Search

Offshore migration of Pacific salmon Oncorhynchus spp. is partly triggered by increasing body size and high motility in the early stages of life. The survival of juvenile salmon may depend on their growth rate during the first few months in the sea, and this factor partly regulates the dynamics of adult populations. Here, we assessed the effects of water temperature and food availability on the growth of juvenile chum salmon O. keta. In addition, by combining the measurements of metabolic performance for growth and activity (Absolute Aerobic Scope: AAS) with a bioenergetics model, we estimated the energy allocation for different activities in the juveniles. Under high temperatures (14 °C), juveniles reared at low food levels (1% body weight) allocated less than half their energy for growth than those reared at high food levels (4% body weight). These findings suggest that high temperature and low food level constrain the growth of juveniles, providing an insight into the effect of the recent increase in warm and low-nutrient water masses on survival of juveniles and catches of adult chum salmon on the Pacific side of Honshu Island, Japan. [ABSTRACT FROM AUTHOR]

In chum salmon (Oncorhynchus keta) homed to the Sanriku region, Japan, most of the fish are matured in bays and spawn near river mouths in coastal short rivers; therefore, their upriver migration is extremely short, but their behavioural characteristics have remained unknown. Upriver migration in the Otsuchi River, a typical coastal river, was evaluated from behavioural and physiological aspects. Homing salmon tracked in Otsuchi Bay held in the inner bay for less than 1 day to more than 10 days before river entry. The varied holding duration was negatively correlated with plasma 17α, 20β‐dihydroxy‐4‐pregnen‐3‐one (DHP) concentration, an indicator of maturation. After river entry, however, most fish were captured in weirs near the river mouths within 2 days regardless of the DHP concentration. Of the 34 fish released in the river, on the contrary, eighteen and five fish were seen next day in the main spawning sites located at c. 1.5 km upstream and in the branch creek, respectively, and 85% of the fish held position there until their death. The mean survival time of released fish was 5.8 days. Plasma DHP level suggested that preparations for spawning were already completed at the timing of the release. Taken together, homing salmon completed spawning preparation in the bay, and then they moved to their spawning sites immediately after river entry and spawned there during their short remaining life. This upriver migration contrasts with those of other populations, such as early migrants and long river migrants, whose maturation is completed during upriver migration. [ABSTRACT FROM AUTHOR]

Fish of the genus Thunnus are unusual because they are regional endotherms. In this study, archival tag data were used to demonstrate behavioural and physiological thermoregulation in juvenile yellowfin tuna, Thunnus albacares (35−52 cm fork length). Tags inserted into the peritoneal cavity were recovered from 23 yellowfin tuna caught mainly around Ishigaki Island, Japan, in 2009−2012. Peritoneal and environmental temperatures and depth data revealed that juvenile yellowfin tuna made daytime bounce dives repeatedly with a median dive duration of 16 min, and they remained at the surface during night-time. Body temperature daily averages were slightly higher than those of ambient temperature, and the body temperatures varied according to ambient temperature changes associated with vertical movements. Heat exchange coefficients (λ) during dives were estimated by a heat budget model. The thermal inertia component of λ was estimated in an experiment with dead juvenile yellowfin tuna. The value of λ exceeded the measured thermal inertia and varied between ascents and descents, thus providing evidence that physiological thermoregulation contributes to λ in juvenile yellowfin tuna and varies according to environmental temperature. Physiological thermoregulation helps maintain relatively stable body temperatures during dives in which tuna behaviourally escape cool deep waters and enter warm surface waters. Comparison of the obtained physiological parameters of λ and heat production (̇ T ˙ m ) with those previously recorded in Pacific bluefin tuna indicate that yellowfin tuna has a higher λ and lower T ˙ m than Pacific bluefin tuna, which may restrict yellowfin tuna to warm tropical and subtropical surface waters. [ABSTRACT FROM AUTHOR]

Catches and recruitment of the Japanese eel, Anguilla japonica, have declined for unknown reasons in recent decades, so examination of possible factors contributing to the declines is needed for resource management and conservation. One factor that has not been examined is the effect of habitat loss and alterations to aquatic community structure resulting from modifications of riverbanks and lakeshores. This study examined if there may be a relationship between riverbank and lakeshore revetment constructions and declines of Japanese eel catches in rivers and lakes in Japan, because levels of revetment may be a useful indicator of the degree of modifications to each aquatic system. Long-term trends in catches showed that although eel catches decreased in most locations and in Japan overall, large differences were observed in the rates of declines among locations. Catch reduction rates in 18 rivers and 9 lakes were positively correlated with the revetment rates of the shorelines of the rivers and lakes, suggesting that higher reductions in eel catches occurred in areas with high levels of revetment or other types of modifications. Evidence about the effects of revetment on aquatic community structure also suggests that revetment shorelines would negatively affect juvenile growth habitats for eels. Other types of habitat modifications or the creation of barriers to upstream movement by eels have also likely affected fisheries catches of Japanese eels, so detailed research is needed to determine how modifications have reduced eel production and if some kinds of habitat restorations may help to conserve this species. [ABSTRACT FROM AUTHOR]

