Economic impact, management and mitigation of red tides in Korea

Harmful Algae 30S (2013) S131–S143

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Economic impact, management and mitigation of red tides in Korea Tae Gyu Park a,*, Weol Ae Lim a, Young Tae Park a, Chang Kyu Lee a, Hae Jin Jeong b a b

Southeast Sea Fisheries Research Institute, National Fisheries Research & Development Institute (NFRDI), Tongyeong 650-943, Republic of Korea School of Earth and Environmental Sciences, College of National Sciences, Seoul National University, Seoul 151-747, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Clay Fish kills Harmful algal blooms (HABs) Paralytic shellfish poisoning (PSP) Red tide

Over the past three decades, a total of USD $121 million in economic losses (fish/shellfish kills) has occurred in the Korean aquaculture industry due to harmful algal blooms (HABs). Paralytic shellfish poisoning (PSP) has also been noted almost every year, closing shellfish farms, and 46 people were poisoned including five people killed by consuming wild mussels. Since 1980, PSP has been officially monitored and managed, and the nationwide control of fish/shellfish kills by HAB species began in 1995. Management and control strategies include both precautionary and emergency measures. Precautionary management includes establishing an observation network and prediction system, an early warning system, and mitigating damage to aquafarms. Along with regular HAB monitoring including species, chlorophyll a, and associated water quality and meteorological parameters, automatic HAB alarm systems equipped with chlorophyll a and turbidity sensors are used in aquafarms as early HAB warnings. Emergency management is essential after a HAB outbreak to prevent fisheries damage. This method includes supplying oxygen to fish, stopping feeding, transferring fish to a safe area, and clay dispersal. Clay dispersion is the prime mitigation technique for HABs in Korea, because clay is natural, nontoxic, inexpensive, and easy to use in field operations. Clay is dispersed over the sea surface using a clay dispensing device to efficiently remove HABs. A third generation (3G) clay dispenser has been developed recently, combining an electrolytic water generator and a clay dispenser, significantly reducing the amount of clay used, resulting in high removal efficiencies. Since using this device, the economic losses from HAB fish kills have dropped >80% in Korea, although the frequency of HABs has increased since 1980. Clay is a natural component, but using too much clay may cause negative impacts on marine organisms and environments. In addition, clay dispersal is not an effective method to control poisoning of fish/shellfish from algal toxins that accumulate in fish and shellfish at low density toxic blooms. Future studies of HAB control should include control of HABs using minimum amounts of clay and practical use of biological control agents. ß 2013 Elsevier B.V. All rights reserved.

1. Introduction Harmful algal blooms (HABs) have serious impacts on public health, aquatic organisms, aquaculture and tourism industries and marine coastal environments in many countries (Hallegraeff, 1993; Anderson, 1997; Smayda, 1997; Lee et al., 2013). The first fish/shellfish kills by HABs occurred in 1981 via Karenia mikimotoi, resulting in losses of USD $1.7 million. Since then, Gyrodinium sp. caused losses of USD $5 million in 1992, and Cochlodinium polykrikoides resulted in losses of USD $7 million in 1993, USD $60 million in 1995, USD $4–18.6 million per year in 2000–2003, 2007 and in 2012 in the Korean aquaculture industry (NFRDI, 2012). Prior to the early 1980s, HABs usually occurred during the summer in Korea (June–August), but since then have frequently

* Corresponding author. Tel.: +82 55 640 4772; fax: +82 55 641 2036. E-mail address: [email protected] (T.G. Park). 1568-9883/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hal.2013.10.012

occurred in spring and autumn. Bloom durations were mostly <1 week in the 1980s, but the duration of HABs has often lasted >1 month since 1995. Effects and impacts vary. When patches of highly dense Cochlodinium polykrikoides (>1000 cells mL 1) enter aquacages in the sea or aquatanks on land, fish in the cages or tanks die within 2 h. Since official paralytic shellfish toxin (PST) monitoring has begun, PST exceeding the permitted PST standard was first detected in shellfish in 1982 (NFRDI, 2006) and human poisoning first occurred in 1984. Paralytic shellfish poisoning (PSP) has occurred almost every year in Korean waters since 1982. Therefore, effective management and control of HAB outbreaks are a primary concern of the aquaculture industry. Many potential control methods have been used for mitigation and control of HAB outbreaks. Chemical control methods have been evaluated, including the use of phytotoxicants such as surfactants (Kutt and Martin, 1974), aerial dusting with copper sulfate (Rounsefell and Evans, 1958), and the use of chemical flocculants (Anderson, 1997). Various biological control methods

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Fig. 1. Economic losses (fish/shellfish kills) in the aquaculture industry by harmful algal blooms (HABs) in Korea over three decades.

have also been suggested such as competition for nutrients by bacteria/dinoflagellates (Kutt and Martin, 1974), use of pathogens (Bratback et al., 1993), algicidal bacteria (Imai et al., 2002; Mayali and Azam, 2004), viruses (Brussaard, 2004), parasitic dinoflagellates (Park et al., 2004a,b, 2013a,b), and protistan grazers (Jeong et al., 2001a,b, 2008). Clay or a clay mixture with flocculants/ surfactants has been investigated in laboratory and field studies over the past 30 years in several countries. Japan (Shirota, 1989a,b), Korea (Na et al., 1996; Choi et al., 1998; Kim, 2000; Kim et al., 2002; NFRDI, 2010), and Australia (Atkins et al., 2001) have attempted clay dispersal to control HABs in the field. The objective of this study is to provide a general overview of fisheries damage and the strategies for managing/controlling HABs in Korea through its nationwide HAB monitoring system, automatic HAB alarm systems, clay dispersal, algicidal bacteria, and protistan grazers. 2. Economic impact on the Korean fisheries industry

From 1993 to 2012, generally summer Cochlodinium polykrikoides blooms (maximum 20,000–48,000 cells mL 1 over >79 km2 area) and fisheries damage occurred, with no or fewer C. polykrikoides blooms (maximum <1700 cells mL 1 over the same area) from 2008 to 2011 (Fig. 2; Table 1). Although there have been no fish kills from 2008 to 2011, nontoxic dinoflagellate Akashiwo sanguinea blooms occurred at a maximum of 3000 cells mL 1 over a 100 km2 area of the southeastern coast in November 2011. A. sanguinea occurs commonly on the southern coast of Korea from May to December, yet the bloom in November 2011 was the largest autumn bloom recorded in the last 10 years. The bloom lasted 3 weeks and slowed oyster (Crassostrea gigas) growth rates, because the dinoflagellate is too large (50 mm) for filter feeding (Y.B. Hur, personal communication). An odd smell was also reported from some oyster farms during the bloom (Y.B. Hur, personal communication). In addition, sales of sushi restaurants around the bloom area decreased due to the negative sushi image and a decline in tourism (Y.B. Hur, personal communication).

2.1. Fish/shellfish kills 2.2. Incidence of PSP Korea and Japan have the highest fisheries product consumption per capita in the world. For the last 5 years in Korea, 9.6 million tons or 447 million fish have been cultured in an average year, worth USD $766 million (Korean Statistical Information Service: http://kosis.kr/index/index.jsp). Fish farms consist of 75% marine floating net cages and 25% inland aquafarms. Rockfish, red sea bream, flatfish, black porgy, and parrot fish are mainly cultured in these aquafarms. As noted above, the first fish kills by HABs occurred in 1981 due to Karenia mikimotoi and resulted in a USD $1.7 million loss (NFRDI, 2010) followed by Gyrodinium sp. blooms killing fish (USD $5 million loss) in 1992. Since then, Cochlodinium polykrikoides has become a major fish/shellfish killing HAB species in Korea. The economic losses peaked in 1995 when USD $60 million of fish were killed, almost a 10% loss of all cultured fish produced that year. Economic losses of >USD $10 million occurred in 1995, 2003, and 2007. These kills included mostly fish except for a massive abalone (Haliotis discus hannai) kill in a land-based aquafarm in 2003 (Fig. 1; Table 1).

