Unravelling the life-history patterns and habitat preferences of the Japanese eel (Anguilla japonica) in the Pearl River,China

Eels have fascinated biologists for centuries due to their amazing long-distance migrations between freshwater habitats and very distant ocean spawning areas. The migratory life histories of the Japanese eel, Anguilla japonica, in the waters of south China are not very clear despite its ecological importance, and the need for fishery regulation and management. In this study, strontium (Sr) and calcium (Ca) microchemical profiles of the otoliths of silver eels were measured by X-ray electron probe microanalysis based on data collected from different habitats (including freshwater and brackish habitats), in the large subtropical Pearl River. The corresponding habitat preference characteristics were further analysed using redundancy analysis (RDA). A total of 195 Japanese eels were collected over 6 years. The collected individuals ranged from 180 to 771 mm in total length and from 8 to 612 g in body weight. Two-dimensional pictures of the Sr:Ca concentrations in otoliths revealed that the A. japonica in the Pearl River are almost entirely river eels, spending the majority of their lives in fresh water without exposure to salt water, while the catadromous migration time has delayed about 1 month in the Pearl River estuary in the past 20 years. RDA analysis further indicated that juveniles and adults preferred water with high salinity and high tide levels. Youth preferred habitats with high river fractals. Our findings contribute to a growing body of evidence showing that the eels are extremely scarce currently and conservation measures against them are imminent, including the protection of brackish and freshwater areas where they live in south China.     

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.     

Head‐shape polymorphism in Japanese eels Anguilla japonica in relation to differences of somatic growth in freshwater and brackish habitats

The age, total length (L_T), head shape and skull shape were investigated for 379 Japanese eels Anguilla japonica sampled in freshwater and brackish areas of the Kojima Bay–Asahi River system, Okayama, Japan, to learn about the differentiation process of head‐shape polymorphism. The relative mouth width (ratio of mouth width to L_T) of A. japonica > 400 mm L_T collected in fresh water was significantly greater than that of fish collected in brackish water. Growth rates of mouth width and the distance from the snout to the midpoint of the eyes (the ratio of width and distance to age, respectively) were not significantly different between freshwater and brackish‐water samples, whereas the somatic growth rate (the ratio of L_T to age) of freshwater samples was significantly lower than that of brackish‐water eel samples. These results suggest that the factors affecting head and somatic growth of A. japonica are not identical. According to these results and feeding patterns in each habitat reported by another study, it is suggested that somatic growth appears to play a significant role in the differentiation process of the head‐shape polymorphism in A. japonica, with the slow‐growing fish in fresh water becoming broad‐headed and the fast‐growing fish in brackish water becoming narrow‐headed.     

据于矢耳石Sr/Ca值的长江日本鳗鲡迁移格局多样性分析(英文)

长江口是中国日本鳗鳗苗的主要产区和仅存的成鳗渔业水域。日本鳗自长江河口至上游金沙江近3000km干流及许多支流中都有分布,但其迁移行为却不为人了解。该文分析了2008年9～11月采自长江靖江段(31o30′N,120o42′E)的153尾银色鳗样本的生物学特征,测定了其中27尾标本的矢耳石Sr/Ca值。结果显示,153尾样本中有雌性85尾、雄性68尾,雌雄性比1:0.8。雌性由3～7(平均5.52)龄组成,平均体长(669±80)mm,体重(555±229)g,丰满度1.77±0.22,性腺指数(GSI)1.32±0.31。雄性由3～5(平均4.38)龄组成,平均体长(518±51)mm,体重(234±76)g,丰满度1.62±0.18,GSI0.21±0.11。雌性的这些生物学参数均显著大于雄性(P<0.05)。依据矢耳石线鳗标志轮平均Sr/Ca值(7.99±1.05)×10-3进行判断,有17尾(即62.96%个体)为"淡水型",10尾(即37.04%个体)为"河口型"。16尾雌性中有13尾(即81.25%)为"淡水型",3尾为"河口型"。11尾雄性中仅36.36%为"淡水型",63.64%为"河口型"。对每个生长层组的Sr/Ca值分析表明,雌雄间2龄时无显著差异,但3龄、4龄和洄游龄组都有显著或极显著的差异,预示着2龄时两者的栖息水域比较一致,但后来出现了明显栖息地分化。     

Interspecific and sexual differences in riverine distribution of tropical eels Anguilla spp.

