Kor J Fish Aquat Sci 43(4), 331-339 한수지, 43(4), 331-339, 2010 사육수비교환방식을이용한포장사육지에서의흰다리새우, Litopenaeus vannamei (Boone, 1931) 의초고밀도양식 조영록 김봉래 1 장인권 국립수산과학원서해수산연구소해역산업과 1 국립수산과학원중앙내수면연구소 Super-intensive Culture of Whiteleg Shrimp, Litopenaeus vannamei (Boone, 1931), in HDPE-lined Ponds with no Water Exchange Yeong Rok Cho, Bong Rae Kim 1 and In Kwon Jang* Aquaculture Industry Division, West Sea Fisheries Research Institute, National Fisheries Research & Development Institute, Incheon 400-420, Korea 1 Inland Fisheries Research Institute, National Fisheries Research & Development Institute, Gapyeong 477-815, Korea Shrimp farming is the most important mariculture industry on the west coast of South Korea. However, it has suffered from mass mortality due to viral disease outbreaks and coastal pollution due to water discharge. This study developed an intensive shrimp culture method for outdoor ponds, without water exchange, which minimizes the chance of viral transmission from the environment, reduces coastal pollution by water discharge and enhances shrimp production. A culture trial was conducted in two high-density polyethylene (HDPE)-lined ponds with a 550 m 2 surface area. The ponds were stocked with postlarvae of Litopenaeus vannamei, the major farmed shrimp species in Korea, on July 10, 2007, and cultured for 90 days with no water exchange. The stocking density of the postlarvae (B.W. 0.0015 g) was 272 ind./m 2, which is eight times higher than in traditional pond culture in Korea. At harvest, the total production of ponds 1 and 2 was 1,362kg (2.48 kg/m 2 ) and 1,282 kg (2.33 kg/m 2 ), respectively. This is 20~22 times higher than the mean farmed shrimp production (0.112 kg/m 2 ) in Korea and about eight times higher than in traditional ponds with a good harvest. Although there was no water exchange throughout the culture period, the mean concentrations of unionized ammonia and nitrite-nitrogen were as low as 0.038 and 6.0 mg/l, respectively. The feed conversion rate (FCR) was 1.38, which is 20~45% lower than that of traditional pond cultures. The high efficiency of the diet in this study is thought to be due to a well-managed feeding strategy and well-developed bioflocs used as diet additions for the shrimp. The final body weight of the shrimp at harvest was low (12.2~12.5 g), compared with that of traditional pond culture. This may have resulted from the combination of a short culture period, high