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

Abstract

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.

Habitat segregation and migration in tropical anguillid eels, Anguilla bengalensis bengalensis and A. bicolor bicolor

“…Wide varieties of migratory history and habitat use were found in A. bicolor bicolor, although the preferred degree of salinity and migratory patterns differed from those found in other countries. Although 76% of A. bicolor bicolor had freshwater residence and 24% had estuarine residence in this study, 100% of the species had either estuarine or marine residences with no freshwater residence in a coastal lagoon of Indonesia 36,37 and estuarine or marine residences were also used in 100% of A. bicolor pacifica with no freshwater residence in a river in Vietnam 31 . There was 23% in freshwater residence and 77% was in estuarine or marine residences in A. bicolor pacifica in the Philippines 31 .…”

“…The diverse migratory history and habitat use corresponded to the distribution range from downstream to upstream as found in the previous study 18 . Such diverse migratory behaviours and habitat use were also found in A. bicolor bicolor in Indonesia [36][37][38] and in A. bicolor pacifica in the Philippines 31 and Vietnam 34 .…”

“…Analysis of the otolith microchemistry is broadly used to trace of the migratory history of the temperate eels A. anguilla 20 – 23 , A. rostrata 24 , 25 , A. japonica 26 – 29 and A. australis and A. dieffenbachaii 30 and the tropical eels A. marmorata 28 , 31 – 35 , A. bicolor pacifica 31 , 36 , A. bicolor bicolor 36 – 38 and A. mossambica 33 . The concentration of Sr in seawater is approximately 100 times higher than freshwater.…”

“…Classification of migratory pattern and habitat use. Several specific otolith Sr:Ca ratios are used as the benchmark for a freshwater life history, although, these values vary between regions and authors 3,22,25,26,[35][36][37] . Different researchers have used different Sr:Ca ratios to differentiate a freshwater environment, such as < 2.0 × 10 −332 , < 2.5 × 10 −334 and < 4.0 × 10 −328, 31,33 .…”

Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798

“…Otolith Sr:Ca ratios revealed that many eels stayed their whole lives in marine waters without freshwater lives after recruitment to coastal waters in the temperate eels A. anguilla ( Tzeng et al. 1997 , 2000 ; Tsukamoto et al., 1998 ; Arai et al., 2006 , 2019 ; Daverat et al., 2006 ; Marohn et al., 2013 ), A. rostrata ( Jessop et al., 2002 , 2008 ), A. japonica ( Tsukamoto and Arai, 2001 ; Arai et al., 2003b , c , 2008 ; Shiao et al., 2003 ; Yokouchi et al., 2012 ) and A. dieffenbachii and A. australis ( Arai et al., 2004 ) and the tropical eels A. marmorata ( Shiao et al., 2003 ; Briones et al., 2007 ; Chino and Arai, 2010 a ; Lin et al., 2012 ; Arai et al., 2013 ; Arai and Chino, 2018 ), A. mossambica ( Lin et al., 2012 ), A. bicolor bicolor ( Chino and Arai, 2010b , c ; Arai and Chino, 2019 ; Arai et al., 2020 ) and A. bicolor pacifica ( Briones et al., 2007 ; Arai et al., 2013 ; Arai and Chino, 2019 ). Furthermore, alternative migratory histories of the estuarine residents were also found in these studies, indicating that eels frequently shifted their habitats between marine and freshwater environments in different salinity regimes.…”