In the past 5 years, 5577 shellfish farms were registered in Korea (Korean Statistical Information Service: http://kosis.kr/ index/index.jsp) approximating 49,261 ha each year for the period. Oyster, mussel, sea squirt, and warty sea squirt farms accounted for 21% (1,182 farms), 3% (171 farms), 8.9% (500 farms), and 2.5% (140 farms) of all shellfish farms, respectively. PSP mostly occurs on the southeastern coasts where about 19.4% (1106 farms) of the farms are located. PSP generally affects shellfish, mostly mussels/oysters; thus, the National Fisheries Research & Development Institute (NFRDI) estimated that the toxin’s presence resulted in harvest closures for about 300–400 mussel/oyster farms in an average year during the PSP period (March–May). Paralytic shellfish toxin (PST) is occasionally detected from other shellfish such as sea squirts (Halocynthia roretzi), warty sea squirts (Styela clava), and Manila clams (Mytilus coruscus) and causes closure of some farms when the PST level is high. PSP incidence has been officially reported since the 1980s in Korea. The first PSP-related accident was

Table 1 Occurrence of C. polykrikoides blooms for the last 20 years in Korea (shown in 2 year intervals). Except for 2003 (abalone kills in land based aquafarms), economic losses (fish kills) dropped by >80% since 1996 when clay dispersal began to be used (also see Fig. 1). HABs alerting level 1: appearance, 2: watch, 3: warning. Item

1991

1993

1995

1997

1999

2001

2003

2005

2007

2009

2011

Duration (days) Maximum cells (mL 1) Number of events Area (km2) Alerting level Economic loss (USD million)

18 3000 2 12–79 1 0

21 9800 10 >79 3 7

46 30,000 20 >79 3 60

25 20,000 13 >79 3 1.2

47 43,000 17 >79 3 0.2

37 32,200 10 >79 3 7

56 48,000 20 >79 3 18.6

55 25,000 8 >79 3 0.09

46 32,500 16 >79 3 10.4

14 1660 6 >79 3 0

7 100 3 <12 1 0

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Fig. 2. A map showing the frequent outbreak region of C. polykrikoides blooms and paralytic shellfish poisoning (PSP) events in Korean waters Fisheries damages (USD $121 million loss) caused by C. polykrikoides blooms occurred from 1993 to 2012. Since 1980, PSP has occurred almost every year, resulting in the harvest closures of shellfish from about 300 to 400 mussel/oyster farms in an average year during March to May.

reported in 1984 when a family of six was poisoned including one death after consuming wild mussels (NFRDI, 2006). In 1986, a second incident occurred during disassembly of a ship hull when workers consumed wild mussels, resulting in 25 poisonings including two deaths (NFRDI, 2006). Since then, 15 people have been poisoned including two deaths (NFRDI, 2006). These human poisoning incidents by PST were all caused by consuming wild mussels. 3. Precautionary management 3.1. Monitoring network for HABs Regular monitoring of HABs by the Korean government began in wide areas when Cochlodinium polykrikoides blooms caused massive economic losses in 1995 (Lee et al., 2013; Park et al., 2013a; Jeong et al., 2013a,b; Kim et al., 2013). In the same year, the Korean government established the ‘‘HABs Emergency Center’’ under the NFRDI and has started to support monitoring, mitigation, and control of HABs according to notification no. 1996-655 of MIFAFF (Ministry of Food, Agriculture, Forestry, and Fisheries), subsequently revised to notification no. 2008-22. The NFRDI HABs Emergency Center and local fisheries offices under the Ministry of Food, Agriculture, Forestry, and Fisheries (MIFAFF) and the National Maritime Police Agency (NMPA) are responsible for monitoring HABs (parameters noted below). The NFRDI and over 30 local fisheries offices monitor HABs from February to November in various locations off Korean coasts. Monitoring occurs once or twice per month beginning prior to the expected HAB occurrence. Once HABs occur, the frequency of monitoring increases to more than once per week, and NMPA monitors the movement of the red tide using helicopter aerial surveillance. NFRDI monitors cell density of the harmful algae, dominant phytoplankton/zooplankton, chlorophyll a, water temperature, salinity, pH, dissolved oxygen, transparency, nutrients (total nitrogen, total phosphorus, and silicate-silicon) and weather (wind, rainfall, typhoon, etc.). This information is regularly uploaded to the NFRDI homepage (http://www.nfrdi.re.kr/redtideInfo). In addition, daily or monthly monitoring of HABs has been conducted by some universities (Jeong et al., 2013a,b; Kang et al., 2013; Yih et al., 2013).

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Both conventional and molecular approaches are used to monitor and identify HAB species. Species are identified and enumerated through microscopy, and molecular detection tools such as species-specific real-time polymerase chain reaction (PCR) are used to detect and quantify Cochlodinium polykrikoides and toxic dinoflagellates (Park et al., 2009a,b; Park and Park, 2010). Use of real-time PCR compensates for the drawbacks (difficulty in distinguishing morphologically similar species) of microscopic observation and makes it possible to conduct accurate, reliable, and high throughput quantification of the target species. This technology was developed in the early 1990s (Higuchi et al., 1992) and has been used for disease diagnosis, quantifying gene expression, and identifying species. Since Bowers et al. (2000) applied this approach to harmful dinoflagellates for the first time, it has been applied to various HAB species (e.g., Zhang and Lin, 2005; Park et al., 2007a,b). This method is useful to detect and quantify morphologically similar species, small sized cells (ca <10 mm), and cysts in natural sediments, which are difficult to identify using standard light microscopy (Zhang and Lin, 2005; Park et al., 2007a,b). 3.2. PSP/ASP/DSP monitoring network In 1979, the U.S. Food and Drug Administration advised ‘‘management of shellfish farms affected by PST producing algae’’, i.e., PST shellfish monitoring (NFRDI, 2006). Since 1980, PST monitoring in Korea was initiated in monthly sampling for mussels/oysters, and monitoring of PST in sea squirts/warty sea squirts/Manila clams has been conducted since 2010. Currently, PST is monitored twice per month from February to June when PSP generally occurs in Korean waters. When PST levels exceed the permitted PST standard (80 mg per 100 g of shellfish meat), the frequency of monitoring increases to more than weekly. The PST level/profile in shellfish is analyzed using a mouse bioassay and/or liquid chromatography–mass spectrometry detection according to notification no. 1993-91 of MW (Ministry of Health & Welfare). Recently, domoic acid (DA) and lipophilic shellfish toxin (LST) in shellfish have been also monitored along with the PST analysis. Although amnesic shellfish poisoning (ASP) and diarrheic shellfish poisoning (DSP) have not caused a human poisoning outbreak and have not exceeded regulatory levels in Korea, the DA and LST levels are being monitored for a potential threat of ASP/DSP (Kim et al., 2010), as the ASP-causing diatom Pseudo-nitzschia spp. and DSPcausing dinoflagellate Dinophysis spp. often appear in Korean waters (NFRDI, 2010). The PST level survey results are publically available in near real-time for public health safety. This information is also provided to countries such as the U.S.A., Japan, and the E.U. through a sanitation agreement to manage designated waters used for shellfish export. The PSP information is available on the NFRDI homepage (http://www.nfrdi.re.kr/bbs?id=shellfish). The NFRDI HABs Emergency Center, the food hygiene division of NFRDI, and local governments are responsible for monitoring PST in shellfish and PST-producing algae such as Alexandrium spp. and Gymnodinium catenatum. Once the PST level exceeds the permitted standard, NFRDI quarantine officials determine the range and duration of shellfish farm closure and prohibit sale of product. Local governments implement the decision in the field. The PST of Alexandrium species in Korea has carbamoyl toxins including saxitoxin, neosaxitoxin, and gonyautoxins (GTX1–GTX4) as well as N-sulfocarbamoyl toxins GTX5, GTX6, and C1–C4 (Kim and Kim, 2005), typical PST profiles of Alexandrium spp. in many other countries (e.g., Anderson et al., 2012). G. catenatum from Korea and other countries also produces carbamoyl and N-sulfocarbamoyl toxins but a high ratio of N-sulfocarbamoyl toxins is observed (Park et al., 2004a,b; Bolch and de Salas, 2007; Hallegraeff et al., 2012). These toxic dinoflagellates are mostly distributed on southeastern

