The essential habitat role of a unique coastal inlet for a widely distributed apex predator.

Essential habitats support specific functions for species, such as reproduction, feeding or refuge. For highly mobile aquatic species, identifying essential habitats within the wider distribution range is central to understanding species ecology, and underpinning effective management plans. This study examined the movement and space use patterns of sevengill sharks (Notorynchus cepedianus) in Caleta Valdés (CV), a unique coastal habitat in northern Patagonia, Argentina. Seasonal residency patterns of sharks were evident, with higher detectability in late spring and early summer and lower during autumn and winter. The overlap between the residency patterns of sharks and their prey, elephant seals, suggests that CV functions as a seasonal feeding aggregation site for N. cepedianus. The study also found sexual differences in movement behaviour, with males performing abrupt departures from CV and showing increased roaming with the presence of more sharks, and maximum detection probability at high tide. These movements could be related to different feeding strategies between sexes or mate-searching behaviour, suggesting that CV may also be essential for reproduction. Overall, this study highlights the importance of coastal sites as essential habitats for N. cepedianus and deepens our understanding of the ecological role of this apex predator in marine ecosystems.

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

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.