A telephone survey of eel fishermen regarding external lesions and mortalities of American eels (Anguilla rostrata) from Lake Ontario and the St. Lawrence River basin, Canada.
Signs of decreasing landings and recruitment have been observed during the last decade in American eels (Anguilla rostrata) from the St. Lawrence River basin, Canada. A study was undertaken to examine whether important manifestational diseases among commercial catches could be associated with these declines. During this survey, 56 fishermen from Lakes Ontario, Saint-François and Saint-Pierre, the Richelieu River, the Québec City area and the St. Lawrence Estuary were interviewed. Most fishermen from every area reported decreasing catches since the 1980s. Eel mortalities were reported from 1990 to 1992 in Lake Saint-François, Lake Saint-Pierre and the Québec City area, but in apparently lower magnitude than in the past decades. The lesions observed on dead eels in Lake Saint-François indicated that the cause of the mortalities might be associated with the upstream hydroelectric dam. Eels with 'scratches all over the body' were only observed in the St. Lawrence Estuary. Fishermen from almost every area reported that they had observed eels with 'crooked tail', 'humpback' and 'cut tail' in low frequency in 1992. The questionnaire data were validated by comparing some of the results of the questionnaire survey and a processing plant survey in 1992. The use of questionnaires for assessing fish diseases in commercial species is discussed. The results of this survey do not indicate that the health of the American eels from the St. Lawrence River basin is severely impaired. However, the magnitude of diseases and mortalities was probably underestimated.
Utilisation des poissons pour évaluer les effets biologiques des contaminants dans l’estuaire du Saint-Laurent et le fjord du Saguenay
Les organismes aquatiques de l’estuaire du Saint-Laurent (ESL) et du fjord du Saguenay (FS) sont exposes a des melanges complexes de composes toxiques pouvant avoir un impact sur les populations, seuls ou en interaction avec d’autres facteurs environnementaux. L’objectif de cet article est de resumer l’information sur les effets biologiques des contaminants dans l’ESL et le FS obtenue a l’aide de poissons sentinelles. Trois etudes de cas demontrent la complementarite de l’information obtenue avec differentes especes. Les etudes sur l’anguille d’Amerique (Anguilla rostrata) montrent comment des poissons migrateurs peuvent etre un vecteur important de contamination pour les predateurs de haut niveau trophique de l’ESL. Des lesions preneoplasiques au foie chez les anguilles en migration, probablement liees a une exposition aux hydrocarbures aromatiques polycycliques (HAP) sur leurs aires de croissance, sont un exemple d’effet a long terme de contaminants qui ne persistent pas dans les tissus des poissons. La presence d’adduits a l’ADN a ete demontree chez les poulamons atlantiques (Microgadus tomcod) residents dans l’ESL et permet de comparer les niveaux d’exposition aux HAP genotoxiques entre l’ESL et d’autres estuaires de la cote Atlantique. Les etudes sur le poulamon ont aussi revele une interaction entre leur periode de jeune hivernal et les produits organiques persistants (POP) menant a une augmentation transitoire des concentrations de POP dans le foie et a une dysfonction hepatique. Des echantillonnages sur le terrain, couples a des expositions en cages in situ et a des experiences en laboratoire, ont permis de mettre evidence chez des plies canadiennes (Hipoglossoides platessoides) exposees a des sediments contamines de la baie des Anglais, des alterations immunitaires causant une augmentation de la susceptibilite aux maladies infectieuses. Les futures etudes devraient poursuivre le developpement de biomarqueurs pour differents groupes de contaminants et les utiliser chez des especes de poissons cles, a des stades sensibles de leur cycle de vie, en combinant differentes approches experimentales multistresseurs a des etudes de surveillance sur le terrain.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.