | 10.9 | 21.5 | 30.7 |
| Caddisfly, order Trichoptera | 9.5 | 16.6 | 5.5 | 4.6 | 21.7 | 4.9 | 5.3 | 2.9 | 3.9 | 0.0 | 10.3 | 6.5 | 15.0 | 1.4 | 3.9 | 4.5 | 27.2 | 6.8 | 6.8 | 13.5 | 5.2 |
| Family Brachycentridae | 2.4 | 3.7 | 13.4 | 2.2 | 1.5 | 12.3 | 2.6 | 1.7 | 5.7 | 1.2 | 1.8 | 7.2 | 11.8 | 8.9 | 6.9 | 8.7 | 3.1 | 6.6 | 4.9 | 3.4 | 8.7 |
| Family Hydropsychidae | 5.4 | 21.6 | 23.8 | 15.8 | 5.9 | 30.4 | 1.3 | 13.7 | 34.0 | 5.1 | 16.3 | 33.5 | 23.1 | 21.9 | 22.8 | 6.2 | 4.5 | 19.1 | 9.4 | 14.2 | 27.2 |
| Flies, order Diptera (misc.) | 1.9 | 3.5 | 1.7 | 2.4 | 5.9 | 3.0 | 0.6 | 4.9 | 7.6 | 1.0 | 4.1 | 4.2 | 2.0 | 5.6 | 13.7 | 1.1 | 7.3 | 8.2 | 1.5 | 5.1 | 6.3 |
| Family Chironomidae | 3.7 | 5.4 | 2.6 | 8.6 | 11.6 | 4.8 | 6.1 | 8.7 | 4.1 | 2.7 | 10.2 | 6.1 | 4.6 | 9.7 | 10.1 | 5.2 | 4.4 | 5.9 | 6.8 | 5.1 | |
| Misc. aquatics | 5.0 | 0.0 | 1.4 | 5.4 | 1.3 | 1.7 | 1.9 | 6.2 | 0.0 | 11.3 | 0.7 | 0.0 | 1.4 | 13.6 | 1.2 | 0.2 | 2.4 | 0.0 | 4.0 | 3.9 | 0.7 |
| Family Asellidae | 29.2 | 4.5 | 3.5 | 5.9 | 0.9 | 18.1 | 0.5 | 0.7 | 1.4 | 2.3 | 2.2 | 0.3 | 10.2 | 2.0 | 0.3 | ||||||
| Family Elmidae | 2.9 | 5.8 | 13.1 | 6.1 | 5.0 | 5.9 | 12.5 | 10.6 | 14.1 | 0.5 | 8.4 | 7.4 | 0.8 | 13.9 | 29.2 | 2.7 | 5.2 | 9.9 | 4.5 | 10.1 | |
| Terrestrials | 19.9 | 28.7 | 1.4 | 31.2 | 21.7 | 1.8 | 29.5 | 22.6 | 4.7 | 51.6 | 7.5 | 12.9 | 11.1 | 4.7 | 0.0 | 19.9 | 15.8 | 1.0 | 26.3 | 17.7 | 3.4 |
Dendrogram of cluster analysis results for subyearling salmonid diets and available forage at lower Grout Brook. Bray–Curtis similarity index values are provided along the ordinate. An asterisk indicates a linkage between significantly related groups. Dotted polygons enclose similar diet or forage groups. A species code (ATS = Atlantic Salmon, BRN = Brown Trout, RBT = Rainbow Trout) and a diel code (Day, Night) indicate group members. Food availability was examined using a Surber sampler (bottom sampling) and drift nets.
