RESEARCH ON THE RELATIONSHIP BETWEEN PREY ABUNDANCE AND FISH AGGREGATION

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1 RESEARCH ON THE RELATIONSHIP BETWEEN PREY ABUNDANCE AND FISH AGGREGATION Kaori Yasuoka 1, Yoshihiro Suenaga 2, Takashi Hoshino 3, Hisao Kakegawa 2 and Hee-Do Ahn 4 1 Graduate School of Engineering, Kagawa University Kagawa, JAPAN s6d45@stmail.eng.kagawa-u.ac.jp 2 Faculty of Engineering, Kagawa University 3 Kuroshio Marine Technology, Co., LTD. 4 Korea Ocean Research and Development Institute ABSTRACT Many types of porous materials have been developed and settled to create fishery grounds for the enhancement of production in marine areas. There are many issues to be clarified about suitable installation of porous materials such as the relationship between clogging conditions and the biological effects of them. In this research, a set of newly developed technologies to restore the environment for juvenile fish is introduced. It consists of a carbonated porous material using an industrial by-product to fouling prey abundance for marine habitat restoration, and an artificial reef that embodies multiple functions. We investigated the contents in juvenile rockfish stomachs to verify whether they could feed on prey abundance. The set of technologies was applied to coastal areas and was verified to improve the biological environment. INTRODUCTION In the past, seaweed beds were used for spawning and nursery grounds of valuable marine animals. The area of Bisan Seto is located east of Seto Inland Sea where red tide and oxygen-depleted water occurs frequently, and where fisheries catch has been declining in recent years. In our study we settled artificial reefs using porous materials for the catching ground or for the nursery of juvenile fishes in order to enhance fish aggregation, exuberance of seaweeds, and density of prey abundance. We conducted field observations to verify any biological effects of a disposed artificial reef using porous material made from an industrial by-product. The disposed artificial reef functions as a current regulator generating the upwelling flow from the sea bottom. The porous material which was made of air-cooled blast furnace slag could be the high enhancement function against other materials like stones, steels and concrete blocks which are presently used as artificial reefs in Japan. We adapted the carbonation technology to make the porous material with an expected reduction of CO 2 and a means of recycling industrial by-products. 79

2 MATERIALS & METHODS The new type of porous material (φ3-4mm) measures 428mm width by 5mm length by 15mm thick per unit (Fig. 1). The material has a bigger void and appropriate strength as a fishery resources enhancement substrate. The materials were carbonated to match the alkalinity equal to seawater (ph = 8-9 in Seto Inland Sea) using a special device to carbonate it (Shiraki et al., 24). We compared the density of the fouling prey abundance of the Figure 1. Using materials (Slag φ3-4mm) carbonated porous material with non-carbonated porous material. Figure 2 shows the artificial reef with the current control function (Miyagawa, 24). In order to verify the biological effects of this artificial reef made of concrete and steel materials, it was installed on plural sea beds (Kakimoto, 24; Suenaga et al., 24; Yasuoka et al., 26) in the coastal marine areas of Kagawa Prefecture, Japan. As the porous materials pulled up after installation, we counted the fouling prey abundance over the surface and inner part of the porous materials. Carbonation porous material (φ 1-2) Carbonation porous material (φ 15-2) Carbonation porous material (φ 3-4) Carbonation porous material (φ 3-4) Concrete (Basic block of structure) Steel (Support frame of the structure) Figure 2. Setting of carbonated porous material on the artificial reef The aggregation of juvenile fish in and around the artificial reef was verified using the cover net method (Fujiwara and Kagawa, 26; Yamaga et al., 24) is shown in Figure 3. The cover net mesh measures 6mm and the net size is 15m length by 6m height. It was installed with a floating buoy at the upper end and anchor weighted at the lower end, and it was possible to connect the double end of the net. We covered juvenile fish with seaweed so the caught fish might not escape from the cover net, then pulled the net after cutting off the root of the seaweed, to the research ship. It could be considered quantitative evaluation of fish aggregation in and around the artificial reef. We also investigated the gastric contents in juvenile rockfish to verify whether they fed on prey abundance. There was no data of gastric contents in juvenile fish in the Bisan Seto area. 8

3 Figure 3. The cover net method to catch juvenile fish in and around the artificial reef Figure 4 shows the marine areas of Kagawa Prefecture and Yamaguchi Prefecture for installation of the proposed artificial reefs. Japan Sea Nago bay Hagi Yamaguchi Pref. Japan Sea Megijima Yashima bay Aji Takamatsu Kagawa Pref. Figure 4. Proposed areas of artificial reefs 81

4 RESULTS AND DISCUSSION Figure 5 depicts the wet weight and population of fouling prey abundance per unit 13 months after installation of the artificial reef. Fouling prey abundance on the surface and inner part of the carbonated porous material was 5-45 times compared to other substrates and proved that carbonation was effective for fouling prey abundance. Wet Weight(g) Carbonated 7.54 concrete steel stone (a)wet weight of the prey abundance Population Noncarbonated Noncarbonated 183 Carbonated 18 concrete 419 steel 361 stone (b)population of the prey abundance Figure 5. Fouling prey abundance of each substrate on the Yamaguchi Pref. artificial reef Pr ey abundance (g/m3) Porosity(%) Prey Abundance (g/m 3 ) 着生量 (g/ m3 ) ,171 13,6 1 1,644 5,731 1, Progress 経過月 ( month ヶ月 ) Figure 6. Fouling prey abundance on four kinds of porosity (Yashima Bay, Kagawa Pref.) Figure 7. The changes in the fouling prey abundance of carbonated porous material with 3% porosity. 82

