Maturation patterns in female European eel: age and size at the silver eel stage

Abstract

The maturation pattern in the female European eel Anguilla anguilla was studied by investigating age and size patterns of silver eels in different aquatic environments in Sweden, covering limnic, brackish and marine waters. The results neither supported the hypothesis that there is a critical size or age when eels enter the silvery stage, nor that size and age at maturity are positively related. Age at maturity, however, was observed to be negatively related to growth rate in all localities, i.e. the female reproductive tactic apparently is to become sexually mature at the earliest possible opportunity. Furthermore, it was recognized that a significant amount of variation was due to habitat differences, since the female eel maturation pattern deviated systematically between sampling sites, as it did also when the effect of growth rate was eliminated. Thus, the ability of the female eel to adjust maturation to an optimal size and age can be questioned, because the panmictic nature of the eel means local adaptations are unlikely. Growth rate dependent differences suggest that variations in maturation patterns between eel environments are linked more to the opportunity for nutrient accumulation than to other aspects of growth. 1996 The Fisheries Society of the British Isles

The influence of growth rate on the size of migrating female eels in Lake Ellesmere, New Zealand

“…However, as this size is not the maximum mean size recorded for the species (1085 mm from Lake Aniwhenua), it could be argued also that influences other than size-maximizing are important in determining the onset of maturation. The data do not support the hypothesis of Svedäng et al (1996) that selection pressure for females will be for maturation at the earliest opportunity. If this were so it would be expected that Lake Ellesmere females would have matured at a similar size to that achieved in the 1970s (say, 610 mm) when they would have been c. 6 years younger than they average presently.…”

“…Further, he suggested that both sexes have evolved to mature at the minimum size, a conclusion at variance with Helfman et al (1987), and that geographic variation in patterns of maturity could be explained in terms of the duration of larval drift theory of Helfman et al (1987). From a review of maturation patterns of female eels in Sweden, Svedäng et al (1996) suggested that ' the most likely female reproductive tactic is to become sexually mature at the earliest possible opportunity '. Consequently the increased fecundity associated with increased size is less important than early maturation with associated reduced prespawning mortality.…”

“…Fish typify this variability, with age at maturity having a generally negative relationship with growth rate (Alm, 1959); such a relationship has been demonstrated for female Anguilla anguilla L. (Vøllestad, 1992;Svedäng et al, 1996). While male eels exhibit a time-minimizing strategy and mature as quickly as possible (Helfman et al, 1987;Larsson et al, 1990;Vøllestad, 1992;Jellyman & Todd, 1998), females might demonstrate the converse, sizemaximizing (Helfman et al, 1987;Vøllestad & Jonsson, 1988;Larsson et al, 1990) to maximize fecundity.…”

Some morphometric and biochemical features of ready‐to‐migrate silver and pre‐migratory yellow stages of the shortfin eel of south‐eastern Australian waters

“…Although, shortfin eels need to attain a minimum size and an age prior to migration, both these criteria are extremely variable amongst populations, and there is considerable overlap in size and age of silver and yellow shortfin eels. In the European eel there is no critical size or age when they become silvery, nor are size and age at maturity positively related (Svedäng et al, 1996). The present data suggest that shortfin eels tend to be ready to undertake the spawning migration when L T >70 cm, but the reverse may not always be true.…”

“…Pankhurst (1982a) showed that the most distinguishable external criterion between yellow and silver European eels was the eye index (I E ); the I E of migrating silver European eels being >6·5, irrespective of size and age. This criterion for distinguishing silver and yellow eels has been successfully used by other workers (Svedäng et al, 1996). Todd (1981a) found the eye area to increase linearly in relation to L T , and the relationship to be significantly different amongst silver and yellow, and males of shortfin eels in New Zealand waters.…”