Diagnosis of structural changes in fish gills—can biopsy replace necropsy?

Abstract

The diagnostic value of gill biopsy was evaluated in the attempt to reduce terminal sampling for gill monitoring. Comparisons were made between biopsy and necropsy of gill samples to observe if there were any differences between the two sampling strategies. Lesions were quantified using two methods: determination of prevalence (percentage of fish showing the lesion) or number of lesions per filament. For both quantification methods, the number of lesions was lower and variation was greater in biopsy samples. These differences were statistically significant except for large plaques quantified as number of lesions per filament. Spatial distribution of lesions on a gill arch was investigated to determine the significance of biopsy sampling sites. The gill was divided into nine areas and lesions were counted for each area. Lesions were found in higher prevalence at the top and bottom of the gill arch (not in the middle) and mostly at the proximal and middle part of the filament. The distal part of the filament (one third to two thirds of a whole filament) was taken for biopsy samples which resulted in underestimation of lesion prevalence and numbers of lesions per filament. The biopsy technique cannot replace terminal sampling without further research to determine size of the biopsy, biopsy collection site (on the gill arch) as well as minimum number of biopsied fish.