Downstream migration of juvenile fish in the Krasnaya River (Vietnam)

The first material is presented on downstream migration of fish juveniles in the Krasnaya River, one of the largest rivers in South-East Asia. The spatial-temporal structure of distribution of downstream-migrating fish juveniles in the Krasnaya River has traits usually characteristic of migration in rovers with turbid water. Concentration of fish was extremely variable by the time of the day, river length, and depth. In length-wise distribution, there was a tendency to increasing concentration of downstream migrants from the upper stretches to the downstream stretch. In the upper reaches, Perciformes prevailed, in the middle part, Cypriniformes prevailed, and in the lower reaches of the river Clupeiformes prevailed. During 24 h, Clupeiformes and Perciformes migrated more intensively at night and Cypriniformes migrated more in the daytime. By depth horizons, Clupeiformes made downstream migration predominantly at the surface and Cypriniformes and Perciformes did at the bottom. These differences may be caused by species differences in fish biology, local specific traits of stream hydraulics, anthropogenous factors, etc.     

Diel and tidal changes in the distribution and feeding habits of Japanese temperate bass Lateolabrax japonicus juveniles in the surf zone of Ariake Bay

Japanese temperate bass Lateolabrax japonicus juveniles recruit to the surf zone and grow by feeding on commonly occurring coastal copepods. However, little is known about diel and tidal patterns in their migration and feeding habits. We sampled wild juveniles during the neap and spring tides, over periods of 24 h, with small seine nets in the sand flat of the eastern part of Ariake Bay, Kyushu, western Japan. In both the neap and spring tides, abundance of juveniles significantly increased during the daytime, being highest around the time of high tide. The relative gut fullness indices of juveniles drastically increased in the morning (0700–0900) and during the flood tide in the daytime, while major prey copepod (Paracalanus spp.) density in the ambient water was relatively constant. We summarized that L. japonicus juveniles would migrate to the surf zone after sunrise to feed on copepods, and then emigrate from the surf zone after sunset. The migratory behavior of L. japonicus juveniles would be influenced by light (daytime) and feeding activity influenced by both light (morning) and tidal condition (flood tide). The intertidal region of the tidal flat was recognized to be one of the important habitats for L. japonicus during their early life history.     

Migration route selection of juvenile Chinook salmon at the Delta Cross Channel, and the role of water velocity and individual movement patterns

We examined movement tracks of ultrasonic-tagged juvenile Chinook salmon (Oncorhynchus tshawyscha) smolts at the juncture of two migratory pathways. This migratory juncture occurs where the Delta Cross Channel splits from the Sacramento River in California’s Sacramento–San Joaquin Delta. Smolt tracks were analyzed to compare the importance of river flow and individual parameters in migratory route selection. The two routes differ significantly in smolt survival probabilities (Perry et al. N Am J Fish Manag 30:142–156, 2010), thus a clearer understanding of the variables contributing to route selection will be valuable for management of this declining species. A comparison of the two migratory groups showed that fish remaining within the Sacramento River: 1) Encountered the migratory juncture when river water velocities were much higher than those in the Delta Cross Channel (p?<?0.0001), 2) showed more direct swimming paths (p?=?0.03) and 3) migrated at higher speeds (p?=?0.04). Logistic regression models showed that the ratio of mean water velocity between the two routes was a much stronger predictor of ultimate route selection than any other variable tested. However, parameters for both the lateral position of smolts within the river and smolt size added predictive power to the final model. Our results suggest that river flow remains the most important variable for predicting smolt migration route, but note that knowledge of individual smolt attributes and movement patterns can increase our predictive ability.     