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Table 2 Management and control of harmful algal blooms (HABs) to protect aquaculture farms. Strategy

Precautionary management

Emergency management

Impact prevention

Integrated multi-trophic aquaculture (IMTA) Integrated coastal zone management (ICZM) HABs monitoring network Control of fish density in farm

Forecast of red tide spread Activate warning level of HABs Supply oxygen to fish Transfer fish to safe area Pumping bottom seawater to surface (fish) Stop feeding fish

Control

HABs Emergency Center Reduction of feed supply (fish) Prepare extra fresh seawater (fish) Prohibition of sale for PSP products

Clay dispersal around aquafarm

coasts of Korea where PSP occurs frequently (Park et al., 2004a,b; Kim and Kim, 2005). 3.3. Impact prevention strategy for fish/shellfish kills The NFRDI issues four types of messages (notification no. 200822 of MIFAFF) alerting the public to the threat of HABs depending on the density of harmful algae, viz. (1) appearance, (2) watch (e.g., >300 cells mL 1 Cochlodinium polykrikoides, >1000 cells mL 1 Karenia mikimotoi or >500 cells mL 1 Gyrodinium sp. over a 12– 79 km2 area), (3) warning (e.g., >1000 cells mL 1 C. polykrikoides, >3000 cells mL 1 K. mikimotoi, or >2000 cells mL 1 Gyrodinium sp. Over >79 km2 area) and (4) warning cancelation. These messages are delivered in near real-time to the HAB-related organizations and fisherman through fax, telephone, the internet and smartphone. There are two comprehensive countermeasures to manage, mitigate, and control HABs (Table 2; Fig. 3). First, mitigation methods include restricting marine pollution and improving water quality in aquafarms under laws such as the Clean Water Law and the Sea Pollution Prevention Law. Methods for improving water quality on aquafarms and in the marine environment include controlling fish density, and employing integrated multi-trophic aquaculture (IMTA) (e.g., co-culture of fish, macroalgae, and sea cucumbers) and integrated coastal zone management (ICZM). IMTA is a mitigation approach to reduce the accumulation of excess nutrients/organic matter generated by intensive aquaculture activities (Barrington et al., 2009) and ICZM is a process of harmonizing different policies and decision making structures bringing coastal stakeholders together to take concerted action towards achieving sustainability (McKenna et al., 2009). Natural seaweed and seagrass beds serve as seawater purifying areas by absorbing inorganic nutrients in aquafarms. Imai and Yamaguchi (2012) suggested that seaweeds have algicidal bacteria that kill harmful algae, and that these bacteria play an important role in preventing HABs. Jin and Dong (2003), Wang et al. (2007), Tang and Gobler (2011) have suggested allelopathic substances from seaweeds. The second countermeasure includes improving HAB prediction through ecophysiological, genetic, and toxicological studies and controlling HABs using clay flocculation.

Fig. 3. Schematic diagram for management, mitigation and control of harmful algal blooms (HABs) by precautionary and emergency management strategies for preventing fisheries damage (IMTA, integrated multi-trophic aquaculture; ICZM, integrated coastal zone management).

Management and mitigation strategies can be divided into two stages, viz. (1) precautionary management before HAB events and (2) emergency management after their occurrence (Table 2). Precautionary management includes establishing an observation network and prediction system, an early warning system, and mitigating damage to aquafarms such as control of fish density and food supply and adding non-HAB seawater. Emergency management includes forecasting of red tide movement and expansion, aquaculture animal transfer to safe areas, and the use of clay. These strategies can also be divided into impact prevention methods to protect aquacultured animals and HAB control methods to eliminate a bloom (Table 2). Automatic HAB alarm systems are one of the impact prevention methods. Alarm systems have been developed by NFRDI for fisherman or HAB responsible agencies to help with early HAB warnings or as a tool to mitigate and minimize fisheries damage. The traditional method detects harmful algae using microscopy, which is not appropriate for 24-h monitoring. Particularly, massive fish kills have often occurred when Cochlodinium polykrikoides blooms have been introduced into aquafarms in the middle of night or at dawn. Automatic HAB alarm systems equipped with chlorophyll a and turbidity sensors have been developed and used on inland aquafarms and marine floating net cages since 1999; fisherman are warned with a sound or signal from the device anytime a red tide is introduced into the aquaculture site. Two types of alarm systems are available for aquafarms. First, an automatic HAB alarm system for an inland aquafarm is set up near the seawater intake pump to warn of a red tide and is equipped with warning receiver and a controller in the fisherman’s office. When a red tide is introduced into the intake pump, the alarm system stops the pumping of seawater into the aquafarm and supplies oxygen to protect the aquacultured animals. Second, an automatic HAB alarm system for marine floating net cages also sounds immediately when a red tide is introduced into an aquafarm and simultaneously operates a bottom seawater pump to spray HAB-free seawater from the bottom to the surface to dilute the red tide. This alarm system is set up at the front, rear, left, and right sides of the aquafarm and continuously monitors the arrival of a red tide. In recent years, freshwater blooms caused by green algae or cyanobacteria such as Microcystis spp., Pediastrum spp., Chlamydomonas spp., etc. have also occurred in Korea (NIER, 2008). Freshwater blooms also include four types of alerting messages depending on the density of cells, viz. (1) appearance (500– 5000 cells mL 1), (2) watch (5000–106 cells mL 1), (3) warning (>106 cells mL 1) and (4) warning cancelation (<500 cells mL 1). These messages are delivered to the public by National Institute of Environment Research (NIER). For drinking water purification and management, ozone and activated carbon treatments are intensified and algal toxins (microsystin) are monitored by using high performance liquid chromatography (HPLC), enzyme linked

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immunosorbent assay (ELISA), molecular method or bioassay (NIER, 2008). 3.4. Impact prevention strategy for PSP The NFRDI food hygiene division issues five types of messages (notification no. 1993-91 of MW) to alert the public of a PST threat, for closure of shellfish farms and passage of the blooms depending on the PST level. These are: (1) before PST detection; (2) early PST level (<40 mg per 100 g of shellfish meat); (3) mid-level of PST (40–80 mg per 100 g); (4) PST concentrations in excess of the permitted PST standard (>80 mg per 100 g); and (5) alerting cancelation. Before PST is detected in shellfish, it is monitored once per month and an annual PST meeting for preventing human poisoning is held in January. During the early detection of PST, monitoring frequency increases to once per week, and local governments guide harvests early and sell the shellfish. During a mid-level PST threat, PST is monitored weekly and local governments stop harvesting shellfish. Once the PST level exceeds the permitted standard, monitoring increases to twice per week and NFRDI decides the range and duration of shellfish farm closures. Thereafter, local governments close farms and ban the sale of product. Once the PST is detected below the permitted standard for 2 weeks (three in a row), the sale prohibition is lifted. Currently, there is no practical method for directly removing a PST-causing algae such as Alexandrium spp. or Gymnodinium catenatum from the sea, yet monitoring and prohibiting sales minimizes the threat of PST to public health. 4. Emergency management 4.1. Physicochemical control: clay dispersal 4.1.1. Clay flocculation Clay dispersal is used as the prime mitigation technique for HABs in Korea (Figs. 3 and 4). Among several mitigation techniques that have been applied, dispersing yellow clay is recognized as the best practical method, as clay is relatively inexpensive and easy to apply in the field without notable effects on aquatic organisms and water quality (Yu et al., 2004; Seo et al., 2008). Clays are natural materials found in aquatic systems, they are abundant, and work quickly (within minutes to hours). Numerous HAB mitigation methods have been examined in Korea, including yellow clay (e.g., Na et al., 1996; Choi et al., 1998; Kim, 2000; Sun et al., 2004a), marine bacteria that kill Cochlodinium polykrikoides (Kim et al., 2008), microscreen filtration and ozone (Kang et al., 2001), ultraviolet radiation (Jung, 2000), parasitic dinoflagellates (Park et al., 2004b, 2013b), and microzooplankton predators of bloom

