I dont know if this is a thing but these fish seem to go on their side and look like a leaf ? Maybe a anti predation thing ? My fish knowledge is zero . But I have seen them in this position before and they do swim away.
This observation is for the fish at the left of the three fish in the first image.
Fotografías tomadas en las instalaciones de la FACIMAR junto con las técnicos Judith Gonzalez, Rebeca Sánchez Cardenas, Maria Maldonado y Juan Maldonado. Arrastre Juan Maldonado y personal de FACIMR
Laboratorio de biología, acuacultura, química e ictiología
Sighting and photos (c) westozpathfinder.
Field Notes - Little fish on the mud flats. Very good at blending in!