Satellite tracking of Swedish Ospreys Pandion haliaetus: autumn migration routes and orientation

Autumn migration routes and orientation of Swedish Ospreys Pandion haliaetus were studied by satellite tracking of 18 birds. Of these, 13 could be followed during the entire migration (6 females, 5 males and 2 juveniles). Most birds migrated across western and central Europe to winter in tropical West Africa. However, one juvenile flew to Cameroon and one female used a very easterly route and reached Mozambique. On average, the birds travelled a total distance of about 6700 km, with little variation except for the female wintering in Mozambique, who travelled more than 10 000 km. Of 21 stopovers (of >1 day), only five were made south of 45°N; three of these in Africa. Females departed before males and juveniles and flew to a stopover site they probably were familiar with. After 3–4 weeks there, they continued to their wintering grounds. Also males and juveniles usually made one or more stopovers. Adults seemed to travel to a known wintering site, where they remained stationary, whereas juveniles were more mobile after reaching tropical regions, probably looking for good wintering sites. Males generally left the breeding area in directions similar to the mean migratory direction, whereas a few females departed in diverging initial directions. Apart from these diversions, adult Ospreys followed very straight migratory routes, with overall mean directions of 185–209° and with mean angular deviations of 6–33°. Some juveniles also departed in diverging directions. Moreover, young birds tended to show a larger variability in orientation. Thus, the Ospreys kept a fairly straight direction and did not avoid geographical obstacles such as mountain ranges and desert areas. However, they seemed reluctant to cross large water bodies. There was no correlation between angular deviation and length of the migrational segment, indicating that the principles of orientation by vector summation may not be valid for Osprey migration. Moreover, the geographic direction of migration did not vary in accordance with variations in the magnetic declination, suggesting that the Ospreys did not orient along magnetic loxodromes.     

Migratory patterns of lower Feather River natural and hatchery-origin Oncorhynchus mykiss

Oncorhynchus mykiss are known to exhibit variable life histories expressed under genetic control or through the influence of the environment. Altered environmental conditions and the presence of large numbers of non-indigenous hatchery fish can affect the rate of anadromy observed in California’s Central Valley O. mykiss. The lower Feather River, a large regulated river in Northern California, supports one of the largest populations of O. mykiss in the Central Valley, heavily subsidized by hatchery production. I examined downstream migratory behavior, movement rate, and success of natural-origin juveniles, natural-origin adults, and hatchery-origin fish using acoustic telemetry. I found that all three groups displayed a mix of migratory and non-migratory life histories. Hatchery-origin adults exhibited more migratory behavior (anadromy) than natural-origin juveniles and natural-origin adults, 41 %, 24 %, and 13 % respectively. Downstream migrants were also significantly larger than non-migratory fish. Migration timing was similar among the three groups, but natural-origin juveniles displayed the most rapid rate of movement. The relative success of each group was highest for fish migrating through the Sacramento-San Joaquin Delta to the Pacific Ocean. Hatchery-origin adults migrating through the lower Feather River showed the lowest relative success (33 %). The dominance of non-migratory life histories pattern among lower Feather River O. mykiss may be in response to the suitable habitat provided by hypolimnetic releases below dams and the current hydrological regime within the highly altered Sacramento-San Joaquin watershed.     

The gut microbiome composition and degradation enzymes activity of black Amur bream (Megalobrama terminalis) in response to breeding migratory behavior

Black Amur bream (Megalobrama terminalis), a dominant species, resides in the Pearl River basin, known for its high plasticity in digestive ability. During spawning season, M. terminalis individuals with large body size and high fertility undergo a spawn migratory phase, while other smaller individuals prefer to settlement over migration. It is well known that gut microbial community often underpins the metabolic capability and regulates a wide variety of important functions in fish. However, little was known about how the gut microbiomes affect fish breeding migration. To investigate the variations in the gut microbiome of M. terminalis during the migration, we used high‐throughput 16S rRNA gene sequencing to reveal the distinct composition and diversity of the whole gut microbiome of migrated and nonmigrated population during period of peak reproduction, respectively. Our results indicated that nonmigrated population in estuary had a higher alpha diversity than that of migrated population in main stem. Additionally, an obvious abundant taxa shift between the gut microbiota community of nonmigrated and migrated M. terminalis was also observed. Change of dominant gut taxa from nonmigrated to migrated population was thought to be closely related to their degradation enzymes. Our results suggested that amino acid metabolism and lipid metabolism in migrated population were higher than that in nonmigrated population, providing a line of evidence for that M. terminalis change from partial herbivorous to partial carnivorous diet during breeding migration. We further concluded that, in order to digest foods of higher nutrition to supply energy to spawning migration, M. terminalis regulate activities of the gut microbiome and degradation enzymes, considered to be a key physiological strategy for reproduction.     

