Hirokazu Urabe

Selective foraging on terrestrial invertebrates by rainbow trout in a forested headwater stream in northern Japan

The important contribution of terrestrial invertebrates to the energy budget of drift-foraging fishes has been well documented in many forested headwater streams. However, relatively little attention has been focused on the behavioral mechanisms behind such intensive exploitation. We tested for the hypothesis that active prey selection by fishes would be an important determinant of terrestrial invertebrates contribution to fish diets in a forested headwater stream in northern Japan. Rainbow trout, Oncorhynchus mykiss, were estimated to consume 57.12 mg m−2 day−1 (dry mass) terrestrial invertebrates, 77% of their total input (73.89 mg m−2 day−1), there being high selectivity for the former from stream drift. Both the falling input and drift of terrestrial invertebrates peaked at around dusk, decreasing dramatically toward midnight. In contrast, both aquatic insect adults and benthic invertebrates showed pronounced nocturnal drift. Because the prey consumption rates of rainbow trout were high at dawn and dusk, decreasing around midnight, the greater contribution of terrestrial invertebrates to trout diet was regarded as being partly influenced by the difference in diel periodicity of availability among prey categories. In addition, selectivity also depended upon differences in individual prey size among aquatic insect adults, and benthic and terrestrial invertebrates, the last category being largest in both the stream drift and the trout diets. We concluded that differences in both the timing of supplies and prey size among the three prey categories were the primary factors behind the selective foraging on terrestrial invertebrates by rainbow trout.

Contribution of woody debris to trout habitat modification in small streams in secondary deciduous forest, northern Japan

Field studies to examine the influence of woody debris on rainbow trout (Oncorhynchus mykiss) abundance through habitat modification were conducted in two small streams, the Horonai and Uenae streams, running through secondary deciduous forest in south-western Hokkaido, northern Japan. Reach-based woody debris volume (total woody debris volume per 100 m2 of study reach) was significantly correlated with the total basal area of riparian stands along the margins of the Horonai stream, but no significant relationship was evident between them for the Uenae stream. This inconsistency between the streams was considered to be a result of the difference in stream size (width, depth and discharge). Woody debris was the principal agent for pool formation, although it had a far smaller volume than that found in streams draining old-growth coniferous forest in North America, where most of the previous studies have been carried out. Untransported debris pieces of larger volume more effectively contributed to pool formation than smaller transported pieces. The volume of individual debris scour pools was positively correlated with the volume of woody debris associated with each. Similarly, reach-based pool volume increased with total woody debris volume, but the relationship was less clear in the Uenae stream, having more abundant transported woody debris than the Horonai stream. The biomass of rainbow trout in individual pools, which were regarded as the most preferred habitat type for stream salmonids, was correlated with pool volume. A positive relationship also existed between reach-based standing crop and pool volume. These results revealed that secondary deciduous forest, like old-growth coniferous forest, plays an important role in enhancing the carrying capacity for rainbow trout by supplying woody debris which promoted preferred habitat formation.

Redd superimposition by introduced rainbow trout,oncorhynchus mykiss, on native charrs in a japanese stream

Spawning redd superimposition of introduced, spring-spawning rainbow trout,Oncorhynchus mykiss, on native, fall-spawning Dolly varden,Salvelinus malma, and white-spotted charr,S. leucomaenis, were examined in a small stream in Hokkaido, Japan. The stream reaches in which Dolly Varden and white-sported charr redds were observed in fall 1997 greatly overlapped with the reaches in which rainbow trout redds were recorded in spring 1998. Spawning microhabitats were also similar between trout and the two charr species. Thirteen and 3% of Dolly Varden and white-spotted charr redds, respectively, were superimposed by rainbow trout redds. The eggs or alevins in the disturbed charr redds were potentially damaged because charrs were not likely to have emerged from the redds before the superimposition occurred. In sufficiently great abundance, introduced rainbow trout may negatively impact native charr populations by dislodging the latter’s spawning redds.

