Impact of Fish and Pearl Farming on the Benthic Environments in Gokasho

FISHERIES SCIENCE 2002; 68: 258–268

Original Article

Impact of ﬁsh and pearl farming on the benthic environments in Gokasho Bay: Evaluation from seasonal ﬂuctuations of the macrobenthos

Hisashi YOKOYAMA*

National Research Institute of Aquaculture, Fisheries Research Agency, Nansei, Mie 516-0193, Japan

ABSTRACT: In order to clarify the influence of mariculture on the benthic fauna, samples of the macrobenthos were collected from Gokasho Bay, where intensive fish culture and pearl oyster culture have been carried out. Monthly samples collected from the fish farm and pearl farm sites during June 1995 to July 1996 revealed that the community structure of the two sites showed distinct differences with seasonal fluctuations. At the fish farm site, azoic conditions were found from July to November; after December, the diversity increased markedly through successive recruitments of small-sized species such as the polychaetes Capitella sp. and Pseudopolydora paucibranchiata, and the amphipods Aoroides spp.; macrofaunal density, biomass and species richness peaked from March to April. At the pearl farm site, a higher diversity, including larger-sized species, and no clear seasonal fluctuations in abundance was found, and the community structure was similar to that at the control site. These results show the large impact by fish farming on the macrofauna, whereas pearl farming causes less effect on the benthic fauna. It is suggested that the difference in the level of organic input between the two sites results in the differences in the dissolved oxygen content of the bottom water, sulfide content of the sediments and, subsequently, the macrobenthic assemblages.

KEY WORDS: aquaculture, Capitella, fish farm, macrobenthos, oxygen depletion, pearl farm, seasonal fluctuation, sulfide.

INTRODUCTION Macrobenthos has been used to assess the impact of fish farming in Finland,4 Scotland,5 Environmental degradation around mariculture North America,6,7 Australia,8 China9 and Japan,2 farms has been conspicuous in many Japanese and to assess the impact of mussel farming in coastal areas. Several reports have shown that Sweden10 and New Zealand.11 These studies show organic wastes discharged from Japanese fish that the reduction in species richness and/or farms cause the deoxygenation of the surrounding species diversity,5–11 the appearance of dense pop- waters,1 and changes in the sediment chemistry ulations of the opportunistic polychaete Capitella and macrofauna.2 At pearl farms, deposition of species complex (especially species I), which often feces and pseudofeces from the cultured pearl result in an increase in total macrofaunal abun- oysters and fouling organisms also cause environ- dance,2,6,7,10 a decrease of large-sized species6 and mental degradation.3 Clear evaluation and mini- the disappearance of echinoderms,10,11 are typical mization of the impacts of farming are necessary effects of mariculture farming on the macroben- from the standpoints of both farm management thos. These effects are identical to those of or- and nature conservation. ganic effluents from other industries and munici- pal sewage, which have been reviewed by Pearson and Rosenberg,12 because nutrients from these dif- *Corresponding author: Tel: 81-599-66-1830. Fax: 81-599- ferent sources have the same potential to cause 1962. Email: [email protected] eutrophication problems such as the deoxygena- Received 2 April 2001. Accepted 29 August 2001. tion of the bottom water2,5,9 and the occurrence of Macrobenthos at mariculture farms FISHERIES SCIENCE 259