Effects of moonlight on the vertical migration patterns of demersal zooplankton

The diel vertical migration patterns of demersal zooplankton, those organisms which habit bottom substrates but periodically emerge to swim freely in the water column, water determined throughout the lunar cycle. Demersal zooplankton were quantitatively sampled on a subtidal sand flat in the Gulf of California every 2 h for 24-h periods at new, full, first, and last-quarter moons, both as they emerged into the water column and as they returned to the benthos. Demersal zooplankton rarely migrated during daylight. Three general patterns of migration were observed. (1) Polychaetes and cumaceans emerged from the benthos at dusk, regardless of the phase of the moon. Polychaetes returned to the benthos throughout the night while cumaceans returned near dawn. (2) Species of amphipods and isopods exhibited significant avoidance of moonlight, delaying emergence until moonset or returning to the benthos at moonrise. (3) Species of copepods, mysids, shrimp, Branchiostoma (cephalochordate), and tanaids emerged into the water column throughout the night. The timing of migration was highly variable and did not correlate with the presence or absence of moonlight. Large zooplankton migrated less frequently into the water column during moonlit periods than small forms, suggesting that nocturnal predation by visually oriented planktivorous fish may be an important selective pressure.Demersal zooplankton emerged into artificially darkened emergence traps in significantly higher numbers during daylight and during full and quarter moons than into undarkened control traps, demonstrating that absence of light is a major cue stimulating migration. Reentry traps resting on the bottom captured higher densities of demersal zooplankton than either emergence traps or reentry traps suspended off the bottom. Thus, many demersal zooplankton remain near the bottom, rarely swimming far into the water column. Some trap avoidance was observed and current methods for collecting demersal zooplankton are evaluated. Since most demersal zooplankton remained in the water column only a short time, dispersal, particularly over short distances, may be a major advantage of migratory behavior. Migration facilitates rapid recolonization of disturbed or defaunated sites, disrupts and mixes bottom sediments, and results in daily variation in the microdistribution, patchiness, and species composition of the benthic fauna.     

Dramatic intraspecific differences in migratory routes,stopover sites and wintering areas,revealed using light-level geolocators

Migratory divides are contact zones between breeding populations that use divergent migratory routes and have been described in a variety of species. These divides are of major importance to evolution, ecology and conservation but have been identified using limited band recovery data and/or indirect methods. Data from band recoveries and mitochondrial haplotypes suggested that inland and coastal Swainson''s thrushes (Catharus ustulatus) form a migratory divide in western North America. We attached light-level geolocators to birds at the edges of this contact zone to provide, to our knowledge, the first direct test of a putative divide using data from individual birds over the entire annual cycle. Coastal thrushes migrated along the west coast to Mexico, Guatemala and Honduras. Some of these birds used multiple wintering sites. Inland thrushes migrated across the Rocky Mountains, through central North America to Columbia and Venezuela. These birds migrated longer distances than coastal birds and performed a loop migration, navigating over the Gulf of Mexico in autumn and around this barrier in spring. These findings support the suggestion that divergent migratory behaviour could contribute to reproductive isolation between migrants, advance our understanding of their non-breeding ecology, and are integral to development of detailed conservation strategies for this group.     

Geolokation

Tracking bird migration with light‐level geolocators Bird ringing started to revolutionize our understanding of bird migration about 100 years ago. Since about 25 years satellite tracking of bird movements has increased our knowledge about migration strategies and migratory routes of large birds considerably. Nowadays miniature light‐level geolocators enable tracking of even small birds, though geolocators require recapture to obtain the data. Light‐level geolocators save basically the experienced sunrise and sunset events of the bird. By determining midday, midnight and length of day and night one can estimate longitude and latitude of birds' whereabouts and hence, map their migration.     