density of shrimp, and low temperature. This study suggests that a super-intensive shrimp pond culture method using biofloc technology with no water exchange can minimize viral transmission via water exchange, reduce coastal pollution, and enhance shrimp production. Key words: Litopenaeus vannamei, Intensive shrimp culture, Biofloc, No water exchange 서 우리나라의새우양식은 1-3 ha 크기의축제식양식장에서반집약적인방식으로이루어지고있다. 새우양식생산량은 2001 년 3,256 mt 을정점으로해마다감소하여 2009 년에는 1,893 mt 에불과하였으며, 지난 10 년간단위면적당생산량도전국평균 1.12 mt/ha 로서정상적인예상수확량인 3.0 mt/ha 에비해크게낮았다 (Jang et al., 2009b; Kim, 2010). 생산성저하의주요원인중하나는전염성질병, 특히 WSSV (white spot syndrome virus) 에의한대량폐사때문인것으로알려져있다 론 Corresponding author: jangik@nfrdi.go.kr (Jang et al., 2007a, b, 2009a). WSSV 는 1992 년중국에서최초로발견된이래 (Chen, 1995) 아시아, 인도 - 태평양을거쳐 1990 년대말에는미주를포함한전세계의새우양식장으로전파되어심각한손실을가져온바이러스로서현재까지도새우양식산업에가장큰장애요인이되고있다 (Chou et al., 1995; Wang et al., 1995; Lo et al., 1996; Flegel, 1997; Lighter, 1999). 이에대한대책의일환으로선발육종을통하여질병내성과성장률이다른새우류에비해높은것으로알려진흰다리새우, Litopenaeus vannamei (Boone, 1931; Briggs et al., 2004; FAO, 2006) 가 2003 년미국하와이로부터국내로이식된이래양식이확산되어 2007 년에는흰다리새우가전국양식새우생산량 331
332 조영록 김봉래 장인권 112 의 62.5% 를점유하게되었다 (Kim et al., 2004; Jang et al., 2008). 그러나국내에서양식되는대부분의흰다리새우는해마다외국으로부터바이러스에비감염된 SPF (specific pathogen free) 의어미새우혹은 postlarva 를수입하여양식장에입식되기때문에양식장에서의바이러스감염은대부분양식중에환경으로부터의수평감염 (horizontal transmission) 을통하여일어난다 (Jang et al., 2007a, b). 일반적으로축제식새우양식장에서는사육수의주기적인환수를통하여수질을유지하는데, 이러한대규모의환수는바이러스수평감염의주된경로가될뿐아니라부영양염의배출수에의한연안환경의악화를가져온다 (Brock et al., 1997; Flegel, 1997; Lo et al., 2000; Lotz and Lightner, 2000). 사육수의비교환 (no exchange) 혹은저교환 (limited exchange) 방식은병원체의유입과환경오염의문제를해결하는근본적인방법이지만, 사육수내의영양염, 특히질소노폐물의축적이라는부담이따른다. 미생물총 (biofloc) 기술을적용한축제식새우양식장에서의사육수비교환방법은병원체의감염을차단할뿐아니라사육수의질소노폐물을효율적으로제거함으로써양식생산성을크게향상시킬수있어최근많은연구가이루지고있다 (McIntosh, 2000; Samocha et al., 2000, 2007; Browdy et al., 2001). 사육수를교환하지않을경우, 양식생물의생체량과사료공급량이증가함에따라양식장의수용력은부하가걸리게되며, 양식생산량은슬러지의처리여부에따라서크게달라진다 (Hopkins et al, 1999). 실제로대부분의축제식새우양식장들은양식이진행될수록사료찌꺼기, 배설물, 플랑크톤사체등으로인하여사육지바닥에유기물의슬러지가축적된다. 슬러지의축적은사육수의부영양화에도영향을미칠뿐아니라혐기상태의슬러지층을형성하는데이것은결국 COD, BOD 의증가와새우에치명적인황화수소, 암모니아등을발생시킨다. 따라서이러한환경은새우에게적합한서식장소를축소시키며새우의성장, 질병, 생존율에심각한영향을미친다 (Boyd and Clay, 2002). 여러양식장에서는중앙배수식의구조혹은별도의침전조설치등을통하여효율적으로슬러지를제거하려는다양한시도가있어왔다. 그러나 McIntosh (1999) 는사육수를교환하거나슬러지를제거하지않고, 모든유기물을수중에부상시킴으로써사육수의수질을유지하면서고밀도로새우를양식하는방법을개발하였다. 이러한조건은일반적인축제식양식장에서식물플랑크톤우점의광합성적자가영양 (photoautotrophic) 혹은자가영양 (autotrophic) 상태의사육수를타가영양 (heterotrophic) 우점의상태로전환시켜주는데, 충분히번식한타가영양세균은암모니아성질소를세균단백질로동화시킴으로써질소화합물을제거한다 (Avnimelech et al., 1994; Avnimelech, 1999; Hopkins et al., 1999). 