WAMSI Node 4.2.4 - Demersal fish assemblage sampling method comparison and power analyses - Summary

Researchers: Harvey, Euan, Dr (Point of contact) ,  Langlois, Tim, Dr (Resource provider of)

Brief description The principal goal of this research was to investigate the most efficient, cost-effective and meaningful way of describing the structure and biodiversity of seabed habitats along the coastline of WA. This study used existing data obtained from three regions (Ningaloo, Abrolhos and Capes) to compare techniques including comparing diver vs camera-based techniques for fish. The first study compared the assemblage composition, relative abundance and size of fishes sampled using baited video and diver swum transects at the Houtman Abrolhos Islands and Ningaloo. At both locations stereo BRUV recorded many more targeted fishes than stereo DOV and in greater relative abundance (e.g. Lethrinus nebulosus, Plectropomus leopardus). Many non-targeted species were also attracted to bait (e.g. Coris auricularis) and were recorded in greater abundances on stereo BRUV than stereo DOV. Stereo DOV transects recorded a greater abundance of some small-bodied Pomacentridae, Labridae and Scaridae species than did stereo BRUV, particularly at Ningaloo. This study demonstrates that choice of stereo-video sampling technique for surveys of reef fish can have a large impact on the structure of fish assemblages surveyed. A link to the paper "Assessing reef fish assemblage structure: how do different stereo-video techniques compare?" by Dianne L. Watson, Euan S. Harvey, Ben M. Fitzpatrick, Timothy J. Langlois and George Shedrawi is given in a URL below (see online resource) and provides further details The second study compares baited video stations with diver swum transects across three biogeographic regions, where both methods use stereo-video techniques to provide accurate estimates of individual fish length and define the sample unit area (Langlois et al. submitted). Cost-benefit analyses showed that baited stereo-video methods were generally more cost-efficient than diver operated stereo-video transects for detecting change in the fish assemblage. The study suggests that baited stereo-video stations are, in general, a better method for monitoring fish communities than diver operated stereo-video transects. A link to the paper "Cost efficient sampling of fish assemblages: comparison of baited video stations and diver video transects" by T.J. Langlois, E. S. Harvey, B. Fitzpatrick, J. J. Meeuwig, G. Shedrawi, and D. L. Watson is given in a URL below (see online resource) and provides further details. **Langlois, T. J., E. S. Harvey, B. Fitzpatrick, J. J. Meeuwig, G. Shedrawi, and D. L. Watson. 2010. Cost efficient sampling of fish assemblages: comparison of baited video stations and diver video transects. Aquatic Biology 9:155-168. **Watson, D. L., E. S. Harvey, B. Fitzpatrick, T. J. Langlois, and G. Shedrawi. 2010. Assessing reef fish assemblage structure: how do different stereo-video techniques compare? Marine Biology 157:1237-1250.

Lineage Statement: Concomitant stereo-diver operated video (stereo-DOV) and stereo-baited remote underwater video (stereo BRUV) systems were used to samples these areas, providing a comparison of multispecies measures of the state of the fish community, including length measurements of the fish observed. Ningaloo: one-off baseline survey; Abrolhos Islands: one year of multi-year survey; Capes: one year of multi-year survey

Lineage Statement: Within each region the sampling design was made orthogonal to the two sampling methods to be compared. The design of the studies within each region differed, reflecting the known environmental gradients and particular objectives of the individual investigations. Occasionally problems occurred with the stereo-video methods in a sample, which meant that it was not possible to obtain length measurements of fish, resulting in the sample being discarded. At Ningaloo, two random locations adjacent to two established no-take marine reserves were sampled. Within each location six sites were randomly chosen to represent the diversity of habitat types found within each location. Six replicate 50 m x 5 m stereo DOV transects and six replicate stereo BRUV deployments were analysed from each site in 1-10 m water depth. Due to problems with the stereo-systems one stereo BRUV replicate and 2 stereo DOV transects were missing from the data set. At the Houtman Abrolhos Islands, a location adjacent to a partial-take marine protected area at each of the three island groups was sampled (Pelsaert and Easter, Wallabi). These three locations were considered as a fixed factor as there is a known gradient in the fish communities between the island groups (Watson et al. 2007). Within each location three sites were randomly chosen and stereo DOV transects were conducted along the same reef slope as stereo BRUV deployments, ensuring similar habitats along the reef edge were sampled. Five replicate 100 m x 5 m stereo DOV transects and five replicate stereo BRUV deployments were analysed from each site. Due to problems with the stereo-systems one stereo BRUV replicate and 4 stereo DOV transects from two sites were missing from the data set. All surveys were conducted on the same coral reef slopes in 8-12 m water depth. In the Capes region three random locations were sampled within the proposed Capes Marine Park. At each of these locations three random sites were sampled. Nine replicate 25 m x 5 m stereo DOV transects and nine replicate stereo BRUV deployments were analysed from each site. All surveys were conducted in comparable rocky reef habitats in 8-18 m water depth.

