Pulsed supplies of small fish facilitate short-term intraguild predation in salmon stocked streams

Pulsed supplies of prey organisms generally increase predator food intake. However, whether this holds true when predators and pulsed prey are in same guild (i.e., intraguild [IG] predators and prey) is unclear. IG prey may increase IG-predator food intake by providing a food source, but they may decrease food intake through competition. To test these hypotheses, we compared the food intake of white-spotted charr (Salvelinus leucomaenis) (IG predator) in streams that were stocked or unstocked with hatchery-reared masu salmon (Oncorhynchus masou) fry (IG prey) in streams in Hokkaido, Japan. One day after stocking, masu salmon fry occupied nearly 60% of the stomach contents by wet weight of white-spotted charr in stocked streams, and mean stomach content weight was six-times higher than in unstocked streams. However, predation of white-spotted charr on stocked fry was rare on other days. Acquisition of predator-avoidance behavior by stocked fry and/or a lack of accommodation by white-spotted charr to the sudden emergence of a new prey source (e.g., the charr in study sites were basically insectivorous, and their foraging behavior might have been ineffective for piscivory) may explain this time-limited intraguild predation. In days other than the first day post-stocking, food intake by white-spotted charr did not differ between stocked and unstocked streams. No effects of interspecific competition on white-spotted charr food intake were observed; this may be due to the body-size advantage of white-spotted charr and/or the low density of stocked masu-salmon fry.

Genetic Structure and Population Demography of White-Spotted Charr in the Upstream Watershed of a Large Dam

White-spotted charr (Salvelinus leucomaenis leucomaenis) is an anadromous fish that has been severely harmed by human land-use development, particularly through habitat fragmentation. However, the anthropogenic impacts on populations of this species have not been evaluated, except those on small dammed-off populations. Using multiplexed ISSR genotyping by sequencing, we investigated the genetic structure of white-spotted charr in four tributaries in the upper section of the Kanayama Dam in the Sorachi River, Hokkaido Island, Japan. There were no distinct genetic structures (FST = 0.014), probably because some active individuals migrate frequently among tributaries. By model-flexible demographic simulation, historical changes in the effective population size were inferred. The result indicates that the population size has decreased since the end of the last glacial period, with three major population decline events, including recent declines that were probably associated with recent human activities. Nevertheless, populations in the watershed upstream of the Kanayama Dam are still expected to be at low risk of immediate extinction, owing to the large watershed size and the limited number of small check dams. An effective conservation measure for sustaining the white-spotted charr population is to maintain high connectivity between tributaries, such as by providing fishways in check dams during construction.

A multistate mark–recapture approach to characterize stream fish movement at multiple spatial scales

We studied movement of a native salmonid, white-spotted char (Salvelinus leucomaenis), in a 1-km tributary in northern Hokkaido, Japan, in May–July 2018. Based on physical mark–recapture of 501 unique individuals and detection by mobile PIT antenna over monthly intervals, a majority of fish (70%–80%) stayed within 60 m of previously released locations, demonstrating what appeared to be restricted movement patterns. However, fixed PIT antenna data showed that as much as 17% of marked individuals emigrated from the study area during the 2-month study period. Probability of emigration did not depend on where in the 1-km segment individuals had been released, indicating that emigration likely represented long-distance movement. Once emigrants made a decision to emigrate, they left the tributary within 1–3 median days by moving downstream in a unidirectional manner, based on detections at a total of three antenna arrays deployed throughout the tributary. Our multiscale analysis provided strong support for co-existence of short- and long-distance movement patterns, and we conclude that movement data at multiple spatial scales complement each other to characterize population-scale movement.