The results in this study confirmed that a major challenge when attaching PSATs to European eel is to achieve a suitable tagging method that reduces tag loss and minimizes the physical damage to the fish. On a positive note, the results also demonstrated that it is possible to tag eels with external tags that are retained for six months. However, tag retention may be lower in ocean migration field studies than in laboratory studies, because the drag may be larger in migrating eels than in fish kept in tanks.
Eels are known to bury and hide in mud, gravel, vegetation and among stones and other structures both when waiting for prey to come near during feeding, or during periods of inactivity [2, 10]. Results from keeping eels in a structured habitat in this study indicated that such hiding behavior increased the chance of losing external tags. A similar conclusion was reached in a tagging-effect study of the eel-like fish Gadopsis bispinosus
, which inhabits interstitial spaces among cobbles and boulders. However, in this study, most tag losses occurred with fish tagged using the Jellyman and Tsukamoto and the Økland-Westerberg anchor methods, which did not have optimal mechanical implementation. Both methods can be improved by using better fit devices. The main conclusion is that external attachment seems robust and can withstand much mechanical abuse during the first few weeks after tagging, but that later losses occur due to rejection of the attachment. In ocean migration studies, tagged eels may be transported and released off shore instead of in shallow areas, or near shore, where it is more likely that they will seek the seabed and hide and move in structured habitats. Alternatively, it could be argued that a recovery period on the seabed before starting migration may be beneficial and decrease the predation risk.
In terms of tag retention, the Økland method showed the best result, with no tags lost during the six-month study period. A similar result was obtained in another laboratory study when the same tagging method was used, with 100% tag retention in 15 eels over a five-month period (FØ and HW, unpublished results). However, eels tagged with the Økland method showed the strongest behavioral reaction to being tagged. Eels tagged with this method also had wires attached through the swimming muscle that moved up through the muscle over time. European eels tagged with the Økland method in a field study showed a slower progression than would be expected if they were to reach the Sargasso Sea in time for the spawning period . It is not known whether this was partly related to the fact that they had wires attached through the swimming muscle. As long as the effect of introducing wires through the swimming muscle is not known, we recommend using the Westerberg method instead of the Økland method, although tag retention was poorer. However, it should be noted that with the Westerberg method, eels that eventually lost their tags still retained their tags as long as 114 to 134 days. From a fish welfare perspective, the Westerberg method seemed more favorable. Eels tagged with this method showed the least behavioral reaction to being tagged. Further, the tag was attached only in the skin, and did not interfere with the swimming muscle to the same extent as the Økland method. Where the tag attachments were lost, the skin healed completely. Hence, both for fish retaining and those losing the tag, the injuries at the attachment sites were small when using the Westerberg method. Where the Økland method is still used, we recommend using silicone pads with rounded shapes under the plastic plate at the side of the fish to minimize skin erosion. If plastic plates are used on their own, it is important that they are rounded and have no sharp structures that may increase skin erosion.
The Jellyman and Tsukamoto method may potentially be better than suggested by the results in this study, as it seemed the wire locks used were not of good quality. However, field studies using this method in longfin eel (Anguilla dieffenbachia) have also shown premature loss of transmitters, with tags being retained from only a few days up to 60 to 90 days for most fish, with a maximum attachment period of 161 days [11–13]. The main problem with this method was keeping the plates aligned flat on the side of the eels without being dragged upwards and digging into the muscle. The advantage with this method is that the wire can run under the skin and not through the swimming muscle. We recommend further refining, testing and development of the method before using it in large-scale field studies.
The Økland-Westerberg anchor implant method was the least invasive of the methods tested, but also the one with the lowest tag retention. The weakness of the method was the loose attachment point between the wire from the tag and the anchor in the fish, which caused sideways movements of the anchor until the wire from the tag slipped off the anchor bar. It should be possible to improve this design, but until that is done and tested, we would not recommend using the method for long-term field studies or in structured environments.
During the experiment, body mass of the individuals was only slightly reduced, and specific growth rate did not differ between tagged and control fish. However, if lipid were replaced by water, loss of body lipid may not be reflected as reduced body mass. Hence, it cannot be fully excluded that carrying a tag increased energy expenditure in the experimental fish. Measurement of body lipid content in live fish may be included in future tagging-effect studies.
The behavioral reactions observed during the first few days after tagging may indicate that data recorded during the first days in field studies of European eel may not reflect natural behavior, and that data from this period should be excluded from analyses. We recommend that field data be examined carefully in the first days of tracking and compared with later behavior to identify whether there are any appreciable differences. The behavior recorded with eels rolling around their own body axis, ‘panic’ reactions, backward swimming, and exploring the tag with the tail or biting it were believed to be an effect of tagging, as these behaviors were never seen in the control group or other untagged fish. Further, these behaviors disappeared progressively during the days following tagging. We are not aware of tagging-effect studies in other fish species that have reported such strong behavioral reactions to tagging as seen in these externally tagged European eel. On the contrary, European eels tagged with surgically implanted transponder tags showed a reduced activity level after tagging compared with control fish . There are also field studies of other fish species that have reported a behavioral effect of handling and tagging by either decreased or increased activity levels during the few days after tagging [15–17]. Further, European eel biting at tag surgery sites has previously been observed .
The spectacular and frequent vertical migrations observed during the ocean migration of both European eel and longfin eel that cover several hundred meters depth [1, 13] are not likely to be tagging effects on behavior, but are most likely their natural migration behavior. This is because the vertical migration behavior was consistent over extended periods of time during the ocean migration and not the type of short-term phenomenon as the behavioral reactions to tagging observed in the present study. Further, the systematic diel movements recorded during the ocean migration is not expected to be a result of a tagging effect. Finally, a similar vertical behavior was recorded for European eel tagged with much smaller and surgically implanted transmitters in the body cavity .