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 Relationship and Structural Analysis of Resident Pacific Salmon Oncorhynchus Masou in Mijiang River, China

Masu salmon Oncorhynchus masou has been listed as national second-class protected species in China, and many of its local populations have been under threats of decline because of environmental deterioration by human activity, such as habitat loss, water pollution, or dams. Genetic characteristics should be clarified in wild masu salmon population for an effective protection and recovery. In the present study, we explained genetic relationship and structure of masu salmon population in Mijaing River by sequencing the COI gene and analyzing microsatellite makers. Results of COI sequences analysis revealed 15 gene variable sites in approximately 696 bp, which defined 12 haplotypes, including some haplotypes related to geographic regions. The NJ tree indicated genetic relationship that we preliminarily inferred, that is, the masu salmon from Mijiang River belonged to the cherry salmon family (O. masoumasou). Results based on 14 microsatellite loci showed high haplotype diversity and low nucleotide diversity, indicating low genetic diversity of wild masu population in Mijaing River. The observed genetic structure was influenced primarily by bottlenecks due to overfishing and habitat loss. Since masu salmon has become highly vulnerable in China, we recommend to strengthen the protection of resident masu salmon population.