A total of 261 individuals of the four tropical eel species, Anguilla celebesensis, Anguilla marmorata, Anguilla bicolor pacifica and Anguilla interioris, were collected from 12 locations around Sulawesi Island, Indonesia, to gain knowledge about the riverine distribution of tropical eels. Anguilla marmorata was predominant in the lower reaches of Poso River (94·4% of total eel catch in the sampling area), Poso Lake (93·3%), three small inlet rivers of Tomini Bay (100%) and Laa River (92·3%). Anguilla celebesensis occurred frequently in the inlet rivers of Poso Lake (63·5%). Anguilla bicolor pacifica and Anguilla interioris were rare (1.5 and 0.4%, respectively). Otolith Sr:Ca ratio electron‐probe micro analysis (EPMA) for individual migratory histories revealed that 15 A. celebesensis caught in Poso Lake and its inlet rivers were categorized into 14 river eels (Sr:Ca < 2·5) showing upstream migration seemingly at their elver stage and only one sea eel (Sr:Ca ≥ 6·0) that stayed in the marine habitat for the majority of its life after recruiting to Sulawesi Island before its late upstream migration. In A. marmorata, 19 examined eels from Poso Lake and its inlet rivers were all river eels, while 17 eels from the lower reaches of Poso River were two river eels, six sea eels and nine estuarine eels (2·5 ≤ Sr:Ca < 6·0) that mostly lived in the brackish water. The sex ratio of A. celebesensis was highly skewed towards a dominance of females (99%). In A. marmorata, females were predominant in Poso Lake (95·2%), its inlet rivers (94·7%) and Laa River (100%), while males were more frequent in the lower reaches of Poso River (76·5%) and small inlet rivers of Tomini Bay (94·1%). These results indicate that the riverine distribution pattern of tropical eels differs among species and between sexes.     

Dispersal of yellow phase Japanese eels <Emphasis Type="Italic">Anguilla japonica</Emphasis> after recruitment in the Kojima Bay-Asahi River system,Japan

The density, size and age distribution were investigated for 233 eels, Anguilla japonica, sampled in fresh and brackish water areas of the Kojima Bay-Asahi River system, Okayama, Japan, to evaluate the possible patterns of dispersal of eels that recruit to this area. Migratory histories of 183 eels were categorized into 5 types depending on the Sr and Ca concentrations in their otoliths: (1) brackish water residents (74 fish, 40.4%), which settled in saline water and remained until capture; (2) freshwater residents (46 fish, 25.1%), which settled in freshwater and remained until capture; (3) upstream shifters (3 fish, 1.6%), which settled in saline water and moved upstream into freshwater; (4) downstream shifters (53 fish, 29.0%), which settled in freshwater and moved downstream into saline water; (5) multiple habitat shifters (7 fish, 3.8%), which shifted their habitats between freshwater and saline water more than twice. For eels captured in the brackish water area, fish density decreased with distance in the downstream direction, while the size and age of eels increased. For eels captured in the freshwater area, size and age were greater than those in the upper-most brackish site. These observations suggest that eels in this system initially accumulate in the lower reaches of the river and then disperse in both upstream and downstream directions following their growth.     

Variation in the sex ratio, size and age of longfinned eels within and among coastal catchments of south‐eastern Australia

Longfinned eels Anguilla reinhardtii were captured by both fishery‐dependent and independent sampling methods from three rivers in New South Wales, south‐eastern Australia. Sex ratios, catch per unit effort and population age and total length structure were examined in three zones (fresh water and upper and lower tidal) in the Hacking, Hawkesbury and Clarence Rivers. Females were found in relatively high proportions in all zones, ranging from 97% in a freshwater (non‐tidal) site down to 59% in a tidal site. Males were found primarily in tidal zones (only two of the 677 longfinned eels caught in non‐tidal fresh water were males), with the greatest proportions being found in the brackish upper tidal areas. The mean number of fish captured per trap was higher in the fresh water and upper tidal zones than in the lower tidal zones. The mean ±  s . e . age, 17·9 ± 0·3 years, and age range, 5–52 years for females were significantly higher than those of males 12·2 ± 0·4 years; range 5–22 years, which is typical of other anguillid species. Longfinned eels captured in fresh water were found be significantly larger and older than those in tidal zones due to the almost exclusive predominance of females.     

