A baited underwater video technique to assess shallow-water Mediterranean fish assemblages: Methodological evaluation
Introduction
In studies of fish populations in shallow water, abundance estimates are usually obtained by scuba divers using underwater visual census (UVC) methods (e.g. Harmelin-Vivien et al., 1985, García-Charton et al., 2000). Advantages of UVC methods include: that they are non-destructive (making them especially well suited to work in Marine Protected Areas– MPAs); relatively rapid and cheap to perform; do not require subsequent lab-work; and are useful to record a great variety of variables (e.g. relative abundance, density, size structure, species composition, habitat characteristics). However, it is well known that UVC has methodological shortcomings which include constraints of scuba diving (imposed regulations of scientific diving, physical limitations and environmental constraints), observer's diving experience and training level, and species-specific variability in the attraction or escape response to the presence of divers. The latter two introduce bias, which affects the accuracy of estimates (Christensen and Winterbottom, 1981, Sale and Douglas, 1981, Brock, 1982), and has lead to criticism of UVC techniques (McCormick and Choat, 1987, Lincoln-Smith, 1988, Greene and Alevizon, 1989, St. John et al., 1990, Watson et al., 1995). To reduce the risk of bias it has been suggested that multiple methods should be used concurrently to obtain overall estimates of fish abundance (Conell et al., 1998, Willis et al., 2000, Cappo et al., 2004). Furthermore, recent studies have demonstrated that the accuracy of a single survey method can be variable for sampling multi-species fish assemblages (Hickford and Schiel, 1995, Jennings and Polunin, 1995, Kulbicki, 1998, Willis et al., 2000).
Remote video systems provide an alternative or complementary method to UVC with which to estimate the relative abundance of marine species. For many authors the principal advantages of remote video methods are that images can be checked by several observers as many times as necessary, and that they provide useful records of abundance, richness, size and behaviour (e.g. Willis and Babcock, 2000, Willis et al., 2000, Cappo et al., 2003). Video records also allow better standardisation of data collection over long time series and permanent records of the fish observed are obtained without destroying the fauna. Furthermore, video can offer a solution to some of the biases caused by attraction or repulsion of some fish to scuba divers (Francour et al., 1999), though baited systems described below have their own biases associated with fish attraction patterns to bait (e.g. Cappo et al., 2004). But most importantly, remote video solves the problem of difficult access to divers due to adverse environmental conditions and depths in excess of 40 m. With adequate lighting and housing materials, and control over timing of image acquisition, video techniques can be used for long durations at any time of day or night and at any depth (Cappo et al., 2004).
In recent years there has been a sharp increase in the number of studies using video devices to study shallow-water marine flora and fauna due to the advent of relatively cheap digital devices and the availability of better image processing software. Some authors have used stationary (Francour et al., 1999) and roving cameras (Tessier et al., 2005, Tessier and Chabanet, 2006), but the predominant video method for studying fish populations is by stationary “baited underwater video — BUV” (e.g. Ellis and DeMartini, 1995, Gledhill et al., 1996, Priede and Merrett, 1996, Hill and Wassenberg, 2000, Willis and Babcock, 2000, Willis et al., 2000, Yau et al., 2001, Jones et al., 2003). The use of bait was a logical solution to the problem of low fish counts associated with fish passing by chance within the camera view (e.g. Posey and Ambrose, 1994), as bait can greatly increase the sampling area by attracting fish from potentially large areas (Cappo et al., 2003). The use of BUV to census carnivorous fish (Willis and Babcock, 2000, Willis et al., 2000) is of particular interest in the case of MPAs, as it provides non-destructive data on predatory species that are often targeted by fisheries and exhibit “reserve effects” (we define “reserve effects” as any response at the species or community level that is the consequence of the reduction or cessation of fishing, and also see Harmelin et al., 1995). BUV may also have the added advantage of recording non-predatory species that are attracted due to curiosity as well as those species that would be passing through the field of view (Cappo et al., 2003, Cappo et al., 2004). Important limitations of BUV techniques include not knowing the attraction area for the bait (which will also vary depending on currents, wave action, topography, fish appetite, feeding activity and bait type amongst others), and limitations on the number of fish that can visit the bait as there may be more fish than can fit in the bait area, or competitive exclusion of some species by others attending the bait (Willis and Babcock, 2000, Bailey and Priede, 2002, Cappo et al., 2003).
Prior to this study, BUV had not been used to study fish populations in the Mediterranean. Our main objective was to assess the performance of a BUV system for estimating abundance and diversity of Mediterranean reef-associated fish assemblages. We also assess optimum deployment times based on species accumulation and abundance response times of different species. Although not part of the BUV assessment, we compare our BUV estimates with UVC estimates from a different study that was conducted at the same locations, but at different times (Harmelin-Vivien et al., in review). The results are not strictly comparable due to the temporal differences in sampling times, but providing this is borne in mind the availability of data from both techniques in the same locations provide a unique opportunity, as comparison of BUV with other commonly used techniques is the best available way to gauge the performance of this method (Cappo et al., 2003).
Section snippets
Study areas
This study was conducted between June and November 2004 at six MPAs and adjacent areas (hereafter called locations) in the western Mediterranean: Cabrera, Tabarca, Medes and Cabo de Palos–Islas Hormigas (hereafter called Cabo de Palos) in Spain, and Cerbère–Banyuls (Banyuls) and Carry-le-Rouet (Carry) in France (Fig. 1). Sampling at each location was carried out in nine sectors. All sectors were located thousands of metres apart, and within each sector 3 zones were selected to perform three
Results
A total of 51 species belonging to 33 families were recorded from BUV deployments at all of the locations combined, and of these approximately 60% were present at three or more locations. Half of the species recorded from all locations could be considered to be “common” in video surveys as they appeared in more than 20% of the deployments (Max % presence) at at least one of the locations (Table 1). Within locations the proportion was lower with 16–29% of species present being “common”,
Discussion
This study has demonstrated that the BUV system used is suitable for attracting a wide range of Mediterranean reef-associated fish species, and that the power of attraction is consistent between deployments at all of the locations studied. Results suggest that the BUV is an effective tool for recording the diversity of fish species with relatively low deployment times. However, although there were a few exceptions, abundance estimates were low for many species and zero values were frequent,
Acknowledgements
This study was carried out within the EC project BIOMEX (QLRT-2001-0891). We wish to thank all the BIOMEX consortium for fruitful discussions throughout the project. We greatly acknowledge help with video technology from Antonio Alpañez, and the assistance of Jesús Nicolás for making the video caging structure. Field assistance by Aitor Forcada, Mikel Zabala, Oscar Esparza, and Laurence Le Diréac'h is greatly acknowledged. We also thank the staff of Cabrera, Tabarca, Medes, Cabo de Palos–islas
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