Introduction
Anguillid eels are targeted, high-value species for aquaculture in Asia and Europe. Unfortunately, eel farming is still capture-based, exclusively relying on wild-caught glass eels and thus, the sustainability of this industry is challenged by the present critically low stock abundance, which is especially the case for European eel, A. anguilla (Pike et al., 2020). Hence, it is urgently needed to further develop and establish captive breeding techniques and technologies for this critically endangered fish species. Advances in Japanese eel culture (Okamura et al., 2013) have formed the baseline for eel research, promoting recent development of hatchery protocols for European eel (Tomkiewicz et al., 2019). While offspring culture techniques for the European eel encompass the endogenous feeding stages (Sørensen et al., 2016), the transition to exogenous feeding is still challenged by lack of insights regarding the feeding stages and dietary requirements for the unique leptocephalus larvae. Research has been conducted to identify natural eel larval feeding sources (Miller, 2009) and physiology (Knutsen et al., 2021), but despite increasing knowledge on larval feeding ecology, the natural first-feeding regimes of Anguillid pre-leptocephali remain an enigma. Scientific inquiry has focused on identifying potential first-feeding diets with the first exogenously feeding experiments of hatchery-reared European eel larvae only recently attempted (Butts et al., 2016; Politis et al., 2018), in combination with enhanced rearing techniques aiming at improving larval survival (Tomkiewicz et al. 2019; Politis et al., 2021). In continuation of this work, research in the innovation project ITS-EEL has focused on developing prototype diets, exploring feeding and gut-priming regimes as well as testing larviculture procedures by applying progressively advancing culture techniques and technology. The presentation will overview these progressive advancements regarding the requirements of larval European eel in terms of abiotic (such as temperature, salinity, light, pH) and biotic (such as gut-priming, feeds, feed amounts, feeding regimes, microbial control, water quality) factors, from a morphological and molecular point of view.
Material and methods
European eel gametes were obtained through assisted reproduction using routine procedures as previously described by Kottmann et al. (2020). After fertilization, the floating layer was transferred to 60 L black conical incubators, supplied with conditioned filtered seawater (Politis et al., 2018) at a flow through rate of ~350 mL min−1, while gentle aeration was added. Temperature was set to 18°C, salinity to 36 psu and light to low intensity of ~10 lx for better embryonic development (reviewed in Tomkiewicz et al., 2019). Larvae hatched at ~56 hpf, which were then reared in ~80 L tanks, connected to a recirculating Aquaculture System (RAS), until reaching the feeding stage. Thereafter, larvae were moved into ~8 L Kreisel tanks, connected to a new RAS, where temperature was kept at ~20°C and salinity lowered to ~18 psu (Politis et al., 2021). Here, larvae were fed prototype liquid diets, provided to the bottom of each tank. During feeding, water flow was stopped and lights turned on at high intensity of ~20 µmol m-2 s-1. After ~30 min feeding time, the remaining food was flushed away, water circulation was restarted and the light turned off again. Larvae were fed using this procedure five times per day.
Results and Discussion
Following constantly updated assisted reproduction protocols, the amount and quality of European eel larvae reaching the first-feeding stage has steadily increased enabling research efforts around larval feeding culture. Knowledge about larval European eel requirements and preferences, regaridng abiotic (temperature, salinity, light, pH) and biotic (gut-priming, feeds, feed amounts, feeding regimes, microbial control, water quality) factors has enhanced survival. Moreover, the European eel larval rearing systems and diets have been progressively upgraded and refined by incrementally integrating gained knowledge, resulting in the first ever European eel leptocephalus culture, surviving up to ~140 dph. Notably, European eel larvae grew from ~3 mm (~1.5 mm²) at hatch, to 7-8 mm (3-4 mm²) at the first-feeding stage (10 dph). The majority of larvae started feeding at 10-12 dph, while the larvae that did not feed or start feeding too late, entered the so called “point-of-no-return” and perished at 20-24 dph. In contrast, larvae that initiated exogenous feeding success-fully, survived beyond this point and transformed into the characteristic leaf-like leptocephalus shape. So far, applying advanced rearing and feeding procedures have led to European eel larvae growing to a length of ~15 mm and body area up to ~20 mm². These results are comparable to Japanese eel larviculture (Tanaka et al., 2001; Okamura et al., 2013), while growth rates are lower compared to estimate patterns for wild leptocephali (Fig. 1). Overall, our results, enabling the first ever European eel leptocephalus culture, present a new promising step towards closing the life cycle of this critically endangered species in captivity. Now, the challenges ahead involve progression of diets, advancement of rearing techniques and improvement of larval survival throughout the leptocephalus stage, to reach the glass-eel stage.
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