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species (e.g., Jeong et al., 2003, 2008). Nevertheless, no other control methods have been used extensively in the sea except yellow clay. Due to the effectiveness and practicality of clay, clay dispersal has become a part of Korea’s management scheme (notification no. 2008-22 of MIFAFF). This strategy has been used effectively in Korea to minimize the damage from C. polykrikoides blooms on commercially valuable fish mariculture. The principle behind this control strategy is mutual flocculation between clay particles and algal cells, leading to the formation of larger, more-rapidly sinking agglomerates, which settle to the ocean floor (Kim, 2000; Sengco et al., 2001). Surface characteristics of red tide organisms affect their coagulation with clays. The cell cortexes are comprised of carbohydrates, lipids, proteins, silica frustules, etc. (Werner, 1977). Because of the excretion and adsorption of cells, there is on the cell surface a layer of mucilaginous surface adherents which are organic acids, aminoacids, carbohydrates, etc. (Maruyama et al., 1987). These may hydrate or ionize under the seawater condition and result in the surface carrying a charge (Maruyama et al., 1987). Addition of clays destroys the stability and the algal cells are aggregated into flocs, resulting in their settling (Yu et al., 1994). Clay minerals dispersed on the seawater surface are quickly destabilized due to their high ionic strength. Destabilization is a process by which the repulsive forces on the clay surface are neutralized by an excess of counterions. As electrostatic repulsion decreases, attractive forces between particles dominate, and flocculation occurs when the clay particles collide and coalesce to form larger particles. The descending flocculated particles then interact with cells, which either flocculate with the clay particles or are captured as the flocculated particles sweep through the medium (Avnimelech et al., 1982; Yu et al., 1994). Algal cells are trapped among the clay particles, sometimes resulting in cell mortality, and physicochemical interactions between the clay particles and algal cells may also lead to cell lysis and death, which is not caused by the release of potentially cytotoxic substances from the clays (Shirota, 1989a,b; Sengco, 2001). 4.1.2. First generation (1G) clay dispenser Korean yellow clay is mostly composed of montmorillonite and kaolinite minerals (Hwang et al., 2000) and has been used to control Cochlodinium polykrikoides blooms since 1996. Yellow clay dispersion is mostly restricted to finfish farms when cell density reaches the HAB warning level (e.g., >1000 cells mL 1 C. polykrikoides). In 1996, approximately 60,000 tons of dry yellow clay was dispersed by barges over 260 km2 at a loading rate of 400 g m 2 (NFRDI, 2010). Since then, 1000–97,000 tons of yellow clay have been dispersed in Korean waters each year to 2012. Seawater is pumped and mixed with clay in a chamber, and then the seawater slurry is pumped over the sea using a first generation

Fig. 4. Large scale clay dispenser ship and clay dispersal using third generation (3G) clay dispenser (electrolytic clay dispenser, ECD). Photo by NFRDI.

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(1G) clay dispenser. Removal rates of C. polykrikoides were calculated to be 90–99% at up to 2 m of depth. Clay removal efficiencies of 80–99% have also been achieved with various HAB species in laboratory and some small-scale field experiments near fish farms (Choi et al., 1998; Sengco and Anderson, 2004). The removal efficiency of clay is affected by water flow. For example, controlled experiments simulating natural flow using a laboratory flume with phosphatic clay showed that removal efficiency of Heterocapsa triquetra was enhanced in low flow (<2 cm s 1) and greatly reduced under higher flow (<13 cm s 1) conditions (Archambault et al., 2003). In Japan, clay was dispersed in the vicinity of fish cages during a Cochlodinium bloom, and the applied concentration ranged from 110 to 400 mg L 1 (Shirota, 1989a,b). In the Korean C. polykrikoides bloom of 1996, virtually no fish mortality was caused by clay dispersal in Korea, the bloom was eliminated, and the bloom did not return for the remainder of the season (NFRDI, 2004). The 1996 operation was considered a major success, as it resulted in reduced Cochlodinium polykrikoides concentrations near the surface, and >80% decline in fish mortalities compared to a similar red tide in 1995 which was left untreated. Consequently, yellow clay has been used in subsequent years (NFRDI, 2010) to control outbreaks along the southern coast of Korea, with some improvements in the methods of clay preparation and dispersal. 4.1.3. Second generation (2G) clay dispenser In most cases, natural yellow clay contains 10–29% fine size clay minerals (<50 mm) depending on the region it is collected. When whole yellow clay is ground to <50 mm in size, 83 and 87% removal efficiency of Cochlodinium polykrikoides per 1000 cells mL 1 occurred with a 10 g L 1 input after 30 and 60 min, respectively (NFRDI, 2004). Removal efficiencies of 67% and 77% were shown for natural (unground) wild yellow clay after 30 and 60 min exposure, respectively. Removal efficiency is mostly determined by particle size, mineral type (Sengco et al., 2001; Kwak et al., 2006), and the zeta potential of the clay minerals (Yu et al., 2004) although recent work by Pan et al. (2011) suggests any sediment can be used to remove algae. Although ground yellow clay results in a 10% higher removal efficiency of C. polykrikoides, grinding clay costs 60-fold more than using wild clay. Thus, a 2G clay dispenser has been developed and used by HAB-control responsible agencies since 1997. This dispenser is under practical application by local governments responsible for mitigation and control of HABs. This specially designed machine was developed to increase the removal efficiency of harmful algae by crumbling wild yellow clay into a fine size (<50 mm) and subsequently dispensing it into the affected area. The operating principle of the machine is that raw seawater is pumped in and forced through angled channels along the walls of the mixing chamber, which creates a dry yellow clay/ seawater slurry. The mixing chamber includes three rotator blades that rotate at high speed to crumble and mix the dry yellow clay and seawater simultaneously. The finished slurry flows into a second chamber through an overflow slot at the top of the mixing chamber. It is then pumped directly over the sea surface from boats