5 Figure 6 shows the fouling prey abundance on four kinds of porosity. The porous material with 3% porosity had the highest density of fouling prey abundance against other porosities (%, 2% and 5%). Figure 7 shows that over time the density changes in the fouling prey abundance of carbonated porous material with 3% porosity increased. The wet weight after 2 months from installation was over 46,g/m 3. There are few data on gastric contents of juvenile fish in the Bisan Seto area of Seto Inland Sea, Japan. Therefore, we tried to catch juvenile fish in and around the installed artificial reefs to confirm whether they are bait for prey abundance. From the resulting catch using the cover net method, and dissecting fish stomachs, it was verified the juvenile fish feed the fouling prey abundance on the surface and inner part of the porous material. Carbonated porous material enhanced marine resources. Additionally, it could be thought that the fouling prey abundance would like the space which consisted of slag diameter φ4-3mm with 3% porosity. Table 1 shows the aggregate population of juvenile fish during 1-hour surveys in Yashima Bay between the structure and control area. We compared the fish aggregation in each area of Yashima, Megijima and Aji. Table 1. Aggregate population of juvenile fish Species In and around the Control structure (Reef of stone) Sebastas inermis (rock fish) 5 4 Hexagrammos otakii 2 - Sebastiscus Marmoratus 15 - Spratelloides gracilis 4 - Acanthogobius flavimanus 3 5 Total Figure 8 shows the result of juvenile fish aggregation comparison in each area. Each research site had a high quality of fish aggregation compared to the control area. Population 個体数 of fish juvenile 3,5 3, 2,5 2, 1,5 1, ,128 1,184 Figure 8. Comparison of fish juvenile aggregation in each area 536 構造物周辺 内部 Structure 対象区 Control area Yashima Bay Megijima Aji 121 屋島地先 女木島地先 庵治地先 Research 調査海域 Area 83

6 In May 26, we conducted the fish catch in and around the structure using the cover net method. From the result of that catch (Table 2) there are many rockfish in and around the structure. We examined the contents of the rockfish s stomach to confirm the predation condition of bait. Table 2. The fish catch by cover net method Fish Spieces Megijima Aji Sebastas inermis (rock fish) 134(22) 35(2) Pseudoblennius percoides 6( 5) 17( 4) Pholis nebulose 1( 1) - Tridentiger trigonocephalus 1( 1) - Ruderius ercodes 1( 1) - Zoarchias veneficus - 18( 6) Total 143(3) 34(3) ( ) Number of fish which analyzed gastric contents Table 3 shows the body length, body weight and predation of sampling juvenile fish, confirming the same fouling organisms, and clarifies the high availability of prey abundance by juvenile fish. Table 3. The average predation by juvenile fish Gastric Contents Body length (mm/ind.) Body weight (g/ind.) Predation (g/ind.) Megijima Aji Megijima Aji Megijima Aji Sebastes inermis (rock fish) Pseudoblennius percoides Pholis nebulose Tridentiger trigonocephalus Ruderius ercodes Zoarchias major Carbonated porous material with porosity of 3% (Steel Slag φ:4-3mm) had the highest density of fouling prey abundance against other porosities (%, 2% and 5%). Additionally, in porous material with 3% porosity the plugging was not being generated after 2 months from installation and it could be an appropriate void for fouling prey abundance. By using the cover net method to catch the juvenile fish, we verified that many juvenile rockfish aggregate in and around the fishery resources enhancement structure. Also, if the juvenile rockfish bait the fouling prey abundance on the proposed porous materials, it would become an effective habitat for the juvenile fish. This research showed that the approach of developing a new type of artificial reef using carbonated porous material is effective in the quantitative evaluation of juvenile fish aggregation in a shallow marine area. 84

7 CONCLUSIONS We proved that carbonation of porous material is effective for high availability of fouling prey abundance for juvenile fish. By analysis of gastric contents, we found that many juvenile rockfish bait the fouling prey abundance, and that fouling prey abundance on the surface and inner part of the carbonated porous material was over 5-45 times compared to other substrates such as stone, concrete, steel and other non-carbonated porous materials. We will continue to investigate comparison of juvenile fish aggregation and fouling prey abundance using carbonated porous materials for long-term marine enhancement. REFERENCES Fujiwara, M. and S. Kagawa. 26. The Fish Juvenile Recruitment by Cover Net Method, Technical Report of Kagawa Prefectural Fisheries Research Institute, 26, 4-7. Kakimoto, A., 24. Fish Aggregation Function of the Artificial Reef. Technical Report of Research of Artificial Reef Technology, 5(2), Miyagawa, M., 24. Research on the Function of the Structure Enhancing Marine Resources. Proceedings of Korea-Japan Maritime Seminar, 24, Shiraki, W., et al. 24. Development of Carbonated Porous Material for the Improvement of Marine Environment, Technical Report of the Region New Birth Consortium Research and Development Project, Kagawa Industry Support Foundation, 25, Suenaga, Y., Y. Kawahara, N. Yamamoto, and Y. Tanaka. 24. Development of New Technology for the Restoration of Marine Biological Environment (in Japanese with English abstract). Journal of Environmental Systems and Engineering, Japan Society of Civil Engineers, 755/VII-3, Yasuoka, K., T. Hirosako, T. Hoshino, Y. Suenaga, and H. Kakegawa, H. 26. Techniques for Improvement of Marine Environment Including the Biological Production Using the Industry By-Product, Proceedings of OCEANS 6, PMT Yamaga, K., M. Fujiwara, S. Kagawa, and N. Ryuman. 24. Fish Aggregation of Artificial Reefs, Technical Report of Kagawa Prefectural Fisheries Research Institute, 24 (2),