Fish assemblages associated with three types of artificial reefs: density of assemblages and possible impacts on adjacent fish abundance

Abstract--We evaluated the effectiveness of wooden artificial reefs (ARs) as fish habitat. Three types of ARs, made of cedar logs, broadleaf tree logs, and PVC pipes, respectively, were deployed in triplicate at 8-m depth off Maizuru, Kyoto Prefecture, Sea of Japan, in May 2004. Fish assemblages associated with each of the nine ARs were observed by using SCUBA twice a month for four years. Fish assemblages in the adjacent habitat were also monitored for two years before and four years after reef deployment. In the surveyed areas (ca. 10 [m.sup.2]) associated with each of the cedar, broadleaf, and PVC ARs, the average number of fish species was 4.14, 3.49, and 3.00, and the average number of individuals was 40.7, 27.9, and 20.3, respectively. The estimated biomass was also more greater when associated with the cedar ARs than with other ARs. Visual censuses of the habitat adjacent to the ARs revealed that the number of fish species and the density of individuals were not affected by the deployment of the ARs. Our results support the superiority of cedar as an AR material and indicate that deployment of wooden ARs causes no reduction of fish abundance in adjacent natural reefs.

The primary goal of the present study was to confirm the efficacy of wooden ARs, especially those made of cedar tree logs as fish habitat. For this purpose, fish assemblages associated with ARs made from cedar trees were compared to those made from broadleaf trees and those made with polyvinyl chloride (PVC) pipes. There is a debate whether ARs merely attract fishes from adjacent areas or whether they do improve fishery productivity (Grossman et al., 1997; Pickering and Whitmarsh, 1997). We therefore tested the possibility that ARs attract fishes from adjacent areas and thus concentrate fish abundance at the ARs, rather than fish abundance is spread over the fishing ground as a whole. A visual census had been conducted twice a month for more than two years before the deployment of these ARs in adjacent areas; hence the fish fauna was compared in the area before and after the deployment of ARs.The recent trend for ARs in Japan has shifted from concrete to wooden construction. This has been partly due to funding shortages, but also because fishermen have found that wooden ARs attract fish more rapidly than those made of concrete or steel. Indeed, most coastal prefectures in Japan deploy wooden ARs with or without governmental subsidies under the supervision of local fishermen's cooperatives. The materials and shape of wooden ARs differ depending on each fishery cooperative. As much as 70% of the land area in Japan is forested, half of which is plantation forests of conifers, such as Japanese cedar (Cryptomeria japonica) and hinoki cypress (Chamaecyparis obtusa). Although these forests require occasional thinning, many of them lack such maintenance because of the decline in the market price of timber. Therefore, the construction of wooden ARs also has the socioeconomic potential to stimulate the demand for forestry materials.Twice monthly visual censuses of fish assemblages associated with each AR were conducted for four consecutive years after AR deployment. All census observations were made by the first author with SCUBA equipment. The area in and around each AR was observed for about three minutes and the species, size, and number of fish were recorded. A census commenced from one of the lateral sides of an AR and extended out to about 1 m from each side. The observer then swam around and above the AR, and the fish inside the AR were recorded. Fish were considered as associating with an AR if they were swimming or dwelling within 1 m of the AR (Sherman et al., 2002), and thus fish in an area of about 10 [m.sup.2] were counted for each AR. Fish standard length (SL) was estimated with the help of a scale marked on a clipboard and was recorded. Length estimates were occasionally calibrated by capturing and measuring fish. These calibrations revealed that visual SL estimates were within 10% error of the actual measured SL. Water temperature and visibility during observations ranged from 10.1[degrees] to 28.8[degrees]C and from 1 to 5 m, respectively. Biomass calculation for each AR was conducted according to the method of Santos et al. (2005) and Friedlander et al. (2007). The estimated average length of each species for each sample was converted to mass by using the length-mass relationshipThese three types of ARs were constructed in triplicate and deployed at a depth of 8 m off the Maizuru Fisheries Research Station (MFRS), Nagahama, Maizuru, Kyoto (35[degrees]29'N lat. and 135[degrees]22'E long.) on 21 May 2004 (Fig. 2). The shore in this area is a concrete bank and its subtidal zone consists of natural rocks, concrete blocks, both partly covered by live oyster (Crassostrea gigas) and their dead shells, and sandy silt with some macroalgal vegetation. The substrate in the research area consisted of muddy silt with no macroalgae vegetation. Each AR was sunk with 240 kg of sand bags (60 kg attached to each corner of the AR). ARs were set 15 m apart.