Telemetric observations on the spawning migration of the eel (Anguilla anguilla) west of the European continental shelf

Synopsis In the Northern Bay of Biscay and west of the Iberian continental shelf five silver eels (Anguilla anguilla L.) have been tagged with ultrasonic transmitters and tracked 13 to 23 hours over a depth of 200 to 2500 m. Their mean direction from release to the final position of tracking was 288° and significantly closer to the direction of the Sargasso Sea (250° west) than silver eels tracked earlier in the North Sea (341°), possibly 260°. Four of the transmitters were equipped with pressure sensing devices capable of indicating depths of at least 400 m. Three eels tracked at night, during full moon, preferred mean depths of 125, 166 or 215 m. One eel chose a depth of 100 m during moonlight and 50 m after the setting of the moon. Major depth changes, usually occurring one per hour, ranged up to a maximum of 200 m at a maximum vertical speed of 0.6 m sec^–1; this is close to the eels' normal horizontal speed. At dawn all but one dived to a depth of 400 m or more. The eels generally swam below the thermocline and often crossed it.     

A preliminary study of the movements of yellow and silver eels, Anguilla japonica, in the estuary of the Fukui River, Japan, as revealed by acoustic tracking

The behavior and movements of yellow and silver phase Japanese eels were observed using acoustic telemetry in the Fukui River estuary and the adjacent waters of Tachibana Bay, Tokushima Prefecture, Japan. The eels were tagged with ultrasonic transmitters and released in the bay, about 300 m from the river mouth in August and November, 1999. All four yellow eels released at the river mouth in August returned to the river. All eels swam further upstream and each stopped at similar locations as the others, which were possibly used as refuges. Each refuge appeared to be a relatively small area (less than 10 m) adjacent to a series of concrete blocks along the shore (100–300 m). These areas were repeatedly utilized by all the yellow eels tracked during the study. The yellow eels spent most of their time in these refuges during daytime and moved predominantly at night. In contrast, a silver eel released in November demonstrated rapid movement towards the sea without stopping after release.     

Migration,residency and habitat utilisation by wild and cultured Japanese eels (Anguilla japonica) in a shallow brackish lagoon and inflowing rivers using acoustic telemetry

This study monitored post-release movements of 20 wild Japanese eels (Anguilla japonica) [mean ± S.D. 520.8 ± 92.3 mm total length (TL), 217.9 ± 146.3 g body mass (BM)] in a brackish water lagoon in northeastern Japan using acoustic telemetry to elucidate how wild Japanese eels use different river, estuary and marine environments. In addition, 12 cultured Japanese eels (TL = 578.9 ± 18.0 mm, BM = 344.9 ± 25.5 g) were released to understand the comparative behaviours of wild and cultured eels. Both types of eels were simultaneously released in the southern inner part of the lagoon in September 2016 where there are freshwater influences from a river. Following release, eight of the wild eels (40%) were largely sedentary near the released point (river mouth) and stayed at the site for overwinter. Nonetheless, several individuals showed behavioural plasticity of habitat use: three wild eels moved towards the northern part of the lagoon with stronger influence from the sea during May–July 2017. Two wild eels showed clear repeated movements from the lagoon to a river at night and returned to the lagoon by dawn for more than a week every day, and one wild eel migrated upstream for overwintering. Signals from 55% of the wild eels could be detected for more than 6 months, whereas those from all of the cultured eels were lost by December 2016, indicating a short resident time of large cultured eels (BM > 200 g) released in a brackish water area. One wild silver eel migrated to the outer sea during the ebb tide at night in November 2016, probably triggered by the decrease in water temperature (from c. 20°C to c. 13°C), and seven cultured eels similarly moved to the outer sea during October–November 2016. The results revealed the similarities (e.g., nocturnal movements) and differences (e.g., stay period and seasonal movements) in the behavioural characteristics of wild and cultured eels and indicated that habitat connectivity among river, estuary and coastal waters is crucial for enabling eels to efficiently utilise these productive habitats through their behavioural plasticity.     

