Seasonal variations of thyroid hormone levels in wild fish

Variations in plasma thyroxine (T4) and triiodothyronine (T3) contents of pike, pikeperch, perch, bream and chub from a large reservoir (Pareloup, South of France) were investigated before and during the progressive emptying of the reservoir. Fish were caught by gill nets, except at the last time of emptying when controls were collected with landing nets. Both iodothyronine levels showed desynchronized seasonal cycles resulting in higher T3/T4 ratios in fish stressed by gill-netting in early summer. The aforementioned results principally emphasized the prevailing influence of the technique of catching on eliciting stress responses and subsequent thyroidal impairment in all five species. However, the much higher amounts of T4 and/or T3 observed in specimens caught in the later stages of draining gave evidence to suggest that the progressive alteration in habitat might also have indirectly affected the thyroidal status of these fish, this latter result leading to the consideration of fish plasma thyroid hormones as possible endocrine biomarkers of chronic perturbation.