You can make your own dataset by merging images from Imagenet(1307 images), SUN database(162 images) and collect some from internet.
Also, look into this paper: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.185.2615&rep=rep1&type=pdf
They used EcoGrid dataset (http://homepages.inf.ed.ac.uk/gnadaraj/ecoImages/) which has fish images. Particularly, see this link (http://fins.actwin.com/species/index.php) for same dataset.
Or, if you find a fish taxonomy , then you can download publicly available images for each fish class from internet and use them for your experiments.