이때타가영양세균은유기탄소 (organic carbon) 를에너지원으로이용하기때문에 carbon 과 nitrogen 의비율 (C/N ratio) 을조절함으로써암모니아를효율적으로제거할수있다. 타가영양세균들은유기물에결합하여미생물총 (microbial floc 혹은 biofloc) 을형성하는데새우는미생물총을 섭식함으로써사료의대체먹이로이용하는것으로알려져있다. 또한타가영양세균의성장은매우높고따라서암모니아제거효율도자가영양에비해높아서고밀도의새우양식에적합하다 (Browdy et al., 2001; Burford and Lorenzen, 2004; Ebeling et al., 2006; Ebling, 2008). McIntosh (2000) 는다양한크기의사육지에서의경험에근거하여저밀도보다고밀도의양식을권장하며준집약적 (semi-intensive) 축제식양식장의 35 마리 /m 2 보다크게높은 125-140 마리 /m 2 의밀도에서오히려안정적으로수질이유지된다고보고했다. 본연구는우리나라에서새우양식의주종인흰다리새우를대상으로비닐포장사육지 (lined pond) 에서사육수를교환하지않고일반축제식양식장보다약 8 배높은초고밀도로양식함으로써배출수에의한환경오염의감소와바이러스수평감염의예방및생산성을크게향상시킬수있는기술을개발하고자한다. Fig. 1. Layout of ponds 1 (upper) and 2 (lower). Pond lined with HDPE plastic sheet. Pond 2 provided with DPD system for aeration. 재료및방법 사육지와사육수준비사육실험은국립수산과학원서해수산연구소양식연구센터 ( 충남태안군근흥면 ) 내에시설된 2 개의사육지에서실시되었다. 사육지의크기는 32 20 m ( 수면적 30 18.3 m, 550 m 2 ) 이며사육지바닥과내벽은모두검정색의 HDPE (high density polyethylene) 재질로포장하였다 (Fig. 1). Pond 1 은
127 사육수비교환방식을이용한포장사육지에서의흰다리새우의초고밀도양식 333 중앙을따라콘크리트격벽을설치하여사육수가순환할수있도록하였으며사육수순환과산소공급을위하여 paddle 형수차 (1 HP) 와에어인젝터 (1 HP) 를각 1 대씩을설치하였다. Pond 2 에는바닥에저층분사용에어공급관 4 개를설치하고각관은에어블로와 (air blower) 에연결하였다. 에어공급관은 polyethylene 재질로서 30 cm 간격으로구멍 ( 2 mm) 을뚫어공기가분사될수있도록하였다. 사육수는자연해수를단계적으로 100μm 까지여과하여 sodium hypochlorite ( 염소농도 20 mg/l) 로소독을실시한후 1 주일간폭기하여염소가충분히제거된상태에서시비를하였다. 시비는요소, 인산염, 규산나트륨으로 N:P:Si 의비율이 2.0:0.1:1.5 의농도로실시하였으며, 별도배양된 Chaetoceros spp. 를 10 2 cell/ml 농도로접종하여사육수를안정화시켰다. 초기의사육수는사육수비교환방식에따른증발량을고려하여지하수를혼합하여약 25 psu 로조절하였다. 새우의사료섭식량을조사하기위하여사육지의가장자리에목재로가교를시설하여사육지별 2 개의먹이망 (feeding tray, 50 50 cm) 을설치하였다. 종묘입식과사육관리 2007 년 7 월 10 일양식연구센터에서생산된바이러스비감염 (SPF, specific pathogen free) 흰다리새우 (L. vannamei) 의유생 (PL 10-15, B.W. 0.0015 g) 을각사육지별 150,000 마리 (272.7/m 2 ) 씩입식하여 10 월 8 일까지 90 일간사육하였다. 먹이는새우용 EP 사료 (crude protein 38%) 를 4 회 / 일공급하였으며사료양은주간성장률및먹이망의조사에근거하여매주결정하였다. 주간성장률 (weekly growth rate, WGR) 은투망을이용하여 pond 별 50-100 마리의새우를 3 회채집한평균체중에근거하여계산하였다. 사육수는전기간동안수질향상을위해서는교환하지않았으나, 증발수의보충및식물플랑크톤의과잉번식시일부를교환하여평균교환율은 0.1%/day 이하였다. Fig. 2. Change of water temperature in ponds 1 and 2 during a 90-day grow-out trial. Fig. 3. Change of dissolved oxygen in ponds 1 and 2 during a 90-day grow-out trial. 수질환경측정수온, 용존산소 (DO), 염분은 YSI-85 model (Yellow Springs Instrument Co., Ohio, USA) 을이용하여매일 2 회측정하였으며, ph 는 YSI-6600 ph meter 로매일 1 회, TAN (total ammonia nitrogen), NO 2-N, alkalinity 는수질분석용 calorimetric kit (Merck Co., Germany) 를이용하여매일 1 회측정하였다. 