Artificial Maturation and Reproduction of the European Eel

Abstract

Arjan Palstra and Guido van den Thillart 13.1 Introduction Mature eels cannot be obtained from the wild situation since nobody has ever observed migrating and spawning silver eels in the ocean. Our existing knowledge comes from the artificial induction of maturation by hormonal injections with carp or salmon pituitary extract (CPE/SPE; Fontaine et al. 1964) in female European eels and with human chorionic gonadotropin (hCG) in males (Fontaine 1936). In 1974, Yamamoto and his colleagues continued their investigations on artificially induced maturation in Japanese eels and were able to produce larvae but these died within a few days (Yamamoto and Yamauchi 1974; Yamauchi et al. 1976). In the 1990s, efforts resulted in fertilized eggs and non-feeding larvae. Research on the induction of maturation and larval rearing of the Japanese eel, Anguilla japonica, is mainly conducted at the National Research Institute of Aquaculture (Nansei, Japan). The maturation procedure for females involves injec-tion of salmon pituitary extract (20 mg per week) for 8–13 weeks (Kagawa et al. 2005). At 24-h after the last injection, 17, 20 β-dihydroxy-4-pregnen-3-one (DHP) is injected (2 μg g −1 body weight) to stimulate final maturation and induce ovulation within 14–23 h (Ohta et al. 1996). On the other hand to mature male eels, 10 to 14 weekly injections of human chorionic gonadotropin (1 IU g −1 body weight) are suf-ficient to induce spermiation (Kagawa et al. 2005). Up to 89.6% fertility and 47.6% hatchability was obtained (Ohta et al. 1996). Because of the difficulty of achieving simultaneous ovulation and spermiation, particular attention has been paid to finding an optimum diluent for cryo-preservation of spermatozoa (Ohta and Izawa 1996). The main emphasis now is on larval rearing. Getting larvae past the first 9 days proved thus far impossible with live food (rotifers), so a complex artificial diet was developed in the form of a paste. The initial problem was to get the larvae to feed at all. Now that this has been achieved, the problem is to find an optimum diet to ensure G. van den Thillart et al. (eds.), Spawning Migration of the European Eel, 309 © Springer Science + Business Media B.V. 2009 310 A. Palstra and G. van den Thillart survival and optimal growth. Growth is slower than in wild larvae up to the first meta-morphosis to the leptocephalus stage, thereafter it slows even more. The larvae are, much smaller than wild leptocephali, when they reach the coast of Taiwan (Tsukamoto 1992; Fricke and Tsukamoto 1998). Jan and Inge Boetius were the first to fertilize the eggs of European eel (Boëtius and Boëtius 1980). A few years later, a Belorussian group (Bezdenezhnykh et al. 1983, 1984) obtained larvae from the European eel, but these larvae showed the same problems as with the Japanese eel, they died within a few days after hatch-ing. The Japanese researchers used their protocol to induce ovarian development for Japanese eel also on European eels (Chiba et al. 1994). However, until now they have been unable to reproduce European eel with same success as for the Japanese eel, showing that maturation of the European eel differs to a great extent. Lokman and Young (2000) used the Ohta et al. (1996) protocol on New Zealand freshwater eels (A. dieffenbachii and A. australis). They obtained larvae of A. australis and kept them alive for a few days. Recently, Palstra et al. (2005) used the Ohta protocol and produced 4 day old European eel embryos (Fig. 13.9). Pedersen (2003, 2004) applied variations of the same protocol and obtained a few larvae that stayed alive for 2 days. However, those larvae showed delayed hatching and abnormal mor-phology. In 2006 and 2007 Tomkiewicz and colleagues (DIFRES, Denmark), were able to produce larvae, which could be repeated several times (presented at EAS meeting Istanbul 2007). They have been able to keep them alive up to 22 days, still in the pre-feeding stage. In 2007 Kurwie and her colleagues were able to produce larvae of A. australis and kept them alive up to 20 days (presented at website, http:// www.aquarticles.com/articles/breeding/index.html). Also in 2007, Oliveira and colleagues produced 1-day old larvae of A. rostrata (not published). However, for all three species it has not been possible yet to initiate feeding. The same breeding protocol with Anguilla japonica is currently more successful; in 2006 Japanese scientists produced feeding larvae on a weekly basis (Dr. Takao Yoshimatsu, National Institute of Aquaculture, Japan, presented at IPA workshop Bilthoven 2006). In addition Tanaka (2003) was already able to get some lepto-cephali larvae through metamorphosis and to obtain glass eels at approximately 250 days after hatching. Still, despite the large investments in time and energy, the ques-tion of controlled reproduction of eels is not resolved. The fact that many fertilized eggs do not develop, and most larvae die prematurely indicate that the problems may lay in the quality of eggs and possibly also in the quality of sperm.