Lineage Statement: *Diver-operated stereo-video systems (stereo DOV)* The stereo DOV methods used in the present study is the same as that used by Harman et al. (2003) and Watson et al. (2005). Stereo DOVs are comprised of two SONY TRV900E video cameras which, similarly to stereo BRUVs, are mounted 0.7 m apart on a base bar inwardly converged at 8 degrees. The stereo DOV system was equipped with a synchronising diode mounted in front of the cameras, and floats to achieve neutral buoyancy. Stereo DOV was conducted by two SCUBA divers with one operating the stereo-video system and the other measuring the distance swum with a chainman cotton counter (bio-degradable cotton). SCUBA divers swam at a slow speed (approx 3 sec/m) and a distance of approximately 30 cm above the substrate. Stereo DOV transects were 50 x 5 m at Ningaloo, 100 x 5 m at the Abrolhos and 25 x 5 m at the Capes and in all instances replicate transects were separated from the previous by at least 15 m. In the laboratory, pairs of stereo DOV images were captured and viewed in PhotoMeasure. For each transect, every individual observed on video was identified to species level and simultaneously measured (snout to fork length; FL). Obtained with the length measure were measures of the three-dimensional location of the individual, i.e. distance in front of and to the left and right of the stereo DOV system, enabling standardisation of the area surveyed. Individuals further than 8 m in front of, or 2.5 m to the left or right of the stereo DOV system were not counted or measured. Lengths of individuals of each species were converted to weight and summed for each species for each transect providing an estimate of the biomass over the area of each transect. A single species, Chromis westaustralis, was excluded from length analysis as measurements of these individuals from stereo DOV footage were difficult due to their small size and schooling behaviour.

Lineage Statement: *Baited remote underwater stereo-video (stereo BRUV)* The stereo BRUV method used in the present study is the same as that used by Harvey et al. (2002) and Watson et al. (2005, 2007). Detailed information on the design and photogrammetric specifics are presented in Harvey and Shortis (1996, 1998). Stereo BRUV systems were comprised of two SONY HC 15E video cameras mounted 0.7 m apart on a base bar inwardly converged at 8 degrees to gain an optimized field of view with visibility of 10 m distance (Harvey and Shortis 1996). Positioned in front of the cameras was a synchronising diode and bait basket. Each system was deployed by boat and left to film on the sea floor for a period of one hour. With multiple systems in use, a single stereo BRUV could be deployed at one site, followed by one at a second site and so on, maximising sampling efficiency. Previous research has shown that a filming time of at least 36 minutes is essential for obtaining measures of the majority of fish species, and furthermore that the full 60 minutes is advisable to obtain measures of numerous targeted fish species (Watson et al. 2005). Stereo BRUV systems had 800 grams of pilchards (Sardinops sagax) in a plastic-coated wire mesh basket, suspended 1.2 m in front of the two cameras. The pilchards were crushed to maximise dispersal of the fish oil. Adjacent replicate stereo BRUV deployments at Ningaloo, the Houtman Abrolhos and the Capes were separated by at least 250 m to avoid overlap of bait plumes and reduce the likelihood of fish moving between stereo BRUV stations within the sampling period. The analysis of the data from each tape was facilitated through a custom database (BRUVS1.5.mdb©, Australian Institute of Marine Science 2006). This database enabled us to manage data collected from the field operations and tape readings, capture the timing of events and reference images of the seafloor and fish in the field of view. We recorded the maximum number of any one species seen at one time during the recording (MaxN). Estimates of MaxN are considered conservative, particularly in areas where fish occur in high-densities (Cappo et al. 2003, Cappo et al. 2007). The program PhotoMeasure (www.seagis.com.au) was then used to make length measurements from stereo-video images. To avoid making repeated measurements of the same individuals, measures of length (snout to fork, FL) were made at the time of MaxN. This MaxN ‘time’ is not instantaneous; rather it corresponds to the amount of time all individuals remained in the field of view of the stereo BRUVs. To ensure good measurement accuracy and precision, as well as a standardized sampling unit, measures of fish length were limited to within a maximum distance of 5 m from the cameras (Harvey et al. 2002a) resulting in a sample unit area of 25.5 m2. The software calculates both distance from the cameras and length at the same time, using this information measurements and counts of abundance of species further than 5 m from the cameras were discarded. During the study period, visibility at the Capes, Houtman Abrolhos and Ningaloo, consistently greater than 5 m. Lengths of individuals of each species were converted to weight and summed for each species for each stereo BRUV sample. These summed weights are considered to be a relative estimate of biomass as although the sample unit area of the stereo BRUV systems has been standardised, different fish species may have been attracted different distances into the sample unit of the stereo BRUV by the bait. However, in the results section of this study the relative biomass estimates by the stereo BRUV systems will be referred to simply as biomass estimates.

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T.J. Langlois

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E.S. Harvey

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B. Fitzpatrick

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J. Meeuwig

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G. Shedrawil

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M. Westera

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D.L. Watson

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This study was conducted with logistical assistance from the Department of Fisheries Western Australia (DoF). Financial assistance was received from The University of Western Australia (UWA), from the West Australian and Australian Governments Natural Heritage Trust initiative (NHT) through the Northern Agricultural Catchment Council (NACC) and from the Western Australian Marine Science Institute. We would like to thank R. Scott in the UWA workshop for construction of the stereo-video systems. We greatly appreciate the help given by Dr. B. Hutchins in identifying numerous fish species.

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