Avoiding turtle by-catch in the Mediterranean
The Mediterranean is a dangerous place for turtles with between 10 and 20 % of the population killed annually. One frequent cause of death is when turtles are caught on longline fishing hooks.
Fishermen typically cut the line without hauling the turtle on-board, but this turns out not to be the best approach. Satellite tracking of a group of loggerhead turtles released into the sea after being by-caught by Spanish longliners revealed around 40% of them died during the following three months.
The problem is not so much the hook as the line. Professor Lluís Cardona, member of the Research Group on Large Marine Vertebrates of the University of Barcelona, explains: “The turtle swallows the line, and it goes down the alimentary tract until the end comes out through the cloaca”.
“In the case of incidental by-catch turtles must be hauled on board, using a brail net, and the fishing line must be cut level with the mouth. That will reduce incidental mortality by half and the impact on population will be acceptable”, says Cardona.
As turtles have a long life span a small variance in mortality rates can have a significant impact on populations.
The Loggerhead turtle (Caretta caretta) is the most common turtle inhabiting Mediterranean grounds and one of the most threatened species around the world. In Spanish waters there are turtles of Atlantic and Eastern Mediterranean origin.
The Spanish longline fleet has been reported to by-catch around 10,000 turtles in the Mediterranean annually. However, more than 95% of them are still alive when longlines are collected.
This study recommends reducing turtle by-catch by weighting lines so they draw more deeply, not to use cephalopods as baits and to modify the design of fishing hooks.