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

Abstract

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 9 10 -3 (mean 3.2 9 10 -3 ) in A. marmorata and from 2.53 to 6.32 9 10 -3 (mean 4.3 9 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.

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

“…However, the habitat preference differed among the sites (countries) (Figures and Table ). These findings indicate that the tropical A. bicolor bicolor and A. bicolor pacifica have a flexible migration strategy with a high degree of behavioural plasticity and an ability to utilise the full range of salinity similar to that of the tropical A. marmorata (Arai et al , ; Arai & Chino, ; Chino & Arai, ; Lin et al , ; Shiao et al , ) and the temperate A. japonica (Chino & Arai, ; Shiao et al , ; Tsukamoto & Arai, ; Tzeng et al ), A. anguilla (Arai et al , ; Shiao et al , ), A. rostrata (Jessop et al , ; Lamson et al , ), A. australis and A. dieffenbachii (Arai et al , ). These eels migrate flexibly among freshwater, brackish water and seawater environments.…”

“…Sagittal otoliths were extracted from each fish, embedded in epoxy resin (Struers Epofix; http://www.struers.com) and mounted on glass slides. The otoliths were then ground and polished as described by Arai et al (). They were then cleaned in an ultrasonic bath and rinsed with deionized water prior to examination.…”

“…Otolith microchemistry studies have revealed that some anguillid yellow and silver eels never migrate into freshwater but spend their entire life history in the ocean. The application of otolith Sr:Ca to trace the migratory history of eels has also revealed otolith signatures intermediate to those of marine and freshwater residents in Anguilla anguilla (L. 1758) (Arai et al , ; Shiao et al , ), A. japonica (Chino & Arai, ; Shiao et al , ; Tsukamoto & Arai, ; Tzeng et al , ), Anguilla rostrata (LeSueur 1817) (Jessop et al , ; Lamson et al , ), Anguilla australis Richardson 1841 and Anguilla dieffenbachii Gray 1842 (Arai et al , ), A. marmorata (Arai et al , ; Arai & Chino, ; Chino & Arai, ; Lin et al , ; Shiao et al , ), Anguilla bicolor bicolor McClelland 1844 (Chino & Arai , b) and Anguilla bicolor pacifica Schmidt 1828 (Arai et al , ; Briones et al , ), all of which appear to reflect brackish water residents, or showed clear evidence of switching between different salinity environments. Thus, it appears that a proportion of the eels move frequently between different environments during their growth phase.…”

Facultative amphidromy involving estuaries in an annual amphidromous fish from a subtropical marginal range

“…As an example, otolith microchemical analysis using G. maculatus revealed that diadromous individuals were only found in rivers with poor productivity, whereas no individuals seemed to move downstream in rivers with productive floodplains (Carrea et al ., ). Similarly, studies of tropical Anguilla spp., such as the Indian shortfin eel Anguilla bicolor pacifica (Schmidt 1928) demonstrated that >80% of individuals were considered to be estuarine residents (Arai et al ., ) and sometimes showed higher growth rates in saline waters than in freshwaters (Cairns et al ., ).…”

Conserving migratory species under human impacts and climate change

“…However, the AUC values reported here are much lower than those for freshwater fish in other regions and indicate further work is required to accurately predict the response of the Fijian fish fauna to disturbances . For instance, we found that the occurrence rates of species with flexible habitat use (e.g., Anguilla spp., Arai, Chino and Le (2013)) and some species that were rarely recorded in the field surveys (e.g., Mugilogobius notospilus, & Glossogobius sp. 2) were poorly predicted by other species within the same trait or taxonomic group.…”