Most legitimate magazines and websites are just that; Legitimate. I have had similar experiences with agencies that work with or for a government and a lot of the times these people just don't know about image usage rights. However, that is not an excuse for them to do wrong. The picture is a nice one and I am sure they would like to continue using it for the time being, especially since they went to the trouble of giving a name. As was mentioned in an earlier thread, send an email to them stating that while you are happy your image is on the site you are curious as to how they received acess to your image as it was only published in a magazine once and see what they have to say. You may even want to include a very lo rez copyrighted image of the shark with the letter. The result will be twofold. They will see you truly did take the image, and it is copyrighted work. That will leave the decision maker on their side with a choice of what to do next. You will probably recieve a flurry of accolades for your work, numerous apologies and perhaps offered some compensation. Give them a week to respond. If you hear nothing, send the letter again with a FAIR invoice for the usage of the image on their site. Wait a few more days. If you get no response, send them an invoice attatched with ANY COPYRIGHT LAWS YOU CAN FIND. Works everytime!
I played cat and mouse with a magazine last year for about two months and the editor actually said he took the picture which was cleary mine. A standard letter with stated copyright laws, attached invoice and an invitation to resolve the matter quickly will get them hoppin'. Good luck, hope this helps