I have been visiting Turneffe Island for a couple of years to fish and dive-great fun! But each year I come home with over 75 bites from large brown flies. These bites blister and itch like crazy. Not even 100% deet stops these blood sucking pests. I only seem to encounter these flies while fishing on the flats or in a boat not at the resort. What kind of flies are these and any repellant suggestions? HELP!!