A novel method to obtain accurate length estimates of carnivorous reef fishes from a single video camera

Abstract

In the last years, technological advances enhanced the utilization of baited underwater video (BUV) to monitor the diversity, abundance, and size composition of fish assemblages. However, attempts to use static single-camera devices to estimate fish length were limited due to high errors, originated from the variable distance between the fishes and the reference scale included in the scene. In this work, we present a novel simple method to obtain accurate length estimates of carnivorous fishes by using a single downward-facing camera baited video station. The distinctive feature is the inclusion of a mirrored surface at the base of the stand that allows for correcting the apparent or "naive" length of the fish by the distance between the fish and the reference scale. We describe the calibration procedure and compare the performance (accuracy and precision) of this new technique with that of other single static camera methods. Overall, estimates were highly accurate (mean relative error = -0.6%) and precise (mean coefficient of variation = 3.3%), even in the range of those obtained with stereo-video methods.En los últimos años, los avances tecnológicos permitieron intensificar el uso de estaciones de video fijas para estudiar la abundancia, composición de especies y estructura de tamaños de los ensambles ícticos. Sin embargo, la utilización de una única cámara estática para estimar el tamaño de los peces puede conducir a determinaciones poco exactas, provocadas por la distancia variable entre los peces y la escala de referencia incluida en la escena. En este trabajo presentamos un método novedoso para obtener estimaciones exactas del largo de peces carnívoros, que utiliza una estación de video fija con una única cámara orientada verticalmente. La característica que distingue a este sistema es la inclusión de una superficie espejada en la base del soporte de la cámara que permite corregir la longitud aparente o "naive" en función de la distancia entre los peces y la escala de referencia. En este trabajo describimos el procedimiento de calibración y comparamos el rendimiento de esta técnica (exactitud y precisión) con la de otros métodos que utilizan una única cámara estática. Las estimaciones obtenidas fueron exactas (error relativo promedio = -0,6%) y precisas (coeficiente de variación promedio = 3,3%), en el rango de valores obtenidos con estéreo-video.

Accuracy Assessment of Go Pro Hero 3 (Black) Camera in Underwater Environment

“…The use of Photogrammetry to observe precise and accurate measurements underwater is not novel and has been used in the past for a number of applications such as the non-destructive monitoring of marine fauna populations and the estimation of fish sizes (Harvey et al, 2003;Costa et al, 2006;Trobbiani et al, 2015). These underwater applications' overall goal is usually the reliable determination of length measurements.…”

“…The use of planar test fields is common in Marine Ecology (e.g. Costa et al, 2006;Bruno et al, 2011;Trobbiani et al, 2015). However, as evaluated in Boutros et al (2015) planar test fields have two main weaknesses.…”

“…Therefore, it is an attractive technology for a number of disciplines such as marine ecology. Possible applications include the monitoring of marine fauna populations and the estimation of fish sizes (Harvey et al, 2003;Costa et al, 2006;Trobbiani et al, 2015). Other possible underwater photogrammetric applications includes structural analysis of sub-sea structures (Dare and Fraser, 2000), ocean mapping (Bodenmann et al, 2010), mapping and detailing of historical shipwrecks (Hollick et al, 2013) and inspection of coral reef structures (Burns et al, 2015).…”

Accuracy Assessment of Go Pro Hero 3 (Black) Camera in Underwater Environment

“…In this paper we will especially focus on the usability of action cameras for Underwater Photogrammetry. The use of Photogrammetry to observe precise and accurate measurements underwater is not novel and has been used in the past for a number of applications such as the non-destructive monitoring of marine fauna populations (Shortis and Harvey, 1998) and the estimation of fish sizes (Harvey et al, 2003;Costa et al, 2006;Trobbiani et al, 2015). These underwater applications' overall goal is usually the reliable determination of length measurements.…”

“…Therefore, it is an attractive technology for a number of disciplines such as marine ecology. Possible applications include the monitoring of marine fauna populations (Shortis and Harvey, 1998) and the estimation of fish sizes (Harvey et al, 2003;Costa et al, 2006;Trobbiani et al, 2015). Other possible underwater photogrammetric applications includes structural analysis of sub-sea structures (Dare and Fraser, 2000), ocean mapping (Bodenmann et al, 2010), mapping and detailing of historical shipwrecks (Hollick et al, 2013) and inspection of coral reef structures (Burns et al, 2015).…”