Validation of annulus in otolith and estimation of growth rate for Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> in tropical southern Taiwan

The age of Japanese eels (Anguilla japonica) is often estimated from otoliths, but this method has not been fully validated, particularly in tropical areas where the annulus in otolith is considered to be less distinct than in temperate areas. To validate the annuli in Japanese eel otoliths from southern Taiwan, known-age (2 year-old) cultured eels from an eel farm and wild eels from Kao-Ping River were collected. It was found that 26 out of 31 cultured eels (83.9%) showed two clear annuli and the remained 5 eels showed either one or three annuli. The mean (± SD) age of the cultured eels was 1.97 ± 0.4 years. Meanwhile, a clear peak in the mean monthly marginal increment ratio of the otolith in wild yellow and silver eels occurred once a year during winter (November to March). The annual deposition of presumed annuli in otoliths of Japanese eel was validated and the age and growth rate estimation for Japanese eels in the tropical southern Taiwan is deemed feasible. The growth rate of cultured eels was significantly faster than that of wild eels, but it did not differ significantly between sexes for wild silver, yellow or cultured eels. The von Bertalanffy Growth Function parameters (K, and t ₀ ) of the wild eels were estimated as 0.114 ± 0.028 year⁻¹, 1178 ± 171 mm and −0.8 ± 0.2 years, respectively.     

Age and growth of migrating tropical eels,Anguilla celebesensis and Anguilla marmorata

The age and growth of migrating tropical eels, Anguilla celebesensis and Anguilla marmorata from central Sulawesi, Indonesia, were examined. Migrating eels (63 A. celebesensis and 38 A. marmorata ) were obtained from weirs near the Poso Lake outlet and non‐migrating eels (35 A. celebesensis and 119 A. marmorata ) were captured by baited hooks, eel pots, scoop net and electro‐fishing in the Poso River system, Laa River system, Baluga River, Tongku River and Padapu River from February 2009 to October 2010. In both species, the proportion of eels with opaque otolith edges showed a single peak in July, suggesting that one annulus (a pair of translucent and opaque zones) was formed each year in their otoliths. Mean ± s.d . and range of total length (L _T) and age was 785·2 ± 114·9 (585–1083) mm and 7·5 ± 1·6 (5–11) years in migrating female A. celebesensis and 1132·2 ± 173·7 (800–1630) mm and 11·6 ± 3·3 (7–23) years in A. marmorata . The age of migrating female eels was negatively correlated with annual growth rate, 100·7 ± 17·2 (68·1–145·0) mm year^?1 in A. celebesensis and 97·9 ± 19·3 (66·6–131·6) mm year^?1 in A. marmorata , but there was no significant correlation between the L _T and annual growth rate in either species. The annual growth rates of these female tropical eels were typically higher than those of temperate anguillid species, suggesting a latitudinal cline in growth rate in the genus Anguilla reflecting the environmental conditions of their growth habitat.     

Annual variability in upstream migration of glass eels in a southern USA coastal watershed

We investigated the environmental factors that affected temporal variability of eel recruitment and upstream migration in a freshwater coastal river along the southeastern US. Glass eels Anguilla rostrata were collected through ichthyoplankton sampling in the lower Roanoke River, North Carolina. Monthly samples were taken from fixed stations from May 2001 through June 2003. There was no evidence of consistent seasonal migration patterns for glass eels in Roanoke River. From May through December in 2001, glass eels were captured only during August. In 2002, glass eels arrived in February and remained in ichthyoplankton samples through October, with the exception of samples from September. Peak catch occurred in March at 4.02 ± 1.2 and declined through June to 0.18 ± 0.07 (#/1,000 m³). By August, the mean density increased to 0.96 ± 0.82 and to 3.59 ± 2.77 by October. In 2003 from January through June, glass eels were captured only during February and March. Glass eels were routinely collected when river discharge rates were <150 m³ s⁻¹. River discharge rates >650 m⁻³ s⁻¹ resulted in no glass eels in our samples. Upstream migration during 2002 was not correlated with water temperature or related to lunar phase. Glass eel freshwater upstream migration was initiated when water temperatures exceeded a threshold range of 10°C to 15°C; however, glass eels continued to migrate when water temperatures approached 30°C. The overall negative effect of river discharge suggests that changes in the water release schedules of upstream hydroelectric facilities during glass eel migration could strongly influence their recruitment success.     

