Opportunistic spawning of tropical anguillid eels Anguilla bicolor bicolor and A. bengalensis bengalensis

Abstract

Information on the spawning migration, spawning ecology and life history of tropical eels in the Indo-Pacific region is very limited. The physiological and morphological characteristics of tropical freshwater eels, Anguilla bicolor bicolor and A. bengalensis bengalensis collected in Malaysia were examined in relation to their downstream migration patterns. A total of 455 eels were collected over monthly intervals between February 2014 and January 2016 and we examined both gonadosomatic index and gonad histology features. In both species, close positive relationships between advanced maturation stages and eye, fin, gonadosomatic indexes were found in males and females. A. bengalensis bengalensis was found to be larger and heavier than A. bicolor bicolor at the time of seaward migration. The final stage of maturation for seaward spawning migration occurred throughout the year in A. bicolor bicolor, although that of A. bengalensis bengalensis was estimated to six months due to the limited number of samples. These results suggest that year-round spawning in the open ocean occurs in the tropical eel. This non-seasonal spawning ecology is notably different from that of temperate eels, which are known to follow a well-defined spawning season, with spawning migrations generally taking place during autumn months.

High genetic diversity and lack of pronounced population structure in five species of sympatric Pacific eels

“…(). The mixing of individuals can also be attributed to the prolonged spawning and recruitment of tropical anguillids, which has been suggested to last nearly the entire year, facilitating spawning groups that originate from different generations (Arai & Abdul Kadir, ; Kuroki et al., ; Shen & Tzeng, ). The spawning ecology of tropical eels differs significantly from those found in temperate waters, enabling interbreeding between silver eels from different areas and cohorts (Acou, Laffaille, Legault & Feunteun, ; Kotake et al., ; Verreault, Mingelbier & Dumont, ).…”

“…The spawning ecology of tropical eels differs significantly from those found in temperate waters, enabling interbreeding between silver eels from different areas and cohorts (Acou, Laffaille, Legault & Feunteun, 2008;Kotake et al, 2007;Verreault, Mingelbier & Dumont, 2012). A year-round spawning ability may help to maintain high population sizes in the Indian Ocean (Arai & Abdul Kadir, 2017). However, the continuous and targeted fisheries, the unregulated removal of recruits for eel farming and the lack of population assessment may nevertheless lead to stock depletion.…”

Ages at maturation of tropical freshwater eels,Anguilla bicolor bicolorandA. bengalensis bengalensis

“…Until recently, considerably less research has examined the spawning migration of tropical eels in comparison with that of temperate eel species. Recently, Arai et al (2016) and Arai and Abdul Kadir (2017b) found that a spawning period of a tropical eel Anguilla bicolor bicolor that extends throughout the year. The findings described in this report indicate that tropical eels have life history characteristics that differ markedly from those of temperate eels.…”

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

“…Information regarding the life history of the tropical eels has been accumulating gradually (Arai & Abdul Kadir, , b; Arai et al , ; Lin et al , ; Shiao et al , ). Hatching was estimated to occur throughout most of the year, with recruitment at a constant age (Arai et al , ).…”

“…Hatching was estimated to occur throughout most of the year, with recruitment at a constant age (Arai et al , ). Year‐round recruitment of tropical glass eels to the river mouth (Arai et al , ) followed year‐round spawning (Arai & Abdul Kadir, ; Arai et al , ) and a stable recruitment age (Arai et al , ). Such a life‐history strategy differed from that of temperate eels, which have a limited spawning season followed by a limited period of recruitment.…”