Effects of the excessive organic enrichment of the Loire Estuary on the downstream migratory patterns of the amphihaline grey mullet Liza ramada (Pisces, Mugilidae)

Abstract : The thin-lipped grey mullet Liza ramada is an amphihaline species that spawns in the Bay of Biscay in winter. The Loire population enters the estuary in spring. Juveniles utilize the estuary as a nursery ground whereas adults migrate upstream to feed in freshwater areas. Downstream migration of the maturing adults takes place in late summer. During this period mass mortalities of grey mullet could occur in the LOire Estuary. High levels of organic matter are observed in the estuary in summer as a result of both river eutrophication and sewage or industrial discharges. Organic matter biodegradation is intense in the turbidity maximum zone of the estuary, where anoxic events can occur. Dissolved oxygen levels fluctuate with water turbidity, which depends on tidal current strength and on the erosion/sedimentation situation. The summertime anoxic conditions of the whole water column are closely related to spring-tidal mixing. During neaptides more suitable oxygen conditions are encountered and shoals of grey mullet can migrate seawards. A tagging experiment was conducted to estimate fish speed during the seaward migration. Field observations were compared with tank experiments on survival thresholds for turbidity and dissolved oxygen concentration. Our results support the hypothesis that the behaviour of mullet during its downstream migration fluctuates in relation to water quality. Anoxic conditions in the turbidity maximum zone represent a hydrological barder that can induce upstream retention or mass mortalities. Consequently, the Loire stock of grey mullet underwent 'water quality related' mass niortalities that might have a significant effect on commercial fishing.
Type de document :
Communication dans un congrès
Keith R. Dyer; Robert J. Orth. Changes in fluxes in estuaries: Implications from Science to Management, Sep 1992, Plymouth, United Kingdom. Olsen & Olsen, Fredensborg, Denmark, pp.349-356, 1994
Liste complète des métadonnées

https://hal.archives-ouvertes.fr/hal-01853805
Contributeur : Pierre-Guy Sauriau <>
Soumis le : dimanche 5 août 2018 - 09:03:24
Dernière modification le : mardi 5 février 2019 - 12:12:34

Identifiants

  • HAL Id : hal-01853805, version 1

Citation

Pierre-Guy Sauriau, Jean-Paul Robin, Jocelyne Marchand. Effects of the excessive organic enrichment of the Loire Estuary on the downstream migratory patterns of the amphihaline grey mullet Liza ramada (Pisces, Mugilidae). Keith R. Dyer; Robert J. Orth. Changes in fluxes in estuaries: Implications from Science to Management, Sep 1992, Plymouth, United Kingdom. Olsen & Olsen, Fredensborg, Denmark, pp.349-356, 1994. 〈hal-01853805〉

Partager

Métriques

Consultations de la notice

39