Decline in non-native freshwater eels in Japan: ecology and future perspectives

The occurrence, distribution, and biological characteristics of non-native freshwater eels were analyzed using 5524 eels collected from 16 sites in Japan between 1997 and 2005. Three hundred seventy-four fishes (6.8%) were identified as non-native European eels, Anguilla anguilla, while the remainder (93.2%) were native Japanese eels, A. japonica. The European eel was found at 7 sites (44%), including 3 rivers, 2 freshwater lakes, one brackish lake, and one sea bay, suggesting a wide rage of habitat use. This variability of habitat use was also evidenced by the otolith microchemistry, which showed that they had lived in not only freshwater but also in seawater habitats. The sites with European eel were localized within the vicinity of southern Japan where a number of these eels were cultivated in the early 1970’s, suggesting that some had escaped from the culture ponds or were released intentionally into nearby natural waters. The large body size (mean total length: 803 mm), pigmented skin, enlarged eyes, and relatively matured gonads (mean gonad somatic index: 1.9) found in non-native European eels indicated that most had metamorphosed into the migratory silver phase, suggesting their ability to initiate spawning migration. However, the proportion of European eels in Mikawa Bay in 1997 was more than 12%, which decreased markedly to less than 2% after 2001, corresponding to the recent decline in import of European glass eels for aquaculture. This suggests that the population of European eels will decrease in Japanese waters in the future.     

Comparative ecology of the European eel, Anguilla anguilla (L., 1758), in a large Iberian river

A total of 1,816 eels were sampled in 1988, from seven sampling areas. Four areas were located in brackish water and the remaining three were located in freshwater reaches of the Tagus river basin. Eels were more abundant in the middle estuary and decreased both in the upstream and in the downstream directions, with a predominance of males in higher density areas. Smaller individuals preferred more peripheral areas, such as margins and upper reaches in the brackish water zone, and the tributaries of the freshwater habitats. It was assumed that this distribution pattern resulted from three main factors: (i) the dominance of larger specimens; (ii) the need to avoid predators and; (iii) the search for better trophic conditions. The condition of the individuals generally decreased toward the upper reaches, apparently due to a corresponding decrease in feeding intensity. The presence of the Belver dam in the main river, 158 km upstream from the sea, seemed to impose major alterations to the described patterns. The concentration of specimens below this impassable obstacle yielded a reduction in the proportion of females and a decrease in the condition and survival of the eels, contributing to a reduction in the spawning success of this population. Suggestions to diminish the effects of the dam, and to preserve the fishery are also presented.     

Occurrence of sea eels of <Emphasis Type="Italic">Anguilla japonica</Emphasis> along the Sanriku Coast of Japan

 The age and migratory history of the Japanese eel, Anguilla japonica, collected along the Sanriku Coast of Japan, were examined using otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that there were eels with several general categories of migratory history, including sea eels that never entered freshwater and others which had entered freshwater for brief periods but returned to the estuary or bay. This first evidence of the occurrence of sea eels in this northern area indicates that Japanese eels of the Sanriku Coast do not necessarily migrate into freshwater rivers. Received: May 15, 2002 / Revised: August 4, 2002 / Accepted: August 15, 2002 Acknowledgments We thank Messrs. S. Yamane and K. Morita, and crews of the Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, for their assistance in collecting the eels. This work was supported in part by Grant-in-Aid No. 13760138 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Correspondence to:Takaomi Arai     

Migration and habitat use of the tropical eels Anguilla marmorata and A. bicolor pacifica in Vietnam

