Diet Composition, Feeding Periodicity, and Prey Selection of Slimy Sculpin During Winter

In the northern latitudes, winter is considered a critical period for stream fishes (Cunjak 1996). Surprisingly, however, winter is also the period when the least amount of information is available on the ecology of stream fishes (Anderson et al. 2016). The difficulty of conducting field research during extreme winter conditions contributes to the paucity of information during the winter (Alfredsen and Tesaker 2002). Because of these issues, Cunjak (1996) stressed that winter should be considered part of the field season and not just a time to analyze data collected during the summer. Because lower water temperatures and reduced photoperiod slow metabolic activity, during the winter, food consumption is less than during other times of the year (Metcalfe and Thorpe 1992). Reduced metabolic activity also influences prey capture efficiency (Watz and Piccolo 2011).

Slimy Sculpin Cottus cognatus is a benthic fish species that inhabit both lentic and lotic freshwater ecosystems from Virginia to Alaska in North America (Lee et al. 1980). This species prefers cold water (Craig and Wells 1976) and is considered an obligate benthivore (Brandt 1986). Where Slimy Sculpins are often found, similar to other species of Cottus, they comprise an important part of the fish community (Adams and Schmetterling 2007). This is the case in Grout Brook, a second-order tributary of Skaneateles Lake in central New York where Slimy Sculpins represent the dominant resident fish biomass. Because they are common in many cold water ecosystems throughout North America, there is a great deal of information on the life history and ecology of this species. However, there is limited information on the ecology of this species during winter. Petrosky and Waters (1975) examined Slimy Sculpin diets during all seasons, but they only provided an annual representation of their diet. Consequently, the objectives of this study were to describe the winter feeding ecology of Slimy Sculpins in Grout Brook, specifically examining feeding periodicity, diel variation in diet composition, prey selection, and food consumption.

Slimy Sculpins were collected at 4-h intervals over a 24-h period in January in Grout Brook in central New York. Grout Brook drains an area of approximately 2,455 ha and has an average width of approximately 4.5 m. The substrate consists mainly of gravel, cobble, and boulder. Stream discharge during winter ranges from 0.1 to 0.27 m³/s (Johnson and Douglass 2009). The stream also supports juvenile Rainbow Trout Oncorhynchus mykiss that are produced from migratory adults that ascend Grout Brook to spawn in the spring from Skaneateles Lake, a New York Finger Lake, and a small resident population of Brown Trout Salmo trutta.

Fish were captured using a backpack electroshocker and immediately placed in 10% buffered formalin. Target sample size at each 4-h interval was 25 fish. This sample size took about 20 min to collect during each interval. Sampling for each successive 4-h interval was conducted immediately above the endpoint for the previous interval to avoid fish disturbance during subsequent collections. Surber samples were used to assess food availability from the benthos. For each sample the substrate was agitated for approximately 3 min. Ten Surber samples (0.09 m²; mesh size 0.75 mm) were taken concurrent with fish collections and preserved in 70% ethanol. The habitat where Surber samples were taken was similar to the habitat where Slimy Sculpins were collected. Before their stomachs were removed, Slimy Sculpins were measured (total length in millimeters) and weighed (to nearest 1.0 mg). Stomachs were weighed full and empty to help quantify feeding periodicity. Feeding periodicity at each 4-h interval was established as the weight of the stomach divided by the weight of the fish. Prey taxa were generally identified to order and family (Chironomidae). Slimy Sculpin diet composition was determined based on dry weights (24 h at 105°C) derived for each prey taxon. Diet composition was determined for each 4-h interval. A 24-h diet was also estimated using diet composition estimates at each 4-h interval that were weighted by the respective periodicity values for the same interval. The mean of these values was used to determine the 24-h diet. For example, for a specific prey taxa (i.e., Chironomidae) rather than simply summing the contribution of chironomids in the diet at each 4-h interval and dividing by the number of intervals, the contribution at each interval was weighted by the periodicity value for that period. This essentially would increase the contribution of chironomids during an individual 4-h period when the weighting exceeds 1 and decrease it when the weighting was below 1.