Evaluation of Habitat Quality for Stream Salmonids Based on a Bioenergetics Model

Abstract We examined the effectiveness of energetic potential (net energy intake [NEI]) estimated from bioenergetics models as an index of habitat quality for stream salmonids in seven streams within four watersheds in Hokkaido, northern Japan. In addition, we confirmed the utility of the NEI as an index of habitat quality by comparing it with several other habitat variables, including pool volume, pool area ratio, and prey density, that are often used as indices of habitat quality for stream salmonids. The mean NEI at each study reach was closely related to salmonid abundance, although the physical environment and drifting prey density differed considerably among study sites. In contrast, the relationships between habitat variables and fish abundance were weaker (drift density) or nonsignificant (pool volume and area). These results suggest that the NEI is more widely applicable as an index of habitat quality for drift-feeding fish, although its validity should be tested in additional systems.

Linking microhabitat availability and local density of rainbow trout in low-gradient Japanese streams

Quantification of reach-based microhabitat availability for rainbow trout (Oncorhynchus mykiss), considering their microhabitat requirements in two low-gradient streams, northern Japan, was attempted to test for habitat space limitation of local trout density. Underwater observations revealed that fish selected microhabitats of moderate current velocity, relatively greater depth and shorter distance to overhead cover in both streams, although habitat features used and available differed slightly between the streams. Habitat space for fish potentially available in the channel environment was evaluated using principal component analysis (PCA) of both available and used microhabitat. A close relationship was evident in both streams between reach-based microhabitat availability and fish density, which was assessed by a three-pass removal method. Direct estimates of fish microhabitat availability using PCA can contribute to accurate predictions of local fish density and provide insight into the mechanisms responsible for fish–habitat relationships in streams.

Erratum to: An overview of salmon enhancement and the need to manage and monitor natural spawning in Hokkaido, Japan

The chum and pink salmon catches in Hokkaido, Japan have increased dramatically since the 1970s and the 1990s, respectively. In contrast, masu salmon catches have been steadily decreasing. Despite intensive hatchery development in Hokkaido, naturally spawning salmon populations persist based on results from a recent river survey. This paper focuses on the challenges of maintaining hatchery salmon populations while protecting natural chum, pink and masu salmon populations in Hokkaido. Two important initiatives related to meeting this ambitious goal are managing hatcheries in a way that minimizes negative interactions between natural and hatchery salmon populations, and initiating new efforts at restoring and rehabilitating degraded freshwater habitats. In addition, in order to maintain a balance of demand and supply in the domestic market through the exportation of extra salmon, Hokkaido has decided to enter full assessment to gain Marine Stewardship Council (MSC) certification of the Hokkaido chum salmon trap net fishery. This would involve a fundamental shift in fisheries management as practiced in Japan, specifically elevating the importance of managing the fishery in a way that conserves natural salmon populations. A key component of a new salmon management strategy is the establishment of a zone management framework based on the designation of stream units to spatially separate natural salmon from hatchery salmon to minimize negative effects of hatchery fish and to utilize effectively hatchery salmon for commercial fisheries. This effort is allied with similar initiatives in other Pacific Rim countries that are focusing on management reform to restore natural ecosystem function and maintain the coexistence of wild and hatchery salmon.

Thermal changes in the gastric evacuation rate of the freshwater sculpin Cottus nozawae Snyder

We experimentally measured thermal changes in the gastric evacuation rate (GER) of the freshwater sculpin Cottus nozawae Snyder under three water temperature regimes (2°, 7°, and 12°C). Laboratory experiments showed that the GER was accelerated with increasing water temperature. This result suggests that the daily food rations of fish are more likely to be underestimated at higher water temperatures if estimation is simply based on the stomach content weight alone. By comparing the GER for various fish species from subfrigid to temperate streams, we found a general pattern that the GER increases with water temperature, regardless of taxonomic group or foraging mode. However, the reaction norms of the GER against water temperature showed considerable interspecific variation. This means that stomach content weight is not comparable as a simple measure for determining the daily rations among fish species when water temperature regimes are different. To consider the temperature-dependent pattern of such a physiological phenomenon is important in understanding the feeding ecology of fishes and their roles in material cycles through food webs in aquatic ecosystems.

