Towed underwater camera systems, of various configurations, have been used since the turn of the 20th century to acquire video and photographic still images of the seafloor (Bicknell et al. 2016) They are deployed on a cable from a surface vessel, have no propulsion mechanisms, and generally have forward-looking oblique and/or downward-looking cameras that either record images which are stored and subsequently downloaded, or transmit data directly to the surface in real-time via a coaxial or fibre optic cable (Bowden and Jones 2016, Durden et al. 2016a). Towed underwater cameras not only augment data from collected specimens (Chapter 8, 9); they also provide an important non-invasive sampling alternative where extractive methods are either unnecessary or unsuitable, such as in sensitive deep-sea habitats (e.g. Althaus et al. 2009, Williams et al. 2015, Sherlock et al. 2016), or for repeated sampling in marine reserves (e.g. Lawrence et al. 2015). Towed platforms also have the added advantage of providing cost-effective permanent data capture along transects that can be up to several kilometers in length and can be used to traverse highly heterogeneous seafloor topography (Shortis et al. 2007, Sheehan et al. 2016). The quality of imagery acquired by towed systems depends largely on sea conditions and water clarity, both of which may vary considerably depending on geographic location, season of sampling and extent of tidal influence. In depths greater than around 30 m, lighting and camera specifications become increasingly important to image quality. The quality and versatility of equipment and the maintenance of a consistent flying altitude above the seabed are also critical factors affecting image quality and usability.
Conventional underwater still photography and video imagery were initially applied by marine ecologists to collect basic qualitative data (e.g. simple visual assessment of seabed conditions to assess habitat type or dominant species), or often low-accuracy quantitative data estimated through the use of parallel lasers to define the scale of the images (see Harvey et al. 2002, Shortis et al. 2008, Durden et al. 2016a). Recent technological advancements have emerged that permit collection of high-resolution benthic imagery using versatile multifunctional towed platforms carrying a variety of camera systems (e.g. stereo-image measurement systems) and a range of other sensors (e.g. high-resolution multibeam and side-scan sonars, motion sensors, conductivity temperature and depth sensors, and subsea acoustic positioning systems) (Kocak et al., 2008, Rattray et al. 2014, Bowden and Jones 2016, Durden et al. 2016a, Logan et al. 2017). This technology, coupled with advances in camera resolution, positional accuracy, digital data processing and visualisation techniques, has enabled more quantitative and spatially-referenced studies of the seafloor. Calibrated stereo-imaging in particular has facilitated more reliable length measurements of mobile species, such as epibenthic invertebrates and demersal fish, and more accurate estimates of biomass and population distributions (Harvey et al. 2002, Shortis et al. 2009). Towed underwater imaging systems can be applied to acquire baseline data, evaluate benthic diversity, map benthic habitats, identify vulnerable communities, assess changes in biota, and support spatial and ecological modelling/monitoring.
For further information on the advantages and disadvantages of towed camera systems compared to other benthic imagery and sampling platforms, refer to Comparative assessment of seafloor samping platforms in Przeslawski et al. 2018).