Slower growth of farmed eels stocked into rivers with higher wild eel density

Abstract

Farmed anguillid eels are frequently stocked into natural fresh waters to enhance eel resources, but little is known about what happens to these eels or their interactions with wild eels after stocking. A recent study observed a depressed survival and growth rate of farmed Japanese eels when they were reared with wild eels, which indicated that wild eels might interfere with the survival and growth of farmed-and-stocked eels through intraspecific competition. To contribute to improving eel stocking efficiency, the growth of farmedand-stocked Japanese eels was compared among four rivers with different wild eel densities using mark-and-recapture studies. Based on the 2-year recapture survey after stocking, it was found that the density of the farmed-and-stocked eels was not significantly different among rivers. The daily growth rates of farmed-and-stocked eels in the rivers with lower wild eel density were significantly higher than those of the eels stocked into the rivers with higher wild eel density. The farmed-and-stocked eels moved significantly greater distances downstream than wild eels that showed sedentary behaviour. This and previous studies indicate that significant questions remain about the effectiveness of stocking farmed eels into water bodies where naturally recruited wild eels are present.

Agonistic behaviour of wild eels and depressed survival and growth of farmed eels in mixed rearing experiments

“…To evaluate the possible effects of wild eels on farmed eels that were stocked in natural water bodies, Wakiya et al (submitted) conducted experimental stocking of farmed eels in four rivers with various densities of wild eels. It was found that the growth rates of farmed eels were greater in rivers with lower wild eel density, but significant differences in survival rate were not detected (Wakiya et al ., submitted). Although a greater understanding of the interactions between wild and stocked farmed eels is needed, it appears that stocking farmed eels in areas with abundant wild eels would not be the most effective strategy.…”

Coupling dynamic energy budget and population dynamic models to inform stock enhancement in fisheries management

“…This model provides dynamic description about the strength of density dependence with respect to environmental variations. In addition, our model embodies the density dependence of a combination of biological traits, including growth, fecundity and post‐recruitment mortality, which have been demonstrated separately by previous studies (Achord et al, 2003; Crozier et al, 2010; Raid et al, 2010; Stenevik et al, 2022; Wakiya et al, 2022). The density‐dependent effects presented by our mechanistic model were comparable to relevant field observations.…”