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A low-cost towed video camera system for underwater surveys: comparative performance with standard methodology

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Environmental Monitoring and Assessment
Authors:
  • Red de Trabajo en Pesquerías y Ecosistemas Costeros, Centro para el Estudio de Sistemas Marinos (CESIMAR-CONICET)
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Abstract and Figures

Technological advances in the field of underwater video have led to an exponential increase in the use of drifting cameras (DC) and remotely operated vehicles (ROVs) to monitor the diversity, abundance, and size structure of marine life. Main advantages of DCs relative to ROVs are their lower costs and the much simpler logistics required to operate them. This study compares the performance of a new low-cost DC system equipped with a novel measuring device with that of a standard DC bearing an array of laser pointers. The new DC, which can be operated from a small boat, carries a pair of parallel steel “whiskers” that are dragged on the seabed within the field of view of the camera, providing a scale for measuring and estimating the density of benthic biota. An experiment conducted using an array of objects of known sizes laid on the bottom showed that its performance in terms of both size and density estimation was similar to that of the standard technique based on laser pointers. Measurement errors had a negligible negative bias (− 2.3%) and a standard deviation that ranged between 13 and 8% for objects from 25 to 110 mm in size. The whiskers offered a simplified method for density estimation that avoids the need to calculate the width of the field of view, thus reducing the video processing time by around 60% with respect to the standard method. Briefly, the new system offers an efficient low-cost alternative for benthic ecology studies conducted on soft or non-irregular bottoms.
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A low-cost towed video camera system for underwater
surveys: comparative performance with standard
methodology
G. A. Trobbiani &A. Irigoyen &L. A. Venerus &
P. M . Fio r da &A. M. Parma
Received: 24 June 2018 /Accepted: 22 October 2018 /Published online: 29 October 2018
#Springer Nature Switzerland AG 2018
Abstract Technological advances in the field of under-
water video have led to an exponential increase in the
use of drifting cameras (DC) and remotely operated
vehicles (ROVs) to monitor the diversity, abundance,
and size structure of marine life. Main advantages of
DCs relativeto ROVs are their lower costs and the much
simpler logistics required to operate them. This study
compares the performance of a new low-cost DC system
equipped with a novel measuring device with that of a
standard DC bearing an array of laser pointers. The new
DC, which can be operated from a small boat, carries a
pair of parallel steel Bwhiskers^that are dragged on the
seabed within the field of view of the camera, providing
a scale for measuring and estimating the density of
benthic biota. An experiment conducted using an array
of objects of known sizes laid on the bottom showed that
its performance in terms of both size and density esti-
mation was similar to that of the standard technique
based on laser pointers. Measurement errors had a neg-
ligible negative bias (2.3%) and a standard deviation
that ranged between 13 and 8% for objects from 25 to
110 mm in size. The whiskers offered a simplified
method for density estimation that avoids the need to
calculate the width of the field of view, thus reducing the
video processing time by around 60% with respect to the
standard method. Briefly, the new system offers an
efficient low-cost alternative for benthic ecology studies
conducted on soft or non-irregular bottoms.
Keywords Remote underwater video .Drifting camera
system .Density estimation .Size measurement .
Laser pointers
Introduction
The use of underwater video to study marine life has
gained wide acceptance in recent years (e.g., Mallet and
Pelletier 2014; Whitmarsh et al. 2017). This is mainly
because they overcome some of the drawbacks of more
traditional removal sampling techniques such as trap-
ping and trawling, which may be too costly (large fish-
ing vessels are often needed), and limited by the rough-
ness of the seabed or by fishing restrictions within
marine protected areas. Remote video techniques allow
collecting information about populations and communi-
ties in a non-destructive manner, while avoiding some of
the limitations and biases of other non-intrusive
methods such as underwater visual censuses (UVCs).
They are free from depth and diving time constraints
imposed by diving safety (Harvey and Shortis 1996;
Harvey et al. 2001a,2002), are not restricted by the
presence of dangerous fauna (Meekan and Cappo
2004), and avoid biases caused by behavioral responses
Environ Monit Assess (2018) 190: 683
https://doi.org/10.1007/s10661-018-7070-z
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10661-018-7070-z)contains
supplementary material, which is available to authorized users.
