In situ measurement of salinity during seaward migration of Atlantic salmon post‑smolts using acoustic transmitters with data‑storage capabilities and conventional acoustic transmitters
Journal article, Peer reviewed
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Background: Development of miniature acoustic transmitters and data-storage tags has provided new insights into ecology of free-ranging aquatic animals. In this study, we used a data-storage-type and a conventional acoustic transmitter, both equipped with a salinity sensor, to measure the in situ salinity experienced by Atlantic salmon postsmolts during the first phase of their marine migration in a Norwegian fjord. The data-storage transmitter conveyed stored salinity data accumulated over a period of up to 27 h prior to moving within the range of a receiver, while the conventional transmitter conveyed only real-time salinity data. Five post-smolts tagged with a conventional transmitter were manually tracked from a boat, and 15 post-smolts tagged with the data-storage transmitter were monitored by six transects consisting of 29 stationary receivers deployed from the river and throughout the fjord. Results: All tagged post-smolts primarily showed rapid seaward movements. They occasionally stayed in water with salinity below 20.0 psu in the inner part of the fjord, most likely because they were swimming close to the surface, where the salinity was low due to freshwater supply from the river. In the outer fjord, where full-salinity sea water (26.0–32.0 psu) was recorded in the entire water column, half (3 of 6) of the recorded fish still experienced low salinities (<20.0 psu) for periods between 2.25 and 54 h. Conclusion: Both types of salinity transmitters provided data on ambient salinity of the post-smolts during the seaward movement. In the outer fjord, the post-smolts likely visited one or several river mouths. It is not known whether this behaviour is normal for Atlantic salmon post-smolts during migration, but it might be advantageous in terms of reducing infestation risk from salmon lice, which have low survival in low salinities. The data-storage transmitters provided data on the ambient salinity history of the tagged fish, even when the fish were outside the detection range of receivers. By using this type of transmitters, we were able to collect salinity data during a four times longer period than with conventional transmitters.