reduced conditions in the sediment, as indicated oysters Pinctada martensii covered 79 000 m2 of the by highly negative redox potentials2,5,9,10 and in- bay, producing 800 kg of pearls.16 creased levels of sulfides.2,9 The fish farms and pearl farms are distributed in Gokasho Bay is a typical embayment in Japan, separate parts of the bay; the fish farms are con- where fish and pearl farms are densely distributed. centrated in an inlet, Hazama-ura, whereas the Yokoyama and colleagues have investigated the pearl farms are distributed chiefly at the inner part impacts of mariculture on the bottom environ- of the main basin (Fig. 1). ments and the macrobenthos in this bay. Yokoyama et al. have pointed out a clear change of the fauna has taken place during the past 50 years, Sampling sites which was concluded to be because of the effects of fish farming.13 Yokoyama compared the benthic Sampling was conducted at three stations in the assemblage in Gokasho Bay to those in other bay (Fig. 1). The fish farm site (station A: water localities suffering from hypoxia, and pointed out depth, 18.4 m) is located at the center of the fish that the community structure at the fish farm farm area in Hazama-ura, whereas the pearl farm site closely resembles those in areas under the site (station B: water depth, 14.1 m) is in an area influence of sewage and industrial effluents.14 with a high density of pearl culture. Sampling at Yokoyama et al. preliminarily reported the seasonal these farm sites was conducted within 40 m from fluctuations of the macrobenthos in the fish and the edge of a fish cage and from the edge of a pearl farms in Gokasho Bay.15 In the present paper, culture raft, respectively. The substrate at station A a detailed explanation of the seasonal fluctuations was silty sand (Mdf=2.6–2.9, silt-clay content = of the macrobenthos is made by analysing monthly 23–38%), and the substrate at station B was sandy collections of samples in Gokasho Bay obtained silt (Mdf=6.0–6.2, silt-clay content = 81–82%).13 from the fish and pearl farm sites and a control The control site (station C: water depth, 23.1 m) site with no facilities for farming, to evaluate the is located in the center of the bay, where there are impact differences between fish and pearl farming. no facilities for farming. Grain-size distributions of the sediment at station C were intermediate (Mdf=2.7–3.6,silt-clay content = 10–38%) between MATERIALS AND METHODS those of stations A and B.13

Mariculture in Gokasho Bay Sampling and data analysis Gokasho Bay has a ria style coastline with an area of 22.2 km2 and a mean depth of 12.7 m (Fig. 1). Monthly surveys were conducted during the In this bay, fish farming has developed steadily period from June 1995 to July 1996. Three replicate since the introduction of yellowtail culture in 1962. samples of the macrobenthos were collected from Since 1976, the production of cultured fish, includ- three stations using an Ekman grab (sampling area ing yellowtail Seriola quinqueradiata and red 0.04 m2) and a 0.5-mm mesh sieve. A sediment seabream Pagrus major, in the bay has been over sample (upper 3 cm layer) was obtained by a corer 1500 metric tonnes. In 1995, fish cages covered attached to the inside wall of the Ekman grab17 for 36 000 m2 of the bay, where 1760 metric tonnes of analysis of acid volatile sulfide (AVS-S), which was 16 fish were produced by feeding them raw fish, determined using a H2S-absorbent column moist pellets and dry pellets. Raw fish consists of (GASTEC, Kanagawa, Japan). A water sample from sardine, sand lance, mackerel, jack mackerel, etc., just above the bottom was also obtained by the which have been caught from Japanese coastal same corer, and dissolved oxygen (DO) was mea- waters. Moist pellets are prepared from minced sured by the Winkler method. raw fish, minced krill and compound powder, After sorting, the number of individuals per approximately in a ratio of 3 : 2 : 5 (Hazama farm) species was recorded. After blot drying with tis- or in a ratio of 6 : 0 : 4 (Sazara farm). Dry pellets are sue paper, the wet weight, including shell to the made by pressing compound powder composed of nearest 1 mg, was also recorded. To evaluate fishmeal (50–60% total weight), wheat flour, corn the species diversity of assemblages, the Shannon– flour, soybean dregs, and other additives. Weaver function H¢, the species richness index 18 Prior to the development of fish farms, Gokasho H¢max and the evenness index J¢ were adopted for Bay was famous for its high productivity of cul- individuals of smaller than 1 g (wet weight). tured pearls. In recent years, the production of Species diversity has components of species rich- pearls has decreased to a level as low as one-eighth ness and evenness; this relation is expressed as 18 of its peak in the 1960s. In 1995, rafts of pearl H¢ = H¢max ¥ J¢. 260 FISHERIES SCIENCE H Yokoyama

Fig. 1 Map of Gokasho Bay showing sampling stations (A, ﬁsh farm site; B, pearl farm site; C, control site) with depth contours (m) and areas of ( ) ﬁsh and ( ) pearl farms.