암모니아농도의증가시탄소원으로공급된당밀은일일평균으로환산하였다. Table 1. Summary of selected water quality parameters in ponds 1 and 2 during a 90-day grow-out trial (mean and range) Water Temp.(C) 25.3 Pond 1 19.0-30.9 25.3 Pond 2 19.8-30.7 DO (mg/l) 5.81 3.84-7.49 5.75 3.36-7.17 Salinity (psu) 25.3 21.8-28.1 25.6 22.2-28.5 ph 7.44 7.15-8.23 7.43 7.01-8.19 TAN (mg/l) 2.7 0-10.0 2.43 0-10.0 NO 2-N (mg/l) 6.0 0-21.0 5.3 0-24.0 Alkalinity (mg/l) 126 95-170 125 95-170 Water exchange (%/day) <0.1 <0.1 Fig. 4. Change of salinity in ponds 1 and 2 during a 90-day grow-out trial. 결 사육수수질환경의변화 2007 년 7 월 10 일부터 10 월 8 일까지 90 일간흰다리새우를사육한 2 개의 HDPE-lined pond 의수질환경요인의평균값과 과
334 조영록 김봉래 장인권 112 범위는 Table 1 에나타나있다. Pond 1 과 2 의수온은사육시작부터점차상승하여 8 월 20 일경 30 이상을보인후에급격하게하강하기시작하여 9 월초에 21 까지내려갔다. 이후 25 전후까지상승하였지만 10 월초에 20 이하로떨어져 10 월 8 일사육을종료하였다. Pond 1 과 2 의평균수온은모두 25.3 (range 19.0-30.9 ) 이었다 (Fig. 2). 용존산소 (DO) 농도는사육기간마지막의 1-2 일간을제외하고는모두 4.0 mg/l 이상을유지하였으며 pond 1 이평균 5.81 mg/l (3.84-7.49 mg/l), pond 2 가 5.75 mg/l (3.36-7.17 mg/l) 이었다 (Fig. 3). 사육수비교환방식에따른증발량을고려하여실험시작시사육수는일반해수에지하수를혼합하여약 25 psu 이었다 (Fig. 4). 사육초기염분은약간상승한후 7 월 20 일이후의장마로인하여 7 월말에는 22 psu 까지하강한이후점차상승하여 9 월에는 28 psu 이상까지높아졌다. Pond 1 과 2 의염분은각각평균 25.3 psu (21.8-28.1 psu), 25.6 psu (22.2-28.5 psu) 를보였다. ph 는초기 1 개월동안에는 8.2 에서약 7.2 까지지속적으로떨어진이후 8 월부터는 7.1-7.5 범위를유지하였다. Pond 1 과 2 의평균 ph 는각각 7.44, 7.43 으로전체적으로성장적합범위를크게벗어나지는않았다 (Fig. 5). Fig. 7. Change of nitrite-nitrogen concentration in ponds 1 and 2 during a 90-day grow-out trial. Fig. 8. Change of alkalinity in ponds 1 and 2 during a 90-day grow-out trial. Fig. 5. Change of ph in ponds 1 and 2 during a 90-day grow-out trial. Fig. 6. Change of TAN(total ammonia nitrogen) concentration in ponds 1 and 2 during a 90-day grow-out trial. TAN 은 7 월하순부터상승하기시작하여 8 월하순에는 10 mg/l 까지증가한이후 9 월초순에는 1.0 mg/l 까지감소하였다. 이후 TAN 은 3.0 mg/l 전후에서유지되며큰증가를보이진않았다 (Fig. 6). Pond 1 과 2 에서의 TAN 평균농도는각각 2.70, 2.43 mg/l 이었다. 아질산염 (NO 2-N) 농도는 8 월 26 일까지는 1.0 mg/l 이하의낮은농도를유지하다가이후급격하게상승하기시작하여 9 월 6 일에는 pond 1 과 2 가각각 21.0, 24.0 mg/l 까지올라간이후점차낮아지는경향을보였다. 평균아질산염농도는 pond 1 과 2 에서각각 6.0, 5.3 mg/l 이었다 (Fig. 7). 아질산염의증가패턴은 TAN 농도보다약 2 주일정도지연되어나타나는경향을보여주었다. 알칼리도 (alkalinity) 는시작시 140 mg/l 이었으나 7 월말에는 100 mg/l 까지점차감소하였다. 이후각사육지에적당량의탄산칼슘 (CaCO 3) 을살포하여 8 월말까지약 170 mg/l 까지상승하였으며이후점차감소하는경향을보였다 (Fig. 8). Pond 1 과 2 의평균알칼리도농도는각각 126, 125 mg/l 이었다. 사육수는 8 월말, 9 월중순경식물플랑크톤의과잉번식으로인하여일부교환한것을제외하고는수질관리를위한교환은없었으며전체평균환수율은 0.1%/day 이하였다.
