Precision Micrometer Measurement of Mouth Gape of Larval Fish

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Abstract
A precision micrometer device is described which standardizes measurement of mouth gape of larval fish and provides a greater degree of accuracy and speed than the conventional manual method. We compared gape measurements of larval lake whitefish (Coregonus clupeaformis) and lake herring (Coregonus artedii) using the gape micrometer versus the manual method. The micrometer measurements revealed a greater increase in gape with body length and resulted in a greater proportion of the variance in gape being explained, indicating that the gape micrometer is more sensitive and accurate than the manual method. Coefficient of variation of gape measurements on 238 larval yellow perch (Perca flavescens) decreased with body size from 0.5–4.0% at 0.8–1.2 cm standard length to 0.2–0.5% at 3.0 cm. The device has the added advantage that it could be adapted to connect to a microcomputer for direct data capture.