Effects of natural foods on food selection and growth rates of cobia (rachycentron canadum) larvae

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CAN THO UNIVERSITY COLLEGE OF AQUACULTURE AND FISHERIES EFFECTS OF NATURAL FOODS ON FOOD SELECTION AND GROWTH RATES OF COBIA (Rachycentron canadum) LARVAE By NGUYEN CHI A thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Aquaculture Can Tho, December 2013 CAN THO UNIVERSITY COLLEGE OF AQUACULTURE AND FISHERIES EFFECTS OF NATURAL FOODS ON FOOD SELECTION AND GROWTH RATES OF COBIA (Rachycentron canadum) LARVAE By NGUYEN CHI A thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Aquaculture Supervisor Assoc. Prof. Dr. TRAN NGOC HAI Dr. LY VAN KHANH Can Tho, December 2013 APPROVEMENT The thesis “Effects of natural foods on food selection and growth rates of cobia (Rachycentron canadum) larvae” defended by Nguyen Chi, which was edited and passed by the committee on 12-27-2013. Sign of Supervisor 1 Assoc. Prof. Dr. TRAN NGOC HAI Sign of Supervisor 2 Dr. Ly Van Khanh Sign of Student NGUYEN CHI Acknowledgements First of all, I would like to express my honest thanks to the Rectorate of Cantho University and the lecturers of College of Aquaculture and Fisheries for supporting me to study after 4.5 years. I would like to thank Assoc. Prof. Dr. Tran Ngoc Hai, Dr. Ly Van Khanh and Dr. Le Quoc Viet who have instructed me enthusiastically to finish this graduating thesis. For other valuable help and guide, thanks are extended to all my friends, Mr. Pho Van Nghi, Mr. Nguyen Van Thang, Ms. Nguyen Thi Diem Chi, Mr. Nguyen Tuan Cuong and students in AAP course 35. I also send my gratefulness to my advisor Dr. Duong Thuy Yen for her constant support and my beloved classmates in Advanced Aquaculture Program for great encouragement during 4.5 years in CAF. Finally, I thank my family and all my friends who have supported and encouraged me to study and finish my course. I honestly thank all of you! NGUYEN CHI i Abstract The aim of study was to investigate the pattern of food selection and growth rates of cobia (Rachycentron canadum) larvae reared in pond water. The study was based on the stomach analysis of cobia larvae during the larval stage from hatching to 10 days old. Stomach contents were compared to environment composition and electivity indices were calculated. Natural pond water that contained various live organisms such as 3 taxons of rotatoria, copepoda and nauplius were released to the cultured tanks. Four days after hatching, the larvae commenced feeding and showed little selectivity on zooplankton for 10 days. Brachionus plicatilis, Brachionus pala and Nauplius, the main prey organisms were found to be preferred by larvae during from day 4 to day 7, and subsequently replaced by bigger size prey such as Schmakeria clubia and Microsetella norvegica during from day 8 to day 10. After 10 days of rearing, the fries reached the size of about 5.10 – 5.50mm. Daily length gain (DLG) of larvae fluctuated from 0.09 to 0.16mm/day and specific growth rate (SGR) ranged from 2.13 to 3.58%/day. Electivity indices on Nauplius and Schmakeria clubia range from 0.27 to 0.40 and from 0.42 to 0.47, respectively, indicating that Nauplius and Schmakeria clubia were found to be most preffered food for the cobia larvae during for 10 days old. The change on prey selectivity might be due to the mouth size, sluggish movement of this fish at their initial stages. ii TABLE OF CONTENT Acknowledgements ............................................................................................................... i Abstract ................................................................................................................................. ii TABLE OF CONTENT....................................................................................................... iii List of Tables .........................................................................................................................v List of Figures ...................................................................................................................... vi List of abbreviations ........................................................................................................... vii CHAPTER I ...........................................................................................................................1 1.1 Introduction ..............................................................................................................1 1.2 Objectives of the study.............................................................................................2 1.3 Contents of research ................................................................................................2 CHAPTER II .........................................................................................................................3 2.1 Biological features of Cobia (rachycentrum canadum) ..........................................3 2.1.1 Classification and taxonomy................................................................................3 2.1.2 Habitat and distribution .......................................................................................4 2.1.3 Food and Nutrition ...............................................................................................4 2.2 Overview about the status of hatchery and farming production of cobia ................6 2.2.1 Overview about the status of hatchery and farming production of Cobia in the world ...............................................................................................................................6 2.2.2 Overview about the status of hatchery and farming production of Cobia in Vietnam ...........................................................................................................................8 2.3 Food selection of fish ..............................................................................................9 