Diel patterns of food consumption were similar for subyearling Atlantic Salmon and subyearling Rainbow Trout, showing a sharp increase from 2000 hours to 2400 hours, with feeding activity higher at night than during the day (Figure 2). After midnight, feeding activity generally decreased until 0800 hours (Atlantic Salmon) and 1200 hours (Rainbow Trout) before increasing again. Although food consumption was highest at midnight, it was only significantly different (P < 0.0001) from 1600 hours for Atlantic Salmon and Rainbow Trout (Figure 2). Unlike Atlantic Salmon and Rainbow Trout, subyearling Brown Trout exhibited no discernible peak in food consumption over the 24-h period (Figure 2). Daily food consumption and daily ration were estimated as 4.6 mg (12.6%), 4.5 mg (10.2%), and 5.3 mg (13.3%) for subyearling Atlantic Salmon, Brown Trout, and Rainbow Trout, respectively.
Feeding periodicity estimates (error bars indicate SE) for subyearling Atlantic Salmon (ATS), Brown Trout (BRN), and Rainbow Trout (RBT) in Grout Brook, New York, over a 24-h period in late July.
Habitat
Cluster analysis of habitat use revealed six distinct groups that separated by diel period and sampling area, and available habitat conditions were distinct. The first two axes of the CCA accounted for 79.5% of the biotic entity–habitat relationship (Figure 3). All six habitat variables had significant influence on the distribution of the biotic entities, and inflation factors were < 1.5. Diel variation in habitat use was most pronounced at the Grout Mill site and was greatest for yearling Rainbow Trout. In lower Grout Brook, yearling Brown Trout and Rainbow Trout exhibited the least diel variation in habitat use, whereas subyearling Atlantic Salmon and Rainbow Trout exhibited the most. Juvenile Brown Trout (both subyearlings and yearlings) exhibited the least amount of diel variation in habitat use of the three species examined. Differences between sites and between day and night periods were generally associated with the use of overhead cover, water depth, and substrate size. The multiway ANOVA revealed significant differences between sites and between day and night periods (Table 4) but not between species or age-groups for any habitat variable (not shown).
| Variable | Component | Entity | Mean | Significance group |
|---|---|---|---|---|
| Depth (cm) | L_D_FISH | 23.1 | A | |
| (F = 39.29, P < 0.01) | L_N_FISH | 20.0 | B | |
| L_A | 18.7 | B C | ||
| M_D_FISH | 17.9 | C | ||
| M_N_FISH | 15.5 | D | ||
| M_A | 14.2 | D | ||
| Substrate size | M_N_FISH | 6.0 | A | |
| (F = 100.06, P < 0.01) | M_A | 6.0 | A | |
| M_D_FISH | 6.0 | A | ||
| L_N_FISH | 5.5 | C | ||
| L_A | 5.5 | C | ||
| L_D_FISH | 5.5 | C | ||
| Percent cover | ||||
| Overhead | L_D_FISH | 8.3 | A | |
| (F = 98.70, P < 0.01) | M_D_FISH | 8.3 | A | |
| L_N_FISH | 4.9 | B | ||
| L_A | 2.0 | C | ||
| M_N_FISH | 1.2 | C | ||
| M_A | 0.4 | C | ||
| Substrate | M_D_FISH | 9.7 | A | |
| (F = 242.73, P < 0.01) | M_N_FISH | 6.7 | B | |
| M_A | 4.8 | C | ||
| L_D_FISH | 2.1 | D | ||
| L_N_FISH | 0.7 | E | ||
| L_A | 0.7 | E | ||
| Surface turbulence | M_N_FISH | 2.8 | A | |
| (F = 14.80, P < 0.01) | M_A | 2.6 | A | |
| L_D_FISH | 2.5 | A | ||
| M_D_FISH | 2.3 | A | ||
| L_N_FISH | 1.5 | B | ||
| L_A | 1.2 | B | ||
| Velocity (m/s) | M_N_FISH | 0.3 | A | |
| (F = 32.01, P < 0.01) | L_D_FISH | 0.3 | A B | |
| M_D_FISH | 0.3 | B | ||
| M_A | 0.3 | C | ||
| L_N_FISH | 0.2 | C | ||
| L_A | 0.2 | D |
First two axes of the canonical correspondence analysis hyperspace; lower Grout Brook samples occurred in the negative quadrants of axis 1 and Grout Mill samples in the positive quadrants of axis 1. Triangles indicate daytime centroids and circles indicate nocturnal centroids for species–age groups. Each species–age centroid is labeled by an “A” for Atlantic Salmon, a “B” for Brown Trout, or an “R” for Rainbow Trout and by a “0” for subyearling or a “1” for yearling. Available habitat centroids are indicated by a diamond for lower Grout Brook and a square for Grout Mill. Similar entities (based on cluster analysis results) are enclosed in polygons; solid lines enclose daytime entities and dotted lines enclose nocturnal entities. The arrows indicate the direction of increasing habitat values, and the arrow length is an indication of influence on the distribution of biotic entities; dashed arrows represent the elements of cover.