Limitations and mechanisms influencing the migratory performance of soaring birds

Migration is costly in terms of time, energy and safety. Optimal migration theory suggests that individual migratory birds will choose between these three costs depending on their motivation and available resources. To test hypotheses about use of migratory strategies by large soaring birds, we used GPS telemetry to track 18 adult, 13 sub‐adult and 15 juvenile Golden Eagles Aquila chrysaetos in eastern North America. Each age‐class had potentially different motivations during migration. During spring, the migratory performance (defined here as the directness of migratory flight) of adults was higher than that of any other age‐classes. Adults also departed earlier and spent less time migrating. Together, these patterns suggest that adults were primarily time‐limited and the other two age‐classes were energy‐limited. However, adults that migrated the longest distances during spring also appeared to take advantage of energy‐conservation strategies such as decreasing their compensation for wind drift. During autumn, birds of all age‐classes were primarily energy‐minimizers; they increased the length of stopovers, flew less direct routes and migrated at a slower pace than during spring. Nonetheless, birds that departed later in autumn flew more directly, indicating that time limitations may have affected their decision‐making. During both seasons, juveniles had the lowest performance, sub‐adults intermediate performance and adults the highest performance. Our results show age‐ and seasonal variation in time and energy‐minimization strategies that are not necessarily exclusive of one another. Beyond time and energy, a complex suite of factors, including weather, experience and navigation ability, influences migratory performance and decision‐making.     

The strength of migratory connectivity for birds en route to breeding through the Gulf of Mexico

The strength of migratory connectivity is a measure of the cohesion of populations among phases of the annual cycle, including breeding, migration, and wintering. Many Nearctic‐Neotropical species have strong migratory connectivity between breeding and wintering phases of the annual cycle. It is less clear if this strength persists during migration when multiple endogenous and exogenous factors may decrease the cohesion of populations among routes or through time along the same routes. We sampled three bird species, American redstart Setophaga ruticilla, ovenbird Seiurus aurocapilla, and wood thrush Hylocichla mustelina, during spring migration through the Gulf of Mexico region to test if breeding populations differentiate spatially among migration routes or temporally along the same migration routes and the extent to which within‐population timing is a function of sex, age, and carry‐over from winter habitat, as measured by stable carbon isotope values in claws (δ¹³C). To make quantitative comparisons of migratory connectivity possible, we developed and used new methodology to estimate the strength of migratory connectivity (MC) from probabilistic origin assignments identified using stable hydrogen isotopes in feathers (δ²H). We found support for spatial differentiation among routes by American redstarts and ovenbirds and temporal differentiation along routes by American redstarts. After controlling for breeding origin, the timing of American redstart migration differed among ages and sexes and ovenbird migration timing was influenced by carry‐over from winter habitat. The strength of migratory connectivity did not differ among the three species, with each showing weak breeding‐to‐spring migration MC relative to prior assessments of breeding‐wintering connectivity. Our work begins to fill an essential gap in methodology and understanding of the extent to which populations remain together during migration, information critical for a full annual cycle perspective on the population dynamics and conservation of migratory animals.     

Social transmission of behavioural traditions in a coral reef fish

Traditional behaviours involve the non-genetic transmission of social information across age classes or generations. French grunts (Haemulon flavolineatum) exhibit social traditions of daytime schooling sites and twilight migration routes. Individuals transplanted to new schooling sites and allowed to follow residents at the new sites used the new migration routes and returned to the new sites in the absence of resident fish. Control fish with no opportunity to learn showed no such directionality or return. This is the first demonstration of apparent pre-cultural behaviour in free-living fish. Our observations suggest additional classes of behaviour and taxonomic groups in which pre-cultural activities are likely to have evolved.     