G. A. Trobbiani (*):A. Irigoyen :L. A. Venerus :
P. M. Fiorda :A. M. Parma
Centro para el Estudio de Sistemas Marinos (CESIMAR), Consejo
Nacional de Investigaciones Científicas y Técnicas (CCT
CONICET CENPAT), Boulevard Brown 2915, (U9120ACD),
Puerto Madryn, Chubut, Argentina
e-mail: gastontrobbiani@hotmail.com
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
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Thesis
With the global carbon crisis a matter of worldwide concern, efforts to preserve natural habitats that sequester carbon are of utmost importance. However, the processes which enable aquatic plants to survive and thrive are poorly known, as is the extent of their distribution and how they change over multiple scales. The aim of this research was to develop methodologies to help define the relationships between key benthic habitats and bio-physical variables and spatially predict their distribution and abundance within south-west Australian estuaries through towed underwater video. This thesis identified multiple non-destructive methods along with their strengths and limitations, to characterise benthic cover from underwater video, and highlighted optimal methods based on equipment, end goals, time and funding available. Additionally, I emphasize that no one method used in isolation was suitable for the analysis of underwater video from the shallow and turbid habitats from my study sites, but that a combination of methods was required for optimal characterisation. This research is one of the first to model and spatially predict fine-resolution (5% intervals) percent cover of benthic habitats within estuaries from post-processed underwater video using biological and physical datasets with a state-of-the art machine learning method called ‘Random Forests’. This method is often used within terrestrial landscape ecology, but rarely within estuarine systems. Random Forests performed well with 79-90% variation explained by the models for each key benthic habitat and partial plots illustrated strong relationships between physical variables and biotic habitats. The most influential parameters driving biotic habitat distributions were longitude (19%), depth (13%), and latitude (11%), although this relationship varied between estuaries and on the degree of estuary connectivity to the sea (permanently- opened, artificially-opened and normally-closed). Predictive performance of key benthic habitat models was moderate to excellent and associated uncertainty maps of standard deviation of each model was highly variable in areas of habitat fragmentation. Broad-resolution distributions of biotic habitats were found to be important in understanding local-scale physical processes. Seagrasses were the most common biotic habitat in five estuaries, although higher numbers of seagrass species occurred in the permanently-opened Leschenault Estuary (e.g. Ruppia megacarpa, Halophila ovalis and Heterozostera tasmanica), while seasonally-opened (Wilson Inlet) and normally-closed (Wellstead, Stokes and Beaufort) estuaries supported monospecific meadows of R. megacarpa. Red and green macroalgae had inverse latitudinal distributions, with red alga occurring in northern estuaries with higher amounts of seawater incursion and freshwater input. Green alga, especially green film alga were more prominent in the more stagnant, and normally-closed waters of the southern estuaries. Motile commercial fishery species such as crabs (Portunus pelagicus) were common in northern estuaries where access to marine influence was essential for their survival. Encrusting benthic polychaete worms such as Ficopomatus enigmaticus and the black mussel Mytilus edulis were found shallow sections of southern estuaries, which were able to tolerate extreme changes in water quality due to estuary bar closure, and often encrusting the hard substratum of submerged trees and rocks. This study demonstrated advances in modelling techniques of species abundances and distribution from underwater video and highlighted the importance of bio-physical relationships on spatial patterns of different seagrass species and other biotic habitats such as algae beds, polychaete mounds and mussel clumps in estuaries. Estuarine habitats are at the forefront of climate change effects and experience rapid changes (within weeks to months) in their spatial distribution and abundance. I developed a real-time, rapid and accurate method to capture broad-resolution semi- quantitative (barren, low, moderate and high percent cover) changes in benthic habitats using underwater video, as traditional remote sensing methods such as aerial photography and satellite imagery can often take up to weeks and months to post- process for spatial habitat distribution. I tested the accuracy of two benthic habitat assessment protocols: the broad-resolution real-time classification protocol (called the “Rapid Benthic Assessment”) against the fine-resolution post-processed habitat classification. I also tested the validation of the broad-resolution percent cover categories of seagrass from the RBA method using in situ samples of R. megacarpa and H. ovalis. The high correlation between the RBA and the fine-resolution method indicated that a high degree of detail and accuracy was retained by the RBA method. The visualisation of benthic habitats almost impossible to map through traditional remote sensing means was made possible through rapid data acquisition and visualisation from underwater video. This study demonstrated that real-time delineation of estuarine habitats allowed for rapid data analysis and representation within hours of data collection. This research will enable resource management authorities to make informed decisions on monitoring benthic habitats which have global significance within estuarine systems from baseline habitat maps, supplement existing maps and understand how bio-physical attributes shape benthic habitat distributions.
Article
During the VIITAL cruise in the Bay of Biscay in summer 2002, two devices for measuring the length of swimming fish were tested: 1) a mechanical crown that emitted a pair of parallel laser beams and that was mounted on the main camera and 2) an underwater auto-focus video camera. The precision and accuracy of these devices were compared and the various sources of measurement errors were estimated by repeatedly measuring fixed and mobile objects and live fish. It was found that fish mobility is the main source of error for these devices because they require that the objects to be measured are perpendicular to the field of vision. The best performance was obtained with the laser method where a video-replay of laser spots (projected on fish bodies) carrying real-time size information was used. The auto-focus system performed poorly because of a delay in obtaining focus and because of some technical problems.
Article
The author has developed a device and a method to measure the body length of fish from played back images that reflect laser beams recorded on videotape by an underwater video camera with laser discharge equipment attached, and examined the usefulness of this measuring method through experimentation with dummies in a large water tank on land and in the field. The distances at which the laser spots could be recognized on various colors were, in ascending order, blue, as the lowest, followed by gray, yellow, white, red, and filefish skin; when a distance from the camera exceeds 2.5m, significant errors were observed. In the field experimentation, the measurement was made using a device for the body length of kokanee salmons, Onchoryncus nerka, coming upstream in an artificial river, and it was compared with that of body length of salmons caught by a trap. Because the mode of both body length distribution coincided, it was confirmed that the usefulness of the measuring method of the body length by the underwater video camera with a laser beam discharge equipment attached. However, its use was limited to conditions of clear water and short distance to fish.
Article
This chapter gives results from some illustrative exploration of the performance of information-theoretic criteria for model selection and methods to quantify precision when there is model selection uncertainty. The methods given in Chapter 4 are illustrated and additional insights are provided based on simulation and real data. Section 5.2 utilizes a chain binomial survival model for some Monte Carlo evaluation of unconditional sampling variance estimation, confidence intervals, and model averaging. For this simulation the generating process is known and can be of relatively high dimension. The generating model and the models used for data analysis in this chain binomial simulation are easy to understand and have no nuisance parameters. We give some comparisons of AIC versus BIC selection and use achieved confidence interval coverage as an integrating metric to judge the success of various approaches to inference.