Bootstrapping cluster analysis (McKenna 2003) was used to evaluate differences in the diet of Slimy Sculpins among 4-h intervals over the 24-h period. The cluster analysis objectively uses the Bray–Curtis similarity index; UPGMA linkage method; and 1,000 bootstrap samples to test for significant differences between linked groups, based on diet at each 4-h interval. 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 by using Kruskal–Wallis 1-way analysis of variance (Statistix 8.0; Analytical Software, Tallahassee, FL). The linear food selection index of Strauss (1979) was used to quantify prey selection by Slimy Sculpins in relation to the benthos. Coefficient values range from −1 (avoidance) to +1 (preference). A significance level of α = 0.05 was used for all comparisons.

In total, 174 Slimy Sculpins were examined (Table 1; Table S1, Supplemental Material). Mean size among the 4-h intervals ranged from 65.0 mm at 1200 hours to 71.9 mm at 0800 hours. The percentage of fish with empty stomachs ranged from 0 at 0800 hours to 6.7% at 1600 hours (Table 2; Table S2, Supplemental Material). The average number of prey per stomach was highest at 0400 hours, and the average weight of individual prey taxa was highest at 0800 hours (Table 2; Table S2, Supplemental Material). Ephemeropterans were the major prey consumed by Slimy Sculpins over the 24-h period (37.8%; Figure S1, Supplemental Material). Trichopterans (31.2%) were the second most important group consumed by Slimy Sculpins in winter. Dipterans (14.0%; mostly chironomids, 11.2%) and plecopterans (12.9%) were the third- and fourth-ranked prey groups in the winter diet of Slimy Sculpins. There was evidence of diel variation in the importance of several prey families in the diet. Over the 24-h period the contribution of ephemeropterans ranged from 24.1% (0400 hours) to 46.7% (1600 hours), trichopterans ranged from 22.7% (1200 hours) to 43.8% (2000 hours), and dipterans ranged from 4.9% (0800 hours) to 23.5% (1200 hours).

The benthos (Surber samples) in Grout Brook was dominated by coleopterans (29.2%), ephemeropterans (28.0%), and dipterans (18.7%; Figure 1). Trichopterans (12.3%) and plecopterans (10.6%) also made up at least 10% of the benthos in Grout Brook in winter (Figure 1). Over all of the 4-h intervals, Slimy Sculpins consistently selected only trichopterans and chironomids (Table S3, Supplemental Material). Except for 0400 hours when they were slightly avoided, ephemeropterans were positively selected. Conversely, coleopterans and nonchironomid dipterans were always avoided (Table S3, Supplemental Material).

Slimy Sculpins in Grout Brook exhibited diel variation in diet composition during winter. Sculpin diets at 1200, 1600, 2000, and 2400 hours were significantly different than those at 0400 and 0800 hours (Figure S2, Supplemental Material). Moreover, sculpin diets at 0400 and 0800 hours were significantly different. The composition of the benthic fauna (Surber samples) was significantly different from Slimy Sculpin diet during each 4-h interval (Figure 2). Feeding intensity of Slimy Sculpins increased substantially after 1600 hours and remained high until 0400 hours (Figure S3, Supplemental Material). After 0400 hours, Slimy Sculpin feeding intensity decreased to its lowest level at 1600 hours (Figure 3).

To our knowledge, our observations are the first reported on the winter feeding ecology of a stream population of Slimy Sculpins. Previous investigations on the diet of Slimy Sculpins are about evenly apportioned between lake and stream ecosystems. In the Great Lakes the amphipod Pontoporeia (Diporeia) was found to be a major prey item of the Slimy Sculpin (Brandt 1986; Kraft and Kitchell 1986). However, with the decline of Pontoporeia, attributed to a proliferation of invasive dreissenid mussels (Watkins et al. 2007), Slimy Sculpins in some of the Great Lakes have been found to switch their diet to Mysis spp. and chironomids (Owens and Dittman 2003; French et al. 2010). One of the primary prey of Slimy Sculpins in the stream studies that have been carried out during nonwinter periods is chironomids (Petrosky and Waters 1975; Craig and Wells 1976; Chalupnicki and Johnson 2016). The diet composition of Slimy Sculpins in Grout Brook during winter contrasted sharply with all of these studies in that chironomids only contributed 11.2% of the diet. This difference is especially surprising when considering that the Chalupnicki and Johnson (2016) study was also done in Grout Brook and found that chironomids made up 33.1% of the fall diet of Slimy Sculpins. During both investigations, chironomids made up less than 5% of the benthos; so, it is unlikely that the availability of chironomids was due to the difference in diet between seasons. Compared to the earlier study on the fall diet of Slimy Sculpins in Grout Brook (Chalupnicki and Johnson 2016), the sharp decline in the consumption of chironomids during winter was offset by increased predation on trichopterans and plecopterans. During both studies, ephemeropterans were the major prey consumed by Slimy Sculpins in Grout Brook, comprising about 38% of the diet.

