SERIES A (Freshwater) No. 36 1992 IRISH FISHERIES INVESTIGATIONS Christopher Moriarty (Editor) Papers presented to the 7th Session of the EIFAC Working Party on Eel -"J1-7 IRISH FISHERIES INVESTIGATIONS SERIES A (Freshwater) No.36 1992 Roinn na Mara (Department of the Marine) Papers presented to the 7th Session of the EIFAC Working Party on Eel edited by Christopher Moriarty DUBLIN: PUBLISHED BY THE STATIONERY OFFICE TO BE PURCHASED FROM THE GOVERNMENT PUBLICATIONS SALE OFFICE, SUN ALLIANCE HOUSE MOLESWORTH STREET, DUBLIN 2 Price: £5.35 Contents Page INTRODUCTION - Christopher Moriarty v GLASS EEL MIGRATION AND ABUNDANCE I M Domingos: Fluctuation of glass eel migration in the Mondego estuary (Portugal) in 1988 and 1989. Daniel Guerault, Raymonde Lecomte-Finiger, Yves Desaunay, Sylvie Biagianti-Risbourg, Pierre Beillois and Patrick Grellier: Glass eel arrivals in the Vilaine estuary (Northern Bay of Biscay) in 1990: Demographic features and early life history. 5 Jan G P Klein Breteler: Effect of provenance and density on growth and survival of glass eels Anguilla anguilla (L.) in mesocosm experiments. 15 R. Lecomte-Finiger, S. Biagianti-Risbourg, Y. Desaunay, D. Guerault, B. Fourcault, S. Planes and A. Yahyaoui: Age at recruitment of A. anguilla glass-eels on the eastern Atlantic coast as inferred from otolith growth increments. 23 P McGovern and T K McCarthy: Elver migration in the River Corrib system, western Ireland. 25 Christopher Moriarty: Catches of Anguilla anguilla (L.) elver on the Atlantic coast of Europe 1989- 1~0. ~ Maria Assunr;ao Santos and Michael Weber: Growth studies on monthly captured glass eels from the Rio Minho in two recirculation systems. 35 Raymonde Lecomte-Finiger: Age and birth date of elvers collected in Moorea (French Polynesia) (poster). 41 Lotti Ben Abdallah: Influence of some abiotic factors on the abundances of glass eel Anguilla anguilla (L.) in the estuary of the River Loire, France (abstract). 42 ECOLOGY C. Belpaire, H Van Driessche, F Y Gao and F Ollevier: Food and feeding activity of glass eel Anguilla anguilla (L.) stocked in earthen ponds. 43 Maria Bninska and Marian Leopold: The effect of eel on fish stock composition in lakes- preliminary results. 55 Rose M Callaghan and T Kieran McCarthy: Variations in population structure and growth rate of eels in the Dunkellin river system, western Ireland. 59 J L Costa, P R Almeida, CAssis, F Moreira and M J Costa: A study of methods of estimating the size of eel populations in small streams. 68 Christopher Moriarty: Management of the Corrib eel fishery, Ireland. 70 I A Naismith and B Kights: The distribution, density and growth of the European eel Anguilla anguilla (L.) in the River Thames catchment. 71 W Russell Poole, Julian D Reynolds and Christopher Moriarty: Age and growth of eel Anguilla anguilla (L.) in oligotrophic streams. 72 METHODOLOGY R H Hadderingh, J W van der Stoep and J M P M Habraken: Deflecting eels from water inlets of power stations with light. 78 K Holmgren, H Wickstrom and K Fredga: Growth of eel in a meoscale experiment. 88 Aline Caillou, Christian Francisco, Raymonde Lecomte-Finiger and Jean-Marie Salmon: Lipofuscin used as an age indicator in the European eel Anguilla anguilla (L.). Comparison between lipopigment, fluorimetric measurements and otolithometric data (poster). 102 Peer Doering, Jiirgen Ludwig and Gerhard Gmel: Prelimary results of otolith amalysis with eels of known age (abstract). 103 Antoine Legault: A study of some selectivity factors in eel ladders (abstract). 104 MARINE STAGES F - W Tesch: Insignificance of tidal currents for silver eel migration as studied by eel trac.kings and current measurements. 105 F - W Tesch and U Niermann: Stock density of eel larvae Anguilla anguilla (L.) on the European continental slope, based on collections made between 1985 and 1989. 110 iii _ .. _----- --------~ .....• -------------~--- Irish Fisheries Investigations Series A (Freshwater), No. 36 (1992). Page Takakazu Ozawa, Futoshi Kakizoe, Osame Tabeta, Takashi Maeda and Yasutaka Yuwaki: Larval Introduction growth and drift of the Japanese eel Anguilla japonica estimated from leptocephali collections (abstract). 114 W-N Tzeng and Y-C Tsai: Otolith microstructural growth patterns and daily age of the eel Anguilla by japonica elvers from the estuaries of Taiwan (abstract). 115 CHRISTOPHER MORIARTY AQUACULTURE Department ofthe Marine, Fisheries Research Centre, Abbotstown, Dublin 15, Ireland S Appelbaum and V Birkan: The effect of grading on the growth and distribution pattern in young eels Anguilla anguilla (L.) reared in recirculating systems. 116 Inge Boiitius, Karl Otto Wahlstrom and Curt Gelin: Experimentally induced sexual maturity in farmed European eel Anguilla anguilla (L.). 121 G. Golombo and G. Grandi: Further experiments in the effects of sex steroids on the gonad sex The Seventh Session of the Working Party on Eel of the European Inland Fisheries Advisory Commission 126 differentiation of European eel. was held In the Royal Hospital, Kllmalnham, Dublin at the kind invitation of the Government of Ireland. A Soeren Hendriksen: Production of eel in recirculation systems in Denmark 1985-1991. 