Organic matter load in the fish and pearl farms calculated from data obtained from the Fisher- men’s Cooperative Association of the Hazama farm In Hazama-ura, two different fishermen’s coopera- (pers. comm., 2001). Organic carbon and nitrogen tive associations manage the Hazama farm and the loads into Hazama-ura were determined by total- Sazara farm, respectively, which are contiguous ing the values obtained from these two farms. with each other. Data on the composition Organic matter loads originating from pearl and amount of fish feed thrown into fish cages at farming in the form of feces and pseudofeces of the Sazara farm from 1995 to 1996 were obtained pearl oysters and fouling animals attaching to the from Yamagata et al.,19,20 who classified fish feed pearl oysters and culture apparatus were estimated into three materials: (i) raw fish; (ii) krill; and (iii) from data provided by Uemoto et al.,21 who sur- compound powder. Monthly values of organic veyed a pearl farm in Ago Bay in 1975. Hydro- carbon and nitrogen loads into the Sazara farm graphic and physical conditions in Gokasho Bay were determined from contents of organic carbon are similar to those in Ago Bay and the methods and nitrogen in these materials.19,20 Organic carbon used for pearl culture have not changed since and nitrogen loads into the Hazama farm were also the survey of Uemoto et al.21 Therefore, the values Macrobenthos at mariculture farms FISHERIES SCIENCE 261 per day) per day) 2 per day) 2 per day) 2 2 N (g/m C (g/m N (g/m C (g/m

Fig. 2 Seasonal fluctuations of the () organic carbon and () nitrogen loads in the form of fish feed into the Fig. 3 Seasonal fluctuations of the ( ) organic carbon fish cages in the study area. Data based on Yamagata and ( ) nitrogen loads in the form of feces and pseudo- 19,20 feces of pearl oysters and fouling animals into a pearl et al. and the Fishermen’s Cooperative Association of 21 the Hazama farm (pers. comm., 2001). farm in Ago Bay. Data based on Uemoto et al. obtained from Ago Bay are applicable to the pearl Table 1 Organic matter load from fish farming in farm in the present study area. Hazama-ura in Gokasho Bay 1995 1996