Blood changes in japanese eels,Anguilla japonica,experimentally infected with typical or atypicalAeromonas salmonicida

• 1.1. Changes in the blood and in the rate of oxygen consumption of Japanese eels injected intramuscularly in the head with a lethal dose of typical or atypical Aeromonas salmonicida at 20°C were investigated.
• 2.2. Eels infected with the bacteria became moribund within 4 to 6 days, and then died within 1 day.
• 3.3. The O₂ consumption rate and blood parameters changed markedly with infections. The responses of hosts to infection by the two kinds of bacteria differed with regard to the following four points: blood pH, plasma Cl⁻, lactic acid, and the numbers of granulocytes and lymphocytes.
• 4.4. The responses of eels infected with atypical A. salmonicida were larger and more rapid than those of eels infected with typical A. salmonicida.

     

Diurnal Fluctuations in Relative Water Content, Nitrate Reductase and Proline Content in Water-stressed and Non-stressed Wheat

Diurnal variations in relative water content (RWC), nitrate reductase (NR) and proline content (PC) were studied at 3 h intervals during a 24 h cycle in the flag leaf of wheat (Triticum vulgare, v. Kalyansona) grown under stressed and non-stressed conditions. RWC was lower in stressed plants than in non-stressed ones throughout the 24 h period. Although it was lowest at 12 h, it recovered by 15 h. Non-stressed plants maintained higher NR activity compared to those under stress. The enzyme activity during night was about the same as during day time in both types of plants. Compared to non-stressed plants, stressed ones had lower NO₃^? content. Proline accumulation occurred under stress conditions and had a maximum at 12–15 h. Non-stressed plants exhibited higher PC during night than day time. Changes in temperature and relative humidity were noted during the period and their influence on RWC, NR and proline was discussed.     

Diel locomotor activity of brook charr, as determined by radiotelemetry

Locations by radiotelemetry during eight 24–h sampling periods in 1991, 1992, and 1993 were used to evaluate the diel activity patterns of adult brook charr Salvelinus fontinalis in two lakes of the Laurentian Shield (Québec, Canada). Based on the minimum distance travelled between two consecutive locations, adult brook charr were more active at dusk and at night than during the day. During daylight periods, individual fish displayed strong site selection, remaining in the same area along the shore from day to day. The fish left their inshore position at dusk, were highly mobile during the night, and returned to their focal point at sunrise, exhibiting diel homing behaviour. These results are supported by an increase in gillnet captures of adult brook charr in Lac Melchior at night. Data obtained by gillnet fishing in lakes Bondi and Simpson showed that juvenile (1+) brook charr were active throughout the diel cycle and that young-of-the-year were active mainly during the day, suggesting that there is an ontogenetic change in the diel activity of brook charr. Despite its limitations (restrictions on the number and size of tracked individuals, post-manipulation mortality, lifetime of transmitter battery), radiotelemetry allows for collection of multiple behavioural observations (distances travelled, instantaneous activity, site selection and homing) during the activity cycle of individual fish, and thus provides a finer degree of spatial resolution than alternative methods such as gillnetting.     

Home ranges, movements, and activity of wolves (Canis lupus) in the Dalmatian part of Dinarids, Croatia

Home-range sizes, movements, and daily activity of wolves (Canis lupus L. 1758) were studied in Dalmatia, Croatia in 1998–2001. The total home ranges (100% MCP) of two packs were 160 km² and 141 km², mean=150.5 km². Core areas (50% kernel) were 26.2 km² and 3.3 km², respectively. Differences in core area sizes were influenced by human activity—hunting and sheep grazing. Compared with random locations, wolf locations were closer to the nearest water source (mean=937 m) and farther from houses (mean=653 m). Wolves were significantly more active during the night than during the day (activity indexes were 0.53 vs. 0.35), and night activity was higher during summer (0.58), and lower during winter (0.48). A correlation was found between distances traveled and activity index (r=0.58, p=0.003). Home range, seasonal variations in home-range size, habitat use, and activity of wolves in Dalmatia were oriented to make the compromise from danger of proximity to humans and also to benefit from human-related food sources.     