The patterns of use of marine and freshwater habitats by the tropical anguillid eels Anguilla marmorata and A. bicolor pacifica were examined by analysing the otolith strontium (Sr) and calcium (Ca) concentrations of yellow (immature) and silver (mature) stage eels collected in Vietnamese waters. In A. marmorata, the change in the Sr:Ca ratios outside the high Sr:Ca core was generally divided into three patterns: (1) typical catadromous life history pattern; (2) constant residence in brackish water; and (3) habitat shifting between sea and brackish waters with no freshwater life. In A. bicolor pacifica, no eels had a general life history as freshwater residents. The eels were also divided into three patterns: (1) constant residence in sea water; (2) constantly living in brackish water; and (3) habitat shifting from brackish to sea water with no freshwater residence. The mean Sr:Ca ratio value after recruitment to coastal waters ranged from 1.73 to 5.67 × 10^?3 (mean 3.2 × 10^?3) in A. marmorata and from 2.53 to 6.32 × 10^?3 (mean 4.3 × 10^?3) in A. bicolor pacifica. The wide range of otolith Sr:Ca ratios in both species indicated that the habitat use of these tropical eels was facultative among fresh, brackish, and marine waters during their growth phases after recruitment to coastal areas. Tropical eel species may have the same behavioural plasticity as temperate anguillid species regarding whether to enter freshwater or to remain in estuarine and marine environments.     

Profiles of mRNA expression for prolactin,growth hormone,and somatolactin in Japanese eels,Anguilla japonica: The effect of salinity,silvering and seasonal change

For understanding the functions of the growth hormone (GH)/prolactin (PRL)/somatolactin (SL) family of hormones, we examined pituitary mRNA expression of these hormones in anguillid eels in relation to salinity difference, silvering, and seasonal change. Female Japanese eels (Anguilla japonica) were collected in the brackish Hamana Lake and its freshwater rivers from July to December. To clarify the effect of salinity, the habitat use history of the eels were determined using otolith microchemistry. Expression levels of mRNA of each hormone were determined using real time PCR. Although GH and PRL have been known to be osmoregulatory hormones, there were no consistent differences in expression levels of these hormones between different salinity habitats. In contrast, SL mRNA expression was higher in eels from freshwater rivers than from the brackish lake. GH mRNA expression clearly decreased during silvering, whereas PRL and SL mRNA expression did not change. We also showed that PRL mRNA and SL mRNA decreased in the brackish lake and PRL mRNA increased in freshwater rivers from autumn to early winter. These findings provide basic knowledge for a further understanding of the role of these hormones.     

Yellow-stage American eel movements determined by microtagging and acoustic telemetry in the St Jean River watershed, Gaspé, Quebec, Canada

The hypothesis that a part of the yellow American eel Anguilla rostrata sub-population of the St Jean River in eastern Quebec feeds in the brackish environment during summer and returns to the river to overwinter was tested. Three years of microtagging and the acoustic tagging and tracking of 40 American eels demonstrated that a part of the downstream migrants exploited the estuary as a summer feeding area. Upstream movement of some microtagged American eels provided support for the hypothesis that a part of those American eels returned to the river to overwinter. In addition to the demonstration of amphidromous behaviour of yellow eels, the study revealed that American eels in the estuary were active at night but homed to specific daytime resting sites.     

Osmoregulatory mechanisms of the Australian freshwater crocodile, Crocodylus johnstoni, in freshwater and estuarine habitats

The estuary of the Limmen Bight River in Australia's Northern Territory is home to an unusual salt water-adapted population of the Australian `freshwater' crocodile, Crocodylus johnstoni. Crocodiles were captured from tidal reaches of the estuary ranging in salinity from 0.5–24‰ and from several permanent fresh water reaches more or less remote from saline waters. C. johnstoni is an effective osmoregulator in moderately saline waters and has osmoregulatory mechanisms very similar to its more marine-adapted relative, the estuarine crocodile Crocodylus porosus. Fasted C. johnstoni in brackish water appear to lose little sodium in cloacal urine, relying on their lingual salt glands for excretion of excess sodium chloride. The lingual glands show clear evidence of short-term and long-term acclimation to salt water. Like estuarine crocodiles, C. johnstoni drinks fresh water and will not drink sea water. Gross sodium and water fluxes in brackish water are very similar to those in other crocodilians, suggesting differences in integumental permeability are not a major influence on osmoregulatory differences between crocodilians. The data reinforce the hypothesis that crocodylids differ fundamentally from alligatorids in the structure and function of the renal-cloacal-salt gland complex and are of interest in current debate over the evolutionary and zoogeographical history of the eusuchian crocodilians. Accepted: 25 February 1999     