Circadian flight schedules in night-migrating birds caught on migration

Many species of migratory birds migrate in a series of solitary nocturnal flights. Between flights, they stop to rest and refuel for the next segment of their journey. The mechanism controlling this behaviour has long remained elusive. Here, we show that wild-caught migratory redstarts (Phoenicurus phoenicurus) are consistent in their flight scheduling. An advanced videographic system enabled us to determine the precise timing of flight activity in redstarts caught at a northern European stopover site during their return trip from Africa. Birds were held captive for three days in the absence of photoperiodic cues (constant dim light) and under permanent food availability. Despite the absence of external temporal cues, birds showed clear bimodal activity patterns: intense nocturnal activity alternating with diurnal foraging and resting periods. The onset of their migratory activity coincided with the time of local sunset and was individually consistent on consecutive nights. The data demonstrate that night-migrating birds are driven by autonomous circadian clocks entrained by sunset cues. This timekeeping system is probably the key factor in the overall control of nocturnal songbird migration.     

Diel migration patterns of demersal reef zooplankton

Demersal zooplankton reside in or near the reef substrata and usually migrate into the water column at night. There is no single pulse of migratory activity. The zooplankton rise at variable rates throughout the night, with a peak activity usually during the second hour after sunset. This temporal pattern is a reflection of the behavior of the dominant (80–90% of night samples) cyclopoid, Oithona colcarva Bowman.Not all zooplankton taxa exhibit the same diel migratory patterns. Harpacticoids, another Oithona sp., copepod nauplii, barnacle nauplii, and appendicularians are most abundant during the day. Isopods show a peak of activity also during the second hour after sunset while polychaetes are most abundant during the first hour. The behavior of the other groups studied (the cyclopoid Corycaeus sp., other cyclopoids, ostracods, amphipods, tanaids, decapods, mysids, and chaetognaths) was less easily defined.The migration of many species in a pulse during the period of least planktivore activity and migration during the day of small species and juvenile members of larger species suggests that visual predators have an important influence on the migratory behavior of reef zooplankton.     

Why do migrating robins,Erithacus rubecula,captured at two nearby stop-over sites orient differently?

The orientation of robins captured during autumn and spring migration at two different sites, Falsterbo and Ottenby, in southern Sweden was investigated by cage experiments during the twilight period after sunset. The robins were tested under clear skies with skylight from sunset visible, and under simulated total overcast. The robins from the two sites differed in orientation, especially during autumn migration. While robins from Ottenby generally oriented in their expected migratory direction, the birds from Falsterbo under clear skies oriented towards the sunset direction with a narrow scatter in individual mean headings. Under simulated total overcast the robins from Falsterbo perferred northerly directions in autumn. Short-distance recoveries, one or only a few days after ringing, show that robins in autumn regularly fly 20–80 km from Falsterbo on northerly courses, indicating that they have temporarily reoriented from their normal migratory direction when confronted with the Baltic Sea. In contrast, most robins arrive at Ottenby by extensive flights across the Baltic Sea, and rapidly continue their sea crossing in the normal migratory directions. Mean fat deposits in autumn robins were significantly larger at Ottenby than at Falsterbo. These results indicate that migrating birds may show markedly different orientational dispositions depending on body condition and on their situation with respect to preceding and impending migration over land and sea, respectively.     

Differences in larval and juvenile devleopment among monthly cohorts of ayu,plecoglossus altivelis, in the shimanto river

The developmental processes of larval and juvenile ayu.Plecoglossus altivelis, in the estuarine and freshwater sections of the Shimanto River were compared among cohorts hatched in November, December and January. Formation of fins and ossification of the centra, which were completed by about 35 mm BL, hardly differed among the three cohorts. In specimens over 35 mm BL, body depth and head length were smaller in the November cohort than in the other cohorts, with pigmentation in the earlierhatched cohort being completed at a grater BL. The November cohort also remained at the “whitebait” stage until reaching a larger size (about 45 mm BL), before transforming to juveniles and commencing upstream migration. In contrast, the January cohort transformed to juveniles and started upstream migration at a smaller size (under 40 mm BL). The difference in transformation size among the cohorts is probably caused by ambient water temperatures existing at the beginning of upstream migration of each.     

