Assessment of Ichthyofauna at Oyster Rafts in Hiroshima Bay, Japan, Using Underwater Video Cameras

Aquacult. Sci. 66（4），267－274（2018） Assessment of ichthyofauna at oyster rafts in Hiroshima Bay, Japan, using underwater video cameras ＊ Atsushi TSUYUKI and Tetsuya UMINO Abstract: There is little information on fish communities around oyster farms, though oyster rafts provide habitats for fish and are important fishing grounds. We investigated the ichthyofauna at oyster rafts supporting Crassostrea gigas in Hiroshima Bay, using underwater video cameras. The fish species composition around the rafts was unique, consisting of identified 18 species, with low similarities to the fish community in nearby littoral areas. Several commercially important fishes, including black sea bream Acanthopagrus schlegelii, filefish Thamnaconus modestus, and surfperch Ditrema temmincki, aggregated at the oyster rafts throughout the year. The abundance of A. schlegelii (68%-85% relative abundance) accounting for most of the fish at the rafts, except in summer. The video recordings revealed that A. schlegelii, T. modestus, and D. temmincki preyed on the sessile organisms (probably mollusks, crustaceans and macroalgae) which had abundantly attached to the oyster wires. Thus, the large and persistent aggregations of fishes suggests that the oyster rafts efficiently function as an artificial reef in the regional ecosystem. Key words: Hiroshima Bay; Ichthyofauna; Oyster rafts; Video survey The culture of Pacific oyster Crassostrea gigas oil jetty platforms and marine salmon farm- is an important, large-scale industry in Japan, ing structures may have higher densities and where total production in 2015 amounted to biomass of fish than adjacent natural reefs, 116,332 tonnes (with shells), valued at approx- due to the benefits of artificial constructions imately 38 billion yen (Ministry of Agriculture, that allow fish to aggregate by enhancing the Forestry and Fisheries 2017). This industry is availability and settlement of food organisms concentrated in Hiroshima Prefecture, which (Rilov and Benayahu 2000; Dempster et al. contributes about 60% to total production 2009). Recently, oyster rafts in Hiroshima Bay (Hirata and Akashige 2004). Since the 1960s, have been considered for their potential to there has been significant expansion of oyster function as artificial reefs and their capacity as farming in Hiroshima Bay, with about 9,994 a local fishing ground. For example, such as oyster rafts deployed there as of 2013. Many about 400 individuals of the black sea bream studies of oyster culture have focused on cul- Acanthopagrus schlegelii were observed around ture methods or the effects of environmental an oyster raft in the Buzen Sea (Nakamura and factors on oyster growth, hydraulic features, Nakagawa 2011). Tsuyuki and Umino (2017) and shellfish poisoning. However, relatively used ultrasonic telemetry to demonstrate that little is known about the fish communities at A. schlegelii inhabiting the oyster-farming area oyster farms, as available studies are typically in the bay are highly dependent on the spatial short-term or relate to only a few fish species arrangement of the oyster rafts. Though the (Sakai et al. 2013). raft structure has been suggested to provide Previous studies indicate that areas around habitat for fishes, seasonal changes of fish fauna Received 3 April 2018; Accepted 21 August 2018. Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8528, Japan ＊Corresponding author: Tel, (+81) 82-424-7944; E-mail: [email protected] (T. Umino). 268 A. Tsuyuki and T. Umino associated with the oyster raft has been very anchored at each end of the line; two or more limited so far. Thus, quantitative estimation rafts are also tied together. Visibility measured of fish abundance at oyster rafts in Hiroshima using a Secchi disk and was about 7 m in the Bay would be a key step to identifying their oyster farming area. ecosystem function. The aim of this study is to reveal the seasonal changes in fish abundance Underwater video survey and species composition around the oyster rafts To determine fish species composition and in Hiroshima Bay in comparison with those abundance under the oyster rafts, we deployed observed in the littoral area by using a small small underwater video cameras (GoPro underwater video cameras. Hero3; GoPro Inc., California, USA) at three positions (Fig. 2). All video recordings were Materials and Methods 60 min long and were taken during the daytime (08:00-17 00 h). We counted the number of Study site individual fish observed in each video record- ∶ Hiroshima Bay is an enclosed bay located in ing. To avoid bias during counting of the dam- the western part of the Seto Inland Sea in west- selfish Chromis notatus notatus (i.e. counting ern Japan (Fig. 1). The average depth is 