RESULTS Total amount of fish feed 7527 7632 (t wet wt/year)* Total amount of fish feed 4160 3943 Seasonal fluctuations of organic matter load (t dry wt/year)* in the fish and pearl farms Raw fish (t wet wt/year)* 1017 991 Krill (t wet wt/year)* 166 181 Figure 2 shows the seasonal fluctuations of organic Compound powder and dry pellets 2977 2771 carbon and nitrogen loads into the fish cages in (t wet wt/year)* Hazama-ura during the period from January 1995 Total amount of carbon (t/year)* 2341 2225 to July 1996. During the low-temperature (11.0– Total amount of nitrogen (t/year)* 345 328 † 12.8 C at a depth of 3 m) period from January Produced fish (t wet wt/year) 1330 1231 ∞ 2 † through March, the feeding activity of cultured fish Area of fish cage (m ) 21 000 24 000 was low, resulting in low levels of organic carbon 19,20 2 *Based on Yamagata et al. and the Fishermen’s Coopera- and nitrogen loads (i.e. 74–161 g C/m per day and tive Association of the Hazama Farm (pers. comm., 2001). 2 11–20 g N/m per day). Organic carbon and nitro- † Basedon Tokai Regional Agricultural Administration Office.16 gen loads increased after March, and attained 420 t, Metric tonnes. g C/m2 per day and 63 g N/m2 per day in June 1995. In August, they decreased to 298 g C/m2 per day Hazama-ura was 2225–2341 metric tonnes/year, and 45 g N/m2 per day because the feeding activity and that of nitrogen was 328–345 metric tonnes/ of fish diminished as a result of the high water tem- year. Yearly mean values per m2 of a fish cage are perature, which reached 28.5∞C, and the occur- 254–305 g C/m2 per day and 37–45 g N/m2 per day, rences of harmful phytoplankton. After August, respectively. they increased again, and attained maximum Organic carbon and nitrogen loads from pearl values of 512 g C/m2 per day and 76 g N/m2 per day farming per m2 of a raft ranged 0.6–2.7 g C/m2 per in October. Thereafter, they diminished rapidly day, and 0.06–0.4 g N/m2 per day, respectively with the decrease in water temperature. (Fig. 3). Highest values occurred during summer Organic matter loads from fish farming in and were lowest in winter. Yearly mean values for Hazama-ura in 1995 and in 1996 are summarized organic carbon and nitrogen loads are 1.7 g C/m2 in Table 1. In this area, 7527–7632 metric tonnes of per day and 0.22 g N/m2 per day, respectively. feed were thrown into an area of 21 000–24 000 m2 of fish cages during a year, producing 1231–1330 metric tonnes of fish. The dry weight of fish feed Environmental factors at the study sites used in a year was 3943–4160 metric tonnes. Raw fish accounted for approximately 25% of the dry Dissolved oxygen in the bottom water at the fish weight of fish feed, krill accounted for appro- and pearl farm sites was clearly lower than DO ximately 4%, and compound powder, including at the control site during the period from April to dry pellets, accounted for approximately 71%. September (Fig. 4a). At the fish farm site, near an- The total amount of loaded organic carbon into oxic conditions (0.5 mg/L) occurred in June 1996. 262 FISHERIES SCIENCE H Yokoyama

Fig. 5 Seasonal fluctuations in: (a) the density; and (b) Fig. 4 Seasonal fluctuations in: (a) the dissolved oxygen the biomass in terms of wet weight, excluding animals (DO) content of the bottom water; and (b) the acid heavier than 1 g, of the macrobenthos at: ( ) the fish volatile sulfide (AVS-S) content in the sediment at: () farm site; ( ) the pearl farm site; and ( ) the control site. the fish farm site; () the pearl farm site; and () the Each plot represents the mean ± SD of three replicate control site. samples.

There were large differences in AVS-S in the sed- decreased suddenly in May, and azoic conditions iment between the three sites (Fig. 4b). Acid were observed again in July 1996. volatile sulfide at the control site scarcely exceeded Faunal recovery at the fish farm site occurred by 0.2 mg/g (dry sediment), whereas much higher successive colonizations of species (Figs 6a,7a). values were found throughout the year at the fish The polychaete Capitella sp. was the first species farm and pearl farm sites; AVS-S at the fish farm that colonized this azoic bottom, and attained a site ranged from 0.42 mg/g to 1.8 mg/g. Except for maximum density (16 400 ± 2700 individuals/m2) June and July 1995, these values were always higher in February. The polychaete Pseudopolydora than those at the pearl farm site (0.12–0.83 mg/g). paucibranchiata was the second to colonize, Acid volatile sulfide at the fish farm site showed reaching maximum densities (5670–4870 individu- seasonal fluctuations; during summer and autumn als/m2) in March and April. The gammarid values exceeded 1.3 mg/g except in November; Aoroides spp., which appears to contain two sibling after December, values decreased until April; there- species, and the caprellid Caprella gigantochir had after, values increased again. Acid volatile sulfide similar recruitment patterns; that is, population at the pearl farm site showed no clear seasonal increased after February and maximum densities fluctuations. (15 700 individuals/m2 and 6280 individuals/m2, respectively) occurred in April. An unidentified Nematoda had maximum densities (2780–3280 Seasonal fluctuations of the macrobenthos individuals/m2) at the phase of the faunal extinction from May to June. The polychaete Prionospio Density and biomass of the macrobenthos (small- pulchra was often found to be the last surviving and medium-sized animals, < 1 g wet wt/indi- species during the summer and autumn. Thus, all vidual) at the fish farm site show conspicuous species except P. pulchra had a clear peak in seasonal fluctuations (Fig. 5). At this site, near density. Among them, Capitella sp., which colo- or complete azoic conditions lasted from July to nized earliest in the faunal recovery, had the high- November 1995; thereafter, the density and bio- est maximum density. mass increased rapidly, attaining a maximum Seasonal fluctuations in the species diversity (mean ± SD, 43 700 ± 5200 individuals/m2; 37.2 ± indices reflected the recovering and subsequent 4.5 g/m2) in April 1996. Density and biomass degeneration of the macrofauna in the fish farming Macrobenthos at mariculture farms FISHERIES SCIENCE 263