Although several studies have been done on the diet of the Slimy Sculpin (i.e., Craig and Wells 1976; Kraft and Kitchell 1986; Cusker et al. 1992; French et al. 2010), few studies have examined diel feeding periodicity and none have been conducted during winter. Brandt (1986) found that Slimy Sculpins in Lake Ontario captured at 35 m exhibited nocturnal feeding peaks, but there was no clear feeding peak for Slimy Sculpins caught at 75 m. Chalupnicki and Johnson (2016) found that Slimy Sculpins fed mainly during diurnal periods, with minimal night feeding in Grout Brook in October. Our findings suggest that the diel feeding trends of Slimy Sculpins in Grout Brook in winter were in stark contrast to what were previously observed by Chalupnicki and Johnson (2016). In winter in Grout Brook, peak feeding of Slimy Sculpins occurred at night, with minimal feeding during the day. The difference in diel feeding patterns between seasons is most likely attributed to predator avoidance by Slimy Sculpins or prey activity. In stream fishes, feeding at night is generally considered a predator avoidance behavior (Railsback et al. 2005). Because potential mammalian, avian, and reptilian predators are generally less active during winter, we suspect that night feeding peaks in Grout Brook in winter may be associated with increased prey availability at night.

Chalupnicki and Johnson (2016) found little diel variation in the diet of Slimy Sculpins in Grout Brook in the fall. We observed more diel variation in the winter diet in Grout Brook, with Slimy Sculpin diets at 0400 and 0800 hours being significantly different than those other time periods as well as being different from each other. During both the 0400- and 0800-hour time periods the most apparent differences in diet composition from the other time periods were increased consumption of plecopterans and decreased consumption of chironomids. The abundance of plecopterans in the drift increases at night (Elliott 1967; Stewart and Szczytko 1983), corresponding to the increase in Slimy Sculpin diets we observed at 0400 and 0800 hours. Conversely, different subfamilies of chironomids have been shown to exhibit diurnal, nocturnal, and crepuscular drifting peaks (Ferrington 1984; Tilley 1989). Consequently, it might be assumed that plecopterans are more active and chironomid subfamilies are less active during these periods in Grout Brook in winter.

We found important differences in the winter feeding of Slimy Sculpins compared to studies carried out during other seasons. Perhaps, most importantly, Slimy Sculpins switched from a diurnal feeding pattern in the fall (Chalupnicki and Johnson 2016) to a nocturnal feeding pattern in winter. Less profound, but nonetheless interesting, was that Slimy Sculpins exhibited more diel variation in diet composition in winter than had previously been observed in other studies. Moreover, there was evidence that some prey taxa (i.e., chironomids) were consumed less during the winter than other seasons. These findings emphasize not only the need to conduct diel investigations when examining the diet of fishes but also the importance of winter observations to truly understand diel and seasonal differences in feeding ecology.

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Table S1.. Percent dry weight diet composition of Slimy Sculpin Cottus cognatus at 4-h intervals and over 2400 hours, and the dry weight composition of the benthos in Grout Brook, New York, during winter 2016.

Found at DOI: http://dx.doi.org/10.3996/032017-JFWM-029.S1 (15 KB XLSX)

Table S2.. Dendrogram of cluster analysis results of Slimy Sculpin Cottus cognatus diet at 4-h intervals and benthic prey in Grout Brook, New York, during winter 2016. Values range from −1 (maximum avoidance) to 1 (maximum selection).

Found at DOI: http://dx.doi.org/10.3996/032017-JFWM-029.S2 (43 KB XLSX)

Table S3.. Feeding periodicity estimates (±SE) for Slimy Sculpin Cottus cognatus in Grout Brook, New York, during winter 2016. Different letters along the x-axis represent significant differences in feeding intensity among 4-h intervals.

Found at DOI: http://dx.doi.org/10.3996/032017-JFWM-029.S3 (10 KB XLSX)

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