the salt draws the moisture out. the only way I ever smoked fish was to first soak the fish in a brine overnight or at least a few hours. there are lots of all kinds of recipes but the basics are a liquid, water or juice, apple juice is popular, even maybe coka cola, can even be coconut milk or pineapple juice if you want to expierment and mix in sugar and salt, for sugar, brown sugar is most popular I like raw sugar, brown sugar is usually white sugar with molasses added back in and raw sugar is usually like it says sugar with the molasses still in it, something salty, salt, sea salt, pink Himalayan but soy sauce probably adds the most pleasant flavor, for me anyway..
soak overnight, drain and if you like sprinkle with cracked pepper and put in smoker