130 135 full report of the session, including names of the 52 participants and a list of all presentations has been M Saroglia, C lando Ii, lingle and G Angle: Recent developments in eel farming in Italy. published by FAO (Rome) as EIFAC Occasional Paper No. 25. Reiner Knosche: An effective biofilter system for eel culture in closed recirculating systems. 141 Hao-Ren Lin, Mei-Li Zhang, Lianxi Chen, Glen Van Der Kraak and R E Peter: Effects of sex steroids The present volume contains peer-reviewed papers and abstracts of other presentations to the session. The on gonadotropin synthesis and secretion as well as ovarian development in female Japanese material IS arranged underthe headings of the six sub-groups of the Working Party and covers a very wide 142 silver eel Anguilla japonica (abstract). range of studies of the eel In wild and cultured conditions. The Working Party, which meets in alternate years, has frequently considered its terms of reference and has DISEASES AND PARASITES on each occasion concluded that, because of the enormous geographical range and variety of habitats of the species of AngUIlla, the primary need in research continues to be local studies of their basic biology. J Hoglund, J Andersson, H. Wickstrom and M Reizenstein: The distribution of Anguillicola in Sweden 143 However, there IS scope for international collaboration. Agreement has been reached by the Working Party and its association with thermal discharge areas. on the most satisfactory methods of age determination and tagging and a handbook on these topics was J Bosnakovski, K Necev, K Apostolski and M Hristovski: Appearance of eel diseases in Ohrid Lake. 151 published In 1988 as EIFAC Occasional Paper No. 21. Proposals were made at the Seventh Session for 152 Inge Boetius: Development of Anguillicola infestations in some Danish lakes and inlets (abstract). international co-operative studies on recruitment and on growth. Grateful acknowledgement is made for the assistance of colleagues who formed the Editorial Board for this publication: Inge Boetius, Yves Desaunay, Brian Knights, Russell Poole and Hiikan Wickstrom. iv v 1 Fluctuation of glass eel migration in the Mondego estuary (Portugal) in 1988 and 1989 by I. M. DOMINGOS Dep. de Zoologia e Antropologia, Faculdade de Ciencias de Lisboa, Campo Grande, Bloco C2, 3° Piso, 1700 Lisboa, Portugal ABSTRACT The drastic reduction in the quantity of glass eels entering estuaries in recent years, reinforced the need to review the legislation of the fisheries, as well as to promote studies aiming at the correct management of such an important resource. A study on the fluctuation of glass eel migration was performed in the lower part afthe Mondego estuary in 1988-1989, Although the freshwater flow seems to be a ruling factor in the ascent of glass eels in this estuary, the results obtained show a variable pattern which is not easily interpreted, confirming the complexity and unpredictability of this migration, INTRODUCTION The fishery for glass eels in Portugal has been regulated by law since 1985. This legislation concerns only the estuarine waters. The official fishing season extends from 1 November to 28 February and the only permitted gear is a hand net called "rapeta". Official data on glass eel catches before 1985 are rare mainly because the previous legislation did not oblige fishermen to apply for fishing licences and to report their catches. Numbers of fishermen and quantities captured are therefore mostly unknown. More recently, the application of the law has been given considerable attention from the authorities. Despite all this, the official data available today are still underestimates. Knowledge on abundance and preferences of glass eels during their upstream migration is poorly documented in Portugal. Apart from Mondego (Jorge and Sobral, 1989) and Minho (Weber, 1986) estuaries no studies have been published. In order to obtain information about the current status of the glass eels in river Mondego, a survey was conducted from 1988 to 1990. The objectives were to examine and compare abundance and environmental preferences of glass eels during migration. Special attention was devoted to studying their abundance related to temperature and salinity. MATERIAL AND METHODS The estuary of Mondego is divided into two channels (Fig. 1), north and south which differ in width and depth. This study was conducted in the north channel since the south channel is too shallow, during low tide, to permit the use of big nets. Each month, from January 1988 to February 1990, the quantity of glass eels entering the estuary at new moon, was estimated by passive fishing using a fixed net called "tela" (Fig. 2) always set at night and at the same site during the sampling period. The net, with a height of 5.8 metres, was set at low tide and the fishery was performed during the flood tide.