I hope you don't get them in large numbers! I live on a large river and thousands of cormorants have moved in. I don't know about fish damage but they roost in large numbers. Poo in large volumes and the ammonia is so strong you don't want to get near the Island. It also decimates all the plant life under them. Once the trees die they move to a new spot and start all over again.
We have an are locally called 'The Islands', as the name implies it is a string of several islands, from small to a 3 or 4 acres. They have roosted there starting on the smaller islands. Decimated those, erosion without the greenery started and a couple of small islands were lost. The larger islands suffered badly but they moved to the other ends and started it all over.