i also went crystal lake for the mudsnapper (as a horde) and got it in about 200 casts.
auto loot wont affect your catch rate, nor will it auto accept the BoP confirmation, so be careful if using auto recast...
the sewer rat (despite not seeming it) has a low drop rate, and all of the fish for the one that didnt get away have extremely low drop rates
i would advise crystal lake as there are many people who have reported catching it within a hundred casts (down to as low as SIX!) because the mudsnapper has only 3 varients... compared to the many of others.. also the bay of storms is a trick card
although it has 3 diffrent fish to be caught (assumingly increasing drop rate) the drop rate is determined by weighted fish - their drop rates kinda marge so you have a 0.5% change or something of catching ANY weighted fish - as a result you're acutally lessening the chance you get of catching the one you want as there are more than 3 times as many non max weighted fish and their drop rates are higher than those of max weighted fish
Happy fishing