Impacts of external and surgery-based tagging techniques on small northern pike under field conditions

In Northern Pike Esox lucius negligible effects of external tags on growth and survival have commonly been reported, but no studies exist for age-0 fish. We tested the impacts of fin clipping, PIT tags, and three external tags (T-bar anchor, opercular, and streamer tags) on age-0 Northern Pike in a pond. We also examined the impacts of surgically implanted transmitters on small adult Northern Pike in a natural lake. The loss of PIT tags was lowest among all tags and close to 0%. Of the external tags, T-bar anchor tags performed best. Loss of these tags was initially low (on average 5.7% after 195 d) increasing to about 20.0% by 520 d post tagging. Tagging-induced mortality ofa few percent was present in all tags tested within the first 195 d post tagging in the pond environment but not present afterwards. The PIT tags, fin clips, and external tags did not affect the growth or condition of surviving fish. Surgically implanted transmitters reduced the growth of small adult Northern Pike with total lengths <480 mm, but no effects were found for larger individuals. We conclude that PIT tagging is the most suitable marking technique for age-0 Northern Pike. If external visibility is required, T-bar anchor tags are also useful, as long as tagginginduced mortality and tag loss rates are accounted for at the analysis stage. Our study raises a cautionary note about surgically implanted transmitters in small adult Northern Pike, as they may negatively affect behavior and fitness.