or barges. The seawater/yellow clay slurry has been sprayed around aquafarms and the cell densities of HAB species at the sea surface were greatly reduced (>80% of Cochlodinium polykrikoides) shortly after treatment and water transparency increased (NFRDI, 2004). The greatest advantage of this design is the incorporation of a slurry, crumbling preparation, and dispersal, which reduces clay dispersal costs and enhances HAB removal efficiency (increases the amount of <50 mm clay particles from 10% to 60%). 4.1.4. Third generation clay dispenser: electrolytic clay dispenser (ECD) Some critical problems remain with these other dispensers, which obstruct the field application of clays. Yellow clay sprayed onto the sea surface sinks to the bottom; thus, the use of a large amount of clay (at least 0.1 g L 1 of mineral; NFRDI, 2004) can cause a negative ecological impact, particularly on the benthos by clay accumulating on the seabed leading to feeding disturbance, suffocation, or burial of clams (Shumway et al., 2003; Archambault et al., 2004). Yellow clay has been sprayed on the sea surface for years, and more and more accumulates on the sea bottom, which may produce an anaerobic benthic environment (Park and Lee, 1998). To address this issue, the ECD was developed to significantly reduce the amount of yellow clay used (Fig. 4). This machine was developed by NFRDI and is a combined electrolytic water generator and clay dispenser. The ECD has been deployed by local Korean governments since 2001. This device minimizes the quantity of clay used and enhances the HAB removal efficiency compared to previous (1G and 2G) clay dispensers. The new model includes a chamber where seawater is hydrolyzed via an electrical current to produce sodium hypochloride (NaOCl), and then yellow clay is added to the hydrolyzed seawater to produce a seawater/clay slurry. Dispersing yellow clay using the ECD results in a higher removal efficiency of HAB with less clay (see below). NaOCl is widely used to inhibit seawater biofouling of ship and electric power plant cooling systems (e.g., Chiristian et al., 1995), and is also an effective chemical (optimal concentration of NaOCl is 300–500 ppb total residual chlorine) for killing HAB species (Jeong et al., 2002). The costs for this treatment are also feasible, because NaOCl can be easily produced by electrolysis of natural seawater, so there is an unlimited resource (NaCl) for its production (Jeong et al., 2002). Additionally, the concentration and amount of NaOCl can be controlled easily by adjusting the electricity applied. Furthermore, NaOCl is rapidly diluted when introduced into the sea and is easily converted to NaCl in sunlight (Jeong et al., 2002). The costs of clay dispersal method are summarized in Table 3. In 1995, a Cochlodinium polykrikoides bloom occurred at a maximum 30,000 cells mL 1 for 46 days over >79 km2 area, resulting in significant fisheries damage (Table 1). Similar C. polykrikoides blooms occurred for the next 17 years (1996–2012) at maxima of 20,000–48,000 cells mL 1 (HAB warning level) for 25– 56 days over the same area. The frequency of fish killing HABs (mostly C. polykrikoides) have also increased since 1980. In the 1980s, the number of HABs events per year was usually <10,

Table 3 Costs of the clay dispersal method to control HABs. Costs were indicated as USD $. Details

Purchase cost of clay

Clay storage (104 tons of clay)

Moving costs of clay (1 dump truck, 1 hydraulic shovel)

Hiring costs of 1 clay dispenser and 1 ship

Cleaning costs of ship, etc. (1 sprinkler truck)

Involved people

Each cost

$18 per ton

$960 per day

$1400 per day

$1100 per day

50–120 people per day

To control 205 m2 of HAB area Total cost per day (205 m2 of HAB area)

20 tons per day

$630 per year (660 m2) $630 per year

4–5 vehicles per day

20–40 ships per day

5–10 trucks per day

50–120 people per day

$360

$630 per year

$3840–4800

$28,000–56,000

$5500–11,000

Government employees

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whereas the number increased to 10–30 in the 1990s and 2000s. However, since the use of clay dispenser devices to control HABs and the nationwide monitoring, economic losses (fish kills) have dropped remarkably, > 80% less loss (USD $0.06–$10.4 million losses) on aquafarms compared to those in 1995 (USD $60 million loss), except for 2003 when mass mortalities (USD $18.6 million loss) occurred on land-based abalone culture farms (Fig. 1). 4.1.5. Different removal efficiency by clay type, cell size, and cell physical condition A variety of natural and treated clays have been tested on HAB species in culture (Table 4). As a result, clay/algal flocculation is influenced by mineral type, algal species (particularly small size cell), ionic strength of the medium, and both algal and clay particle concentration (Soballe and Threlkeld, 1988; Sengco, 2001). The size, shape, density, surface charge and chemical composition of the clay particles contribute to the interparticle forces, as well as to the movement and collision between clay particles and cells, and, hence, affect their flocculation efficiency (e.g., Choi et al., 1998; Kim, 2000; Han and Kim, 2001). In sea water, where the ionic strength of solution is high, the collision efficiency is high. The highest collision efficiency occurs when the particle size of clay and algae are similar. When the clay size is much larger than the algae size, the collision efficiency approached zero (Han and Kim, 2001). In fresh water, where the ionic strength of solution is low, the collision efficiency becomes very low. Although some peaks in the collision efficiency diagram were found, the value was very low compared with the case of sea water (Han and Kim, 2001). Additionally, chemical properties such as stickiness of the algal species influence removal efficiency (Sengco et al., 2001). Sengco et al. (2001) tested 25 clays from the US and yellow clay from Korea for their ability to flocculate and remove Gymnodinium breve and Aureococcus anophagefferens. Clay samples consisting mostly of montmorillonite, bentonite, and Florida phosphatic clay displayed removal efficiencies >90% against G. breve at a clay loading rate of 0.25 g L 1, whereas zeolites and kaolinites showed much lower removal efficiencies for the dinoflagellate. A lower removal efficiency was shown for A. anophaefferens due to low stickiness of the organism and small cell size (ca. 2 mm). Sengco et al. (2005) also showed that cell physical condition is another factor affecting removal efficiency, because healthy cells with sufficient nutrients grown in N and P replete conditions may have enough energy to avoid clay particles. Song et al. (2010) showed that sediment mixed with slaked lime, quick lime, and sludge has 78–100% removal efficiency at 24 g m 2 which is a much lower clay amount than the recommended concentration (100–400 g m 2) by NFRDI (2004). Yu et al. (1995) found that kaolinite from China was more effective than montmorillonite to remove Noctiluca scintillans and Prorocentrum minimum (80%) and almost 100% for Skeletonema costatum at a concentration of 1 g L 1. These studies suggest that

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algal cell size/physical condition and clay type are critical factors for removal efficiency. 4.1.6. Enhancement of removal efficiency using clay mixed with polyaluminum chloride (PAC) or surfactants Chemical flocculants or surfactants have not been used in Korean coastal waters, yet their potential use for controlling HABs and their high removal efficiency have been reported. A combination of clay with chemical flocculants such as polyaluminum chloride (PAC) significantly improves clay removal efficiency by increasing adhesiveness and reducing the amount of clay used (Sengco et al., 2001). PAC is an inorganic coagulant commonly used in water treatment to remove fine suspended particles by inducing flocculation, without the dramatic pH shifts that occur when traditional flocculants such as alum are used (Sengco et al., 2001). The PAC flocculant is light, does not settle easily, and has low toxicity to fish; thus it appears to be an acceptable additive to clay treatment for bloom control (Beaulieu et al., 2005). This flocculent reduces the amount of clay needed to remove HABs (Yu et al., 1995; Sengco et al., 2001). For example, phosphatic clay loading of 0.05 g L 1 removes 85% of the Gymnodinium breve, whereas adding 5 ppm PAC to phosphatic clay reduces the target loading to 0.01 g L 1 with a 75% removal efficiency (Sengco et al., 2001). PAC mixed with bentonite clay was also effective for eliminating a cyanobacteria (Microcystis aeruginosa) bloom in Australia (Atkins et al., 2001). Local beach sand has also been used with PAC/ chitosan to remove Chlorella sp. and Amphidinium carterae. Pan et al. (2011) showed that untreated sand is ineffective for flocculating algal cells, but an 80% removal efficiency was achieved in 3 min using 120 mg L 1 sand modified with 10 mg L 1 PAC and 10 mg L 1 chitosan. Chitosan is an edible food additive and is a biodegradable and nontoxic natural polymer (Renault et al., 2009). Chitosan is essential to bridge the small flocculant particles into larger denser flocculent particles, and PAC is helpful in forming small flocculent particles (Pan et al., 2011). Therefore algal cell removal efficiencies increase remarkably when PAC and chitosan are added. However, the dispersal method must be improved when these compounds are applied to the sea, because clays mixed with PAC at the field-treatment scale immediately form large baseballsized aggregates that immediately sink to the bottom, leaving behind most of the cells in suspension (Sengco, 2001). Use of surfactants with clay enhances removal efficiency and reduces the amount of clay used (Sun et al., 2004a,c; Lee et al., 2008a,b). Surfactants can either be chemically synthesized (synthetic) or microbially produced (biosurfactant). Sun et al. (2004c) showed that Cochlodinium polykrikoides can be eliminated with 10 mg L 1 of the chemically synthesized surfactant cocamidopropyl betaine (CAPB), 60% after 5 min, and 90% after 24 h, whereas <10% of Alexandrium tamarense was removed after a 2 h interaction with 50 mg L 1 CAPB, suggesting different removal

