Wild and hatchery-reared brown trout, Salmo trutta, differ in smolt related characteristics during parr–smolt transformation

Abstract

Morphological changes during parr–smolt transformation are generally less apparent in hatchery-reared than wild salmonids. The aim of the present study was to investigate possible differences between wild and hatchery-reared brown trout in regard to physiological characteristics during smoltification. Plasma growth hormone levels, hypoosmoregulatory ability, gill Na +, K +-ATPase activity and condition factor were compared between wild and hatchery-reared fish from the same river stock, in two different streams on the Swedish coast. Plasma growth hormone levels were consistently higher in wild compared with hatchery-reared trout, and the growth hormone levels increased in wild fish from one of the streams after a 24 h seawater challenge test. At the time of parr–smolt transformation, there was a peak in gill Na +, K +-ATPase activity, which coincided with the lowest plasma sodium levels. Wild fish possessed consistently higher gill Na +, K +-ATPase activity and lower plasma sodium levels compared with hatchery-reared fish. The condition factor of wild fish decreased throughout the smoltification period, in both river strains, whereas the hatchery-reared fish had consistently high condition factor. It is concluded that the artificial environment of hatchery-reared anadromous brown trout can depress the natural parr–smolt transformation, and that this may adversely affect the success of seawater migration and long-term survival of the fish.