Fig. 6 Seasonal fluctuations in the density of the main species at: (a) the fish farm site; (b) the pearl farm site; and (c) the control site. area (Fig. 8). During the earliest stage of the faunal 0.04 m2) and a maximum H¢ of 3.0 were observed. recovery in December and January, H¢ was low In May, however, many species suddenly disap- (0.8–1.0) because of the occurrence of only a few peared, resulting in decreases of H¢max (4.1, 17 pioneering species, resulting in low species rich- species/0.04 m2) and H¢ (2.5). Faunal extinction 2 ness values of H¢max (3.0–3.6, 9–12 species/0.04 m, ) continued through July 1996, when azoic condi- and the dominance of Capitella sp., which ac- tions re-occurred. counted for more than 80% of the total abundance Seasonal fluctuations of the macrobenthos at during these months (Fig. 7a), resulting in low the pearl farm site were in marked contrast to those evenness values of J¢ (0.23–0.33). As subsequent at the fish farm site. Azoic conditions were not species colonized, H¢max increased, and with the observed at the pearl farm site. Furthermore, there resulting reduced dominance of Capitella sp., were no clear seasonal trends in any of the density, increases in the values of the evenness J¢ occurred. biomass, and species diversity indices (Figs 5,8). In April, a maximum H¢max of 5.5 (45 species/ Any overwhelming dominance by a single species, 264 FISHERIES SCIENCE H Yokoyama

(a)

(b)

(c)

Fig. 7 Seasonal fluctuations in the numerical composition of species at: (a) the fish farm site; (b) the pearl farm site; and (c) the control site. which accounted for more than 50% of the total species, excluding S. costarum, showed temporal abundance, did not occur at anytime through- fluctuations in densities, resulting in the change of out the year (Fig. 7b), resulting in the evenness dominant species. Community structure and sea- J¢ (0.61–0.87) being maintained at higher values sonal trends at the pearl farm site were similar to than those at the fish farm site (Fig. 8c). The main those at the control site, although higher values for species at the pearl farm site, which were the density (6600–18 200 individuals/m2 vs 2700–8300 bivalve Theora fragilis, the gastropod Lucidestea individuals/m2; Fig. 5a), species diversity H¢ mundula, the polychaetes Prionospio pulchra, (3.8–5.4 vs 2.9–4.8; Fig. 8a), and species richness Lumbrineris longifolia and Spiochaetopterus H¢max (5.6–6.5 vs 4.8–5.7; Fig. 8b) were observed at costarum, and the gammarid Aoroides spp., the control site compared with at the pearl farm occurred throughout the year (Fig. 6b). These site. Also, a more stable composition of species Macrobenthos at mariculture farms FISHERIES SCIENCE 265