**********Although the deployment of structures functioning as ARs may well have started long ago by fishermen in various localities around the globe, research on this subject is relatively recent (Seaman and Sprague, 1991). Two countries, United States and Japan, have relatively long histories of nationwide projects on ARs. In the case of the United States, the main goal of deploying ARs has been to improve catch for recreational fishermen. Common materials used for these ARs have been waste products, such as automobiles, tires, and oil and gas platforms. The use of such products has caused environmental concerns, resulting in a shift toward the construction of ARs with concrete (Collins et al., 2002). In contrast, the purpose of Japanese deployments of ARs have primarily been to improve commercial fishery production, and governmental agencies have invested heavily in the construction of large ARs made of concrete and steel to be deployed in coastal areas.[FIGURE 1 OMITTED]Three types of ARs were prepared. The design of the ARs was modified from that designed by the Atake Forestry Association, Yamaguchi, Japan (http://www.geocities.jp/ abu_kikori/katsudou/gyosyou/gyosyou2.html, accessed on December 2003; also see Fig. 1). The first type of AR (cedar AR) was constructed of 16 log sections (1.5 m long, 6.9-18.4 cm diameter) of Japanese cedar (Cryptomeria japonica) arranged in a parallel cross formation. Each corner was tied with rope and fixed with a stainless steel rod. Diagonal wires helped maintain the rectangular shape. The second type of AR (broadleaf AR) was constructed from six species of broadleaf trees harvested from the Ashiu Forest Research Station, Kyoto University, and assembled with the same dimensions as those used for the cedar AR. The broadleaf tree species used were Japanese cherry birch (Betula grossa), hornbeam (Carpinus laxiflora), Japanese beech (Fagus crenata), Chinese chestnut (Castanea crenata), redvein maple (Acer rufinerve), and macropoda holly (Ilex macropoda). The diameter of broadleaf and cedar logs ranged from 7.5 to 19.2 cm. The third type of AR (PVC AR) was made of hollow PVC pipes (11.8 cm diameter, 3 mm thickness) and was assembled in the same manner as that used for the other two types of ARs.Materials and methodsHabitat complexity plays a major role in the survival of young demersal fishes by providing a refuge from predation (Ferreira et al., 2001; Scharf et al., 2006; Hamilton and Konar, 2007). Fish species richness is highly dependent on the rugosity and variety of growth forms in the habitat, whereas the height of vertical structures is an important predictor of total fish abundance (Gratwicke and Speight, 2005). In this respect, artificial reefs (ARs) are often deployed to improve the quality of habitat (Gorham and Alevizon, 1989). In addition to their role as refuges, ARs host encrusting invertebrates that can be consumed as prey by fishes (Seaman and Jensen, 2000). Fish are often more abundant at ARs than at natural reefs, probably because the vertical structures potentially allow more varied refuges for fish settlement and recruitment than the usual more moderately sloped bottoms of natural reefs (Rilov and Benayahu, 2000; Reed et al., 2006).

Twice monthly visual censuses of fish assemblages associated with each AR were conducted for four consecutive years after AR deployment. All census observations were made by the first author with SCUBA equipment. The area in and around each AR was observed for about three minutes and the species, size, and number of fish were recorded. A census commenced from one of the lateral sides of an AR and extended out to about 1 m from each side. The observer then swam around and above the AR, and the fish inside the AR were recorded. Fish were considered as associating with an AR if they were swimming or dwelling within 1 m of the AR (Sherman et al., 2002), and thus fish in an area of about 10 [m.sup.2] were counted for each AR. Fish standard length (SL) was estimated with the help of a scale marked on a clipboard and was recorded. Length estimates were occasionally calibrated by capturing and measuring fish. These calibrations revealed that visual SL estimates were within 10% error of the actual measured SL. Water temperature and visibility during observations ranged from 10.1[degrees] to 28.8[degrees]C and from 1 to 5 m, respectively. Biomass calculation for each AR was conducted according to the method of Santos et al. (2005) and Friedlander et al. (2007). The estimated average length of each species for each sample was converted to mass by using the length-mass relationship