Stereo-video Systems Research Articles

A comparison of temperate reef fish assemblages recorded by three underwater stereo-video techniques

Three underwater stereo-video techniques were used to sample the relative densities and species richness of temperate reef fish assemblages at three reef locations and two habitats (high- and low-relief reef) within Hamelin Bay, south-western Australia. The three techniques compared were diver-operated stereo-video strip transects, baited remote stereo-video and unbaited remote stereo-video. While unbaited remote stereo-video and diver-operated stereo-video transects recorded greater species richness at high compared to low-relief reefs, baited remote stereo-video recorded similar species richness at the two habitat types. The diver-operated stereo-video system was manoeuvred through caves and under overhangs recording small, cryptic, cave-dwelling species that were not recorded by either remote video techniques (Trachinops noarlungae, Trachinops brauni, Chromis klunzingeri, Trachichthys australis). Both remote video techniques recorded greater species richness and relative density of the most common species of Labridae, Ophthalmolepsis lineolatus. Baited remote video recorded the rarer, large predatory fish species (e.g. Seriola hippos, Glaucosoma hebraicum, Heterodontus portusjacksoni). None of the techniques sampled small cryptic fish families such as Gobiidae or Blenniidae. A combination of survey techniques is recommended for comprehensive fishery-independent studies that aim to sample broad components of fish assemblages.

A comparison of underwater visual distance estimates made by scuba divers and a stereo-video system: implications for underwater visual census of reef fish abundance

Underwater visual census of reef fish by scuba divers is a widely used and useful technique for assessing the composition and abundance of reef fish assemblages, but suffers from several biases and errors. We compare the accuracy of underwater visual estimates of distance made by novice and experienced scientific divers and an underwater stereo-video system. We demonstrate the potential implications that distance errors may have on underwater visual census assessments of reef fish abundance. We also investigate how the accuracy and precision of scuba diver length estimates of fish is affected as distance increases. Distance was underestimated by both experienced (mean relative error = −11.7%, s.d. = 21.4%) and novice scientific divers (mean relative error = −5.0%, s.d. = 17.9%). For experienced scientific divers this error may potentially result in an 82% underestimate or 194% overestimate of the actual area censused, which will affect estimates of fish density. The stereo-video system also underestimated distance but to a much lesser degree (mean relative error = −0.9%, s.d. = 2.6%) and with less variability than the divers. There was no correlation between the relative error of length estimates and the distance of the fish away from the observer.

The accuracy and precision of underwater measurements of length and maximum body depth of southern bluefin tuna ( Thunnus maccoyii) with a stereo–video camera system

The accuracy and precision of in situ stereo–video measurements of the snout to fork length (SNFL) (range 830–1412 mm) and maximum body depth (MBD) (range 228–365 mm) of free-swimming southern bluefin tuna (SBT) ( Thunnus maccoyii) were tested by filming live fish in sea cages immediately prior to harvest. Stereo–video measurements of the SNFL of 54 fish produced an average error of 1.72 mm (relative error of 0.16%), while an average error of 1.37 mm (relative error of 0.51%) was recorded for measurements of MBD from 47 fish. A procedure was developed to maximise the accuracy and precision of measurements of the SNFL and MBD from a single SBT over sequential images to avoid the underestimation of SNFL and overestimation of MBD due to sinusoidal changes in body form associated with fast swimming. The results demonstrate the potential of stereo–video systems to non-destructively make counts and measurements of tuna and other fish in both wild fisheries and mariculture situations, without the need to capture and handle them.

Stereo–video observation of nearshore bedforms on a low energy beach

A new technique utilising underwater stereo–video was tested for its ability to measure nearshore bed morphology at high-frequency and high-resolution. A stereo–video system was used to monitor small-scale ripple morphology (wavelengths 0.05–0.10 m, heights ∼5 mm) in the nearshore of a low energy beach in southwestern Australia. Waves, currents and suspended sediment concentrations were also monitored. Bed morphology could be measured with a mean accuracy of 2 mm with the stereo–video system. Measurements in the field were limited to ripple crest positions since targets over the entire ripple profiles could not be identified in both the left and right stereo images. This limitation could be overcome in the future by use of laser stripes to illuminate the profiles and the generation of epipolar lines within the analysis software to match corresponding targets in each of the images. Ripple crest positions were measured at 1 Hz and the relationship between ripple movement and near-bed hydrodynamics was investigated to test the applicability of the stereo–video system to make high-frequency bedform observations. Ripple crests were found to oscillate at the same frequency as the swell waves. The magnitude of the ripple movement at swell wave frequencies was an order of magnitude greater than ripple movement at wind wave frequencies. Net onshore migration of the ripples corresponded to onshore wave asymmetry, very weak mean cross-shore currents and net onshore suspended sediment flux measured at an elevation of 0.05 m.

