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

Temporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratios

Given the limited information on prey use during the marine residency period for Atlantic salmon, scales were collected from salmon at return to the River Namsen (Norway) for spawning after 1 year at sea, and scale material from the first and second summer marine feeding periods was analysed using stable isotope methods to understand dynamics of their trophic ecology. As the salmon increased in size from the first to second summer, they reduced their feeding niche and specialised more (narrowed the δ13C range) and increased their dependency on higher tropic level (δ15N) prey, likely fish. Changes in δ13C indicated a consistent pattern of movement towards the north and west between summer feeding periods. Hence, salmon during their first year at sea may have a migration route roughly resembling that of previous spawners, as inferred from earlier tagging studies. Feeding conditions and nutrient composition during the last summer at sea, i.e. in the months before returning to the river for spawning, impacted final body size and within-season timing of return. Fish undergoing the largest trophic niche shift (δ13C and δ15N combined) between summer feeding periods, returned earliest. The earliest returning fish had the fastest specific growth rates at sea. Hence, salmon encountering abundant high-quality fish food during the marine migration, particularly during the last months, may reach a size and energetic state whereby it is better to return early to a safer environment in freshwater than risk being eaten by a big predator at sea. Both trophic status (δ15N), resource use (δ13C) and growth rates were significantly correlated between feeding periods. Nutrient composition during the first summer at sea did not impact the fish body length after the following winter, but growth conditions during the first summer evidenced carry-over effects from the first to the second summer of feeding. diet, energetics, feeding, growth, marine migration, Salmo salar Life history ecologyTemporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratiospublishedVersio

Marine food consumption by adult Atlantic salmon and energetic impacts of increased ocean temperatures caused by climate change

Atlantic salmon Salmo salar are experiencing widespread population declines, and reductions in growth and survival in the marine environment are contributing factors. Our aims were to estimate marine food consumption of adult salmon and to determine how energetics would be directly afected by the increased ocean temperatures associated with climate change. We tagged previous spawners on outward migration (body size 76–119 cm) with archival tags and used a bioenergetic model to combine in situ temperature recordings with individual data on body growth. Average energy consumption was estimated to be 331–813 kJ per day, which is equivalent to 5–11 prey fsh with an average body mass of ca. 15 g. Energy content of prey was the most important factor determining food consumption required to maintain growth. Conversely, the increases in ocean temperatures expected with climate change were predicted to have limited physiological efects on energy budgets and limited impact on the food consumption needed to maintain growth. We conclude that climatic warming will impact Atlantic salmon primarily through changes in prey availability and ecosystem structure rather than the direct efects of temperature on physiological performance

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

Impacts of a Changing Ecosystem on the Feeding and Feeding Conditions for Atlantic Salmon During the First Months at Sea

During the last decades, many wild Atlantic salmon populations have declined dramatically. One hypothesis for an observed reduction in salmon marine growth and survival is reduced abundance of prey. However, the effect of spatial and temporal variation in marine prey abundance on post-smolt feeding conditions is poorly understood. Here we use stomach content data from 2572 salmon postsmolts sampled during 25 years in the Northeast Atlantic Ocean to examine spatial and temporal changes in diet and stomach fullness. Sandeel larvae west of Scotland and Ireland and in the northern North Sea, herring larvae in the eastern part of the Norwegian Sea, and amphipods in the western part of the Norwegian Sea were particularly important prey species. There was a reduction of fish larvae in the postsmolt stomachs over a large geographic area when comparing the period 1995–2004 to 2008–2019. This may be result of a bottom-up driven process, as increasing extent of Arctic Water masses and zooplankton abundance were positively correlated with postsmolt stomach fullness in the Norwegian Sea. Furthermore, the interspecific competition for fish larvae between post-smolts and mackerel may have increased with a larger mackerel stock expanding the feeding migrations northwards since 2007. Norwegian Sea, fish larvae, zooplankton, mackerel, oceanography, dietpublishedVersio