127 사육수비교환방식을이용한포장사육지에서의흰다리새우의초고밀도양식 335 Table 2. Summary of stocking and harvest of L. vannamei in ponds 1 and 2 during a 90-day grow-out trial Stocking Harvest survival FCR (%) B.W. (g) Total No of PL density (ind./m 2 ) B.W. (g) total yield (kg) production (kg/m 2 ) Pond 1 0.0015 150,000 272.7 12.5 1,362 2.48 72.6 1.39 Pond 2 0.0015 150,000 272.7 12.2 1,282 2.33 70.1 1.38 Fig. 9. WGR(weekly growth rate) of L. vannamei in ponds 1 and 2 during a 90-day grow-out trial. Fig. 10. Changes of mean body weight of L. vannamei in ponds 1 and 2 during a 90-day grow-out trial. 새우의성장률과생존율 9 월말경부터사육수의수온이 20 이하로하강한관계로사육 90 일째인 10 월 8 일새우를수확하였다. Pond 1 과 2 의새우의생산량과생존율은 Table 2 에나타나있다. 총수확량은 pond 1 과 2 가각각 1,362, 1,282 kg 이며단위면적당생산량은각각 2.48, 2.33 kg/m 2 이었다. 수확시새우의평균체중은각각 12.5, 12.2 g 이었다. 생존율은 pond 1 과 2 가각각 72.6, 70.1% 이며, FCR (feed conversion rate) 은각각 1.39, 1.38 이었다. Pond 1 과 2 의새우의체중변화및주간성장률은 Fig. 9 와 Fig. 10 에나타나있다. 주간성장률 (weekly growth rate) 은 pond 1 과 2 가각각 0.97, 0.95 g/week 로서큰차이가나지않았 다. 주간성장률은초기 1 개월동안은 0.5 g/week 를넘지않았으며체중이 2 g 을이상이되는 6 주 (8 월 20 일 ) 까지는주간성장률은 1.0 g 이하였다. 7 주째인 8 월말경에는 pond 1 과 2 의주간성장률이각각 2.29, 2.33 g 에달했으며이후에도주간성장률은 1 g 이상을나타냈다. 고찰생산량및생존율 550 m 2 크기의비닐포장 (HDPE-lined) 된 2개의사육지에서흰다리새우의 postlarva를 272.2 마리 /m 2 의밀도로입식하고사육수교환없이 90일간양식한결과, 총수확량은 2,644 kg이었으며단위면적당생산량은 23.3-24.8 mt/ha에달하였다 (Table 2). 일반적으로우리나라축제식새우양식장에서는 30-35 마리 /m 2 의 postlarva를입식하여약 120-150일후에수확하는데, 질병등의문제없이정상적으로양식할경우약 3.0 mt/ha를생산할수있으나지난 10년간전국평균생산량은 1.12 mt/ha에머물렀다 (Jang et al., 2007a; Kim, 2010). 본연구는일반축제식새우양식장에비해 8-9배높게 postlarva를입식하여전국평균새우양식생산량의 21-22배, 정상수확된양식장에비해서도 8배가높은생산량을보여주었다. 이와같이축제식새우양식장에서의생산성을높이기위한시도는외국의여러나라에서도많은시도가있었다. Browdy et al. (2001) 은 0.25 ha 크기의 HDPE-lined pond에서 104 마리 / m2의밀도로흰다리새우종묘를입식하고사육수의교환없이단백질함량이다른사료를공급하여양식한결과, CP 30% 과 CP 40% 실험구에서각각 9.0, 10.0 mt/ha를생산하였다. 이들의생존율은 74.5-78.6% 로서본연구의 70.1-72.6% 와비슷하지만단위면적당생산량은본연구의약 40% 에머무르는데이것은입식밀도의차이에기인하는것으로판단된다. McIntosh (2000, 2001) 는중미에위치한벨리즈의새우양식장에서다양한시도를통하여사육수비교환방식을개발하였는데, 1998년이후매년생산성이증가하여 2000년에는총면적 13.44 ha에서평균 15 mt/ha의높은생산량의흰다리새우를수확하였다. McIntosh (2001) 는사육수의교환없이고밀도로새우를사육하는조건에서는사료에어분대신탄수화물이많은밀이나옥수수를사용한사료를혼합공급함으로써타가영양세균 (hetertrophic bacteria) 의발달을촉진시킬수있는것으로보고하였다. 실제로그는 grain-based pellet (CP 18.5%) 를공급하여탄소 : 질소의비 (C/N ratio) 를 20:1까지상승시킴으로써효과적으로사육수의암모니아농도를크게낮출수있었다. 타가영양세균은성장을위하여탄수화물, 즉유기탄소와암모니아를필요로하기때문에타가영양상태를잘발달시키기위해서는높은 C/N ratio를유지하는것이유리한것으로알려져있다 (Avnimelech, 1999; Ebling, 2008). Samocha et al. (2007) 은 40 m 3 의 raceway형수조에서탄소원으로서의당밀을다양한농도로공급하면서고밀도로흰다리새우를중간육성한결과, 높은 C/N ratio는암모니아와아질산염의농도를효과적으로떨어뜨리지만 7.8 m 3 와같은소형수조에서는유의적인차이를발견하지못하여 C/N ratio는수조의
