It was a big one (about 40 cm in length) and yet I nearly missed it!
Great mimicry skills on a coral covered by boring sponges (and note, boring sponges are not boring at all - they just perforate calcareous substrates and grow inside them ;-)
Pic taken in the context of project Green Bubbles, H2020 Marie Sklodowska-Curie grant agreement No 643712.
I suspect they stock the reservoir and this one got up stream when the water was higher, because I seriously doubt there's enough water or oxygen for trout later in the year.
Found stranded in a sand depression.
After taking photos and we had just finished fish and chips we gloved up and took it back to the ocean, but as it was exhausted the waves brought it back in.
So we filled a container with water and put it's head and gills into the water and walked along until we came to the rocks and put it in a tidal gut. It had recovered by then and happily swam away!
Not sure if I have the ID right :)