A bit of quiet between the migrations: the resting life of the European eel during their freshwater growth phase in a small stream

The movements and habitat use of resident yellow eels were studied in a stream stretch having both natural and minimum flow zones. N = 12 individuals (total length 505–802 mm) were surgically tagged with radio transmitters and released at their capture sites. They were located using manual radio receivers during the daytime from 2 to 5 days/week over periods ranging from 200 to 329 days, for a total of 1,098 positions. Eels showed home ranges ranging from 33 to 341 m (median value, 62 m), displayed strong fidelity to sites and demonstrated a great degree of plasticity in habitat use. Eels were slightly mobile throughout the year, but their movements were season and temperature dependent, with a maximum during the spring (mean water temperature, 12 °C) and a minimum in winter (3 °C). Stones and roots (utilization rate greater than 50 % of eels for more than 30 % of location days) were significantly the most frequently used habitats. Between the two flow zones, the natural flow was the most occupied, with a significantly higher proportion of resident eels (66.7 % of radio-tagged yellow eels) and longer occupation (81 % of location days) than the minimum flow zone with less suitable habitats.     

Diving activity of migrating silver eel with and without Anguillicola crassus infection

Infection with the swim bladder nematode Anguillicola crassus has been hypothesised to threaten the spawning migration success of the endangered European eel (Anguilla anguilla). To examine this assumption, we compared the swimming behaviour of one Anguillicola crassus infested eel in the North Sea and three parasite‐free eels in the Baltic using data recovered from data storage tags attached to migrating silver eels. In both areas, eel activity was characterized by frequent diving behaviour throughout the water column during the night, with reduced activity during the day. Despite substantial damage of the swim bladder, the behaviour of the infested eel from the North Sea was within the same range of migrating and diving activity parameters as the three parasite‐free eels from the Baltic Sea. All eels had a similar frequency distribution of descent or ascent speeds and a similar average horizontal migration speed. The diving speeds and dive ranges exclude the possibility that the eels were in continuous hydrostatic equilibrium during their migrations and suggests therefore that the role of the swim bladder in vertical migration is likely to be more complex than currently thought. Our results suggest that eels infested by Anguillicola crassus are capable of diving in a similar manner to uninfested eels during the first stretch of their spawning migration.     

Sleeping cluster patterns and retiring behaviors during winter in a free-ranging band of the Sichuan snub-nosed monkey

Little information is available on the sleeping cluster pattern and retiring behavior of Sichuan snub-nosed monkeys (Rhinopithecus roxellana). Here, we provide observational data on a provisioned free-ranging band in the Qinling Mountains, central China. The results suggest that winter night activity of R. roxellana is a compromise between antipredator and thermoregulatory strategies and an adaptation to ecological conditions of their temperate habitat. Monkeys retired between 1804 and 1858 h in winter. In support of the antipredation hypothesis, all monkeys slept in trees at night, whereas 18.8% of individuals slept on the ground during the day. Also, the study band was more spatially cohesive at night than in daytime, with shorter distances between one-male units. Keeping warm is critical for survival in freezing temperatures. Monkeys often slept in the lower stratum of the tree canopy, avoiding the upper canopy where it is cold and windy. They formed larger sleeping clusters at night than in daytime. The most common types of night-sleeping clusters were adult females and juveniles, followed by adult females with other adult females. These accounted for 60.2% of the total records. The frequency of female–male clusters is two times greater, and that of adult male–juvenile clusters was four times less at night than during the day. The variations in composition of sleeping clusters suggest affiliative patterns at night-sleeping sites differ from those during the day.     

Nocturnal activity and resource utilization in the aphid hyperparasitoid, Dendrocerus carpenteri

Abstract.

• 1 Females of the aphid hyperparasitoid Dendrocerus carpenteri (Curtis) search successfully for hosts during both day and night. Oviposition numbers per host patch did not differ significantly between day and night.
• 2 D.carpenteri females also displayed a nocturnal flight activity, showing that they are not only capable of searching on a given host plant but also of dispersing between host plants.
• 3 Nocturnal oviposition activity was mainly influenced by egg load. Females with a high egg load laid more eggs at night than females with a comparatively low egg load. Thus, D.carpenteri females may use nocturnal foraging to compensate for the lack of oviposition opportunities during day.
• 4 D.carpenteri females which foraged continuously for hosts both day and night (= for 24 h per day) benefitted from an 1.4-fold increase in lifetime reproductive success when compared to females which foraged only by day (= for 16 h per day).
• 5 The benefit of night foraging for this species is a significantly increased reproductive success.

