Swim with the fishes: is tuna tourism just a bit of harmless holiday fun?

Campaigners are concerned at how the tourist treat could affect the species, and how it could become an event like whale watching or shark diving

A tall, slim fin slashes the Mediterranean's surface for a split second and the bait fish is gone, provoking a few gasps and some nervous laughs from the 40 or so snorkellers lined up to enter the open-water pen.

Most wear wetsuits and clutch masks and air tubes, a few are in swim shorts and goggles. Some have already had a beer or a glass of wine. The midday sun is out, The Trammps' Disco Inferno is playing from the catamaran's speakers and now people are climbing down a stepladder into a ring of netting that stretches 35 metres down into the sea near the Spanish port of L'Ametlla de Mar, about 80 miles from Barcelona.