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Artificial Reefs: Nothing More Than Benthic Fish Aggregators Jeffrey J

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Artificial Reefs: Nothing More Than Benthic Fish Aggregators Jeffrey J POLOVINA: ARTIFICIAL REEFS: BENTHIC FISH AGGREGATORS ColCOFl Rep., Vol. 30,1989 ARTIFICIAL REEFS: NOTHING MORE THAN BENTHIC FISH AGGREGATORS JEFFREY J. POLOVINA Southwest Fisheries Center Honolulu Laboratory National Marine Fisheries Service, NOAA 2570 Dole Street Honolulu, Hawaii 96822-2396 ABSTRACT lieve the real benefit of the reefs is that they aggre- The potential for artificial reefs to substantially gate wide-ranging fishes close to shore so they can increase standing stock of marine resources is con- be harvested by fishermen with small vessels and sidered. Three sources - the Japanese artificial reef thus keep the fleet of small vessels economically program; relationships between fishery production viable. and the area of natural habitat for several fisheries; Outside of Japan, artificial reefs have not been and population dynamics - offer evidence that ar- deployed on a large enough scale to evaluate their tificial reefs do not substantially increase the stand- effectiveness in increasing standing stocks. How- ing stock of marine resources. ever, examining the relationship between habitat and fishery production can provide estimates of the RESUMEN level of fishery yield per area of habitat that might be expected from appropriately designed and sited Se considera el potencial de 10s arrecifes artifi- artificial reefs. One example is penaeid shrimp: ciales para aumentar sustancialmente el stoc4 dis- worldwide fishery yields range from 8 to 200 kg/ ponible de 10s recursos marinos. Tres fuentes de hectare of intertidal nursery habitat (Turner 1977). evidencia: el programa de arrecifes artificiales ja- pods, la relaci6n entre producci6n y Area del hAbi- In the case of artificial reefs, yields are measured in production per unit of reef volume. Thus if a square tat natural de varias pesquerias, y la dinimica de meter of intertidal habitat is assumed to also contain poblaciones, indican que 10s sustratos artificiales no about a meter of vertical structure, grass, or man- aumentan sustancialmente el stock disponible de 10s grove, then these yield figures are equivalent to 0- recursos marinos. 0.02 kg/m2. Another example is the coral reef sys- tems, from which .fishery yields have been reported DISCUSSION in the range of 5-20 t/km2 (Marten and Polovina Artificial reefs can be excellent fish aggregators, 1982). If one square meter of coral reef habitat is but they do not effectively increase standing stock. conservatively assumed to be equal to one cubic This position will be supported with three types of meter of reef volume, then production per volume evidence: first, with observations from the Japanese of reef habitat is 0.005-0.02 kg/m3. artificial reef program; second, from the relation- To put this production-per-unit-of-habitat vol- ship between habitat and fishery production; and ume in perspective, I will relate it to an example finally from considerations of population dynamics. from California, where recent annual commercial Between 1976 and 1987, the Japanese spent U. S. landings for rockfishes are about 15,000 metric tons $4.2 billion to construct and deploy 6,443 artificial (MT) (California Department of Fish and Game reefs, covering 9.3% of the ocean bottom from 1987). I will assume that the average figure for fish- shore to a depth of 200 m (Yamane, in press). But ery production per habitat volume from coral reef despite this enormous volume of artificial reefs de- fisheries, 0.01 kg/m3, can be applied to rockfishes. ployed in coastal water, there has not been any I will further assume that artificial reefs can be ten measurable increase in coastal fishery landings (Ja- times more effective than natural habitat that might pan: Statistics and Information Department 1984). include barren or unproductive areas. With a level Studies specifically investigating the impact of the of fishery production of 0.1 kg/m3 of habitat vol- artificial reefs have generally not documented any ume, it would require 15 million m3of artificial reefs significant increases in fish production that can be to increase the annual rockfish catches by 10%. At attributed to the reefs (Kawasaki 1984; Kakimoto the extremely low cost of $10/m3 of reef for con- and Okubo 1985). After three visits to Japan and struction and deployment, this would cost $150 numerous discussions with people involved in all million. To put 15 million m3 of artificial reefs into aspects of the Japanese artificial reef program, I be- perspective, consider a space 100 X 50 m, about the 37 POLOVINA: ARTIFICIAL REEFS: BENTHIC FISH AGGREGATORS CalCOFl Rep., Vol. 30,1989 size of a football field, covered with 1-m cubes. One by the fishery, independent of density. However, if such field would contain 5,000 m3; therefore, 3,000 the fraction of the settled larvae, which ultimately such fields covered with 1-m cubes would equal 15 contribute to the fishery, declines as the density of million m3. Of course the major flaw in