336 조영록 김봉래 장인권 112 크기등사육환경에따라서암모니아제거에영향을미친다는점을시사하였다. 그러나소형수조에서의결과는사육조건이라기보다는근본적으로타가영양세균의종조성과군집의발달여부에기인하는것으로판단된다. 그러나현재까지사육수비교환조건에서의타가영양세균군의종조성과군집의변화등에대해서는전혀연구된바가없다. 본연구에서는 C/N ratio 나타가영양세균의종조성등은분석하지는않았지만 CP 38% 의배합사료를사용하면서암모니아와아질산염의농도변화에따라서당밀을공급함으로써 C/N ratio 를높여주었다. 본연구에서얻어진생산량은일반축제식양식장에비해 8-22 배높았지만수확시새우의평균체중은 12.2-12.5 g 으로우리나라의축제식양식장에서생산되는새우의평균체중인 20 g 보다크게작았다. Browdy et al. (2001) 은본연구지역 (36.5 N) 보다약간낮은위도인미국의사우스캐롤라이나 (34 N) 에서 104 마리 / m2의밀도로사육한결과, 본연구와유사한체중인 11.7-12.6 g 의새우를수확하였다. McIntosh (2008) 는보다저위도국가인태국 (14 N) 에서기후대가다른 3 개지역의양식장을선정하여본연구와동일한방식으로고밀도로흰다리새우를양식한결과, 기온이높은남부양식장에서는평균체중 18.3 g 을얻었으나동북부와중부에서는체중이각각 17.5 g, 16.2 g 으로보고하였다. 이와같이고밀도양식의경우새우의성장률은저밀도에비해낮아질뿐아니라기후, 즉수온에도영향을받는다. 본연구기간동안의평균수온은 25.3 (range 19.0-30.7 ) 로서동일지역의축제식양식장에서의 6-9 월의평균수온 27.5 (range 21.9-34.2 ) (Jang et al., 2007a) 보다약 2 가낮았으며사육밀도는일반축제식에비하여 8-9 배높았다. 또한종묘생산의지연으로일반축제식양식장의입식시기인 5-6 월보다크게늦은 7 월 10 일입식하였기때문에양성기간이 1-2 개월짧았다. 본연구에서의낮은성장률은이러한복합적인이유에기인하는것으로판단되지만우리나라에서의상품크기인 20 g 의새우를고밀도로생산할수있는한계사육밀도를확인하기위해서는다양한밀도와양성기간에따른사육실험이수행될필요가있다. 본연구에서는 CP 38% 의사료를공급하여 1.38-1.39 의 FCR (feed conversion rate) 을얻었는데이것은일반축제식양식장의 1.7-2.0 에비해크게낮으며, Browdy et al. (2001) 가본연구와비슷한조건하에서 CP 30-40% 를공급하여얻은 FCR 1.89-1.92 에비해서도약 38% 가낮은수치이다. McIntosh (2001) 는본연구와유사한고밀도사육환경하에서 2.0 의 FCR 을얻었으나사료의단백질함량이 22% 로매우낮았던점을고려하면사료효율이매우양호했다고할수있다. 본연구에서 FCR 이매우낮았던이유는체계적인사료관리와잘형성된 biofloc 때문으로판단된다. 효율적인사료관리 (feed management) 는새우의생산비를낮출뿐아니라사육수수질악화를감소시키는중요한양식관리요인의하나이다 (Jory, 1995). 과잉공급된사료는새우의소화력을저하시키며잔량의사료는사육지바닥에축적되어수질을악화시킨다. 뿐만아니라사료공급은새우의상태, 새우의분포상태, 저질의상태, 주야간섭식습성등에따라서적절히조절하는것이바람직하다 (Clifford, 1992). 본연구에서는사료의공급량을 1 차적으로새우의주간성장률 (weekly growth rate) 에근거하여결정하였으며, 부가적으로사료공급후먹이망 (feeding tray) 의조사, 새우의탈피주기와날씨의변화등을관찰하여과잉의사료가공급되지않도록매일세심하게사료양을조절하였다. 또다른이유로는잘형성된 biofloc 이새우의추가적인먹이로이용되었기때문일수도있다. 본연구에서 biofloc 이어느정도형성되었는지를판단할수있는정확한지표는없지만, 앞선연구결과들에따르면본연구의사육환경과같은조건에서는 biofloc 이잘발달되는것으로알려져있다 (Avnimelech, 1999; McIntosh, 2000; Browdy et al., 2001). Avnimelech et al. (1994) 는사육수비교환방식의틸라피아양식장에서타가영양세균으로이루어진미생물총 (microbial floc or biofloc) 이틸라피아에의해섭식됨을관찰하였다. 