Facultative amphidromy involving estuaries in an annual amphidromous fish from a subtropical marginal range

Variation in the life cycle of diadromous fishes can be explained by differential food availability between marine and freshwater habitats, since migration is often interpreted as a mechanism for exploiting food resources. Theoretically, a migration pattern of mainly remaining in freshwater occurs in tropical and subtropical habitats where fluvial productivity possibly exceeds marine productivity. However, in Yakugachi River, Amami-Oshima Island, southern Japan, low nutrient concentrations in the river suggest that food availability is limited for the subtropical Ryukyu-ayu Plecoglossus altivelis ryukyuensis. Since Ryukyu-ayu is an amphidromous fish that mainly grows in rivers after spending 2 months in the sea, limited food availability in rivers would force this species to migrate to other habitats with better food availability. Otolith increment and Sr:Ca analyses of 48 adult Ryukyu-ayu collected from the Yakugachi River revealed that all individuals visited estuaries more than three times after moving upstream. Although the specific growth rates of this species in the river had no correlation with the salinity profile in the fluvial period, this movement may be an adaptive choice because the salinity profile significantly affected the body size at maturity. Our results highlighted individual-based variations in amphidromous migration for utilizing estuaries, which could be explained by relatively higher productivity in estuarine than in freshwater and marine habitats.

Effects of experimental duration and density of Japanese dace Tribolodon hakonensis on the strength of trophic cascades on benthic algae

To analyze density-mediated indirect effects through trophic cascades caused by Japanese dace Tribolodon hakonensis on the algal biomass and growth of ayu Plecoglossus altivelis altivelis, (freshwater fish belonging to Salmoniformes), I introduced zero, five, ten, and 20 dace with and without ayu into experimental pools. Ayu fed predominantly on benthic algae, whereas dace fed on both invertebrates and algae. The percentage of algae in the food content of dace was correlated with the number of dace in a pool. In the experiment lasting 8 days, trophic cascades on algae were not clear. In contrast, when the experimental duration was extended to 20 days, the number of gastropods and ephemeropteran nymphs was negatively correlated with the number of dace, consequently increasing the algal biomass and ayu growth through trophic cascades. Compared with pools with five dace, the presence of ten and 20 dace had similar gross effects on the number of algivorous invertebrates and algal biomass and greater gross effects on the ayu growth. The significant increase in ayu growth in pools with ten and 20 dace strongly suggests that small positive effects on algal growth by dace accumulated and extended to ayu during a 20-day experimental period. These results indicate the importance of analyzing interactions between the density and behavior of the top predator and the experimental duration on the intensity of trophic cascades.

Molecular typing by pulsed-field gel electrophoresis of Flavobacterium psychrophilum isolates derived from Japanese fish

Sixty-four isolates of Flavobacterium psychrophilum from ayu, Plecoglossus altivelis altivelis (Temminck & Schlegel), and other fish (n=16) in Japan and the type strain (NCIMB 1947(T)) were typed using pulsed-field gel electrophoresis (PFGE) with endonuclease BlnI and XhoI. These isolates were classified into 20 clusters and 42 genotypes by PFGE analysis. The most predominant cluster of isolates from ayu was cluster XII (n=20), followed by clusters XVII, XVI, XX, XI, IX, X, XIII and XV; the remaining 17 isolates from other fish were divided into clusters I, II, III, IV, V, VI, VII, VIII, XIV, XVIII and XIX. The PFGE genotype of isolates from ayu clearly differed from those of other fish. The isolates from ayu in Gunma Prefecture belonged to clusters XII, XVI, XVII and XX, and the strains of three of these clusters (XII, XVII and XX) were isolated from ayu in 15 of 19 prefectures. PFGE typing enabled more accurate classification of isolates into clusters than previously achieved by analysing the restriction fragment length polymorphism of PCR products. These results suggest that F. psychrophilum isolated from ayu and other fish are genetically different and strains with several PFGE types have spread within Japan.

Intraguild indirect effects through trophic cascades between stream-dwelling fishes

1. To examine the strength and role of indirect effects through trophic cascades by omnivorous fish on algal biomass in streams, we introduced one of four fish species (ayu Plecoglossus altivelis altivelis, pike gudgeon Pseudogobio esocinus esocinus, Japanese dace Tribolodon hakonensis and pale chub Zacco platypus) in experimental pools. We also investigated the indirect effects of gudgeon, dace and chub on the growth of ayu. 2. We chose the four fish species across a continuum of omnivory. Ayu fed mainly on benthic algae, and gudgeon predominantly on invertebrates. Dace and chub fed on both algae and invertebrates, but dace preyed on invertebrates more than chub. 3. The presence of gudgeon, dace and chub reduced the number of algal-feeding invertebrates and increased the algal biomass through trophic cascades. Consequently, ayu growth rate over the experimental period in pools with one of the three fish species was 25.9-44.1 times greater than the growth rate in pools with only ayu. 4. The positive indirect effect on ayu growth was large for gudgeon and dace and small for chub, whereas the addition of ayu reduced ayu growth considerably due to intraspecific competition. 5. The relative intensity of indirect effects on ayu growth through trophic cascades was predictable from the food overlap between ayu and the other fishes. However, the similar strength of indirect effects by gudgeon and dace that fed differently on algae and invertebrates suggests that feeding behaviour, prey preference and trait-mediated indirect interactions were also important in the prediction.