At Grout Mill, habitat selection (determined by CCA distances between fish habitat centroids and available habitat centroids) was greater during the day than at night for subyearling Atlantic Salmon and both age-classes of Rainbow Trout (Figure 3). Ordination indicated that day and night habitat use was strongly related to overhead cover and depth, with selection for more cover and deeper habitat during the day. This was much more pronounced at the lower Grout Brook site than at the Grout Mill site. Habitat variation along CCA axis 1 indicated substantial differences in substrate size and substrate cover between the two sample areas. Habitat selection in lower Grout Brook was greater during the day for subyearling Atlantic Salmon and Rainbow Trout, but no diel habitat selection was evident for subyearling and yearling Brown Trout or yearling Rainbow Trout.
The CCA showed that at the Grout Mill site yearling Rainbow Trout used deeper and faster water with more overhead cover than either subyearling Atlantic Salmon or subyearling Rainbow Trout (Figure 3). At the lower Grout Brook site, water depth and overhead cover were the major habitat variables that differed between yearling salmonids (i.e., deeper water, more cover) and subyearling salmonids. Subyearling Atlantic Salmon occupied slightly faster water velocities than subyearling Rainbow Trout at both sites.
DISCUSSION
Interspecific differences in the feeding ecology or habitat use of Atlantic Salmon, Brown Trout, and Rainbow Trout occurring sympatrically in streams had not previously been reported. Subyearlings of these three salmonid species all fed most heavily on aquatic invertebrates, although terrestrial invertebrates, considered as a single prey group, were the major prey consumed by Brown Trout (26.3% of the diet). Although terrestrial invertebrates have been reported in the diet of juvenile Brown Trout in several studies (Kreivi et al. 1999; Skoglund and Barlaup 2006; Sanchez-Hernandez et al. 2011), some researchers have noted that their diet contained few terrestrials (Oscoz et al. 2005). The consumption of terrestrial invertebrates by Atlantic Salmon (3.3% of the diet) and Rainbow Trout (17.7%) was lower than for Brown Trout. Coghlan et al. (2007) also found that, in sympatry, juvenile Rainbow Trout consumed more terrestrial invertebrates than Atlantic Salmon. During summer, terrestrial invertebrates often dominate the diurnal drift in streams (Hynes 1972), which was the case in Grout Brook. The closer association of the diet of Brown Trout and Rainbow Trout to the composition of the drift than to that of the benthos is reflected in their higher consumption of terrestrial invertebrates compared to Atlantic Salmon. These findings are consistent with previous studies. Johnson (2013) found that the diet of subyearling Atlantic Salmon was more closely associated with the composition of the benthos than the drift. Subyearling Brown Trout in two of three rivers fed most heavily from the drift (Oscoz et al. 2005) as did subyearling steelhead (anadromous Rainbow Trout) in two of three Lake Ontario tributaries (Johnson et al. 2013).
The examination of diel feeding patterns in the field is laborious such that, even for widely studied species like Atlantic Salmon, few studies have been carried out (Amundsen et al. 1999). Although Amundsen et al. (1999) found only minor variation in diel feeding activity of subyearling Atlantic Salmon, the lowest consumption occurred at night. Similar to this study, Johnson (2013) found higher feeding activity by steelhead during nocturnal periods but also a significant spike in feeding activity from 0400 hours to 0800 hours. Dedual and Collier (1995) found no diel variation in stomach fullness of juvenile Rainbow Trout in Silver Creek, Idaho, but similar to our observations in Grout Brook, Riehle and Griffith (1993) reported an increase in feeding at dusk. The lack of a discernable feeding peak or trough by subyearling Brown Trout in Grout Brook was consistent with earlier studies (Glova et al. 1992; Kreivi et al. 1999). The similarity of diel feeding patterns of these three sympatric salmonid species in Grout Brook with previous studies suggests that interspecific interactions did not influence the time of feeding.
