Preheat oven to 350˚ F.
Season catfish fillets with salt. Lightly spray a small baking dish with vegetable oil. Place fillets in dish, and bake for 15 to 20 minutes or until done. Let cool, and flake.
Peel and chop potatoes. In a medium boiler, cover potatoes with water, and add a pinch of salt. Bring to a boil. Lower heat, and simmer until potatoes are soft. Drain.
Mash potatoes in a large bowl, and set aside to cool. When cool, add garlic, egg and tarragon; gently fold in catfish flakes.
Shape into 3-inch cakes. Dip in beaten egg, then in breadcrumbs. Chill cakes in the refrigerator for 30 to 60 minutes to firm them.
Heat olive oil in a skillet over medium-high heat. Fry cakes until golden brown. Place on paper towel to drain.
Top with a teaspoon of lime mayonnaise, and garnish with a sprig of fresh tarragon or cilantro.
To make lime mayonnaise, combine all ingredients and chill.