Table 4 Removal efficiencies of clay and clay mixed with flocculants/surfactants. Type

Removal efficiency

Target algal species

Reference

Yellow clay CAPB CAPB Yellow clay mixed with sophorolipid Phosphatic clay Phosphatic clay Phosphatic clay mixed with PAC Kaolinite type clay

90–99% at 400 g m 2 loading during red tide in 1996 90% at 10 mg L 1 CAPB loading in laboratory <10% at 50 mg L 1 CAPB loading in laboratory 95% at 10 g L 1 loading in laboratory 90% at 0.25 g L 1 loading in laboratory <40% at 0.25 g L 1 loading in laboratory 75% at 0.01 g L 1 loading in laboratory 80% at 1 g L 1 loading in laboratory

NFRDI (2004) Sun et al. (2004c) Sun et al. (2004c) Lee et al. (2008a,b) Sengco et al. (2001) Sengco et al. (2001) Sengco et al. (2001) Yu et al. (1995)

Sediment mixed with slaked lime Wet bentonite mixed with PAC Beach sand mixed with PAC and chitosan

78–100% at 24 g m 2 loading in the sea 100% at 0.1 g L 1 loading in laboratory 80% at 120 mg L 1 loading in laboratory

Cochlodinium polykrikoides Cochlodinium polykrikoides Alexandrium tamarense Cochlodinium polykrikoides Gymnodinium breve Aureococcus anophaefferens Gymnodinium breve Noctiluca scintillans Prorocentrum minimum Cochlodinium polykrikoides Prymnesium parvum Chlorella sp. Amphidinium carterae

Song et al. (2010) Sengco et al. (2005) Pan et al. (2011)

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efficiencies by species. A sophorolipid-yellow clay mixture has also been used to remove C. polykrikoides. Sophorolipids are a group of glycolipid biosurfactants produced by yeast, with low toxicity, high biodegradability, and ecological acceptability (Haba et al., 2000). The structure of sophorolipid consists of a dimeric sugar (sophorose) and a hydroxyl fatty acid, linked by a b-glycosidic bond, and some types of glycolipid biosurfactants exhibit antimicrobial, antifungal, mycoplasmicidal, and antiviral activities (Asmer et al., 1988). Sophorolipid affects the chemical affinity (stickiness) of the clay particles, creating higher binding, thus increasing the cell removal ability with a similar mitigation principle as PAC (Sun et al., 2004b). Compared with other surfactants, sophorolipids show a good algicidal effect and biodegradation efficiency for mitigating HABs (Sun et al., 2004a). Lee et al. (2008a,b) showed that a sophorolipid-yellow clay mixture more efficiently mitigated a bloom (95% removal efficiency after 30 min with 10 g L 1 yellow clay) than that of yellow clay alone (79% after 30 min), similar to the 90% removal efficiency found by Sun et al. (2004a). 4.1.7. Certification criteria for suitability of clay to disperse in the sea Because removal efficiency of HABs varies with the types of clay and the safety of additives is mainly unknown, safety and selection criteria are required for clays or any materials before they are dispersed into the sea. In 2004, certification criteria and procedures (notification no. 2004-63 of MIFAFF) for substances to control HABs were announced to provide a guideline for suitable clays and substances (Table 5). According to the notification, substances proven to be practical and safe for marine environments and organisms can only be used in the sea, and, so far, about 170 natural clays have been deemed suitable for sea dispersal. The major minerals in Korean clays are 50–65% silicon, 16–24% aluminum, 4–9% iron, 0.6–2.5% magnesium, and 5–15% other. NFRDI (2010) showed that the removal efficiency of 98 clays collected from various locations in Korea varied from 32% to 98% within 10 min. The average removal efficiency was 71% and half exceeded 80% efficiency. Clays with >80% removal efficiency are considered suitable to use, because these clays can easily decrease Cochlodinium polykrikoides density from the HAB warning level (1000 cells mL 1) to the watch level (300 cells mL 1). Clays with >70% efficiency, which is the average efficiency in Korea, are considered feasible for use in the sea. The removal efficiency of clay also differs by particle size. Particle sizes of <0.063 mm, 0.063–0.125 mm, and >0.125 mm have 84–97%, 85–96%, and 19–58% removal efficiencies, respectively, suggesting significantly lower efficiency at >0.125 mm clay size (NFRDI, 2010). Clays with higher removal efficiencies (>85%) contain 11% (3–27%) particles >0.125 mm in size, whereas clays with lower removal efficiencies (<80%) contain 26% (7–76%) of all particles >0.125 mm in size, indicating that high efficiency clay contains >2 fold lower amounts of the >0.125 mm size particles (NFRDI, 2010). Clays with >80% removal efficiency mostly consist of <20% particles that are >0.125 mm; thus, clays composed of a lower percentage (<20%) of >0.125 mm size particles are desirable. Generally, acidic clays, containing a high percentage of aluminum and iron, remove HAB cells at high efficiency by

releasing aluminum, but its effect on removal efficiency in coastal seawaters with a pH 8.0–8.2 may be negligible (NFRDI, 2010). Therefore, particle size, cohesiveness, and stickiness of clay (i.e., removal efficiency of clay for HAB species) are considered important criteria for clay use in coastal waters. 4.1.8. Impact of clay on fisheries animals and marine environments As a natural component, clay might be expected to cause fewer environmental impacts than other substances (Anderson, 1997). However, clay/algal flocculation, settling, and deposition likely affect other planktonic species in the water column as well as organisms on the sea floor (Park and Lee, 2006). The environmental concern relates to the effect of sedimented particles on bottomdwelling (benthic) organisms. Another concern is that clay treatment might deposit so much algal biomass that oxygen depletion would become a problem in bottom waters (Anderson, 2009). Shumway et al. (2003) performed short-term feeding experiments in a laboratory using seven benthic species to examine the effects of clay (0.01–10 g L 1) on filter-feeding invertebrates. The more clay that was added, the lower the clearance rate of the seven species, indicating a negative effect on filter-feeding invertebrates as clay concentration increased. For this reason, NFRDI has started to monitor environmental changes in areas where yellow clay has been frequently dispersed during Cochlodinium polykrikoides blooms. Ecosystem impacts due to clay dispersion, particularly the benthos, have been assessed since 1998. No significant differences in biomass or species composition of the benthos such as annelida, mollusca, decapoda, or anthropoda have been observed between the areas of clay dispersal and control areas, although large seasonal fluctuations were observed, indicating that the impact of clay dispersal on the benthic animals might be negligible (NFRDI, 2008). Some researchers have reported that clay dispersion may not be lethal to juvenile fish, shellfish, or invertebrates (Lewis et al., 2003; Seo et al., 2008). Portmann (1970) investigated the effects of clay from pottery operations on the bottom fauna of two bays near Plymouth, U.K. As a result of the area’s pottery industry, Chinese clay was distributed over 48 km2, accumulating in bottom sediments at a very high loading of 188 kg m 2. However, fish and benthic organisms were abundant in the area and many seemed to thrive with the clay substrate. Shirota (1989a) reported that clay/organic flocculent would be beneficial as a food source to some benthic animals such as sea cucumbers. Clay has been dispersed at approximately 100–400 g m 2 on red tide invading farming areas for many years in Korea. If it is assumed that most clay dispersion target areas are approximately 10 m in depth, the amount of dispersed clay in an area will be <0.04% of wet weight (Seo et al., 2008). The movement of suspended clay particles by tidal currents soon after dispersion in the field has been estimated using a theoretical model (NFRDI, 2008). Obido embayment in Tongyong, Geoyongnam province, where clay has been introduced for several years, was selected as a target area for the model experiment. Approximately, 38,000 metric tons (5000 metric tons annually) of yellow clay had been used in the area to minimize fish kills from 1998 to 2005. The model showed that the boundary of clay particle transport with

