Fertilisation of eggs, rate of embryonic development and hatching following induced maturation of the European eel Anguilla anguilla

Abstract

The study reports on 11 female Anguilla anguilla in which sexual maturation was induced using modifications of a technique that has proven successful in Anguilla japonica. In 10 of the females, the outcome of fertilisation attempts is known. One female did not respond to hormonal treatment and failed to break down the germinal vesicle; in the remaining nine females, eggs were fertilised. In the eggs from two of the females, embryonic development proceeded to the hatching stage, and in one case, hatching occurred at about 48-h postfertilisation at an embryonic stage when the tail had just passed the snout. Induction of maturation consisted of weekly injections with salmon pituitary extract (SPE) at 18.2-mg pituitary powder per fish and week; the number of SPE injections ranging from 11 to 29 before the oocytes had reached a stage when final oocyte maturation and ovulation was induced by 1 mg of 17,20 ß-dihydroxy-4-pregnen-3-one (DHP). At the time of DHP injection, the ooplasm and the oil droplets were transparent and the nucleus was in the migratory stage near or at the animal pole; cell diameters ranged from 763 to 930 μm. To avoid sudden overripening of the eggs near the time of final oocyte maturation, the final SPE doses were individually regulated—usually downregulated—relative to the weekly dose, and this resulted in the use of four protocols for final egg maturation. The best protocol, which resulted in hatching, used 13.6 mg SPE followed 22 h 40 min later by DHP treatment when the oocytes had reached the above stage. The second best protocol, in which the embryos developed to the hatching stage, used a weekly SPE dose of 13.6 mg followed by a priming SPE dose of 1.8 mg 2 days later, 11 h 40 min after which DHP was injected.