Avoidance of oxygen-poor zones by fish in the Elbe River

Abstract

For three selected weeks in July, August, and October 1985, catches from anchor net vessels at ten stations along the tidal Elbe River (northern Germany) were analyzed and related to oxygen conditions in the river. The oxygen concentrations decreased from an average of 6.6 ml/L in the estuary to an average of 1.4 mg/L upstream. Fish were found to be concentrated downstream from the low-oxygen zone and at the entrance to a shallow side arm of the main stream. Highest catches of eel were made where oxygen levels were in the range of 1.2-3 mg/L, highest catches of flounder at 3-4 mg/L, and hi hest catches of juvenile and adult smelt in areas with more than 5 mg02/L. Such a situation enables fisiermen to make extraordinarily high catches. It is concluded that despite high levels of toxicants, the tidal Elbe still offers living conditions adequate for the mass of fish species as long as there is a sufficient supply of oxygen. Zusammenfassung Meidung von Sauerstoffmangelzonen dwcb ElbfiscbeWahrend dreier Wochen im Juli, August und Oktober 1985 wurden die Fange eines Hamenkutters auf zehn Stationen entlang der Tideelbe analysiert und in Relation zum Sauerstoffgehalt des Wassers gesetzt. Der Sauerstoffgehalt ging von einem Minelwert von 6,6 mg/L im Astuar auf ein Mittel von 1,4 mg/L nahe Hamburg zuriick. Die Fische konzentrierten sich unterhalb der Sauerstoffman elzone und am Einfang eines Nebenarmes des Hauptstroms. Die grogten Aalfange wurden im Bereict von 1,2-3 mg/L, ie grogten Flunderfange im Bereich 3-4 mg/L und die grogten Fange sowohl juveniler als auch adulter Stinte im Bereich von mehr als 5 m /L gemacht. Dieser Umstand ermoglicht Fischern zeitweise ungewohnlich groge Fange. Es wird gefofgert, dag trotz hoher Schadstoffbelastung die Tideelbe den Massenfischarten dort ausreichende Lehensbedingungen bietet, wo der Sauerstoffgehalt des Wassers ausreichend ist. RCsumC Fuite despoissons des zonespauvres en oxygPne dans I'ElbePendant trois semaines, choisies en juillet, aoGt et octobre, les prises provenant de filets ancrb en dix diffbrentes places le long de I'Elbe, fleuve i marCe de I'Allemagne du Nord, ont CtC analysies et corrClCes avec la teneur en oxygene de I'eau du fleuve. Les concentrations en oxygine ont d k r u de 6,6 g/L en moyenne dans I'estuaire i une moyenne de 1,4 mg/L en amont pr2s de Hambourg. Les poissons se concentraient en aval de la zone auvre en oxygine et i l'entr6e d'un bras sCparC du flux principal. La plus grosse prise d'anguilles a CtC &te dans une zone 06 la concentration en oxy tne atteignait 1,2-3 mg/ lu'adultes, dans une zone 8 5 mg/L. Ces circonstances ont permis aux picheurs de faire parfois 'exceptionelles grosses prises. I1 en est conclu que la partie de 1'Elbe subissant le cows des marCes, malgrC un haut niveau de produits toxiques, offre des conditions de vie adiquates pour la plupart des poissons quand le taux d'oxygkation de I'eau est suffisant. L, la plus grosse prise de flets entre 3 et 4 yg/L et la plus grosse prise d'Cper P ans, aussi bien juveniles

Poor water quality constrains the distribution and movements of twaite shad Alosa fallax fallax (Lacépède, 1803) in the watershed of river Scheldt

“…The modelled logistic response to dissolved oxygen reported in this article suggests that adult shad become seriously stressed at DO less than 3 mg l -1 , a concentration at which the capture probability is less than 10%. This parallels the situation in the Elbe estuary (Germany) where twaite shad were absent in fish samples taken at sites where DO felt below 3 mg l -1 (Möller & Scholz, 1991). Stier & Crance (1985) used 5 mg l -1 DO as threshold level for adult American shad Alosa sapidissima.…”

Environmental factors as forces structuring the fish community of the Elbe Estuary

“…According to Pomfret et al (1991) presence of smelt indicates improving water quality, so this, could be one reason for recent increasing abundances of smelt in the Elbe. Generally, an early onset of oxygen depletion can destroy an entire smelt generation (Moller & Scholz, 1991) or even entire fish populations in the Elbe estuary (Kohler, 1981). Growth rate of smelt larvae was reduced at oxygen concentrations <3 mg 1-' (Sepulveda, 1994).…”

Infection of eel Anguilla anguilla from the River Elbe estuary with two nematodes, Anguillicola crassus and Pseudoterranova decipiens

“…Summer eel fishing during former decades was carried out with baited fyke nets. This method is no longer in use since the regular occurrence of oxygen-poor zones every summer enables fishermen to use anchor nets to catch eel concentrating in very high densities in front of the oxygen-poor zone (Moller & Scholz 1991).…”