In situ measurement of fish body length using perspective-based remote stereo-video

Size–frequency distributions provide important information on population-level processes in fishes, but for some non-commercial species the collection of length data through direct sampling may be unacceptably destructive. Although in situ measurement techniques based on photographic or video image analysis are non-destructive, these generally require diver-operated equipment and consequently are labor-intensive and limited in both working depth and deployment time. A remotely operated, continuously recording video measurement system is described which has a number of advantages over diver-operated equipment, particularly for uncommon species or when it is important that estimates of the size composition of a fish population be unbiased by observer presence. The described system bases measurements on perspective changes when objects are viewed from different distances and has less stringent alignment requirements than other stereo techniques that depend on the intersection of sight lines. Test measurements of PVC pipe sections carried by divers differed from actual values by an average of 1.6% when the pipe axis was in a plane approximately perpendicular to the camera axis. Replicate length measurements made on free-swimming Bluntnose Sixgill Sharks (Hexanchus griseus) also differed by an average 1.6%.