Surface longline targeting mainly swordfish is considered a major threat for loggerhead turtle populationsin the western Mediterranean Sea. For this reason, it is necessary to analyse the relationship betweenthe characteristics of longline sets and loggerhead turtle by-catch. The objective of this study is to look fordifferential spatial distribution of loggerhead turtle and swordfish captures along the longline to gaininsights that might be useful to diminish unintended, incidental catch of turtles without affecting thecaptures of the target species.

Longlines are divided into transects separated by two consecutives buoys. During the 1999 fishing periodof the traditional (without roller) Spanish surface drifting longline fleet (August to November), 59 technicallyhomogeneous fishing operations, composed of 23 sections each, were observed. Chi-square test wasused to compare loggerhead turtle and swordfish distributions among transects with those expected atrandom. Significant differences were obtained only for loggerhead turtles, with 93% of the specimenscaught in the second half of the longline transects, which were retrieved from the sea during daytime.Significant differences were not found for the swordfish along the whole longline or for loggerhead turtlecaptures retrieved during daytime. We conclude that swordfish captures are independent of retrieval timewhereas the incidental catch of loggerhead turtles occurs mainly during daytime, probably becauseloggerhead turtles use vision to locate baits.