consisting of the bivalve Theora fragilis, the gas- found at the control site compared with at the pearl tropod Lucidestea mundula, the polychaetes farm site (Figs 6c,7c). Aricidea sp., Paraonis sp. and Prionospio There were also marked differences in the size membranacea, and other various animals were composition of animals between the three sites (Table 2). All specimens collected from the fish farm site, with the exception of a juvenile sea eel Conger myriaster (0.18 g wet weight), belonged to the category of small-sized animals (< 0.1 g). In contrast, the assemblage at the pearl farm site consisted of a variety of species of various sizes. At this site, 48 medium-sized individuals (0.1–1.0 g) belonging to 13 species, including the polychaetes Haploscoloplos sp., Terebellides kobei, Sthenolepis sp., Streblosoma sp. and Lagis bocki, the bivalves Macoma incongrua, Fulvia hungerfordi and Pillucina pisidium, the holothurian Patinapta ooplax, and an unidentified Ascidiacea, comprised 25.2% of the total weight; nine large-sized individuals (>1.0 g), which were identified as Haploscoloplos sp., the bivalves Macoma incongrua and Paphia undulata, and an unidentified Ascidiacea, accounted for 33.8% of the total weight. The sample collected from the control site contained 51 large- or medium-sized individuals, which accounted for 30.0% of the total weight.

Relationship between assemblages and acid volatile sulﬁde

The AVS-S in sediment at the fish farm site was high during the azoic period in summer and autumn. After December, as the density of the macrobenthos increased, AVS-S decreased until April 1996, when the minimum value and the maximum density were observed; after April, as the density decreased, AVS-S increased again. Thus, a negative correlation was found between the Fig. 8 Seasonal fluctuations in: (a) the Shannon– density of the macrobenthos and AVS-S, although Weaver’s diversity index H¢; (b) the species richness there were a few exceptional data obtained in June, index H¢max; and (c) the evenness index J¢ at: ( ) the fish farm site; () the pearl farm site; and () the control site. July and November 1995 (Fig. 9). In contrast, at the Each plot represents the mean ± SD of three replicate pearl farm site, no correlation was found between samples. these two factors.

Table 2 Wet weight (Wt, g) and number of individuals (N) of animals collected from the three study sites (each sampling area: 1.68 m2) Fish farm site Pearl farm site Control site Wt (%) N (%) Wt (%) N (%) Wt (%) N (%) Large-sized animals* 0 (0) 0 (0) 17.8 (33.8) 9 (0.10) 2.9 (7.2) 2 (0.01) Medium-sized animals† 0.2 (1.3) 1 (0.01) 13.3 (25.2) 48 (0.51) 9.2 (22.8) 49 (0.26) Small-sized animals‡ 13.6 (98.7) 16 472 (99.99) 21.6 (41.0) 9400 (99.40) 28.2 (70.2) 19 042 (99.73)

*>1 g/individual. †1–0.1 g/individual. ‡<0.1 g/individual. 266 FISHERIES SCIENCE H Yokoyama

