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