Table 5 Certification criteria (notification no. 2004-63 of MIFAFF) for use of clay in Korean waters. Analysis item

Certification criterion

Analysis criterion

Note

Removal efficiency

Over 80% removal efficiency

Particle size

Below 20% of >0.125 mm size particle

Removal efficiency at 1% clay concentration within 10 min Analyze particle size ratio after sedimentation and dissolution of over 1 kg clay with seawaters

Allowed to use over 70% removal efficiency clay considering practicability –

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tidal currents could be as far as 11.2, 0.84, 0.09, and 0.04 km2 for 10, 35, 50, and 100 mm sized clay particles, respectively. The theoretical height of clay sedimentation within the embayment for 8 years was estimated to be 1.6–4.0 mm in the outer regions and 40–80 mm near a finfish culture farm where the yellow clay was dispersed. Field surveys on clay sedimentation over 10 stations on the southern coasts of Korea where clay had been frequently introduced since 1996 were conducted in 2005 to confirm the clay residual in the area (NFRDI, 2008). Results of soft X-ray and clay composition analyses showed that a clay bed did not exist except for one station at the inner part of the surveyed area. It was assumed that tidal currents and high waves due to frequent typhoons might play a significant role resuspending and transporting the clay that settled in this area. Yellow clay has been dispersed with NaOCl using the ECD device for many years in Korea. Jeong et al. (2002) examined the effect of NaOCl on phytoplankton, heterotrophic protists, planktonic crustaceans, finfish, shellfish, and macroalgae in the laboratory. Results indicated that other organisms had much higher NaOCl tolerance compared to HAB species; therefore, red tides can be effectively controlled without serious harmful effects to other marine organisms. For example, the lethal total residual chlorine concentration that killed 50% of Gymnodinium catenatum, Cochlodinium polykrikoides, Akashiwo sanguinea, Lingulodinium polyedrum, Prorocentrum micans, Aleaxandrium affine, and Gymnodinium impudicum ranged from 57 to 157 ppb for a 10 min exposure and from 30 to 106 ppb for a 1 h exposure. A concentration < 500 ppb NaOCl killed 100% of the HAB cells, whereas 50% of benign species (such as the diatoms Skeletonema coastatum and Thalassiosira rotula) were killed by 3083–3383 ppb during a 10 min exposure and 3128–3433 ppb after a 1 h exposure. However, the heterotrophic dinoflagellates Polykrikos kofoidii and Oxyrrius marina were killed at a similar NaOCl concentration to the HAB species. For metazoans, the lethal total residual chlorine concentrations that killed 50% of marine animals and plants were 1234–1883 ppb for juvenile gray mullet Mugil cephalus and juvenile black rockfish Sebastes schlegell over a 10 min exposure and 1234–1440 ppb for a 1 h exposure. Those of adult Manila clam Ruditapes philippinarum and spat of the abalone Nordotis discus were >20,000 ppb. Those of macroalgae Griffithsia japonica (Rhodophyta) and Ulva pertusa (Chlorophyta) were 1519–12,365 ppb for a 10 min exposure and 1085–12,558 ppb for a 1 h exposure. Hence, overall study results suggested that 300–500 ppb total residual chlorine is the optimal NaOCl concentration for controlling a red tide without serious harmful effects on other marine organisms. Han et al. (2001) also reported that when rockfish S. schlegell and little neck clam R. philippinarum were exposed to NaOCl at concentrations of 0.5 or 2 ppm (4-fold higher than the concentration for algal mortality) for 1 h, no histological changes were found in the tested animals. The potential toxic effects of a phosphatic clay/PAC mix and the removal efficiency of brevetoxins by clay flocculation were also examined in Karenia brevis. Phosphatic clay mixed with PAC was not acutely or chronically toxic to infaunal amphipods, grass shrimp embryos, and larval sheepshead minnow (Lewis et al., 2003). However, the clay was not efficient at removing the extracellular toxin from K. brevis, indicating that the toxin released from algal cells is not readily flocculated with the clay (Pierce et al., 2004). 4.2. Biological controls: algicidal bacteria and protistan grazers 4.2.1. Algicidal bacteria Relationships between terminating HABs and biological agents such as viruses and bacteria have been studied to understand bloom dynamics and their ecological niche. Previous microcosm and field studies have shown an increase in the abundance of algicidal bacteria during the HAB termination period (e.g., Imai and Kimura,

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2008; Seong et al., 2006; Kim et al., 2008; Lee et al., 2008a,b). Therefore, algicidal bacteria have been proposed as a potential control agent for HABs (Imai and Yamaguchi, 2012). Isolation, the killing mechanism, distribution, and ecological effects have been studied in many countries (e.g., Doucette et al., 1999; Kim et al., 1998; Imai et al., 2002; Seong et al., 2006). In Korea, various marine bacteria that kill HAB species including Cochlodinium polykrikoides have been isolated and studied for control of HAB species (Park and Lee, 1998; Lee and Park, 1998; Kim et al., 1999, 2008, 2009; Seong et al., 2006; Oh et al., 2011; Seong and Jeong, 2011). Several approaches have been suggested for practical use of algicidal bacteria, such as dispersal of algicidal compounds purified from bacteria and the development of seaweed/seagrass beds around aquafarms (Jeong et al., 2005a,b; Imai and Yamaguchi, 2012). Imai et al. (2002) found that a huge number of algicidal bacteria are attached on some seaweeds and suggested that the use of seaweed may reduce algal cell density, and, consequently, would be an ultimate countermeasure to HABs (Imai and Yamaguchi, 2012). 4.2.2. Protistan grazers Grazers affect red tide population dynamics (e.g., Watras et al., 1985; Jeong et al., 2011a,b; Kim et al., 2005). In particular, heterotrophic dinoflagellates and/or ciliates sometimes have considerable grazing impact on populations of red tide organisms, and grazing by these heterotrophic protists is believed to contribute to the decline in some red tides (Holmes et al., 1967; Eppley and Harrison, 1975; Jeong et al., 2005a,b, 2011a; Yoo et al., 2013a,b). However, to prevent large-scale finfish and shellfish mortality, red tides should be controlled before they reach aquaculture cages. Korean scientists have made a great effort to develop a biological method to control red tides using masscultured grazers (Jeong et al., 2001a,b, 2008). Protistan grazers search, capture, and feed on red tide organisms. They can divide by feeding on the prey faster than the prey itself. In turn, they are eaten by higher trophic level predators (Jeong et al., 2010, 2011a,b). Thus, when mass-cultured grazers are introduced into red tide patches, grazers are able to increase their population while eliminating prey cells. To date, effective grazers have been discovered for each of the red tide organisms that cause red tides in Korean waters (Table 6). However, effective grazers on Prorocentrum micans have not been identified. Using mesocosm studies, Jeong et al. (2001a, 2008) showed that introducing the mass-cultured heterotrophic dinoflagellate Oxyrrhis marina and the naked ciliate Strombidinopsis sp. reduced populations of the raphidophyte Heterosigma akashiwo and Cochlodinium ploykrikoides, respectively, to undetectable levels in a few days. Using laboratory and field experimental results on growth and ingestion rates of Oxyrrhis marina on Heterosigma akashiwo, Jeong et al. (2001a) estimated the time for 104 L 1 O. marina to eliminate H. akashiwo cells in a red tide patch of 100 m long, 10 m wide, and 1 m deep containing homogeneously distributed prey of 20,000 cells mL 1. The growth and ingestion rates of cultured O. marina on natural populations of H. akashiwo with concentrations >20,000 cells mL 1 could maintain approximately 40% of the maximum growth and ingestion rates obtained from the bottle incubation (0.57 per day and 50 cells per grazer per day, respectively). If the growth rate of the prey is 0.23 per day as obtained from the mesocosm experiment, O. marina should be able to dissipate the red tide patch within 9 days (Jeong et al., 2001a,b). Similarly, based on the results of the laboratory and field experiments on growth and ingestion rates of Strombidinopsis sp. on Cochlodinium ploykrikoides, Jeong et al. (2008) estimated that the time for 104 L 1 of Strombidinopsis jeokjo (30 cells mL 1) to eliminate C. polykrikoides cells in a red tide patch 100 m long, 10 m wide, and 1 m deep containing homogeneously distributed

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Table 6 Effective protistan grazers on phytoplankton. mmax (maximum growth rate, per day). Imax (maximum ingestion rate, ng C per predator per day). MTD, Mixotrophic dinoflagellate; EUG, euglenophyte; RAP, raphidophyte; DIA, diatom; T, toxic strain; NT, nontoxic strain. Prey species

Predator

mmax

Imax

Reference

Akashiwo sanguinea (MTD)

Strombidinopsis sp.