Estimation of reef fish length by divers and by stereo-video: A first comparison of the accuracy and precision in the field on living fish under operational conditions

Using simultaneous measurements from an underwater stereo-video system the accuracy and precision of length estimates of reef fish made by three experienced diver scientists under field conditions is determined. The trial showed that under optimal conditions the divers’ estimates were very accurate ( mean error =0.87 cm) but lacked precision ( mean S.D. =5.29 cm) . The effects of the low precision are then demonstrated by using these field estimates to model the theoretical statistical power of the scientific divers to detect changes in the mean length of three species of fish from New Zealand coastal waters. The results suggest that the experienced diver scientists would have a much lower statistical power than stereo-video measurements to detect changes in the mean length, especially where low numbers of fish are recorded or where a research program aims to detect a change in the mean length of a fish population of 30% or less.

A Comparison of the Accuracy and Precision of Measurements from Single and Stereo-Video Systems

Underwater tests using plastic silhouettes of fish were used to compare the accuracy and precision of measurements made with a single video camera system to those made from two stereo-video systems (one using digital camcorders, the other using Hi8 camcorders). Test measurements made across a variety of ranges and angles of silhouette orientation in the fields of view showed the length estimates from both the digital and Hi8 stereo-video systems were substantially more accurate and precise than those obtained by the single video camera system, and had the great advantage that the position (range and bearing) and orientation of a fish target could be measured directly. Measurements made with stereo-video were much less restricted by range and subject orientation than those made with single video. The data resulting from these trials are used to propose a set of guidelines to optimize the accuracy and precision of underwater measurements of fish length using single and stereo-video systems.

Development and preliminary evaluation of VISLAN, a surgical planning and guidance system using intra-operative video imaging

VISLAN is an integrated neurosurgical planning and guidance system. New segmentation and rendering techniques have been incorporated. A stereo video system is used intra-operatively and fulfils four roles. First, the video display is overlaid with graphical outlines showing the position of the planned craniotomy or the target (enhanced reality displays). Second, a skin surface patch is reconstructed from the stereo video images using patterned light (mean errors of surface point location are <0.15 mm). Third, a freely mobile, hand-held localizer is tracked in real time (position errors are <0.5 mm and with improved calibration <0.2 mm), with its position superimposed on the pre-operative patient representation to assist surgical guidance. Fourth, markers fixed to the skull bone next to the cranial opening are used to detect intra-operative movement and to update registration. Initial results from phantom experiments show an overall system accuracy of better than 0.9 mm for intra-operative localization of features defined in pre-operative images. The prototype system has been tested during six neurosurgical operations with very good results.

Development and preliminary evaluation of VISLAN, a surgical planning and guidance system using intra-operative video imaging

VISLAN is an integrated neurosurgical planning and guidance system. New segmentation and rendering techniques have been incorporated. A stereo video system is used intra-operatively and fulfils four roles. First, the video display is overlaid with graphical outlines showing the position of the planned craniotomy or the target (enhanced reality displays). Second, a skin surface patch is reconstructed from the stereo video images using patterned light (mean errors of surface point location are <0.15 mm). Third, a freely mobile, hand-held localizer is tracked in real time (position errors are <0.5 mm and with improved calibration <0.2 mm), with its position superimposed on the pre-operative patient representation to assist surgical guidance. Fourth, markers fixed to the skull bone next to the cranial opening are used to detect intra-operative movement and to update registration. Initial results from phantom experiments show an overall system accuracy of better than 0.9 mm for intra-operative localization of features defined in pre-operative images. The prototype system has been tested during six neurosurgical operations with very good results.

Use of a Stereo-video System to Estimate the Energy Expenditure of Free-swimming Fish

We tested the ability of a stereo-video method to provide estimates of the metabolic costs of spontaneous swimming by simultaneously video taping free-swimming fish and measuring their oxygen consumption. Three swimming characteristics (speed, turning rate, and acceleration) obtained from image analysis of video recordings explained, on average, 80% of the variability in spontaneous swimming costs. Colinearity among the swimming characteristics prevented us from estimating their relative importance. We converted the swimming speeds recorded by the stereo-video system to metabolic costs using relationships from the literature between oxygen consumption and speed during forced swimming (swimming with constant speed and direction). Spontaneous swimming costs, measured directly by respirometry, were, on average, six times higher than the costs predicted by the forced swim relationships for the same speeds. This difference suggests that the metabolic costs of turning and acceleration can be substantial. Relationships used to convert movements obtained with the stereo-video method into energy expenditure should therefore be developed using spontaneously swimming fish and should include turning rate and acceleration as well as speed.