Seasonal variations in and effect of incubation water temperature on vertebral number in naturally spawning chum salmon Oncorhynchus keta

Seasonal variations and reaction norms for vertebral number (VN) in response to incubation water temperature were estimated in adult and juvenile naturally spawning chum salmon Oncorhynchus keta. The mean VN of adults varied according to spawning time; the early-spawning population had higher VN values than the late-spawning population. Moreover, the mean VN values in the early-spawning population decreased with seasonal changes, whereas VN values in the late-spawning population remained stable. Chum salmon embryos in three full-sib families were incubated at five different temperatures until hatching, and the VN values of the resulting juveniles were analyzed. The VN reaction norm to incubation water temperature showed a V-shaped curve that was lowest at an intermediate temperature. The mean VN at the same incubation temperature varied among the three families. These results suggest that VN values in chum salmon are influenced by genetic components and incubation water temperatures. VN may be a useful parameter for estimating the environmental conditions during ontogenesis and the genetic background by detecting population changes.

A delayed effect of the aquatic parasite Margaritifera laevis on the growth of the salmonid host fish Oncorhynchus masou masou

Parasitic species often have detrimental effects on host growth and survival. The larvae of the genus Margaritifera (Bivalvia), called glochidia, are specialist parasites of salmonid fishes. Previous studies have reported negligible influences of the parasite on their salmonid hosts at natural infection levels. However, those studies focused mainly on their instantaneous effects (i.e., during the parasitic period). Given the time lag between physiological and somatic responses to pathogen infections, the effect of glochidial infection may become clearer during the post-parasitic period. Here, we examined whether the effect of glochidial infections of Margaritifera laevis on its salmonid host Oncorhynchus masou masou would emerge during the post-parasitic period. We performed a controlled aquarium experiment and monitored fish growth at two time intervals (i.e., parasitic and post-parasitic periods) to test this hypothesis. Consistent with previous observations, the effects of glochidial infection were unclear in the middle of the experiment (day 50; parasitic period). However, even with a natural glochidial load (48 glochidia per fish), we found a significant reduction in growth rates of infected fish in the extended period of the experiment (day 70; post-parasitic period). Our results suggest that examining only instantaneous effects may provide misleading conclusions about mussel–host relationships.

Profiles of circulating insulin-like growth factor-I during smoltification of masu salmon reared under different conditions

We compared profiles of serum insulin-like growth factor (IGF)-I levels during smoltification of masu salmon reared under different environments, hatcheries and growth histories. Masu salmon from the Kenichi River in Hokkaido showed a sharp increase in serum IGF-I from April to May, followed by a peak of gill Na+,K+-ATPase (NKA) activity. Fish at Kumaishi Hatchery had an IGF-I profile similar to that of the river fish, while the increase in gill NKA was lower. At Shimamaki Hatchery, interval feeding during winter appeared to suppress the spring IGF-I peak. At Kumaishi Hatchery, a difference in size during smoltification affected IGF-I levels at release, but the numbers of adults that returned to the release site were not significantly different. In the following year, three release groups differing in winter size and/or spring growth (Large-High, Large-Low and Small-High) were created. Large-High and Small-High fish showed a higher IGF-I peak than Large-Low fish in April, while smolt-to-adult return of Large-High fish was highest. These results suggest that in smolting masu salmon in freshwater, circulating IGF-I level alone is not a predictor of long-term survival in seawater. However, since growth history in freshwater affected the smolt-to-adult return, optimizing rearing conditions is a critical component of hatchery releases for masu salmon.