Estimating the spatial position of marine mammals based on digital camera recordings

Abstract

Estimating the spatial position of organisms is essential to quantify interactions between the organism and the characteristics of its surroundings, for example, predator–prey interactions, habitat selection, and social associations. Because marine mammals spend most of their time under water and may appear at the surface only briefly, determining their exact geographic location can be challenging. Here, we developed a photogrammetric method to accurately estimate the spatial position of marine mammals or birds at the sea surface. Digital recordings containing landscape features with known geographic coordinates can be used to estimate the distance and bearing of each sighting relative to the observation point. The method can correct for frame rotation, estimates pixel size based on the reference points, and can be applied to scenarios with and without a visible horizon. A set of R functions was written to process the images and obtain accurate geographic coordinates for each sighting. The method is applied to estimate the spatiotemporal fine-scale distribution of harbour porpoises in a tidal inlet. Video recordings of harbour porpoises were made from land, using a standard digital single-lens reflex (DSLR) camera, positioned at a height of 9.59 m above mean sea level. Porpoises were detected up to a distance of ∽3136 m (mean 596 m), with a mean location error of 12 m. The method presented here allows for multiple detections of different individuals within a single video frame and for tracking movements of individuals based on repeated sightings. In comparison with traditional methods, this method only requires a digital camera to provide accurate location estimates. It especially has great potential in regions with ample data on local (a)biotic conditions, to help resolve functional mechanisms underlying habitat selection and other behaviors in marine mammals in coastal areas.

Marine Mammal Behavioral Response to Tidal Turbine Sound

“…Video files for tracks collected during acceptable sighting conditions were selected within the PAMGuard Landmark module. Using the same approach as Hoekendijk et al (2015), video footage was run until a porpoise or group of porpoises were visible at the sea surface (aided by the observers' acoustic cue for a surfacing event). The video file was then re-wound frame by frame until the animal was at the highest point in its surfacing.…”

Examining the viability of dorsal fin pigmentation for individual identification of poorly-marked delphinids

“…The need to monitor cetacean populations for management purposes typically requires assessments of abundance 1 , 2 , site fidelity 3 , movement patterns 4 , 5 and social structure 6 . A range of methods have been used to examine these parameters including: distance-sampling where line-transect surveys are used to count animals to assess their density 7 , 8 , and mark-recapture (MRC) analysis using sightings (and re-sightings) of naturally or artificially marked individuals as a sample of the population 9 .…”

Through the looking-glass : Marine mammal monitoring in a changing world

“…In the Wadden Sea, bordering the southern North Sea, seals have been hunted for centuries, which led to the local extinction of grey seals in in the Middle Ages (Reijnders et al, 1995) and nearextinction of the harbour seal in the 20 th century (Reijnders et al, 1992). Following the cessation of the hunt in 1960, populations of both species have shown steady recovery: grey seals recolonised the area (Brasseur et al, 2015) and in the international Wadden Sea number over 8,000 individuals counted in 2023 (Schop et al, 2023) and the number of harbour seals counted reached over 25,000 individuals, resulting in a population estimate of 40,000 individuals in 2014 (Brasseur et al, 2018).…”

“…Both have been studied extensively in this region. The numbers and pup production are recorded annually and hundreds of individual animals have been tracked throughout the years (Reijnders, 1976;Reijnders, 1986;Reijnders et al, 1995;Ries et al, 1998;Härkönen et al, 2006;Reijnders et al, 2010;Brasseur et al, 2015;Brasseur et al, 2018;Aarts et al, 2019). Regular aerial monitoring started in the 1970s and is now done annually, with multiple surveys carried out during the respective breeding and moulting season.…”

“…In this study, datasets from two fundamentally different real-world ecological use cases were employed. The objects of interest in these images were manually counted in previous studies (Brasseur et al, 2018;Cremer et al, 2017;Brasseur et al, 2015;Van der Veer, in prep) without the aim of DL applications.…”