새우도 biofloc 을추가적인먹이원으로이용한다는사실이많이보고되어있는데, biofloc 은타가영양세균으로만구성되기보다는동식물플랑크톤, 유기물등이복합적으로구성되며다양한영양분과미량원소가포함되어새우의성장, 생존율및사료효율에많은기여를하는것으로추측된다 (Moss et al., 1999; Browdy et al., 2001; Burford and Lorenzen, 2004; Avnimelech, 2006; Wasielesky et al., 2006). 사육수의수질환경고밀도양식에서사육수내암모니아성질소의축적은가장큰제한요인의하나이다. 본실험기간동안총암모니아성질소 (TAN) 는 pond 1 과 2 가각각 2.7, 2.43 mg/l 로서이중독성이있는비이온화암모니아 (unionized ammonina) 는평균수온 25, ph 7.4 를기준으로환산하면각각 0.038, 0.034 mg/l 에해당한다. 이러한농도는새우의성장적정한계인 0.03 mg/l 에근접하지만만성적으로영향을미치는한계농도인 0.1 mg/l 에는크게못미친다 (van Wky et al., 1999). 새우는성체보다유생시기가암모니아에대한내성이낮다. 얼룩새우 (Penaeus monodon) 의 postlarva 의경우, 96hr-LC 50 는 0.2 mg/l 이지만 (Chen and Chin, 1988), 4.87 g 의치하는 0.95 mg/l 로높아진다 (Chen and Lei, 1990). TAN 의농도는 7 월말까지 1.0 mg/l 를유지했지만 2 개월째인 8 월에는 2-10 mg/l 로증가한후다시떨어지는경향을보였다. TAN 이높았던 8 월의새우체중은 1-5 g 으로이기간동안의평균비이온화암모니아농도는 0.1 mg/l 보다낮아새우의성장에는영향을미치지않은것으로판단된다. 아질산성질소 (nitrite-n) 의평균농도는 pond 1 과 2 가각각 6.0, 5.3 mg/l 였으며 TAN 이감소하는 8 월말부터급격하게증가하는경향을나타냈다. 아질산염의형성은질화과정 (nitrification) 의첫번째과정으로암모니아의산화에의하여만들어지는데새우의성장에적합한한계농도는 1.0 mg/l 이하로알려져있다 (van Wky et al., 1999). Chen and Chin (1988) 과 Chen and Lei (1990) 는얼룩새우의암모니아독성연
127 사육수비교환방식을이용한포장사육지에서의흰다리새우의초고밀도양식 337 구와마찬가지로아질산염에대한 96hr-LC 50 를조사한결과, 유생과치하가각각 12.6, 171 mg/l 로보고하였다. 그러나흰다리새우는일반적으로얼룩새우보다아질산염에대한내성이높은것으로알려져있다. Cohen et al. (2005) 은사육수비교환방식의흰다리새우의중간육성에서아질산염농도가 26 mg/l 로 1 주일간지속되었으나새우의생존과성장에는영향을미치지않았으며, Mishra et al. (2008) 도 20 mg/l 의아질산염농도가 2 주간계속되었으나 96.2% 의생존율을얻었다. 본실험에서아질산염농도가가장높았던 9 월중 20 mg/l 이상은 2 일간으로새우의성장과생존에심각한영향은미치지않은것으로판단된다. 그러나 9 월동안의평균농도는 13.6 mg/l 로서이기간의새우체중 (5 g 이상 ) 을고려하더라도이러한농도의아질산염의장기간노출에의한영향을배제할수는없다. 본연구에서얻어진새우의저성장이수온과밀도요인외에아질산염의지속적인고농도에기인할가능성도있다. 아질산염의농도가 TAN 의감소와더불어급격히증가하는것은본실험에서자가영양세균 (autotrophic bacteria) 에의한질산화과정이진행되는증거이다. 타가영양상태가잘발달된축제식양식장이라도식물플랑크톤과 Nitrosomonas 와같은세균에의한자가영양에의한 TAN 의질산화과정이함께진행된다 (Boyd and Clay, 2002; Cohen et al., 2005). 본연구에서는사육수를교환하지않고추가적인당밀을공급함으로써질소와유기탄소의높은농도를지속시키고충분한양의산소와폭기를함으로써타가영양상태의발달과 biofloc 의형성을촉진시켰다. 이와같은사육조건하에서높은생산성을얻었으나본연구에서타가영양세균이얼마나발달하고우점하였으며결과적으로형성된 biofloc 이본연구결과에어느정도기여했는지는분명하지않다. Biofloc 이새우의성장에미치는효과와같은일부의연구 (Wasielesky et al., 2006) 을제외하고는타가영양세균의종조성과기능에대한연구는전무하다. 이러한이유중의하나는타가영양세균의대부분은배양불가능한세균 (unculturable bacteria) 으로서접근방법이매우어렵기때문이다. 