Estimates of food consumption and daily ration for juvenile salmonids from field studies have been reported. Kreivi et al. (1999) was the first to estimate daily ration for subyearling Brown Trout in the field, reporting a range of 1.1–14.7%. Our estimate of 10.2% falls within the range they observed. Mean fish consumption and daily ration estimates derived for subyearling steelhead in three Lake Ontario tributaries were 5.3 mg and 12.6%, respectively (Johnson et al. 2013), which was very similar to what we observed in Grout Brook. Our food consumption estimate for subyearling Atlantic Salmon (4.6 mg) was similar to early season estimates derived for salmonids using radioactive tracers (Kennedy et al. 2008).
Intraspecific diel variation in habitat use in Grout Brook was evident for all species and year-classes and differed by site. Whereas yearling Rainbow Trout exhibited the most diel variation in habitat use at Grout Mill, they, along with yearling Brown Trout, exhibited the least in lower Grout Brook. Bradford and Higgins (2001) also found within-river variation in the habitat use of juvenile steelhead in the Bridge River, British Columbia, and attributed it to differences in local habitat conditions. This observation is consistent with our findings; the CCA indicated that available habitat conditions between the two sites were distinct, especially in terms of substrate size and substrate cover. Substrate size at Grout Mill was larger, hence affording more cover, and was largely responsible for observed intra- and interspecific habitat differences in habitat use between sites. Yearling Rainbow Trout displayed the most variation in habitat use between sites and subyearling Atlantic Salmon the least. Stream salmonids require more cover as they grow (Gries et al. 1997), so observed differences in the habitat use of yearling Rainbow Trout between sites may be associated with the lack of substrate cover afforded at the lower Grout Brook site. The lack of substrate cover at the lower Grout Brook site may also have impacted the number of yearling Rainbow Trout that occupied that section. The ratio of recorded habitat observations for subyearling versus yearling Rainbow Trout at the lower Grout Brook site (3.3:1) was higher than at the Grout Mill site (1.8:1), and this may indicate a habitat limitation, likely substrate cover, for yearling salmonids at the lower site. Conversely, the smaller variation in habitat use for subyearling Atlantic Salmon between sites may suggest that they are less reliant on substrate cover and that other habitat variables, such as water velocity (Morantz et al. 1987; Armstrong et al. 2003), influence their microhabitat choice more.
Within a site, intraspecific variation in diel habitat use by juvenile salmonids in Grout Brook was primarily related to differences in water depth and overhead cover. However, these differences were not as pronounced at the lower Grout Brook site as they were at Grout Mill. In particular, yearling Brown Trout and yearling Rainbow Trout at the lower site exhibited little variation in diel habitat use. As mentioned previously, the lack of variation in diel habitat use by yearling salmonids may be due to the lack of habitat complexity in lower Grout Brook. In streams, salmonids often move from deeper water into shallower areas to feed at night (Railsback et al. 2005). This movement is thought to be associated with maximizing fitness by increasing foraging success while decreasing the risk of predation (Metcalf et al. 1999). Johnson (2013) speculated that darkness, as a surrogate for daytime concealment cover, reduced the need for more traditional structure-oriented cover by juvenile Atlantic Salmon during periods of low light intensity. These observations are consistent with the diel feeding patterns we observed for subyearling Atlantic Salmon and subyearling Rainbow Trout in Grout Brook. Feeding activity of both species increased sharply from 2000 hours to 2400 hours, the transition period from dusk to night. Subyearling Brown Trout, which lacked a discernable feeding peak, also displayed the least amount of variation in their diel habitat use. The lack of diel movement of subyearling Brown Trout, coupled with their uniform feeding pattern, is consistent with the theory that increased foraging success requires movement to more profitable, predator-free areas at night. In this instance, why move to another habitat if you are not increasing your food intake? However, contrary to what we found in Grout Brook, variation in the diel habitat use of subyearling Brown Trout has been reported, with fish moving to shallower water at night in a regulated stream (Hubert et al. 1994). Roussel and Bardonnet (1999) also reported the importance of shallow stream-margin habitat to subyearling Brown Trout during the day, with fish moving to deeper habitat at night. Although we did not quantify stream-margin habitat in this study, subyearling Brown Trout in lower Grout Brook generally occupied stream-margin habitat during both day and night periods, whereas subyearling Atlantic Salmon and Rainbow Trout used both stream-margin and midstream habitat.
