HUSBANDRY
(Convener: Colombo; Rapporteur: Seymour)
36. Appelbaum and Birkan reported the effects of grading on the growth and distribution of size in young eels reared in a recirculating system. Grading is expensive in terms of labour and causes stress to fish on eel farms so that the optimal grading frequency to optimize growth is an important issue. The discussion after the paper centred on the frequency of grading used by farmers on existing farms. Gelin reported that eels were graded at intervals of six weeks and Hendricksen reported the same frequency on Danish farms but that fish over 50 g were not graded as these larger fish were more susceptible to stress effects.
37. Santos and Weber reported the growth of captured glasseels from the Rio Minho in two recirculated systems. These were weaned onto food after three days and reared in a hydroponic recirculating system. It was found that the elvers caught in the early part of the season (October to November) showed the best growth and good growth was reported for the latest eels captured in February and March. Parasitic infection with Ichthyophthirius slows the growth of the fish in the other group. Discussion centred on the reasons for this and it was suggested that this might have some relationship to the time of arrival and to feed and other abiotic factors.
38. Knosche presented a paper on the comparison of two biofilter systems used in intensive closed recirculating systems for eel culture. This was seen to be a key factor in the operational efficiency of commercial eel farming plants. He reported on water quality parameters from a farm operating with a conventional fixed film trickling filter and that of a Staehlermatic biofilter which operates as a combined fixed-film activated-sludge reactor. The latter gave lower levels of nitrite and ammonium and was able to take higher loading than the other system. Loadings were reported to be about 1.4 g N/m2/day plus an additional 2.8 g N/m2/day in activated sludge treatment for the Staehlermatic compared with just 0.5 g N/m2/day for the other at 26°C. But it was pointed out that often the efficiency of a biofilter is dependent on the management skills of a farm and it was suggested that 0.5 g N/m2/day was low compared with well managed systems.
EXPERIMENTAL ASPECTS
(Convener: Boetius; Rapporteur: Seymour)
39. Colombo and Grandi reported progress with the effects of steroids on the gonad sex differentiation of European eel. Sex was influenced by environmental factors such as density, temperature, feed. Sex steroids can influence sexual differentiation in the eel and the effect is dose and duration dependent. It appears that eels are susceptible to modification of sex over a large size range of 7 to 20 cm length. Above this size administration of steroids has an adverse effect. Discussion raised a question of whether sex once determined in eels could revert and it was generally concluded that female fish were unlikely to revert owing to the lack of germ cells after development of the ovary. Immature tests of yellow eels, however, were possibly able to revert once differentiated.
40. Lin Hao-Ren et al., reported the effect of sex steroids on gonadotropin synthesis and secretion as well as ovarian development in female Japanese silver eel. Injections and intra-peritoneal implantation were used as administration methods. Testosterone was found to be more effective than 17B-estradiol in inducing GTH synthesis and ovarian development. The reasons for this are not very clear and discussion showed that two different mechanisms may be involved. These were: the conversion of testosterone to estradiol, which act in the liver and in ovarian development and some positive feedback mechanism on the pituitary gland and hypothalamus.
41. Boetius et al., reported on experimental maturation of farmed eel made at the Swedish eel farm, Scandinavian Silver eel AB. Increases in GSI were obtained with serial injections of HCG (Human Chorionic Gonadotropin) and extracts of either salmon or carp pituitaries, when the eels were properly silvered. Spermiation was easy to obtain in males. Results with the females were almost successful and a few did show signs of ovulation. The salinity of the water was discussed. It was questioned whether females are able to obtain full maturation in brackish water (22 ppt) or whether full sea water is necessary to fulfil the osmotic requirements for ovulation and spawning. The depth of spawning may be also significant. While spawning, fish could be exposed to considerable pressure in depths of 600 to 1 000 m.
