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

The "Tracked Roaming Transect" and distance sampling methods increase the efficiency of underwater visual censuses.

Underwater visual census (UVC) is the most common approach for estimating diversity, abundance and size of reef fishes in shallow and clear waters. Abundance estimation through UVC is particularly problematic in species occurring at low densities and/or highly aggregated because of their high variability at both spatial and temporal scales. The statistical power of experiments involving UVC techniques may be increased by augmenting the number of replicates or the area surveyed. In this work we present and test the efficiency of an UVC method based on diver towed GPS, the Tracked Roaming Transect (TRT), designed to maximize transect length (and thus the surveyed area) with respect to diving time invested in monitoring, as compared to Conventional Strip Transects (CST). Additionally, we analyze the effect of increasing transect width and length on the precision of density estimates by comparing TRT vs. CST methods using different fixed widths of 6 and 20 m (FW3 and FW10, respectively) and the Distance Sampling (DS) method, in which perpendicular distance of each fish or group of fishes to the transect line is estimated by divers up to 20 m from the transect line. The TRT was 74% more time and cost efficient than the CST (all transect widths considered together) and, for a given time, the use of TRT and/or increasing the transect width increased the precision of density estimates. In addition, since with the DS method distances of fishes to the transect line have to be estimated, and not measured directly as in terrestrial environments, errors in estimations of perpendicular distances can seriously affect DS density estimations. To assess the occurrence of distance estimation errors and their dependence on the observer's experience, a field experiment using wooden fish models was performed. We tested the precision and accuracy of density estimators based on fixed widths and the DS method. The accuracy of the estimates was measured comparing the actual total abundance with those estimated by divers using FW3, FW10, and DS estimators. Density estimates differed by 13% (range 0.1-31%) from the actual values (average = 13.09%; median = 14.16%). Based on our results we encourage the use of the Tracked Roaming Transect with Distance Sampling (TRT+DS) method for improving density estimates of species occurring at low densities and/or highly aggregated, as well as for exploratory rapid-assessment surveys in which divers could gather spatial ecological and ecosystem information on large areas during UVC

Number (N) of wooden fishes estimated with the Distance Sampling (DS) method by the experienced and non-experienced divers for the total census area (3000 m²) and the confidence interval (N CI 95%) at p < 0.05.

<p>DS estimator: the best fit from the nine estimators tested with the DISTANCE software. D = Density per 300 m2. %CV = Coefficient of variation of the estimation in percentage. N detected (%) = Number of wooden fishes detected by divers and the percentage of the total wooden fish. MDD = Mean distance detection derived from visual estimations and standard deviation (SD) by divers. "*" Actual number of wooden fishes. "**" Actual mean distance.</p

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, 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 Caleta Valdés functions as a seasonal feeding aggregation site for N. cepedianus. The study also found sexual differences in movement behavior, with males performing abrupt departures from Caleta Valdés 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 behavior, suggesting that Caleta Valdés 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.All analyses were perfomed using open-access R programming software.Funding provided by: Council on Australia Latin America Relations*Crossref Funder Registry ID: Award Number: year 2019Funding provided by: Shark conservation fund*Crossref Funder Registry ID: Award Number: year 2019Funding provided by: CAPES-UC*Crossref Funder Registry ID: Award Number: Funding provided by: Fondo Nacional de Desarrollo Científico, Tecnológico y de Innovación TecnológicaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100010751Award Number: FB0002Funding provided by: Agencia Nacional de Investigación y DesarrolloCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100020884Award Number: ICN2019-015Funding provided by: Temaikèn Foundation*Crossref Funder Registry ID: Award Number: year 2019Detection data were collected via passive acoustic telemetry using VEMCO instruments. Temperature and tidal data were downloaded from the web (see manuscript for sources). Shapefiles are self-produced and downloaded from the web (see manuscript and R code for sources)

Density estimations (D) (fish/300m²) and coefficients of variations (CV) for studied species and the total of groupers for different combinations of transects types and census widths in Cabo de Palos and Cabo Tiñoso.

<p>Note that CST values in Cabo Tiñoso were derived from simulations. CST = Conventional Strip Transect. TRT = Tracked Roaming Transect. FW3 and FW10 = Fixed Width of 3 and 10 m for each side of the transects, respectively. DS = Distance Sampling Method. The census area of each survey is detailed in m² considering total transect lengths and 6 and 20 m width for FW´s estimators and 40 m width for the DS estimator.</p

Study sites.

<p>(A) Islas Hormigas and Hormigon in Cabo de Palos Marine Reserve (37.6550° N -0.6497° E). The distance between islands is not to scale to ease the representation. (B) Cabo Tiñoso (37.5370° N -1.1419° E). Straight lines: 50-m Conventional Strip Transects. Curved lines: Tracks of the Tracked Roaming Transects.</p

Scheme of the count method using the Distance Sampling method.

<p>IL = imaginary line. X^1-2 = perpendicular distances of solitary individuals to the transect line. X³ = perpendicular distance of a school of fishes to the perpendicular line. The equipment (Fig 2) is illustrated above divers, like at the surface. Inset: The Tracked Roaming Transect equipment. A = Waterproof case for GPS. B = Body Board Slate. C = Diving reel. D = Weight.</p

Extreme roll angles in Argentine sea bass: Could refuge ease posture and buoyancy control of marine coastal fishes?

The swim bladder provides a mechanism for buoyancy regulation in teleosts. However, in certain species, its location can result in an unstable body position, with associated energetic costs assumed for maintaining posture in addition to the energetic demands from swim bladder volume regulation. Direct observations show that some body-compressed, cave-refuging teleosts that nominally operate near neutral buoyancy may adopt unusual body attitudes within crevices. We hypothesize that these fishes may relax their buoyancy and posture control mechanisms during periods of rest. A prediction derived from this is that resting fish may adopt a wide range of roll angles (i.e., rotation about their longitudinal axis) inside caves. To quantify this behavior and for testing this hypothesis, triaxial accelerometers were deployed on free-living, cave-refuging Argentine sea bass Acanthistius patachonicus, and the relationship between roll angle and a proxy for activity (defined as the vectorial dynamic body acceleration, VeDBA) was analyzed. The results were compared with data available for three other species of fishes with disparate body forms and lifestyles: the pelagic whale shark Rhincodon typus, the dorsoventrally compressed benthic great sculpin Myoxocephalus polyacanthocephalus, and the fusiform and demersal Atlantic cod Gadus morhua. Inactive Argentine sea bass adopted a wide variety of roll angles, including extreme ones exceeding 80°, but had lower roll angles closer to an upright posture primarily associated with higher activity levels. In contrast, the great sculpin and Atlantic cod both rested at a close to upright roll angle but had higher activity levels associated with larger roll angles. Whale shark did not rest for the duration of the recorded period and also showed higher activity levels associated with larger roll angles. We propose that relaxation of buoyancy and posture control may help to reduce the metabolic rate in laterally compressed, cave-refuging fishes during periods of rest within crevices.Fil: Ciancio Blanc, Javier Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto de Biología de Organismos Marinos; ArgentinaFil: Venerus, Leonardo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; ArgentinaFil: Trobbiani, Gastón Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; ArgentinaFil: Beltramino, Lucas Esteban. Universidad Nacional de la Patagonia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Gleiss, Adrian C.. Murdoch University; AustraliaFil: Wright, Serena. Centre For Environment, Fisheries And Aquaculture Scien; Reino UnidoFil: Norman, Brad. Ecocean Inc; AustriaFil: Holton, Mark. Swansea University; Reino UnidoFil: Wilson, Rory P.. Swansea University; Reino Unid