Identification of freeze-thaw artifact in fresh and decomposed black rockfish (Sebastes melanops) and steelhead trout (Oncorhynchus mykiss)

Identification of freeze-thaw artifact in fish can help to determine whether they have been harvested within the appropriate season and monitor adherence to fishing regulations. Recognition of freeze-specific changes will also prevent potential misinterpretation due to decomposition, disease, injury, or species variation. An initial survey using black rockfish (Sebastes melanops) identified which tissues reliably exhibit freeze artifact. Tissues were exposed to different treatments: immediate formalin fixation; refrigeration or storage at room temperature for 24, 48, or 72 hours; or freezing for 1, 8, or 28 days. Three fish underwent a combination of treatments. Tissue changes in each treatment group were compared macroscopically and microscopically. Macroscopic changes in frozen-thawed and never-frozen fish overlapped somewhat; however, microscopic findings of skeletal myocyte cavitation, lens liquefaction, and brain tissue fractures were consistent findings only in frozen-thawed tissues. A validation study was then done to establish the accuracy of microscopic analysis. Brain and paired ocular and skeletal muscle samples from 61 steelhead trout (Oncorhynchus mykiss) were fixed in formalin either fresh or after being frozen for 4 weeks. Weighted kappa values showed both high observer accuracy and interobserver agreement in the identification of freeze-thaw status. Based on these findings, microscopic changes in the skeletal muscle, eye, and brain are considered consistent and easily identifiable indicators of a previous freeze-thaw cycle and should not be confused with a pathologic process.