In situ measurement of salinity during seaward migration of Atlantic salmon post-smolts using acoustic transmitters with data-storage capabilities and conventional acoustic transmitters

Abstract

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.

An acoustic sensor transmitter for in situ assessment of water quality and fish behaviour during chemical treatment of a parasite-infected river system: tag design and practical use

“…1) requirements. This demonstrates the flexibility and capacity provided by contemporary digital electronic fish tags to carry out on-board processing and analysis of sensor signals prior to transmission over the severely bandwidth-limited acoustic channel, a concept that has been utilized successfully in several subsequent telemetry studies [17,[25][26][27]. Even if the recorded sensor data must undergo substantial compression and loss of detail before acoustic transmission, the approach features an advantage over archival tags in that no recapture of the tag is needed to recover the data [25].…”

“…Similar to the findings of Davidsen et al [26], there were no avoidance behaviour in the Atlantic salmon and anadromous brown trout tagged in the study presented here, but in contrast no mortality was recorded which is an advantage of the Al treatment method. In another study, Mitamura et al [27] used a tag based on the design presented here to measure salinity during seaward migration of Atlantic salmon in a fjord in Norway. They utilized both the conventional tracking method and the storage feature of the tag to record the salinity, and found that the statistical data stored on the tag both provided a longer record of data and insight into fish using low salinity areas also in the outer fjord which would be difficult to obtain with a conventional tag and tracking setup.…”

Surface environment modification in Atlantic salmon sea-cages: effects on amoebic gill disease, salmon lice, growth and welfare

“…High salmon lice infestations of wild sea trout Salmo trutta are associated with entry into shallower brackish water or rivers, possibly for self-treatment against lice (Gjelland et al 2014). Once completing their seaward out-migration, wild post-smolt Atlantic salmon also use less saline environments and this may also be a reaction to new salmon lice recruits (Mitamura et al 2017). Despite the potential for Atlantic salmon to self-treat against salmon lice by moving from seawater to freshwater or low salinity environments, this did not occur under the conditions created in FW snorkel cages within the current trial.…”

A Persistent Sea-Going Platform for Robotic Fish Telemetry Using a Wave-Propelled USV: Technical Solution and Proof-of-Concept

“…The acoustic receiver thus comprises the inseparable and essential second half of a telemetry system as its performance and placement, together with the transmitter and the channel characteristics, jointly determine the probability of making detections. Although active (mobile) tracking of fish is regularly used and serves as a viable option in many scenarios (Mitamura et al, 2017), the advent of cost-effective automatic monitoring receivers has made it feasible to deploy entire systems of moored receivers (often referred to as passive acoustic telemetry) that enable continuous year-round monitoring of aquatic habitats over a wide span of geographical scopes (Hussey et al, 2015). Such stationary receiver systems are typically organised as clusters of transects (Jackson, 2011;Chaput et al, 2018) or arrays (Simpfendorfer et al, 2002;Hedger et al, 2008;Kraus et al, 2018), or in less regular structures, e.g., focusing on monitoring of specific landmarks or bottlenecks where the fish are expected to dwell or pass (Urke et al, 2013).…”