FishSense Lite: A Camera-Based, Single Laser Hardware Framework with Software System for In Situ Fish Length Measurement

Fish stock assessments need fish lengths paired with species from far more places than trained surveyors can reach. Recreationaldivers already visit many of those places, but measuring a fish underwater has required trained observers or specialist equipment.We present FishSense Lite, which recovers fish length from a single photograph taken with a recreational dive camera, a dive laserpointer, and a 3D-printed mount. The diver aims the laser at a fish and takes a picture; afterward, the dot’s position in the imagegives the range to the fish, and the range turns the fish’s size in the image into a length. A citizen-science program, trained by thesystem’s builders, taught its own divers and collected lengths paired with species for 73 wild fish across seven open-water dives. Onseven wild fish also measured by an underwater stereo-imaging system, the two instruments agreed without detectable bias (meandifference +0.8%). On rigid targets of known length, FishSense Lite recovered every target to within 1.3%, and 99% of 1,055 frames fellwithin a 15% error budget. From the error sources we measure, we derive a field protocol: calibrate against a dive slate on every dive,photograph fish within 15° of perpendicular and beyond 1 m, and take at least ten frames of each.