Ontogenetic Behavior and Migration of Atlantic Sturgeon, Acipenser oxyrinchus oxyrinchus, and Shortnose Sturgeon, A. brevirostrum, with Notes on Social Behavior

Ontogenetic behavior of Hudson River Atlantic sturgeon and Connecticut River shortnose sturgeon early life intervals were similar during laboratory observations. After hatching, free embryos were photonegative and sought cover. When embryos developed into larvae, fish left cover, were photopositive, and initiated downstream migration. Free embryos may remain at the spawning site instead of migrating downstream because the risk of predation at spawning sites is low. The two species are sympatric, but not closely related, so the similarities in innate behaviors suggest common adaptations, not phylogenetic relationship. Atlantic sturgeon migrated downstream for 12 days (peak, first 6 days), shortnose sturgeon migrated for 3 days, and year-0 juveniles of both species did not resume downstream migration. Short or long migrations of larvae may reflect different styles related to the total migratory distance from spawning sites to juvenile rearing areas. Atlantic sturgeon need to move a short distance to reach rearing areas and they had a long 1-step migration of 6–12 days. In contrast, shortnose sturgeon need to move a long distance to reach all rearing areas. This may be accomplished by a 2-step migration, of which the brief migration of larvae is only the first step. Early migrant Atlantic sturgeon were nocturnal, while late migrants were diurnal, and shortnose sturgeon were diurnal. These diel differences may also be adaptations for long (Atlantic sturgeon) or short (shortnose sturgeon) migrations. Cultured shortnose sturgeon, and possibly Atlantic sturgeon, have a dominance hierarchy with large fish dominant when competing for limited foraging space. Social behavior may be more important in the life history of wild sturgeons than is generally recognized.     

Seasonal distribution of juvenile Atlantic cod in the southern Gulf of St Lawrence

Seasonal changes in the geographic distribution of juvenile (ages 1–4) Atlantic cod Gadus morhua were determined from research surveys conducted in the southern Gulf of St Lawrence during 1986–87, 1990–91, and 1994. From late June to early October, juvenile cod were most abundant in shallow water (<50 m deep). By early November, all age groups (adults and juveniles) had migrated to deep (>100 m) offshore waters for the winter. For most juveniles, this represented a minimum migration of >225 km (each way). Age 3 and 4 cod migrated greater distances than age 1 and 2 fish. Some age 3 and 4 cod migrated as far as 650 km from their summer and early autumn areas. Although there was partial geographic segregation of juvenile and adult fish during summer and winter, all age groups were found together during the migration. No cod were present in the shallow waters (<80 m deep) of the south-western Gulf during April due, in part, to later than average ice-melt. Large numbers of all age-classes were present in most of the south-eastern Gulf by mid-May.     

To migrate, or not to migrate: partial diel horizontal migration of fish in a temperate freshwater reservoir

The diel horizontal migration (DHM) of fish between the inshore and offshore zones of the ?ímov Reservoir (Czech Republic, deep, stratified, meso-eutrophic) was investigated by a combination of horizontal and vertical hydroacoustic surveys at 3-h intervals over 48 h and day/night purse seining in August 2007. An overwhelming majority of fish were aggregated within the epilimnetic layer. Considering only the horizontal surveys, cyclic diel fish movements between inshore and offshore habitats were apparent, while the total fish biomass remained constant between day and night. A higher fish biomass was detected in the offshore zone during daytime by both hydroacoustics and purse seining. In contrast, a higher fish biomass was recorded at night in the inshore zone. Bream Abramis brama, roach Rutilus rutilus, and perch Perca fluviatilis dominated the daytime offshore fish assemblage whereas bleak Alburnus alburnus prevailed at night. Bream and roach decreased in abundance at night while perch completely disappeared from the offshore habitat. The diel differences in size distributions and direct catches suggested the population-wide horizontal offshore migration of bleak and inshore migration of all perch during dusk. On the other hand, partial inshore migration of bream and roach adults was observed during dusk (52 and 80%, respectively). The different proportions of offshore residents among species and size classes suggested that differences in size, and, therefore, predation vulnerability, contributed to the observed migration patterns.