Artificially increased habitat complexity reduces predation-induced mortality for juvenile salmon in simplified environments

Abstract

Many river systems have been artificially modified for various reasons worldwide. Although simplified riverbeds due to artificial modifications often alter the species compositions and interactions, methods for mitigating habitat degradation in such rivers are underdeveloped. In this study, we evaluated the effects of tree branches on the survival of juvenile masu salmon Oncorhynchus masou under predation risk by piscivorous fish (White-spotted charr Salvelinus leucomaenis) in artificially simplified environments. We found that 0.4 m 2 or more of tree branches per 1.0 m 2 of artificial concrete pond doubled the survival of juvenile salmon for 20 days. In addition, body sizes of predator and prey did not have a significant effect on salmon survival, whereas the number of branches did in this experiment. These results indicate that adding structural complexity can mitigate predation pressure by piscivorous fish in artificially simplified environments. This study provides important guidelines for the management and restoration of suitable habitats for stream salmonids in artificially simplified environments where the main predators are piscivorous fish.

Cochineal Dye Concentration and Treatment Time for Otolith Marking of Japanese Smelt Hypomesus nipponensis Embryos

“…To use binominal distribution to evaluate the effects on mark quality, its four categories were integrated into two, wherein mark qualities of 1 (invisible) and 2 (faintly visible) were classified as unusable (1) and 3 (visible) and 4 (visible brightly) were classified as usable (2). The model was designed with reference to Sahashi and Morita (2017) and Miyamoto et al (2021), and it goes as follows: logit (p) = a1 + b1 Concentration + c1 Immersion time + d1 (b1 Concentration × c1 Immersion time), with p = probability of survival or mark quality; a1 = regression constant; and b1, c1, and d1 = regression coefficients. To determine which of the four hypotheses that were mentioned in the introduction were relevant for this COCHINEAL DYE MARKING OF JAPANESE SMELT study-namely, how concentration and immersion time contribute to mark quality and survival-the dredge function from the MuMIn package, R (version 3.4.3) was used to test all possible combinations of the variables, including their interaction.…”

“…The model was designed with reference to Sahashi and Morita (2017) and Miyamoto et al. (2021), and it goes as follows: logit ( p ) = a 1 + b 1 Concentration + c 1 Immersion time + d 1 ( b 1 Concentration × c 1 Immersion time), with p = probability of survival or mark quality; a 1 = regression constant; and b 1, c 1, and d 1 = regression coefficients. To determine which of the four hypotheses that were mentioned in the introduction were relevant for this study—namely, how concentration and immersion time contribute to mark quality and survival—the dredge function from the MuMIn package, R (version 3.4.3) was used to test all possible combinations of the variables, including their interaction.…”

Predation research with electronic tagging

“…For juvenile Atlantic salmon, refuge in gravel interstitia, undercut banks and woody debris is important and thought to limit predation risk and restoration efforts have shown that remediating these habitat features can increase juvenile densities (Pulg et al 2022). Direct tests have demonstrated that increased habitat complexity reduced predation (Miyamoto et al 2021). However, mechanistic data to support restoration actions are often more difficult to obtain.…”