     

Water temperature and upstream migration of glass eels in New Zealand: implications of climate change

Glass eels migrating upstream in a New Zealand river showed a clear preference for water temperatures between 12 and 20°C, with an optimum of 16.5°C. Water temperatures <12°C and >22°C almost completely inhibited migration, which implies that warmer temperatures associated with global climate change might have a detrimental impact on glass eel recruitment in their current ranges. We established this by trapping glass eels of shortfin, Anguilla australis, and longfin, A. dieffenbachii, eels nightly from September to November. Eels caught in 2001 (50,287) outnumbered those caught in 2002 (19,954); shortfin glass eels dominated catches in both years, comprising 91–93% of the catch. Longfins were larger than shortfins, and size and pigmentation in both species increased as the seasons progressed. Temperatures within the migratory season in 2001 showed ∼14-day intervals between maxima that appeared to be associated with the new and full moons.     

Response of total night-time respiration to differences in total daily photosynthesis for leaves in a Quercus rubra L. canopy: implications for modelling canopy CO2 exchange

Measurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night‐time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7 μmol m^?2 s^?1) were approximately four times higher than those in the lower leaves (4.3 μmol m^?2 s^?1) and maximum photosynthesis rates in the upper shaded leaves (8.0 μmol m^?2 s^?1) were about half those in the upper leaves. There was a strong linear relationship between total night‐time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20°C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, V_cmax, were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, J_max (P>0.05), and light use efficiency, ? (P<0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model was used to quantify the significance of the findings on estimated canopy CO₂ exchange for the full growing season. Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night‐time respiration by 23% compared with the value when respiration was held constant. This highlights the need for a process‐based approach linking respiration to photosynthesis when modelling long‐term carbon exchange in forest ecosystems.     

Vital population statistics based on length frequency analysis of the exploited Japanese eel (Anguilla japonica) stock in the Kao‐Ping River,southern Taiwan

Vital statistics such as growth, mortality, and maturity parameters are crucial in understanding the population dynamics of a species. A total of 7 074 Japanese eels (Anguilla japonica) in the lower reach of the Kao‐Ping River, southern Taiwan, were collected with eel tubes in 1998 ～ 2004 and shrimp nets in 2004 ～ 2007. Data from 2004 were excluded due to mixed gear information and escapement of cultured eels; in subsequent years escaped cultured eels were identified and excluded from analyses. The estimated asymptotic length in the von Bertalanffy growth function (84.5–110 cm) was smaller, while the Brody growth parameter (0.30–0.44 year^?1) was higher using electronic length frequency analysis (ELEFAN) than when using Shepherd’s length composition analysis (SLCA). The total instantaneous mortality rate (Z) was around 1 for periods 1998–2003 and 2 year^?1 for 2005–2007 using length‐converted catch curves. The 95% confidence intervals of Z did not overlap for two of the periods, suggesting that the mortality rates were significantly higher during 2005–2007, possibly due to the introduction of shrimp nets. The maturity function differed significantly between sexes, indicating that females become silver eels at a larger size. The Japanese eels in the lower reach of the Kao‐Ping River were likely heavily exploited, thus management and conservation actions are strongly recommended.     

Diel differences in fish assemblages in nearshore eelgrass and kelp habitats in Prince William Sound,Alaska

The importance of a particular habitat to nearshore fishes can be best assessed by both diurnal and nocturnal sampling. To determine diel differences in fish assemblages in nearshore eelgrass and understory kelp habitats, fishes were sampled diurnally and nocturnally at six locations in western Prince William Sound, Alaska, in summer 2007. Abundance of fish between day and night were similar, but species composition and mean size of some fish changed. Species richness and species diversity were similar in eelgrass during the day and night, whereas in kelp, species richness and species diversity were greater at night than during the day. In eelgrass, saffron cod (Eleginus gracilis) was the most abundant species during the day and night. In kelp, the most abundant species were Pacific herring (Clupea pallasii) during the day and saffron cod at night. Diel differences in fish size varied by species and habitat. Mean length of saffron cod was similar between day and night in eelgrass but was greatest during the day in kelp. Pacific herring were larger at night than during the day in kelp. Diel sampling is important to identity nearshore habitats essential to fish and help manage fish stocks at risk.     