this brute settled larvae increases, then density dependence force approach to fishery enhancement is that even may be a factor, and artificial reefs, which provide if this enormous volume of artificial reefs was built additional habitat and reduce the density at some and deployed, any benefits would not be noticeable, stage, may increase fishery production. A relation- because landings annually fluctuate by 10% to 20%. ship between larval settlement and fishery catches is That is exactly one of the lessons the Japanese available for a spiny lobster (Punulirus Cygnus) fish- learned with their $4.2 billion experiment. ery in western Australia (Phillips 1986). This rela- There is also the question of which resources lose tionship is linear, indicating that the same fraction habitat when low-relief habitat is covered with ar- of the settled larvae are captured as adults by the tificial reefs. There is a perception that low-relief fishery, independent of the level of larval settle- habitat is not used by important commerical spe- ment. Thus, even for a spiny lobster which requires cies, whereas high-relief habitat is preferred by shelter, larval settlement is the limiting factor to valuable species. This is often based on the obser- production, and even at high postlarval densities, vation that the adults are seen and caught in high- habitat is not limiting to fishery production. relief habitat. This has been commonly accepted in Artificial reefs are often suggested as a solution to Hawaii, but recently, when correct sampling gear overfishing. Yet they do not help if either growth was used, the juvenile habitat of the very valuable or recruitment overfishing is occurring. In the case deepwater snappers was found to be the low-relief, of growth overfishing, they may aggregate flat-bottomed, sandy habitat, which had been con- younger fish, making them more vulnerable to cap- sidered a biological desert. Large-scale deployment ture and actually increasing overfishing. In the case of artificial reefs on this flat, sandy-bottom habitat of recruitment overfishing, standing stock is a frac- would have attracted shallow-water reef fishes at tion of its unexploited level, and habitat is certainly the cost of destroying juvenile habitat for the more not limiting. Aggregating adults further simply in- commercially valuable deepwater snappers. In Ja- creases catchability, and hence fishing mortality, pan, a study around an artificial reef site found that which further reduces the spawning stock biomass. the artificial reef attracted some species and repelled Reefs are popular as management options be- others and that the effectiveness of the reef could be cause they do not require reductions in fishing effort negative, depending on which species were at- and they aggregate fish, resulting in higher catches tracted or repelled (Kawasaki 1984). in the initial stages. Thus artificial reefs may actually The wide ranges in the levels of fishery produc- be detrimental to the fishery and the stock simply tion per area of habitat suggest that, although habi- because they allow managers to delay making hard tat is necessary, it is not limiting to production. but necessary decisions, such as imposing size limits Current research suggests that for many species, or reducing effort. population size is determined during the larval rather than benthic phase. For example, the adult LITERATURE CITED population of the Caribbean coral reef fish, Thulus- California Department of Fish and Game. 1987. Review of some Cali- soma bfusciatum, is determined primarily by recruit- fornia fisheries for 1986. Calif. Coop. Oceanic Fish. Invest. Rep. 28:ll-20. ment and not by the supply of space on the reef Japan: Statistics and Information Department. 1984. Fisheries statistics (Victor 1983). In another study, the survival ofju- ofJapan 1984. Ministry of Agriculture, Forestry, and Fisheries, Gov- venile reef fishes was estimated for varying levels of ernment ofJapan, 61 pp. Kakimoto, H., and H. Okubo. 1985. Fishery production from artificial recruitment, and survivorship appeared constant - reefs (Jinko gyosh6 ni okeru gyogyd seisan). In Comprehensive re- independent ofjuvenile density on the reef (Sale and search on artificial reefs in the coastal areas of Niigata Prefecture Ferrell 1988). These studies indicate that even for (Niigata-ken engan-iki ni okeru jinkd gyoshd no sogoteki kenkyu to jigyo). Niigata Prefectural Fisheries Experimental Station, pp. 193- coral reef fishes that require reef habitat during ju- 200. (Engl. transl. by T. Otsu, 1987, 14 p., Transl. No. 108; available venile and adult phases, more reefhabitat would not Southwest Fish. Cent. Honolulu Lab., Natl. Mar. Fish. Serv., increase standing stock. NOAA, Honolulu, HI 96822-2396.) Kawasaki, T. 1984. The distribution and behavior of fishes in the arti- Insight into whether a resource can be enhanced ficial reef fishing grounds (Jinko gyosho ni okeru sakana no kodo to with artificial reefs can be obtained from the rela- bunpu). IJIBiological process in the ocean (Kaiyo no seibutsu katei), tionship between larval settement density and fish- R.
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