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.
Highlights
Information on the spawning migration, spawning ecology and life history of tropical eels in the IndoPacific region is very limited
We recommended that further studies are needed to elucidate whether the spawning season of A. bengalensis bengalensis females occurs throughout the year or restricted season
The findings described in this report indicate that tropical eels exhibit life history characteristics that differ markedly from those of temperate eels
Summary
Information on the spawning migration, spawning ecology and life history of tropical eels in the IndoPacific region is very limited. The final stage of maturation for seaward spawning migration occurred throughout the year in A. bicolor bicolor, 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. Leptocephali, drift and are transported along ocean currents at the continental shelf These leptocephali leave oceanic currents after metamorphosing into glass eels and typically migrate upstream as elvers four to eight months after hatching[6] to grow in fresh water habitats during the yellow stage (immature stage). The spawning migration of temperate eels generally occurs during autumn months between August and December[9,10,11] in the Northern Hemisphere and between February and May in the Southern Hemisphere[12], except in a case of the prolonged downstream migration activity between April and December with high temporal variation in the European eel Anguilla anguilla in northeastern Germany[13]
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