Crabs are more profitable than fish, but you can't always sell them, sometimes i have caught over a million. (lol)
The start of the game is all about getting enough imps to always have your buildings maximally staffed. First focus on wood/water/stone production (first 5 small, then 5 medium), and sell excess wood/water to other players, then you'll make a lot more gold than from the troll (i wish i did that, but i was stubborn and impatient). Stone is tough to sell, so that goes directly to troll. So you make gold to buy imps to make more gold to buy more imps to... etc etc. This is your first trial of patience
Other good moneymakers for later on are crabs, ore, leather. If possible, try to wait until the troll gives the highest possible price, the difference can really add up. Sometimes it helps to give the troll a good cursing.