25.6 m, the same individual twice as it moved in and and the northern and western parts of the bay out of a rock hole), we considered the maxi- are subject to riverine inputs; mean annual sea mum number of individuals that appeared on surface temperature is 19°C, and ranges from the monitor during the entire video record- about 9°C in March to 29°C in August; salinity ing. Small-sized fishes (<10 cm TL) were not averages 29 PSU, and fluctuates from 15 to included in the assessment because of the dif- 33 PSU (Gonzalez et al. 2008). The sea bottom ficulty in achieving a rigorous count. Fish spe- of the bay is mainly composed of sand and cies were identified by referring to the work of rocks, and some of seaweed cover. Masuda et al. (1988). Oyster rafts (8 m wide×16 m long) in the The video surveys were conducted season- bay are each hung with 600 oyster wires (called ally: in spring: May 2016, summer: August 2016, ren in Japanese; length 10 m), each with about autumn: October-November 2015, and winter: 37-40 oyster collectors, providing a complex March 2016. To enable a detailed examination underwater structure (Hirata and Akashige of the fish species composition under the oyster 2004). These wired rens are submerged at a rafts, we set up video cameras at three sections depth of either 0-10 m or 7-17 m, which is ideal of the assemblages: at depths of 5-10 m (upper for the growth and maturation of Crassostrea section of oyster wires), 10-15 m (lower sec- gigas based on the availability of phytoplankton, tion of oyster wires), and 15-20 m (sea bottom a suitable water temperature, and minimization beneath the raft). In each season, six or seven of periphyton growth. The rafts are laid out in oyster rafts were surveyed, except in autumn rows 5-10 m apart, tied together with lines, and 2015, when one camera was lost and only five Fig. 1. Location of the study site at Hiroshima Bay, Seto Inland Sea. Shaded boxes on the close-up map show the oyster farming areas at Etajima Bay and Nomishima Island. Ichthyofauna of oyster rafts 269 rafts were surveyed. As a control site, the littoral 2011). This multivariate analysis was per- area of Etajima Bay was surveyed (Fig. 1). There, formed using the statistical package PRIMER we selected four to six typical littoral sandflat 7 (Plymouth Marine Laboratory, Plymouth, subtidal zones, at depths of 5-10 m, and likewise United Kingdom). conducted seasonal video surveys: in spring: May 2016, summer: August 2016, autumn: December Results 2016, and winter: March 2016. However, the video survey in autumn was conducted along Ichthyofauna of the oyster rafts and the littoral area the west coast of Noumishima Island. A total of 16,110 individual fish were identified, representing 28 species from 19 fami- Data analysis lies. The overall species composition differed The video material was used to identify fish between the oyster rafts and the littoral area, abundance and numbers of species. Species but a seasonal difference was not apparent diversity was calculated using the open-ended (Table 1). We identified a total of 24 fish species Shannon-Weaver index (Doi and Okamura in the littoral area, with the dominant species 2011). The different community indices were being Halichoeres tenuispinis, H. poecilopterus, natural log-transformed (ln (x+1)) and com- Ditrema temmincki, and Mugil cephalus cepha- pared four locations (i.e. the three sections at lus. Eighteen of 24 species were identified near the oyster rafts and in the littoral area) in differ- the oyster rafts, where Acanthopagrus schlegelii, ent seasons, using a two-way ANOVA followed Thamnaconus modestus, and Sebastes cf. iner- by Schefféʼs test. mis were dominant. The oyster farming area Nonparametric multivariate techniques were had a greater abundance of A. schlegelii than used to compare the fish assemblage structure. the littoral area, and this species accounted A similarity matrix was constructed using fourth- for 68%-85% of all fish counted near the root transformed data and the Bray-Curtis simi- upper and lower sections of the oyster wires, larity index. A nonmetric multidimensional-scale except in summer, when it comprised 13%- ordination plot was constructed to visually 58%. Oplegnathus fasciatus, Plectorhinchus explore patterns in the fish assemblages associ- cinctus, and Rhyncopelates oxyrhynchus were ated with the oyster rafts and the control site, to observed only in the oyster-farming area. reveal relationships among the fish assemblage The 2-dimensional nMDS indicated a distinc- structures (Clarke 1993; Doi and Okamura tion between the oyster-farm-associated and Fig. 2. Schematic diagram of the oyster rafts. Three video cameras were deployed on three different sections of the assemblage, at depths of 5–10 m (upper section

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