Comparative histological and immunohistochemical studies of rainbow trout under ice slurry and electric stunning methods

Abstract

Humane slaughter methods are essential to ensure fish welfare and preserve product quality. This study compared the macroscopic, histopathological, and immunohistochemical effects on the skin and white muscle of rainbow trout (Oncorhynchus mykiss; n = 30) subjected to three stunning protocols: immersion in ice slurry for 32 min (S1); dry electric stunning at 200 mA for 2 s followed by immersion in ice slurry for 32 min (S2); and a two-step dry electric stunning protocol (400 mA for 0.5 s followed by 200 mA for 1.5 s), also followed by ice slurry immersion for 32 min (S3). No macroscopical lesions were observed in any group in the study. Microscopically, all methods induced tissue alterations whose severity followed a graded pattern (S3 > S2 > S1). Moreover, electric stunning produced more extensive dose-dependent tissue damage. Main histopathological findings included epidermal coagulative necrosis, dermal collagen degeneration with edema and fibrocyte necrosis, hypodermal fat necrosis, vascular and nerve damage, and multifocal necrosis of white muscle fibers. Immunohistochemistry revealed progressive disruption of E-cadherin in the epidermis, disorganization and loss of fibronectin at the dermal–epidermal junction, and an increased Hsp70 detection in muscle fibers consistent with cellular stress. These molecular changes paralleled the histological severity pattern, being most pronounced after high-intensity electrical stunning. Collectively, the results indicate that ice slurry causes mild to moderate structural injury, whereas electric stunning produces dose-dependent and more extensive tissue damage. Optimizing electric stunning parameters is therefore essential to minimize tissue alterations associated with different stunning conditions in aquaculture.