Firstfeeding diets for European eel larval culture: insights at morphological, nutritional and molecular levels.

Abstract

Closing the life cycle in captivity of European eel is a requirement to develop sustainable aquaculture of this species. Due to the decline of the population together with low levels of juvenile recruitment, this species has become critically endangered and subject to trade restrictions (Pike et al., 2020). Thus, hatchery production of offspring is essential in order to lift limitations on aquaculture production and markets for European eel (Tomkiewicz, 2019). Information regarding the eel early life stages is scarce, which has considerably delayed the development of culture techniques and technologies. The main bottleneck for progressing in rearing hatchery-produced eel larvae is the establishment of an adequate first-feeding regime. First studies showed that European eel larvae successfully ingested a paste made of rotifers, Brachionus plicatilis (Butts et al., 2016), but despite successful initiation of first-feeding at 15 dph (cultured at 18°C) and an increased body area, indicating growth compared to starving conspecifics, the larvae did not survive past 24 dph (Politis et al., 2018). However, the latter study showed thatInterestingly though genes encoding the major digestive enzymes were expressed earlier than the actual initiation of first feeding (already at 4 dph), indicating a premature molecular functionality initiation of the gastro intestinal tract. Subsequent studies have focused on the possibility to promote an early maturation of the gut by early introduction of feed, where it was indicated that early availability of dietary nutrients can influence the molecular ontogeny of feeding related mechanisms and processes, but the successful initiation of larval feeding depends on the quality and attractiveness of the diet(s) (Benini et al., 2022). In the present study, three diets were formulated and experimentally tested as potential first feeding prototypes for hatchery produced European eel larvae. Larval mortality was recorded daily from the onset of the first feeding stage at 10 dph until 28 dph, while sampling was conducted at regular intervals to obtain larval biometrics and gene expression patterns to follow the molecular ontogeny of digestive functions and growth of the larvae during the experimental period.