The behavior of juvenile yellowfin tuna Thunnus albacares in southwestern Japan was investigated using archival tag data from five fish (fork length 52.5-92 cm, days at liberty 26-280 days) released near the Nansei Islands (24-29°N, 122-130°E). Vertical behavior was classified into three patterns: 'shallow' (≥50 % of daytime hours at depth of <50 m), 'deep' (≥50 % of daytime hours at ≥100 m), and 'intermediate' (other than 'shallow' or 'deep'). The pooled proportion of the number of days of each behavior was 29, 25 and 46 %, respectively. The proportion of 'shallow' behavior increased with fish size. The proportion of time spent near the surface at nighttime increased in the colder season, when the thermal gradient was relatively small. Surface-oriented behavior (fish remained at a depth of <10 m for more than 10 min) occurred mainly during nighttime and between November and January. Dives exceeding 500 m were occasionally observed (0.02 day), and one fish dived to 1230 m. The results of our study show that yellowfin tuna were typically distributed in the mixed layer or upper thermocline where the water temperature was close to the sea surface temperature and that the vertical behavior was variable. [ABSTRACT FROM AUTHOR]

The article presents a study on the relationship of population connectivity of Ezo abalone Haliotis discus hannai in the northern Miyagi coast of Japan with the formation of harvest refugia. It states the use of hydrodynamic and particle-tracking models which objectives include clarification of process of larval dispersal, quantification of distance of dispersal, and estimation of connectivity. It says that the study calls for investigation of larval dispersal in marine reserve management.

We used Lagrangian numerical simulations to examine the trans-Pacific dispersal processes of loggerhead turtle hatchlings. Ten thousand simulated particles were released from each of the three nesting regions in Japan and tracked for 5 years. Results showed many particles moving eastward, drifting in the Kuroshio Current followed by the Kuroshio Extension Current. However, no particles reached Baja California, a known feeding area, through passive processes, indicating that trans-Pacific transportation requires active swimming by turtles. The duration of the trans-Pacific dispersal was estimated to be at least 1.6-3.4 years, with some turtles drifting in the Kuroshio Countercurrent and remaining in the western Pacific even after 5 years. This indicates that as revealed by previous genetic studies, not all loggerheads always disperse along a trans-Pacific route. The findings showed that survival and expected growth rates varied widely according to ambient temperatures during drifting, which in turn depended on nesting location. [ABSTRACT FROM AUTHOR]

Most juveniles of Haliotis discus hannai have been found to be descendants of wild individuals, although most adults were artificially produced (released) individuals as a result of restocking inside the refugium located near the head of Oshoro Bay, Hokkaido, Japan. To estimate the larval supply from released and wild individuals into the refugium and to compare the suitability of locations as larval sources, we simulated larval dispersal using a coupled hydrodynamic and particle-tracking model. The simulation results indicated that more larvae may be supplied from the wild adults inside the bay to the refugium than from the released adults. These results are consistent with the observed high abundance of wild juveniles in the refugium. Most larvae from the refugium were predicted to disperse out of the bay. We found that larval retention in the bay was at least one order of magnitude higher than that in the refugium. Thus, it may be more effective in terms of self-replenishment and reproduction if the refugium were to be expanded to the bay scale. There were only minor differences among the compared sites at the head of the bay in terms of their suitability as larval sources. Consequently, the establishment of new refugia in this area could be expected to provide an effectiveness equal to that of the current refugium. [ABSTRACT FROM AUTHOR]

The article presents a study on the larval dispersal process for abalone in Sagami Bay, Japan. It notes that inorder to stimulate larval dispersal of large abalone including Haliotosis discus, H. gigantea, and H. madaka, a coupled particle-tracking and hydrodynamic model was used. It mentions that the study aims to elucidate larval dispersal process and settlement sites, assess the existing harvest refugium and estimate the appropriateness of other areas for larval sources. It explores the results of the study wherein it has crucial significance for refugium design at the site of the study.