Variations in the migratory history of the tropical catadromous eels Anguilla bicolor bicolor and A. bicolor pacifica in south-east Asian waters

Fish movements between aquatic habitats of different salinity ranges (fresh, estuarine, marine) by the tropical catadromous eels Anguilla bicolor bicolor and A. bicolor pacifica were examined by analysing the otolith strontium and calcium concentrations of yellow (immature) and silver (mature) stage eels collected in south-east Asian (Indonesia, Malaysia and Vietnam) waters. The ratios suggest that all migratory-type eels, including freshwater, brackish water and marine residents, pass the river mouth. However, the habitat preference was different among the sites (countries). In Indonesia and Vietnam, most A. bicolor bicolor and A. bicolor pacifica were either marine or brackish water residents in this study. Alternatively, most A. bicolor bicolor were freshwater residents in Malaysia; such a typical catadromous migration pattern in these eels has not been found in previous studies. The wide range of otolith Sr:Ca in both subspecies indicates that the habitat use of these tropical eels was opportunistic among fresh, brackish and marine waters during their growth phases following recruitment to coastal areas. The geographical variability of migratory histories suggests that habitat use might be determined by the inter and intraspecific competition and environmental conditions at each site.     

Opportunistic use of estuarine habitat by juvenile bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, from the Elwha River before,during, and after dam removal

Estuaries are used by anadromous fishes, either as the definitive marine habitat or as transition habitat as they move to fully marine waters, and extent of estuary use may vary with habitat conditions and fish attributes. Bull trout (Salvelinus confluentus) are commonly fluvial or adfluvial, though anadromous populations also exist. However, little is known about estuary use, especially by juveniles of this threatened species. We sampled the estuaries of the Elwha River, where a spawning population exists, and the nearby Salt Creek, where none exists, to reveal seasonal timing of estuarine use by juvenile bull trout, size of those using the estuary, and possible use of the non-natal estuary. We captured juvenile bull trout (all ≥100 mm FL, most <300 mm) in the Elwha River estuary in all months except August, but primarily December through May. None was captured in Salt Creek’s estuary despite comparable sampling effort. We also evaluated how dam removal on the Elwha River influenced bull trout estuarine occupancy by sampling before, during, and after dam removal, because this process enlarged the estuary but also increased turbidity and sediment transport in the lower river. Catches were low before dam removal, increased during and immediately after removal, and returned to low levels in recent years, suggesting that juveniles temporarily sought refuge from conditions associated with dam removal. Our findings indicate juvenile bull trout occupy estuarine habitat opportunistically; this information may aid conservation efforts as anadromous populations occur elsewhere in rivers with estuaries altered by human development.     

Location,size and age at onset of metamorphosis in the Japanese eel Anguilla japonica

This study clarifies the location, size and age at the onset of metamorphosis in Japanese eels Anguilla japonica through oceanic surveys, rearing experiments and analyses of the morphology and otoliths of leptocephali and glass eels. Twenty‐eight metamorphosing leptocephali were collected in the mesoscale eddy region to the east of Taiwan during research expeditions in 2004. Rearing experiments showed that the total length (L_T) of leptocephali decreased by an average of 12·5% during metamorphosis and 13·9% during the 2–12 h after death. Thus, the mean back‐calculated L_T at the onset of metamorphosis for 630 glass eels from Taiwan and Japan was estimated at 67·8 ± 2·7 mm (mean ± S.D.). The estimated mean ante‐mortem size of the fully grown pre‐metamorphic leptocephali collected in 2004 was 64·6 ± 3·4 mm, which was consistent with the L_T estimate for glass eels. Otolith analysis showed that the mean age at the onset of metamorphosis was 137 ± 15 days and indicated that Japanese eels may have a recruitment route through the mesoscale eddies to the east of Taiwan in addition to the direct transfer route from the North Equatorial Current to the Kuroshio Current.