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

Abstract

To contribute to improving eel stocking procedures, the survival, growth and behaviour of farmed Japanese eels reared together with wild individuals were evaluated to learn about possible encounters of farmed‐ and stocked eels with wild eels. Two experiments were conducted to evaluate the (a) effect of the presence of wild eels on survival and growth of farmed eels in experimental ponds and (b) behavioural differences between wild and farmed eels placed in a small tank. In the survival and growth experiment, significantly lower survival and growth rates of farmed eels reared with wild eels were observed compared with farmed eels reared without wild eels (control). In the behavioural observations, the authors observed significantly higher occupation rates of a refuge and more frequent biting for wild eels, indicating a dominance of wild eels over farmed eels. Because wild and farmed eels used in the current experiments belong to the same genetic population, these differences resulted within one generation through the aquaculture rearing process.

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

“…Based on a mixed rearing experiment, Wakiya et al . (2022) reported that the growth rate of farmed Japanese eels is reduced when they are reared with wild eels, corresponding to the results obtained in this study. Therefore, wild eels in recipient water bodies likely interfere with the growth of farmed Japanese eels stocked there.…”

“…Wakiya et al . (2022) also observed reduced survival and growth rates of farmed eels when they were reared with wild eels for 2 years with live food (freshwater shrimp), whereas farmed eels showed similar survival and growth rates compared to wild eels when they were reared without wild eels. That study indicated that wild eels might be more competitive than farmed‐and‐stocked eels and they might interfere with the survival and growth of stocked eels through intraspecific competition.…”

“…Sloman and Armstrong (2002) reviewed research on social competition and the resulting physiological consequences for fishes, and found that studies on physiology in relation to individual dominance interactions in natural habitats suggest that hierarchies may cause less intense physiological responses than have been suggested by the results of laboratory studies in simple environments. This suggests that the reduced survival and growth observed in an experimental condition (Wakiya et al , 2022) may not occur to the same degree in natural conditions.…”

“…A recent experimental study (Wakiya et al , 2022) observed agonistic behaviour of wild eels directed towards farmed eels in small tanks where the two types of eels encountered each other. Wild eels dominated the occupation rates of a polyvinyl chloride (PVC) pipe that was placed in each experimental tank as a refuge, and they bit farmed eels more frequently than the opposite.…”

“…That study indicated that wild eels might be more competitive than farmed‐and‐stocked eels and they might interfere with the survival and growth of stocked eels through intraspecific competition. Nonetheless, these experiments (Wakiya et al , 2022) were conducted only in experimental tanks and ponds where conditions are totally different from natural water bodies. For example, farmed eels likely avoid or move away from wild eels after being stocked into natural water bodies such as rivers.…”