There's a lot to unpack here, especially if this happens within one generational cycle:

"Greater salinity resulted in adult fish displaying shallower bodies and longer jaws.

As a second example, Parsons et al. in a 2016 study fed two separate groups of young juvenile fish different diets. The two groups developed into adults with distinctly different head-jaw structures that were tailored to forage for the available food most effectively. Navon et al. in 2021 reported on a similar experiment that confirmed the Parsons et al. results. The investigators also demonstrated diet-induced adaptive changes to body shape and fin-ray number. The observed dissimilarity between the two diet groups mimicked the differences between distinct cichlid species in the wild."