MEPS 538:213-219 (2015)  -  DOI: https://doi.org/10.3354/meps11476

ABSTRACT: Estimating fish sizes from camera images is an important requirement of many fish monitoring programs, typically involving complex and expensive technology such as stereo-video. However, as a fish grows, the relative size of its eye typically decreases, providing a potential means of estimating fish size from a single image. We show that the ratio of head height to eye diameter is a good predictor of body length for 6 species of common New Zealand reef fish representing 6 different families. The regression equations describing such relationships can be used to estimate lengths of individual fish from single photographs or video frames, which in turn can be used to estimate the distance of each fish from the camera (by determining the proportion of the image frame occupied by an object of known length at known distances) in order to standardize the survey area. In a field test, lengths of 90% of 511 individual snapper Pagrus auratus recorded by unbaited video cameras could be estimated from their head height:eye diameter ratios. This method enables fish lengths to be estimated from single still or video images, allowing fish to be monitored with small inexpensive cameras. While this simple and cost-effective approach will increase the accessibility of video monitoring techniques, it will be best suited to areas where fish diversity is low enough to enable equations to be obtained for all common species, or where the focus is on a subset of species (e.g. harvested species).

KEY WORDS: Fish · Length estimation · Monitoring · Photogrammetry · Survey