Of the three salmonid species examined in this study, only Atlantic Salmon and Brown Trout coevolved sympatrically within their native range. Consequently, there have been several studies conducted on the habitat use of these species in sympatry, whereas interspecific studies between Rainbow Trout and Atlantic Salmon or Brown Trout are much less common. Juvenile Brown Trout are more aggressive than juvenile Atlantic Salmon or Rainbow Trout (Van Zwol et al. 2012b), and, in sympatry, Atlantic Salmon habitat is restricted because of interspecific competition with Brown Trout (Heggenes et al. 1999). However, juvenile Atlantic Salmon have been found to outcompete Brown Trout in areas with high current velocities (Armstrong et al. 2003), most likely because of morphological adaptations (i.e., body streamlining, pectoral fin size [Riddell and Leggett 1981]). Conversely, because of the aggressiveness of Brown Trout, intraspecific competition is considered to be important in governing their use of stream habitat, which has been found to be size structured (Heggenes et al. 1999). In Grout Brook, subyearling Atlantic Salmon occupied faster water velocities than subyearling Brown Trout and Rainbow Trout, which is consistent with previous studies (Hearn and Kynard 1986; Bremset 2000; Heggenes et al. 2002). Moreover, the use of deeper and faster water with more cover by yearling Atlantic Salmon, Brown Trout, and Rainbow Trout compared with the subyearling salmonids that we observed in Grout Brook is also well documented (Gries and Juanes 1998; Maki-Petays et al. 2004; Reeves et al. 2010).
This study provides important insight into the resource use of juveniles of these three species of salmonids while occurring sympatrically. Important differences in feeding ecology were observed, with subyearling Atlantic Salmon feeding more from the benthos than the other species and subyearling Brown Trout having a more uniform diel feeding pattern. Although variation in the diel habitat use of juvenile salmonids was observed, it was apparent that habitat availability within even a small stream like Grout Brook has a large bearing on the habitat occupied by salmonids. In this study, the habitat variables used by salmonids that differed the most between day and night periods were water depth and overhead cover, with an increased use of each occurring during the day. Evidence for diel differences in habitat selection varied between sites but, when they were observed, habitat selection was always greatest during the day. These findings support the recommendations of others that the comprehensive evaluation of the habitat needs of stream salmonids requires insight into day versus night habitat usage (Jakober et al. 2000; Bradford and Higgins 2001). Failure to understand these needs can result in management actions that can waste valuable resources (Reeves et al. 2010). The lack of habitat complexity at the lower Grout Brook site compared to the Grout Mill site provided useful information to managers for potential stream habitat enhancement to benefit yearling salmonids. Specifically, our observations suggest that although appropriate stream habitat restoration needs may be site specific, even in a small second-order stream such as Grout Brook a thorough understanding of the intrastream habitat variation and habitat use of salmonids is needed in order to determine the most appropriate management actions.
ACKNOWLEDGMENTS
We thank Tim Wallbridge for assistance in the field, and Marc Chalupnicki and Ross Abbett for data tabulation and analysis. This article is contribution 1906 of the U.S. Geological Survey Great Lakes Science Center. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