Physical conditions such as oceanic turbulence related to food availability are considered to be important factors affecting fish larval survival. Rearing experiments were conducted to elucidate the effects of turbulence on the survival and feeding rates during the initial feeding period of Pacific bluefin tuna Thunnus orientalis. Six levels of turbulence intensity were provided by changing flow rates from pipes set on the bottom of rearing tanks. The result showed a dome-shaped relationship between turbulence level and survival rate, in which the feeding rate appeared higher at a logged turbulence energy dissipation rate of −6.32, and decreased at both higher and lower turbulence levels. Compared with the turbulence intensity in the ocean, the optimal turbulence level for Pacific bluefin tuna larvae corresponded to the turbulence caused by sea surface winds with speeds of 4–12.5 m/s. The estimated optimal turbulence intensity for Pacific bluefin tuna larvae is comparable to that for yellowfin tuna Thunnus albacares. [ABSTRACT FROM AUTHOR]

Immature Pacific bluefin tuna Thunnus orientalis, tagged with archival tags, were released near Tsushima Island in the East China Sea during the winters of 1995 through 1998. Time-series data for ambient and peritoneal cavity temperatures, recorded every 128 or 256 s for 23 fish recovered, were analyzed. The objective of this study was to clarify the process of development of thermoconservation ability with growth in relation to adaptive mechanisms to cooler temperate waters. According to the results, mean ambient temperatures ranged from 14.9 to 20.7°C, which is almost within the optimum temperature range according to previous reports. Mean peritoneal temperatures were higher than ambient temperatures (19.7–27.3°C), but never reached 35°C, which would induce overheating. Although the mean thermal differences between peritoneal and ambient temperatures increased with body size, the rate of increase decreased with body size. A heat budget model suggests that as the insulation of the body develops, the estimated mean values of internal heat production decrease with body size. This is probably due to the allometric scale effect and explains why the thermal difference does not increase quickly with body size. It is likely that Pacific bluefin tuna inhabit cooler temperate waters in mid-latitude regions to avoid overheating. [ABSTRACT FROM AUTHOR]

• The timing of river entry correlate with plasma steroid levels. • Homing chum salmon adjust the timing of river entry to full-maturation. • Homing chum salmon hold in an inner bay until just before copulation. • Full-maturation do not force homing chum salmon to enter non-natal rivers. • The timing of river entry may be helpful to mature successfully. The correlations among gonad maturity and various homing behaviors of chum salmon, Oncorhynchus keta , were evaluated using acoustic tracking of tagged fish in Otsuchi Bay, Japan. There was a negative correlation between the time duration from release of tagged fish until river entry and the plasma 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) levels, an indicator of final maturation. Females with high DHP entered the rivers soon after the release, whereas females with low DHP (<10 ng/ml) took a few days to more than one week until river entry. Similar correlation was also found in males. A pattern of river entry correlated with maturational conditions was also observed in fish entering the rivers of neighboring bays. DHP concentrations of fish caught in the rivers were consistently higher. On the other hand, more than half of released salmon departed from the bay regardless of their plasma DHP level, suggesting that maturational status does not force homing adults to enter the most available nearest rivers. Fish entering the rivers experienced ambient temperatures less than 8 °C, which is approximately 5 °C lower than that of the bay. These results indicate that homing salmon hold their position in the bay until just before spawning, which may be attributable to low temperature avoidance. This characteristic type of river entry may be suitable to geographical features and thermal regimes of this region. [ABSTRACT FROM AUTHOR]

• Timing and duration of herring coastal spawning migration differ by fish age and size. • Older fish spawn earlier and remain in frozen surface Akkeshi Lake waters. • Younger fish swim shallower than older fish through the day and night. • Different fish ages and length may lead to variable risk and cost in migration. Timing and duration of spawning migration of Pacific herring is known to vary by fish age and length. Older fish mature earlier, but there are few verified examples of differences in the pre-spawning migration of individuals varying in age or size. Here we use sonic tagging to examine the fluctuations in age, fish length and maturity throughout the migration of herring in Akkeshi, Japan. We tracked the horizontal and vertical movement of individual fish using acoustic telemetry and biologging methodology. In Akkeshi Lake, older, larger individuals migrated early, reached peak maturity and remained in the lake until early April, after spawning. The timing and duration of the stay in Akkeshi Lake coincided with the freezing and melting of the surface layer of the lake. Fish tracking indicated no differences in herring in different length classes migrated to and from Akkeshi Lake and offshore and stayed in Akkeshi Bay. For the vertical movement, larger, older herring occupied a wide depth range during diurnal vertical movements as they moved to Akkeshi Lake and to offshore. In contrast, the smaller group occurred frequently in depths shallower than 5 m throughout the day and night. Greater depth variation could result in beneficial effects in predator avoidance and increased prey encounter opportunities though vertical migration. In contrast, smaller herring that were more confined to surface waters could experience more frequent sensing of the environmental cues of spawning grounds such as low-salinity water and olfactory stimulation near the surface. Therefore, there are differences in the benefits and risks during migration for each fish length, which may result in superior spawning grounds and individual mortality in this area. [ABSTRACT FROM AUTHOR]

