Variation in the Numbers of Shags Phalacrocorax aristotelis and Common Seals Phoca vitulina near the Mouth of an Atlantic Salmon Salmo salar River at the Time of the Smolt Run

1. In the 3 years 1987-89 the bulk of the Atlantic salmon smolt run in the Lussa River occurred in late April and early May. Peak daily downstream displacement occurred in the early evening as light levels were falling. 2. The numbers of shags and common seals counted in the vicinity of the river mo...

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Bibliographic Details
Published in:The Journal of animal ecology 1993-01, Vol.62 (3), p.565-576
Main Authors: Greenstreet, S.P.R., Morgan, R.I.G., Barnett, S., Redhead, P.
Format: Article
Language:English
Subjects:
Agnatha. Pisces
Animal and plant ecology
Animal, plant and microbial ecology
Animals
Autoecology
Biological and medical sciences
Fisheries science
Fundamental and applied biological sciences. Psychology
Marine fishes
Ocean fisheries
Predation
Predators
River deltas
Salmo salar
Salmon
Salmon smolt
Seals
Seas
Vertebrata
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Summary:1. In the 3 years 1987-89 the bulk of the Atlantic salmon smolt run in the Lussa River occurred in late April and early May. Peak daily downstream displacement occurred in the early evening as light levels were falling. 2. The numbers of shags and common seals counted in the vicinity of the river mouth were strongly correlated with the numbers of migrating smolts. Peak daily shag numbers occurred around midday, confirming their preference for daylight feeding. Peak daily seal numbers coincided with low tide when they were observed hauled out. Aerial surveys confirmed that at the time of the smolt run, shags were concentrating on the area off the Lussa River. 3. The size of smolts leaving the river was such that both shags and seals would be capable of preying on them. 4. Sub-aqua surveys indicated that the smolt run coincided with a plankton bloom and an increase in the numbers of several marine fish and invertebrate species in the area. It is suggested that shags and seals were not responding to the smolt run. Smolts migrated down the Lussa to arrive at the sea at the time of a local burst of marine production which also attracted other fish into the area which in turn attracted the predators. Whatever the causal relationship, smolts arrived in the sea at a time when the number of predators in the area was high.
ISSN:0021-8790
1365-2656
DOI:10.2307/5205