Incidental capture of Caretta caretta in trammel nets off the western coast of Sardinia (Italy): statistical models of capture abundance and immediate survival

1. Trammel nets are one important cause of sea turtle mortality resulting from incidental capture. This study presents findings over a 10-year period during which loggerhead turtle by-catch in trammel nets, set off the central west coast of Sardinia (Italy) in the summer months, was recorded by 17 fishing vessels. 2. Since the by-catch registered represented counts of a rare event, data from the 17 vessels were used in the zero-inflated Poisson (ZIP) model to determine the abundance of turtle by-catch in trammel nets of the entire fleet, while a binomial generalized linear model was used to assess the probability of immediate survival for sea turtles incidentally caught in this gear. 3. The ZIP model quantified in about 45% the probability that 0.6 turtles have been caught by each vessel using trammel nets during summer over the 10-year period. In addition, the model estimated a total of 916 by-catch if the entire small-scale fleet was to use trammel nets in the study area in the summers between 1992 and 2001, with a direct mortality rate of 69%. 4. The probability of immediate survival for sea turtles caught in trammel nets seems to be directly related to the size of the specimens caught; however, this relationship is plausible only when incidental capture in the net occurs shortly before gear retrieval. 5. The study area represents a region where the abundance of sea turtle by-catch per vessel in trammel nets was much higher in July and August than in June and could represent a potential hot spot for the presence of juvenile loggerhead turtles, with high levels of interactions between the species and this fishing gear. Further research should better characterize the incidents of by-catch and assess potential innovative solutions that allow small-scale fisheries to coexist alongside sea turtles. Copyright (C) 2010 John Wiley & Sons, Ltd.