• Vesicles of Sargassum horneri tended to increase with increasing algal body weight. • Contribution rate of vesicles of S. horneri to acoustic reflection was high. • Target strength (TS) of algal bodies increased with increasing algal body weight. • Weight-dependent TS of S. horneri followed this equation: TS w = 11.1 log 10 W -71.1. • Biomass of S. horneri can be estimated by acoustic measurements. A species of macroalgae, Sargassum horneri , which has gas-filled vesicles, plays important roles as nursery grounds, feeding grounds, and hiding grounds for coastal fauna. In addition, it recently emerged as a new coastal fishery resource in Japan. However, its acoustic reflection intensity is unknown, resulting in the impossibility of quantitatively estimating its biomass via an acoustic survey with a quantitative echosounder. We measured the target strength (dB re 1 m2) of S. horneri and clarified its characteristics. The measurements were taken using a 120 kHz quantitative echosounder in the fishing port of Iwate Prefecture in June and July 2014. Seven samples were collected by scuba diving in the coastal waters surrounding the port. Each individual sample was divided every 30 cm, and each divided layer was suspended at a depth of about 4–5 m, and its target strength was measured. Then, we divided S. horneri into vesicles and other parts. Each part was exsiccated at 80 °C for over 10 h. The dry weights of the vesicle and algal body were then measured. The weight of the vesicles of S. horneri tended to increase with an increase in algal body weight. The target strength increased with increasing algal body weight (Target strength = 11.1 log 10 Weight -71.1, R2 = 0.81) and vesicle weight. We considered that the contribution of the gas in the vesicles to the target strength was high, which is similar to the swim bladder of fish, as the acoustic reflection intensity of the vesicle was as strong as what has been reported for the gas in the swim bladder, and the intensity increased owing to the increase in vesicle weight accompanying the increase in algal body weight. Thus, S. horneri biomass could be estimated by acoustic survey using the target strength revealed in our study. [ABSTRACT FROM AUTHOR]

The Sanriku-ria coast of Japan, a homing area for chum salmon, Oncorhynchus keta , is characterized by a large number of small closed bays into which one or multiple short rivers flow. The present behavioral investigation of chum salmon in this region was designed to gain deeper insight into the migration of chum salmon to their natal rivers. Eighty-three fish caught at the middle part of Otsuchi Bay were tracked using an acoustic transmitter in the narrow inlet into which flow three rivers: the Otsuchi, Koduchi, and Unosumai. The majority of 18 fish that entered the Unosumai River, which flows into the southwest side of the bay, directly approached the river along the southern coast. More than half of fish that entered the Otsuchi and Koduchi Rivers, which flow into the northwest side, also migrated into the inner bay via the southerly route, and then entered these rivers frequently after passing the mouth of the Unosumai River. In the inner bay, the salinity of sea surface water suggested that water from the three rivers circulates in a counterclockwise direction at a depth of less than 1.0 m, flowing eastwardly along the southern coast. The observed migratory paths of homing salmon in Otsuchi Bay thus correspond well with the counterflow of surface river water in the bay. The present results suggest that homing migration of salmon in the Sanriku narrow inlet is guided by natal river flows.

Ectotherms adjust their thermal performance to various thermal ranges by altering their metabolic rates. These metabolic adjustments involve plastic and/or genetic traits and pathways depend on species-specific ecological contexts. Chum salmon ( Oncorhynchus keta ) are ecologically unique among the Pacific salmonids as early-run and late-run populations are commonly observed in every part of their range. In the Sanriku coastal area, Japan, early-run adults experience high water temperatures (12-24°C) during their migration, compared with those of the late-run adults (4-15°C), suggesting that the two populations might have different thermal performance. Here, we found population-specific differences in the thermal sensitivities of metabolic rates [resting metabolic rate, RMR, and maximum (aerobic) metabolic rate, MMR] and critical temperature maxima. Using these parameters, we estimated thermal performance curves of absolute aerobic scope (AAS). The populations had different thermal performance curves of AAS, and in both populations high values of AAS were maintained throughout the range of ecologically relevant temperatures. However, the populations did not vary substantially in the peak (AAS at optimal temperature, T optAAS ) or breadth (width of sub-optimal temperature range) of the performance curves. The AAS curve of early-run fish was shifted approximately 3°C higher than that of late-run fish. Furthermore, when the data for RMR and MMR were aligned to the thermal differences from T optAAS , it became clear that the populations did not differ in the temperature dependence of their metabolic traits. Our results indicate that chum salmon thermally accommodate through compensatory alterations in metabolic rates. Our results imply that metabolic plasticity and/or the effect of genetic variance on plasticity might play a pivotal role in their thermal accommodation., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2019. Published by The Company of Biologists Ltd.)