Video transects as a complement to underwater visual census to study reserve effect on fish assemblages

Abstract

The present study focuses on the use of diver-operated video (DOV) as a tool to evaluate the impact of a marine reserve on a fish assemblage, in comparison to underwater visual census (UVC). Samplings were conducted in the vicinity of the marine protected area (MPA) Cerbère-Banyuls (northwestern Mediterranean Sea). Four sites, with different levels of protection, were sampled 4 times, at depths of 5 and 10 m, to study abundance, species richness, structure of fish assemblages and fish aggregation. Results obtained by DOV and UVC provided evidence of significant effects of protection on abundance, species richness and fish assemblages. However, detection of weaker reserve effects using DOV may be limited. Furthermore, certain species are difficult to identify by video (e.g. Symphodus spp.), but DOV may be useful when studying a target and/or easily identifiable species. Contrary to UVC, DOV data can be archived and used for further study. Results concerning fish aggregation within fish assemblages showed specific patterns. Sites outside the MPA were mainly characterized by the absence of fish or few fish aggregations, and sites inside were characterized by high fish density (i.e. dispersed or patched aggregations). Thus, the fish aggregation parameter measured using DOV is an interesting metric to quantify reserve effect. Consequently, although this technique is time-consuming to produce results, DOV presents great potential for estimating MPA effectiveness while allowing simultaneous work on several sites to limit temporal bias.

Diver operated video most accurately detects the impacts of fishing within periodically harvested closures

“…We conclude that DOV was the most suitable technique for detecting the change in fish populations after this harvest. This is contrary to a previous study which suggests DOV may only be suitable as a compliment to UVC (Tessier et al, 2013). The ability of DOV to detect the impacts of a moderate harvest suggests it will be a useful tool for investigating the effectiveness of PHCs as a fisheries management technique.…”

“…Previous studies attributed this difference to reduced clarity of video images and/or a restricted field of view associated with video techniques, preventing them from recording fish at greater distances (Holmes et al, 2013;Pelletier et al, 2011;Tessier et al, 2013). As was the case in this study, visibility often determines the distance at which fish are counted during UVC transects (Samoilys, 1997), however, variation in visibility may alter results.…”

“…However, without knowing the "true" composition of fish within an area, it is not possible to determine which technique is the most accurate. As a result, some studies use gradients in fishing pressure (such as that found inside and outside of a marine reserve) to determine which techniques best detect differences (Langlois et al, 2006;Pelletier et al, 2011;Tessier et al, 2013). Again, without knowing the "true" gradient or difference they cannot conclude which technique is the most accurate.…”

Chasing fish and catching data: recreational spearfishing videos as a tool for assessing the structure of fish assemblages on shallow rocky reefs

“…Spearfishing is very popular along the coasts of the Mediterranean (Lloret et al 2008) and we argue that opening a basin-wide call for videos may allow assessing variation in the structure of shallow fish assemblages at spatial and temporal scales that are well beyond those allowed by ad hoc sampling campaigns operated by scientific divers. However, we stress that this does not imply that spearfishing videos represent a solution to the variety of biases that have been identified for visual sampling techniques (HarmelinVivien et al 1985, Cappo et al 2004, Edgar et al 2004, Cole et al 2007, Stobart et al 2007, Tessier et al 2013.…”

“…In contrast to most studies attempting to assess the validity of remotely or diver operated video as a sampling technique, generally carried out in MPAs , Stobart et al 2007, Tessier et al 2013, this study was carried out along a coast characterized by heavy exploitation of fish assemblages by both commercial and recreational fisheries. On the one hand, this has prevented an assessment of how the effectiveness of spearfishing videos in sampling large predatory fish (rare at our study sites) compares to that of traditional UVC.…”