Koh Hasegawa

Evidence that an ontogenetic niche shift by native masu salmon facilitates invasion by nonnative brown trout

The mechanisms underlying successful invasions by a competitively subordinate species are poorly understood. In Japan, nonnative brown trout Salmo trutta L. have successfully invaded a number of streams that contain native masu salmon Oncorhynchus masou Brevoort, even though young of the year (YOY) brown trout are thought to be competitively subordinate to YOY masu salmon because of their smaller body size due to later emergence time. We conducted a laboratory experiment and field observations to evaluate whether ontogenetic habitat shifts could explain the success of brown trout. In a laboratory experiment, smaller YOY brown trout were competitively inferior to YOY masu salmon. Our field observations suggest that YOY masu salmon shift to deeper and faster velocity habitat before the emergence of brown trout fry. Thus, there is separation of habitat use between the two species. Taken together, our results suggest an ontogenetic niche shift by competitively dominant native masu salmon may reduce niche overlap and the opportunity for competitive interactions, and thus facilitate invasion by a nonnative brown trout.

Food habits of introduced brown trout and native masu salmon are influenced by seasonal and locational prey availability

A knowledge of food habits is important for evaluating interspecific competition and predation between sympatric species. Data on food availability should be combined with data on food habits in this type of survey. Although food availability differs between habitats or seasons, these differences had never been considered in previous studies. We conducted year-round field surveys throughout a stream to compare the food habits of an introduced salmonid, brown trout Salmo trutta, and a native salmonid, masu salmon Oncorhynchus masou. Masu salmon did not constitute a large proportion of the diet of brown trout and vice versa. Thus, predation will likely not affect the population level of either species. The dietary overlap between brown trout and masu salmon varied depending on the presence of Gammaridae and terrestrial invertebrates; i.e., the intensity of interspecific competition for food resources may differ according to food conditions.

Distinguishing local growth from immigration-based size shifts for juvenile chum salmon communities in coastal Hokkaido, northern Japan

The body size of juvenile fish is often used as an index of growth rate, which in turn is influenced by local habitat conditions. We evaluated the size and origin of juvenile Chum salmon Oncorhynchus keta in the coastal areas of three regions (Atsuta, Shari, and Konbumori) of Hokkaido, northern Japan. The origin of the individuals in these communities differed between Konbumori and the other two regions. The former contained juveniles that originated from both the nearest stocked river and other rivers that were outside the area of interest. Conversely, the communities in Atsuta and Shari consisted exclusively of fish from the nearest stocked river. Moreover, the juveniles in Konbumori were larger than those in Atsuta and Shari. The results of our otolith analysis suggest that the larger size of the fish in Konbumori was due to the immigration of large individuals from natal rivers at distant locations. Thus, immigrants were likely to enlarge their body size composition in the area of interest. In summary, if the distance from a natal river is adjusted and daily growth is taken into account, body size can be used as an indicator of growth performance in coastal juvenile chum salmon.

Are hatchery chum salmon fry a native invader? Direct and indirect effects of stocking salmon fry on stream organisms

Native species may show invasiveness toward a recipient ecosystem through increases in abundance as a result of artificial stocking events. Salmonid species are typical examples of native invaders whose abundance is increased after stocking with hatchery fish. This study evaluated the effects of hatchery chum salmon fry on sympatric wild masu salmon fry, benthic invertebrate prey, and algae, after a single stocking event in Mamachi stream, Hokkaido, northern Japan. The results suggested that the stocked hatchery chum salmon fry decreased the foraging efficiency and growth of the wild masu salmon fry through interspecific competition, and depressed the abundance of Ephemerellidae and total grazer invertebrates (Glossosomatidae, Heptageniidae, and Baetidae) through predation. Also, the hatchery chum salmon fry may increase algal biomass through depression of grazer abundance by predation (top-down effect). These results suggested that a single release of hatchery chum salmon fry into a stream may influence the recipient stream ecosystem.

Wild masu salmon is outcompeted by hatchery masu salmon, a native invader, rather than brown trout, a nonnative invader

Artificially grown native species are released into natural environments to increase biological resources or to recover threatened populations. Such stocks typically have enhanced survivability and may outcompete wild conspecifics as so-called native invaders. In addition, it is likely that the competitive effects of native invaders on native species are more intense than those of nonnative invaders. To test these hypotheses, an enclosure experiment was conducted using young-of-the-year wild and hatchery (normally grown to a relatively large size to increase survival after stocking) native masu salmon, Oncorhynchus masou, and nonnative brown trout, Salmo trutta (which attain a smaller size than masu salmon). Competitive effects between these fishes were evaluated in terms of stomach fullness and specific growth rate of the wild masu salmon. The magnitude of the relationship between stomach fullness and growth between the experimental treatments revealed a similar pattern, suggesting that competition for foraging habitat most affected their growth. Wild masu salmon were negatively affected by hatchery conspecifics, and the effects were greater than those caused by brown trout. We propose that these outcomes were caused by competitive dominance as a consequence of body size differences. In conclusion, the results support the hypothesis that size-enhanced hatchery masu salmon have the potential to function as native invaders, and the negative effects of artificial stocks on wild masu salmon could be greater than those caused by a nonnative invader.