(especially species I) is regarded as a short-lived, typically opportunistic species that is adapted to unpredictable environments by virtue of its ability to reproduce rapidly.24 Capitella species have often been reported in fish farm areas in Scotland,5 North America,6,7 and Japan.2 It has been demonstrated that nutrient enrichment is posi- tively related to the individual growth, population growth, and fecundity of Capitella species.25–29 It has also been suggested that micronutrients such as amino acids and fatty acids may limit the growth and reproduction of Capitella sp. I.30,31 The deposition of organic wastes from fish cages into the sediment during an azoic period appears to provide an enhanced food supply to Capitella sp., which feeds on the subsurface sediments containing decayed organic material and associated microbes.32 The density of this species decreased markedly before the beginning of the environmental degradation Fig. 9 Relationship between the density of the macro- in May. The observed decline in the Capitella benthos and acid volatile sulfide (AVS-S) in the sediment at: () the fish farm site; and () the pearl farm site. population after February may be caused by a food shortage, especially if organic-rich sediment containing available food is necessary for maintaining a large population of Capitella sp., and if this food DISCUSSION supply has already been consumed during the high-density phase of this species. The present survey showed azoic conditions at the Pseudopolydora paucibranchiata, the second fish farm site during summer and autumn. It has species to re-populate at the fish farm site, feeds been shown that low oxygen levels is a key factor in selectively on organic–mineral aggregates and sus- eliminating macrofauna in an organic-enriched pended organic particles,33,34 which are unavailable habitat.12 However, it is difficult to explain this from to the Capitella species directly. The population the observed DO values, which were more than growth of P. paucibranchiata needs to be sup- 2.8 mg/L during the summer of 1995. Abo and ported by a steady input of nutrient-rich particles Toda22 and Abo23 have observed that the oxygen in the form of unconsumed food particles and fish content of the bottom water in Hazama-ura was feces, which accumulated at the sediment–water subject to repetitive fluctuations ranging from 5 interface. mg/L to completely anoxic, within a period of The most diverse assemblage at the fish farm 10–15 days during the summer. It is probable that site was found during April, when the gammarid low and lethal, but unrecorded concentrations of Aoroides spp. dominated. At this time, several indi- oxygen, led to defaunation at this site. viduals of Aoroides spp. were also found in the Another conspicuous occurrence at the fish stomach of a sea eel Conger myriaster sampled farm site was the rapid increase in density after from this site. This observation indicates the prob- December. Monthly observations at Gokasho Bay able food chain in the benthic community at the during the period of 1989–1996 have recorded that fish farm site. This community, however, tended DO content of the bottom water at the fish farm toward abrupt extinction in May, probably as a site did not fall below 3 mg/L after October,23 prob- result of the deoxygenation of the bottom water ably because of the disappearance of the thermo- accompanied by increasing temperatures (from cline and a decrease of the organic matter load 13.6∞C in April to 16.4∞C in May) and increasing from fish farming. This finding indicates that a fish farming activities. period of 2 months after the recovery of the oxy- A clear negative correlation between the density gen content is required for successful recruitment. of the macrobenthos and AVS-S in the sediment There may be other factors that also inhibit was found at the fish farm site, although there were colonization. a few exceptions, probably because of the lack of The polychaete Capitella sp. was the first species homogeneity of the sediment. This finding sug- to colonize the azoic bottom, and attained the gests that sulfides in the reduced sediment inhibit largest density among the constituent species at the initial recruitment of benthic animals, and that the fish farm site. The Capitella species complex the recovered DO leads to re-oxygenation of the Macrobenthos at mariculture farms FISHERIES SCIENCE 267

reduced sediment, which facilitates the recruit- larly, the nitrogen load onto the seabed of the fish ment of animals. As re-oxygenation advances, the farm is 52 times greater. It is concluded that such area of suitable habitats for animals expand and, a difference in the level of organic matter load simultaneously, recruited animals accelerate the between the two sites results in marked differences re-oxygenation process of the sediment by their in the levels of AVS-S in the sediment and the bioturbation and ventilation activities, resulting in macrobenthic assemblages. a decrease in the AVS-S content of the sediment. It has been pointed out that a reduction in species richness, a decrease in the densities of ACKNOWLEDGMENTS echinoderms, and an increase in the total abundance with a high ratio of small-sized organisms I am grateful to Mr S Yamamoto, and Drs M are common features of the macrobenthos in the 35,36 Toyokawa and K Abo for their help in the collection vicinity of aquaculture facilities. Similar phe- of samples; Drs H Ariyama and H Sudo for identi- nomena have also been observed in fish farm areas 2 fying specimens; and Dr Y Yamagata for providing in west Kyushu by Tsutsumi and in Gokasho Bay me with information on the organic matter load in during the present study. A comparison of the the fish farm. species composition in Gokasho Bay with a previ- ous survey conducted in 1941 also indicates that the echinoderm Schizaster lacunosus has dis- appeared from the fish farm area in this bay.13 All REFERENCES of these features observed in the present survey 1. Hirata H, Kadowaki S, Ishida S. 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