Recommendations on size and position of surgically and gastrically implanted electronic tags in European silver eel

Background: Information on European silver eel Anguilla anguilla anatomy was collected to gain information on limitations on size and placement of electronic tags. Findings: To reduce the eel’s ability to bite its own sutures, it may be an advantage to make surgical incisions close to the head, but this increases the risk of cutting the liver. Recommended placement of an incision was slightly further from the head than one-fourth of the fish’s body length (LT) to avoid damaging the liver. Long, flexible tags comprising various components can be adjusted to the narrow body cavity and undulating movements of eels. There was space for surgically implanting a 100 mm long tag (11 mm in diameter) in the body cavity of eels with LT ≥380 mm. During gastric tagging, tag length is limited by stomach length. Silver eels with LT 380 to 998 mm had stomach lengths of 47 to 185 mm, indicating that there was space for short gastric tags in the smallest eels, but that there was space for relatively long tags in larger eels. The distance from the snout to the start of the stomach constituted 15 to 23% of LT, indicating how far the transmitter should be inserted during tagging. Conclusion: This information aids the development of tags and tagging methods that consider the unique morphological and behavioral features of eels. Keywords: Intraperitoneal implantation, Intragastric insertion, Tagging, Transmitter, Telemetry, Tagging effectRecommendations on size and position of surgically and gastrically implanted electronic tags in European silver eelpublishedVersio

Positioning of aquatic animals based on time-of-arrival and random walk models using YAPS (Yet Another Positioning Solver)

Aquatic positional telemetry offers vast opportunities to study in vivo behaviour of wild animals, but there is room for improvement in the data quality provided by current procedures for estimating positions. Here we present a novel positioning method called YAPS (Yet Another Positioning Solver), involving Maximum Likelihood analysis of a state-space model applied directly to time of arrival (TOA) data in combination with a movement model. YAPS avoids the sequential positioning-filteringapproach applied in alternative tools by using all available data in a single model, and offers better accuracy and error control. Feasibility and performance of YAPS was rigorously tested in a simulation study and by applying YAPS to data from an acoustic transmitter towed in a receiver array. Performance was compared to an alternative positioning model and proprietary software. The simulation study and field test revealed that YAPS performance was better and more consistent than alternatives. We conclude that YAPS outperformed the compared alternative methods, and that YAPS constitute a vast improvement to currently available positioning software in acoustic telemetry. Additionally, in contrast to vendor-supplied solutions, YAPS is transparent, flexible and can easily be adapted and extended for further improvements or to meet study specific requirements such as three-dimensional positioning.publishedVersio

Development and testing of attachment methods for pop-up satellite archival transmitters in European eel

Background: Four methods for attaching pop-up satellite transmitters to European eel were tested in the laboratory by recording long-term tag retention, growth and survival; short-term behavioral responses; and physical damage from attachments. Results: All eels survived until they lost their tag, or until end of the six-month study. Specific growth rate did not differ between tagged fish and controls. Tag retention varied from 0% to 100% among attachment methods. A tagging method that uses the strength of the eel skin by attaching the tag to the skin at three points is recommended for ocean migration studies based on a long tag retention time, minimal behavioral reactions, negligible damage to the swimming muscle, and minimal physical damage both for fish retaining and losing the tag. Although tag retention was 50% over six months, those losing their tags still retained them for 114 to 134 days. Another method had higher tag retention (100%), but required the use of steel wires that moved upwards through the muscle over time. This method was regarded as less suitable because of a strong behavioral reaction in the first two days after tagging and damage to the swimming muscle. Results from 275 silver eels released on European coasts equipped with pop-up satellite transmitters or similarly sized pop-up data storage tags to study the ocean spawning migration indicated a large premature tag release. This was partly related to mechanical tag loss, but probably mainly to a high predation rate (>20% confirmed predations of eels with pop-up satellite transmitters). Mean time to premature tag release was 14 to 21 days (maximum nine months). Conclusions: Laboratory and field data showed that pop-up satellite transmitters attached to eels can remain attached for six to nine months, but that tag retention is a challenge. Hiding behavior in a structured habitat increased the risk of entanglement and tag loss. In ocean migration studies, consideration should be given to transportation and release off shore instead of in shallower areas where they are more likely to seek the seabed and hide in structured habitats. Behavioral reactions indicate that data recorded during the first two to three days after tagging may not reflect natural behavior.Development and testing of attachment methods for pop-up satellite archival transmitters in European eelpublishedVersio

Comparing dominance relationships and movement of native marble trout (Salmo marmoratus) and introduced rainbow trout (Oncorhynchus mykiss)

Behaviour observations of the endangered native marble trout (Salmo marmoratus,Cuvier, 1829) and introduced rainbow trout (Oncorhynchus mykiss, Walbaum, 1792)in the laboratory and in a tributary to the Idrijca River in Slovenia were combined tostudy the movements and dominance relationships between individuals of the twospecies in an open field test. Under laboratory conditions, no difference between thespecies was detected for neither time spent actively or distance moved. In speciespaired tests, rainbow trout initiated more aggressive behaviours towards marble troutthanvice versa, and rainbow trout were clearly the dominant individuals. After simul-taneous release in the river, marble trout immediately left the release area and spenttwice as long time as rainbow trout until they settled in an area of the river; there-fore, the release site was immediately occupied exclusively by rainbow trout. Thus,the dominant and aggressive behaviour of rainbow trout seen in the laboratorybefore release might have influenced marble trout's subsequent behaviour in theriver, by marble trout leaving the areas occupied by rainbow trout and moving tolocations further away from the release site. In the field, the marble trout occupiedsites individually, whereas rainbow aggregated at a few locations. Rainbow troutshowed higher movement activity in the morning compared to marble trout. Therewas a positive correlation between swimming speed in the laboratory and movementin the field for marble trout but not for rainbow trout. In conclusion, the results in thisstudy support the need to end stocking of rainbow trout in rivers with native marbletrout. To better understand the interaction between the species, and to develop effi-cient management plans to protect the native marble trout, reference behavioursshould first be understood, and future research in sites where the two species do not co-exist is needed. This is especially important for marble trout for which behaviouralresearch and data are lacking.acceptedVersio

An Invited Reply to: A Comment on: Migrating silver eels return from the sea to the river of origin after a false start (2021) by Tambets M et al

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Annual and diel activity cycles of a northern population of the large migratory cyprinid fish asp (Leuciscus aspius)

Little is known about the exact movement parameters of migratory cyprinids listed as conservation priorities. A northern population of predatory asp, listed on the Bern Convention Appendix III, was studied in a near natural lake–river ecosystem by tracking adult fsh tagged with acoustic transmitters for 32 months. Activity levels in terms of swimming speed, swimming distance and movement range were four times higher during the warmer part of the year (water temperatures>12–15 °C, April/May–September), which coincides with their main feeding period, than other times of the year. All fsh had an annual riverine movement range larger than 40 km (max 110 km). Asp activity was afected by light, habitat type and water discharge. For most of the year, activity levels, in terms of number of movements per time unit, were higher during dawn and dusk than during day and night. Under poor light conditions and low temperatures, activity was also relatively high during the day. Fish were more likely to swim upstream around sunrise or during the day than during other diel periods. Knowledge about highactivity periods, which may render the fsh vulnerable to fshing and other impacts, can be used to develop and evaluate fshing regulations. Large annual movement ranges highlight the need for extensive continuous river systems open for migration between essential habitats. This study emphasises the need for region-specifc research on the ecology and behaviour of fsh populations in order to facilitate protection of the populations in the face of negative human impacts.publishedVersio