1.27

343

Jeong et al. (1999)

Amphidinium carterae (MTD)

Oxyrrhis marina Paragymnodinium shiwhaense Pfiesteria piscicida

1.17 1.10 1.22

2.8 0.3 1.1

Jeong et al. (2001a) Yoo et al. (2010a) Jeong et al. (2006)

Alexandrium minutum (MTD, T) Alexandrium tamarense (MTD, T) Alexandrium tamarense (MTD, NT) Ceratium furca (MTD)

Gyrodinium moestrupii Gyrodinium moestrupii Gyrodinium moestrupii Polykrikos kofoidii

1.60 0.68 0.71 0.35

2.6 2.1 1.3 9.8

Yoo et al. (2013a,b) Yoo et al. (2013a,b) Yoo et al. (2013a,b) Jeong et al. (2001b)

Cochlodinium polykrikoides (MTD)

Polykrikos kofoidii Strombidinopsis sp.

1.08 1.38

14.5 353

Kim (2004) Jeong et al. (1999)

Gymnodinium aureolum (MTD) Gymnodinium catenatum (MTD) Karenia brevis (MTD, T)

Gyrodinium dominans Oxyrrhis marina Polykrikos kofoidii Gyrodinium moestrupii

0.92 0.71 1.12 0.80

Lingulodinium polyedrum (MTD)

Polykrikos kofoidii Protoperidinium cf. divergens Strombidinopsis sp.

0.83 0.484 0.83

24.4 12.0 222

Jeong et al. (2001b) Jeong and Latz (1994) Jeong et al. (1999)

Prorocentum minimum (MTD)

Gyrodinium dominans Gyrodinium moestrupii Oxyrrhis marina Strombidinopsis sp.

1.13 1.07 1.20 1.06

1.2 1.4 2.0 267.0

Kim and Jeong (2004) Yoo et al. (2013a,b) Lee (1998) Jeong et al. (1999)

Scrippsiella trochoidea (MTD)

Gyrodinium moestrupii Polykrikos kofoidii Strombidinopsis sp.

1.50 0.97 0.67

3.0 16.6 207

Yoo et al. (2013a,b) Jeong et al. (2001b) Jeong et al. (1999)

Eutreptiella gymnastica (EUG)

Gyrodinium dominans Oxyrrhis marina Protoperidinium bipes

1.13 0.81 0.77

2.7 2.7 14.3

Jeong et al. (2011b) Jeong et al. (2011a) Jeong et al. (2011a)

Heterosigma akashiwo (RAP)

Oxyrrhis marina Pfiesteria piscicida Stoeckeria algicida

1.43 1.10 1.63

1.3 1.1 0.8

Jeong et al. (2003) Jeong et al. (2006) Jeong et al. (2005a,b)

Skeletonema costatum (DIA)

Protoperidinium bipes

1.37

2.9

Jeong et al. (2004)

dinoflagellate prey at a concentration of 1000 cells mL 1 could be estimated. Growth and ingestion rates of cultured S. jeokjo feeding on natural C. polykrikoides populations at 1000 cells mL 1 could thus be maintained at levels approximating 0.715 per day and 51 ng C per grazer per day (72 cells per grazer per day), respectively. If a red tide prey population does not grow, as in the control mesocosm, S. jeokjo should be able to dissipate the red tide patch within 6 days (Jeong et al., 2008). However, if C. polykrikoides reproduces at its maximum rate observed in laboratory culture (m = 0.4 per day), dissipation of the patch may require 14 days. Hence, large scale cultures of S. jeokjo may be useful for eliminating C. polykrikoides cells in fresh seawater supplied to large land-based, flow-through tanks on abalone aquaculture farms. 5. Conclusions and future directions Early warning of HABs is considered an important precautionary management strategy to prevent economic losses on aquaculture farms. One of the effective precautionary management tools for HABs is automatic HAB alarm system equipped with chlorophyll a and turbidity sensors, which has been used for early warning of red tide inflow into aquafarms in real-time. However, this device simply measures the total biomass of phytoplankton as pigment and cannot distinguish between harmful and benign species. One promising strategy for species-specific/toxin-specific detection is use of an environmental sample processor (ESP). This device, a robotic electromechanical/fluidic system, autonomously samples water, concentrated particles, and applies molecular

2.0 0.5 17.1 1.9

Yoo et al. (2010b) Yoo et al. (2010b) Jeong et al. (2001b) Yoo et al. (2013a,b)

probe-based assays remotely (Roman et al., 2007). The ESP currently uses DNA probes in a sandwich hybridization assay, fluorescent in situ hybridization, real-time PCR, and antibody probes in a competitive enzyme linked immunosorbent assay (Greenfield et al., 2008; Scholin, 2010). It has been used to estimate bloom toxicity and to detect several HAB species such as Pseudonitzschia and Alexandrium spp. (Greenfield et al., 2008; Doucette et al., 2009). Once HABs occur and approach an aquafarm, emergency management is essential to protect fisheries. Clay dispersion is currently the prime mitigation technique to directly kill C. polykrikoides in the sea. This dispersal method was significantly improved by development of the ECD, which enhances removal efficiency and reduces the amount of clay used. About 1000– 97,000 tons of yellow clay were dispersed each year for 17 years in Korea, yet notable effects on aquatic organisms and marine environment have not been reported. Although clay has been successfully used to control C. polykrikoides blooms, there are still several problems and limitations. Clay is not effective for controlling toxic algal blooms occurring at low cell densities, which can cause paralytic shellfish poisoning, amnesic shellfish poisoning, diarrhetic shellfish poisoning or ciguatera fish poisoning by the accumulation of fish/shellfish poisoning toxins. Because clay removal efficiency decreases when applied to low density algal blooms (e.g., Sengco et al., 2001), clay is only dispersed in Korea when C. polykrikoides density exceeds 1000 cells mL 1. Additionally, clay dispersal is a very labor intensive method (about 50–120 people per day were mobilized for control of C. polykrikoides blooms in Korean waters) and requires a large

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number of vessels (20–40 ships used per day), both downsides to routine use of the clay dispersal method. Accordingly, continued research into environmentally friendly methods for preventing HABs and a cost-effective countermeasure for mitigating fisheries damage/human poisonings is needed. Acknowledgements We thank Dr. Hak Gyun Kim and Dr. Heon Min Bae of NFRDI for valuable advice on mitigation of HABs in Korea. We also thank Prof. Myung Gil Park of Chonnam National University for valuable comments and suggestions. This study was funded by a grant from the National Fisheries Research & Development Institute (RP2012-ME-021) award to T.G.P. and by long-term change of structure and function in marine ecosystems of Korea program, Korea Institute of Marine Science & Technology Promotion/KOF award to H.J.J.[TS] References Anderson, D.M., 1997. Turning back the harmful red tide. Nature 388, 513–514. Anderson, D.M., 2009. 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