Feasibility study for a pilot program: catching and tagging harbour porpoises in the Netherlands

Summary

In this study the feasibility of tagging harbour porpoises in Dutch waters is evaluated, with the long-term objective to study their large-scale habitat use in the North Sea. Based on a detailed review of available information of tag types and catch methods, and discussions with experts experienced with tagging and handling porpoises it is concluded that tagging of porpoises in Dutch waters is feasible. The study provides recommendations on the catch methods, tag types and methods, potential sites for carrying out tagging, type of additional biological data to collect, as well as a preliminary protocol on how to tag harbour porpoises in Dutch waters. Harbour porpoises are too small to attach tags from a distance (for example by use of a pole or cross-bow), so the animals need to be caught in order to attach the tag. Therefore, different catching approaches were considered: passive and active methods. The active catching strategy ‘surface gillnet herding’ is considered the most feasible in Dutch waters. This method has been applied with success in Denmark and Greenland, and experience is available. It provides the most flexibility in both tag location and timing, maximizing chances for animal presence under suitable environmental conditions. Passive methods were also considered, but they have several drawbacks. As passive methods rely on animals being caught in a net by chance, they require constant monitoring and a tagging team that is on permanent stand-by. Due to the required involvement of experienced Danish researchers and a veterinarian, the project team is not flexible enough to use this method as a starting approach in this pilot project. In the Netherlands, there is currently only one pound net used in the inner Eastern Scheldt, and the probability of catching a harbour porpoises is very small. Catching effort could be increased by constructing more nets, but construction is costly, and once constructed, nets cannot easily be re-located. Because there is no single tag type that will address all research priorities, it is recommended to apply different tag types during the pilot project. It is recommended to start with three specific models: SPLASH10-336, SPOT-196J, and SPOT-F-398. This allows for comparison of configurations (one is mounted at the back, and two at the side of the dorsal fin), location accuracy (one has Fastloc® GPS in addition to Argos) and depth data (one of them does, the others do not collect this). The different tag types allow us to evaluate their relative value in contributing to the main research question on how porpoises use the North Sea. To gain experience in the catching method, it is recommended to follow a phased approach: first, ‘dry-runs’ with a ‘dummy’ porpoise; second, tagging attempts in the sheltered Eastern Scheldt where there is a continuous presence of porpoises. Thirdly, as soon as sufficient experience with the method has been gained, catching can be carried out in other more exposed sites with direct connection to the North Sea. This third phase will concentrate on locations in the Wadden Sea, e.g. near Den Oever, or south-east of Texel. A tagging protocol has been developed for the active catch tagging approach, which considers practical and safety considerations for the animals and personnel. The protocol can be further adapted based on experience from the initial dry-runs, feedback from ethical permitting agencies, and continuous evaluation of tagging attempts as the pilot project progresses.