No, "fish change" are not so usual , and it doesn't depend on you/your fishtrap/your actions : I think it's a server-side change.
Last one I experienced was mid-April : Raging bullhead disappeared from one of my fishing area , and appears in another one (someone else experienced the same thing , with the same kind of fish at the same period and said it
here ). Most of the others fishes-species didn't change at that moment (still got the same Catfish , Red Herring , Pickerel) , except maybe a few more Halibut but I'm not sure.
I don't think there's salt-lake in Salem : the east-coast is the only area where I was able to find some fish (Lobsters , Blueback Tuna , Seabass , Halibut ...)
@M13tz : Lobster spawn in very few areas of the salt-area . If you're catching sea-fishes (tuna , seabass, codfish ...) then you're doing everything good , just not in the good place . Try (again) a few tiles away and you'll find it ... one day.
Bjebr