Suggett, D. J. and Houghton, J. D. R. (1998) Possible link between sea turtle bycatch and flipper tagging in Greece. Marine Turtle Newsletter 81: 10-11
Links Between Sea Turtle By-Catch and Tagging around Kefalonia, Greece
Tagging is an essential part of programmes assessing the dynamics of sea turtle populations. On Kefalonia, a saturation tagging programme has been in place since 1984. Plastic roto-tags are applied to each of the rear flippers to minimise errors resulting from tag loss. To date, 139 female Caretta caretta have been tagged. Tag loss over the 14-year period has been calculated at ~8% yr-1 which is analogous to losses reported by Alvarado et al (1991) for black turtles.
In terms of science, this practise may be deemed relatively successful. It has allowed us to accurately assess the return rate of individual females and to gain a reasonable idea of population fluxes. Additionally, the use of these tags has given us an insight into sea turtle movements outside of the nesting season via sightings by fishing vessels. Unfortunately, such sightings are often the result of entanglement. This has led to some concern amongst fisherman and researchers alike, as recent evidence suggests that the majority of sea turtles caught in nets have been tagged. This finding may be indicative of a successful saturation tagging programme, or alternatively may suggest that turtles bearing plastic flipper tags are more likely to become entangled in nets as demonstrated by Nichols and Seminoff (1998). This claim is further substantiated by recent observations of sea turtles coming ashore with remnants of fishing net tangled around their tags. In terms of conservation, this poses two major problems. The first, and most obvious, is the potential loss of reproductively active females through drowning. The second is the financial burden inflicted on the fishing communities through damage to their nets.
The most commonly used nets around Kefalonia are comprised of three layers of nylon square mesh (32mm). Fishing vessels in the area typically deploy around 1200m of fixed net, although this figure can be as high as 3000m for larger boats. The entrapment of loggerheads in fixed netting around Kefalonia may also result from their opportunistic feeding behaviour with numerous observations being made of turtles feeding on fish previously caught in nets. This may suggest that turtles around Kefalonia may be attracted to fixed netting to feed on entrapped fish. Alternatively, this finding may be purely coincidental with the turtles and fishermen both selecting the same site. It is interesting to note that such sightings around the south of Kefalonia more common in the winter months. Although, this finding is more than likely a function of the increased fishing intensity during this period that to any behavioural trait, it does suggest that at least some of the turtles tagged on Kefalonia display year round residency.
So to conclude, the establishment of communication between the fishing community and researchers on Kefalonia has led to some interesting findings. It has indicated that loggerheads will actively prey upon caught fish and that a certain number of females may display a degree of residency. More worryingly, however, is the increasing body of evidence that links plastic roto-tags to sea turtle by-catch. As such, what is required on Kefalonia is a tagging method that allows individual turtles to be identified by researchers and fishermen alike, but does not increase the chance of entanglement. The discussion into the most appropriate tag design (and material) continues, however, it is clear that alternatives to those currently in use must be sought.
Alvarado J., Figuero, A., Delgado, C., Sanchez M. T., (1991). Retention rate differences between plastic and metal tags on the black turtles of Michoaean, p.134 in Salmon M. & J. Wyneken (Compilers). Proceedings of the Eleventh Annual Workshop on Sea Turtle Biology and Conservation. NOAA Technical Memorandum NMFSSETSC-302, 195pp.
Nichols, W. J., Seminoff, J. A. (1993) Plastic "Rototags" may be linked to sea turtle by-catch. Marine Turtle Newsletter 79, pp20-21.