향후 GS-FLX 등과같이수많은유전자를동시에동정할수있는초고속 pyrosequencing 방법 (Langaee and Ronaghi, 2005; Metzker, 2009) 등을이용하여 biofloc 사육수의세균군집에대한분석이필요할것이다. 본연구에서는사육지내폭기방법에따른수질과새우의성장을비교하고자 pond 1 과 2 에각각축제식에사용되는전통적인수차방식과저층폭기방식을설치하였다 (Fig. 1). 후자는일반적으로 PDP (pressure differential piping) system 으로알려져있는데, 전통적인수차에비해서여러가지장점이있어최근외국의새우양식장에서는일부적용되는추세이다. 수차의경우는 1 HP 로 400-450 kg 의새우를생산할수있지만 PDP 방식은 700 kg 을생산할수있어에너지절약효과가있으며내구성도훨씬길다. 또한 PDP 방법은수차방식의사육지에많이축적되는바닥의슬러지를효과적으로분산시키고산화시킴으로써저질의오염을방지하며유속을감소시킴으로써새우의사료섭식율을높여줄수있는이점이있다 (McNeil, 2004). 그러나본실험에서는일반수차를설치한 pond 1 과 PDP system 을설치한 pond 2 간에수질환경의모든항목에서유의적인차이를보이지않았으며, 새우의생산량과생존율에서도거의차이를보이지않았다 (Tables 1, 2). 이러한이유는아마도본실험에사용된 HDPE 재질로포장된소규모의사육지에서는바닥에슬러지의축적이대형의사육지에비해훨씬적으며유속에의한새우의섭식에미치는영향도작기때문으로판단된다. 그러나폭기시설관리측면에서는두 pond 간많은차이를보여주었다. 실험기간동안 pond 1 의수차는몇차례수리를해야하는긴급상황이발생하였으나 pond 2 에서는이러한문제점이없었다. 본연구와같은초고밀도양식장의경우수차가정지될경우, 용존산소의급속한저하로인하여심각한문제가발생할수있다. 또한몇 ha 크기의대규모축제식양식장의경우에서도많은수차의유지관리비용을고려한다면 PDP 방식의적용도충분히고려할만한것으로사료된다. 사 이논문은국립수산과학원연구과제 ( 갑각류양식기술개발, RP-2010-AQ-040) 의지원에의하여수행되었음. 사 참고문헌 Avnimelech Y, Kochva M and Diab S. 1994. Development of controlled intensive aquaculture systems with a limited water exchange and adjusted carbon to nitrogen ratio. Israeli Journal of Aquaculture- Bamidgeh 46, 119-131. Avnimelech Y. 1999. Carbon/nitrogen ratio as a control element in aquaculture systems. Aquaculture 176, 227-235. Avnimelech Y. 2006. Bio-filters: The need for a new comprehensive approach. Aquacult Eng 34, 172-178. Boyd CE and Clay JW. 2002. Evaluation of Belize Aquaculture, Ltd: A Super-intensive Shrimp Aquaculture System. In: Review report prepared under the World Bank, Network of Aquaculture Centers in Asia-Pacific (NACA), World Wildlife Fund (WWF) and Food and Agriculture Organization of the United Nations (FAO) Consortium Program on Shrimp Farming and the Environment, Bangkok, Thailand, 1-17. Briggs M, Funge-Smith S, Subasinghe R and Phillips M. 2004. Introductions and movement of Penaeus vannamei and Penaeus stylirostris in Asia and the Pacific. RAP publication 2004/10. FAO Regional Office for Asia and the Pacific, Bangkok, Thailand, 1-92. Brock JA, Gose RB, Lightner DV and Hasson K. 1997. Recent developments and an overview of Taura
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