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Describing the mechanisms that drive variation in species interaction strengths is central to understanding, predicting, and managing community dynamics. Multiple factors have been linked to trophic interaction strength variation, including species densities, species traits, and abiotic factors. Yet most empirical tests of the relative roles of multiple mechanisms that drive variation have been limited to simplified experiments that may diverge from the dynamics of natural food webs. Here, we used a field-based observational approach to quantify the roles of prey density, predator density, predator-prey body-mass ratios, prey identity, and abiotic factors in driving variation in feeding rates of reticulate sculpin (Cottus perplexus). We combined data on over 6,000 predator-prey observations with prey identification time functions to estimate 289 prey-specific feeding rates at nine stream sites in Oregon. Feeding rates on 57 prey types showed an approximately log-normal distribution, with few strong and many weak interactions. Model selection indicated that prey density, followed by prey identity, were the two most important predictors of prey-specific sculpin feeding rates. Feeding rates showed a positive relationship with prey taxon densities that was inconsistent with predator saturation predicted by current functional response models. Feeding rates also exhibited four orders-of-magnitude in variation across prey taxonomic orders, with the lowest feeding rates observed on prey with significant anti-predator defenses. Body-mass ratios were the third most important predictor variable, showing a hump-shaped relationship with the highest feeding rates at intermediate ratios. Sculpin density was negatively correlated with feeding rates, consistent with the presence of intraspecific predator interference. Our results highlight how multiple co-occurring drivers shape trophic interactions in nature and underscore ways in which simplified experiments or reliance on scaling laws alone may lead to biased inferences about the structure and dynamics of species-rich food webs.

We exposed prickly sculpin Cottus asper and reticulate sculpin Cottus perplexus to electroshock and sham treatments in a controlled laboratory setting to determine if backpack electrofishing contributed to or exacerbated external signs of gas bubble trauma (GBT) in fish exposed to elevated total dissolved gas (TDG) levels. Fish were exposed to 115, 120 and 125% TDG (measured as percent of saturation) for various amounts of time (24–144 h) then subjected to a 5‐s electroshock or sham treatment of no electroshock. Fish were examined to determine the incidence of GBT pre‐ and post‐treatment, and all aspects (i.e., dorsal, ventral, left and right sides) of each fish were photographed. Across all TDG levels in all trials, no sculpin showed any change in GBT incidence following treatment. Analysis of GBT signs evident in photographs of 68 fish found no evidence of change in mean pre‐ and post‐treatment gas bubble sizes in fish in any TDG trial, nor were any new gas bubbles found following treatment. We are aware of no physiological mechanism by which backpack electrofishing can cause GBT in sculpin, and as such, we believe using this gear should not increase GBT incidence rates in sculpin.

The freshwater sculpin species Cottus gulosus and Cottus perplexus present a challenge to biologists because of their overlapping sympatric distributions, morphological similarity, and historical inconsistency regarding species limits and ranges. We applied linear morphometric analyses in conjunction with meristic and discrete character counts to 1) assess the validity of previously cited diagnostic characters, 2) test multivariate combinations of characters to elucidate novel diagnostic characters useful in distinguishing the 2 species, and 3) apply these diagnostic characters to identify previously unidentified specimens obtained from Washington streams, where the 2 species occur in sympatry. Our analysis of 270 Cottus specimens encompassing the range of both species uncovered little support for the diagnostic utility of previously cited characters, although significant differences between mean measurements for each group were detected in the proportion of individuals with palatine teeth and a median chin pore, as well as in mean mouth-width to body-width ratio. Additionally, we were unable to detect linear transformations of continuous measurements that could be used to reliably identify individuals of these 2 species. These results prevented us from confidently identifying the unknown Washington specimens and illustrate that multi-locus phylogenetic studies are needed to assess species boundaries for these morphologically similar fishes.

The distribution and extent of chemical alarm signaling systems among some families of fishes, including the Cottidae, remains unclear. In laboratory experiments, we tested whether reticulate sculpins, Cottus perplexus, respond to chemical alarm signals released by injured conspecifics. Sculpins decreased movement following exposure to skin extracts from conspecifics, but did not respond to cues of syntopic speckled dace, Rhinichthyes osculus, or allotopic swordtails, Xiphophorous helleri. Additional tests demonstrated that the responses of sculpins to alarm cues were dependent on the hunger level of the test fish. Sculpins deprived of food for 2 days failed to respond to conspecific alarm cues, however, the same individuals fed to satiation did respond to alarm cues.[PUBLICATION ABSTRACT]

Intraspecific variation is increasingly recognized as an important factor in ecological interactions, sometimes exceeding the role of interspecific variation. Few studies, however, have examined how intra- versus interspecific variation affect trophic interactions over time within a seasonally dynamic food web. We collected stomach contents from 2,028 reticulate sculpin ( Cottus perplexus ), 479 cutthroat trout ( Oncorhynchus clarkii clarkii ), and 107 Pacific giant salamanders ( Dicamptodon tenebrosus ) in western Oregon streams and compared diets among predator species and size classes over three seasons. Predator body size and species identity both showed strong effects on dietary niche breadth, proportional prey composition, and prey size, with seasonal variation in the relative magnitudes of intraspecific and interspecific diet variation. Size-associated diet variation was high in summer and fall but was heavily outweighed by species-associated diet variation in spring. This pattern was driven primarily by a 50-fold increase in the consumption of terrestrial thrips (Order: Thysanoptera) by cutthroat trout in spring compared to summer and fall. Mean dietary niche breadth generally increased with body size and was roughly half as wide in sculpin as in trout and was intermediate in salamanders. Predator-specific responses to the seasonality of terrestrial prey availability were associated with interspecific differences in foraging mode (e.g., benthic vs drift-feeding) and contributed to temporal variation in the roles of predator size and identity in trophic niche differentiation. Our results thereby demonstrate that intraspecific and interspecific diet variation can exhibit strong seasonality in stream predators, emphasizing the dynamic nature of food webs and the need to incorporate sampling over relevant temporal scales in efforts to understand species interactions.

Along the length of the Marys River system, Oregon, the distributions of Cottus perplexus, C. rhotheus and C. beldingi overlapped and were strongly related to substrate characteristics. C. perplexus occurred throughout the river system on all substrates, C. rhotheus was found only where rock was present and C. beldingi was restricted to riffles that contained loose, cobble-sized rock with many interstitial spaces. Local use of habitat by each species was related strongly to the presence or absence of other Cottus species. Where it existed alone, C. perplexus utilized both pools and riffles and was found in highest density near cover. Where it coexisted with the riffle-inhabiting C. rhotheus, C. perplexus occurred primarily in pools and in lower density than when alone. C. beldingi and C. rhotheus segregated within riffles. In summer, when currents in all riffle areas flowed slowly enough for C. rhotheus, C. beldingi burrowed into the substrate during the day. In winter and spring C. rhotheus was excluded from portions of some riffles by fast currents, and C. beldingi was found atop the substrate to a greater degree. In an artificial stream, C. perplexus selected riffles when alone, but shifted partially to pools when C. beldingi was added. The addition of C. rhotheus resulted in C. perplexus being more rigidly confined to pools, but C. beldingi was able to coexist in riffles with C. rhotheus, apparently by utilizing the interstitial spaces in the substrate. Although the interactive mechanisms involved were not thoroughly investigated, predation, interference and exploitation competition apparently complement each other in maintaining spatial segregation of these sculpins.