2.4 Types of natural food used for larval nusery ........................................................11 2.4.1 Rotifers...............................................................................................................12 2.4.2 Copepods ...........................................................................................................13 CHAPTER III ......................................................................................................................15 3.1 Time and location ..................................................................................................15 3.2 Materials ................................................................................................................15 3.2.1 Equipment ..........................................................................................................15 iii 3.2.2 Water source ........................................................................................................15 3.2.3 Feed ....................................................................................................................15 3.3 Research methodology ..........................................................................................15 3.3.1 Experimental design ..........................................................................................15 3.3.2 Sampling and data collection .............................................................................16 3.4 Statistical analysis .................................................................................................19 Data will be analyzed for mean value, standard deviation by using Excel software. ......19 CHAPTER IV ......................................................................................................................20 4.1 Water quality parameters ...........................................................................................20 4.2 Growth rates ...............................................................................................................23 4.3 Food selection of cobia larvae....................................................................................25 4.3.1 Planktons in rearing tank .....................................................................................25 4.3.1.1 Species composition of planktons in rearing tank ............................................25 4.3.1.3 Percentage composition of zooplankton in rearing water ................................26 4.3.2 Planktons in stomach of cobia larvae ..................................................................27 4.3.2.1 Species composition of planktons in stomach of cobia larvae .........................27 4.3.2.2 Density and amount of zooplankton in stomach of cobia larvae ......................28 4.3.2.3 Percentage composition of zooplankton in stomach of cobia larvae ...............30 4.3.3 Electivity index of cobia larvae ...........................................................................31 CHAPTER V .......................................................................................................................35 5.1 Conlusions ..................................................................................................................35 5.2 Recommendations ......................................................................................................35 Appendix .............................................................................................................................46 iv List of Tables Table 3.1 Physical and chemical parameter collection……………………………....18 Table 4.1 Water quality parameters during culture period…………………………..20 Table 4.2 Growth rate of cobia larvae during 10 days……………………………….23 Table 4.3 Composition of zooplankton in rearing tank……………………………....25 Table 4.4 Density of zooplankton in rearing tank…………………………………....26 Table 4.5 Composition of planktons in stomach of cobia larvae……………………28 Table 4.6 Density of zooplankton in stomach of cobia larvae………………………29 Table 4.7 Amount of zooplankton in stomach of cobia larvae………………………29 Table 4.8 Electivity index of cobia larvae……………………………………………32 v List of Figures Figure 2.1 15 – 20 kg broodstock cobia……………………………………………….4 Figure 2.2 Global aquaculture production of Rachycentron canadum………………..9 Figure 2.3 Euryhaline Brachionus species used in aquaculture as live food………...13 Figure 4.1 Variation of temperature during the culture period……………………....21 Figure 4.2 Variation of pH during the culture period………………………………..22 Figure 4.3 Variation of water clarity during the culture period………………...........22 Figure 4.4 Variation of TAN during the culture period……………………………...22 Figure 4.5 Variation of nitrite during the culture period…………………………….23 Figure 4.6 Illustration of cobia larvae growth in 10 days……………………………25 Figure 4.7 Density of zooplankton in rearing tank…………………………………...26 Figure 4.8 Percentage of zooplankton in rearing tank………………………………..28 Figure 4.9 Density of zooplankton in stomach of cobia larvae………………………30 Figure 4.10 Amount of zooplankton in stomach of cobia larvae…………………….30 Figure 4.11 Percentage of zooplankton in stomach of cobia larvae………………….31 vi List of abbreviations CMFRI…………………......Central Marine Fisheries Research Institute HUFA………………………Highly unsaturated fatty acid PUFA……………………….Polyunsaturated Fatty Acid ARA………………………..Arachidonic acid EPA………………………...Eicosapentaenoic acid DHA………………………..Docosahexaenoic acid PRC…………………...........Peoples Republic of China HCG………………………..Human chorionic gonadotropin DLG………………………..Daily length gain SGR………………………...Specific growth rate TAN………………………..Total ammonia – nitrogen FAO………………………..Food and Agriculture Organization of the United Nations dph……………………........Day post hatch ppt…………………………Part Per Thousand CFU/g……………………...Colony Former Unit BLi…………………………Initial Body Length BLf…………………………Final Body Length vii CHAPTER I INTRODUCTION 1.1 Introduction