Tracking and mapping sun-synchronous migrations and diel space use patterns of Haemulon sciurus and Lutjanus apodus in the U.S. Virgin Islands

The spatially explicit diel movement patterns of fish using coral reef ecosystems are not well understood, despite the widespread recognition that many common species undergo distinct migrations to utilize different resources during night and day. We used manual acoustic telemetry coupled with global positioning technology to track the detailed spatially explicit daily movements (24 h) of multiple individuals of two common Caribbean fish species, Haemulon sciurus (bluestriped grunt) and Lutjanus apodus (schoolmaster snapper). Movement pathways and day and night activity spaces were mapped and quantified in a Geographic Information System (GIS). Directional sun-synchronous migrations occurred close to astronomical sunset and sunrise. Site fidelity within day and night activity spaces was high. Nine of twelve individuals exhibited overlap of day and night activity spaces and three fish (L. apodus) exhibited complete spatial segregation. Night activity spaces (H. sciurus: 11,309?±?3,548 m²; L. apodus: 9,950?±?3,120 m²) were significantly larger than day activity spaces (H. sciurus: 2,778?±?1,979 m²; L. apodus: 1,291?±?636 m²). The distance between sequential position fixes (step lengths) was significantly greater at night than day, indicative of nocturnal foraging and day resting behavior. Integrating acoustic telemetry, GIS techniques and spatial statistics to study fish movement behavior revealed both individual variability and some broader generality in movement paths and activity spaces suggestive of complex underlying behavioral mechanisms influencing diel movements.     

Spatial and temporal habitat use of kob antelopes (Kobus kob kob, Erxleben 1777) in the Comoé National Park, Ivory Coast as revealed by radio tracking

Spatial and temporal habitat use of kob antelopes (Kobus kob kob) have been investigated in the Comoé National Park (Ivory Coast, West Africa) by use of radio telemetry. A total of 23 kob were equipped with radio collars and radio tracked for up to 15 months. Home ranges of males were smaller and those of females larger than expected from theoretical models. Adult males used smaller areas than adult females and did not show seasonal home range shifts. Daily distances travelled did not differ between sexes. Kob walked less during the night than by day and covered shorter distances in the wet season. Whereas an increase in home range overlap between females resulted in higher rates of association among individuals, association of adults of mixed sexes was not correlated with the degree of home range overlap. Territorial behaviour of males and predator avoidance by females are suggested to explain the sex‐specific differences in home range size of adults and the deviation from the predicted sizes. Predator avoidance is presumed as the main reason for the reduced walking distances at night as well as in the wet season. Reproductive behaviour and feeding ecology are assumed to determine the degree of association of conspecifics.     

Comparison of Day and Night Fyke Netting, Electrofishing and Snorkelling for Monitoring a Population of the Threatened Golden Galaxias (Galaxias auratus)

The littoral zone of small off-stream water storage containing a translocated population of Galaxias auratus was sampled fortnightly at day and night with fyke nets, electrofishing and snorkelling over 3 months. Variation in population data provided by each method, including relative abundance indices, size structure, and habitat preferences, were examined. Aspects of behaviour and activity patterns were also investigated. Night sampling using all methods consistently yielded larger catches than day sampling. The size structure of catches varied, with electrofishing at night and fyke netting during the day having higher proportions of juveniles, whilst snorkelling at night and electrofishing during the day had higher proportions of adults. Fyke netting at night yielded by far the largest catches (~3-fold more than other methods) and also captured balanced proportions of juveniles and adults. Galaxias auratus had a strong diel activity pattern and were most active at night. The majority of the population migrated into the littoral zone during the night and back into deeper water during the day. A small number of juveniles remained in the littoral zone and some adults sheltered in the dense cover of species-rich littoral vegetation during the day. Shores with shallow depth profiles appeared to be preferred due to higher catches in these areas using all methods. Based on the results of this study, fyke netting at night in littoral habitats is recommended for monitoring populations of G. auratus. Fyke netting is likely to be an effective method for monitoring other lacustrine galaxiid species; however, further work is required to investigate the effects of habitat variables and fish community structure on activity patterns of galaxiids, and hence their catchability with various methods, in more extensive lentic environments.