Vietnam has great potential for aquaculture development. It has a 3,260km coastline, 12 lagoons, straits and bays, 112 estuaries, canals and thousands of small and big islands scattering along the coast, more than 1 million km² exclusive economic zone and very large water surface area of brackish water. In recent years, marine aquaculture in Vietnam has been developing very fast and a large number of high quality juveniles is required. Many research to produce sea bass (Lates calcarifer), grouper (Epinephelus spp), spotted Scat (Scatophagus argus), Giant mottled eel (Anguilla marmorata), mudskipper (Pseudapocryptes elongatus) and Cobia (Rachycentron canadum) juveniles in hatchery have been being conducted to develop technology for seed production to apply into practice. Cobia (Rachycentron canadum), the only member of the family Rachycentridae in North America, is a widely distributed species of pelagic fish found worldwide, except the Eastern Pacific; in tropical, subtropical, and warm temperate waters (Shaffer and Nakamura 1989). Cobia aquaculture has been expanding in many tropical countries during the recent past mainly due to its fast growth rates and good meat quality. The success of cobia farming in Taiwan (Yeh, 2000; Su et al., 2000; Liao and Leano, 2005) has led to the rapid expansion of cobia farming throughout Southeast Asia, the Americas and Carribian regions (Benetti and Orhun, 2002; Kaiser and Holt, 2004; Schawrz et al., 2006, 2007; Benetti et al., 2008; Nhu et al., 2010; 2011). Realising the potential of cobia farming in India, the Central Marine Fisheries Research Institute (CMFRI) focused research attention on the broodstock development of cobia and the first successful spawning was obtained in March 2010 (Gopakumar et al., 2011).Several studies have also reported successful larval rearing of cobia in semistatic and recirculating aquaculture systems from both wild caught (Hassler and Rainville, 1975) and captive spawned eggs (Faulk and Holt, 2003) with the use of rotifers, Artemia, and/or wild zooplankton. However, little information is available regarding the nutritional requirements of cobia larvae in recirculating aquaculture systems, 1 and such information is essential to maximize larval growth and survival and further the successful commercial production of this species (Faulk and Holt, 2005). In Vietnam, Cobia is considered the most popular species for culture in offshore cages. This is because of its fast growth, high market value, good meat quality, the established technology in mass production of larvae, the current innovation in intensive and super intensive nursery rearing in ponds, and improved formulated feeds. However, there are still many difficulties remained, such as environmental and diseases problems as well as limitations when relying on natural seed sources, natural food sources, quantity and quality of the seed. Therefore, the study of marine fish seed production in general and the cobia seed production in particular is very necessary and urgent. Based on research and practical demands, this study on “Effects of natural foods on food selection and growth rates of cobia (Rachycentron canadum) larvae” was carried out to apply in practice. 1.2 Objectives of the study To evaluate the effects of natural foods on food selection and growth of cobia larvae at early stage in order to contribute to seed production of cobia fish. 1.3 Contents of research - To evaluate water quality and natural food in tanks. - To evaluate the growth rates of larvae. - To evaluate food selection through food composition in stomach contents at different stages of larvae. 2 CHAPTER II LITERATURE REVIEW 2.1 Biological features of Cobia (rachycentrum canadum) 2.1.1 Classification and taxonomy Cobia (Rachycentron canadum) is classified as the followed (Linnaeus, 1766): Kingdom: Animalia Phylum: Chrodata Class: Actinopterygii Order: Perciforms Family: Rachycentridae Genus: Rachycentron Species: Rachycentron canadum According to FAO (2009), the cobia is characterized by a dark brown dorsally, paler brown laterally and white ventrally; black lateral band as wide as eye extends from snout to base of caudal fin, bordered above and below by paler bands; below this is a narrower dark band. Black lateral band very pronounced in juvenile, but tends to be obscured in adult. The cobia has an elongated body that is strongly rounded with a broad flat head and depressed head. Mouth large, terminal, with projecting lower jaw; villiform teeth in jaws and on roof of mouth and tongue. First dorsal fin with 7 – 9 short but strong isolated spines each depressed into a groove, not connected by a membrane, 28 – 33 rays. Second dorsal fin long, anterior rays somewhat elevated in adults. Pectoral fins pointed, becoming more falcate with age. Anal fin similar to dorsal, but shorter; 1 – 3 spines, 23 – 27 rays. Caudal fin lunate in adults, upper lobe longer than lower (caudal fin rounded in young, the central rays much prolonged). Scales small, embedded in thick skin; lateral line slightly wavy anteriorly. 3 Figure 2.1 15 – 20 kg broodstock cobia (FAO, 2009) 2.1.2 Habitat and distribution Cobia, Rachycentron canadum, is considered one of the most promising candidates for warm – water marine fish aquaculture in the world (Kaiser and Holt, 2004; Liao et al., 2004; Benetti et al., 2007). Cobia, also known as lemonfish or ling, is the only member of the family Rachycentridae, and is found in the warm – temperate to tropical waters of the West and East Atlantic, throughout the Caribbean and in the Indo – Pacific off India, Australia and Japan (Briggs, 1960; Hassler and Rainville, 1975; Shaffer and Nakamura, 1989; Ditty and Shaw, 1992). In the Eastern Pacific its occurrence has been reported as marginal (Fowler, 1944; Briggs, 1960; Collette, 1999). Cobia are eurythermal and euryhaline, tolerating ranges of temperature and salinity between 16.8 and 32.2°C and 5 – 44.5 ppt, respectively (Shaffer and Nakamura, 1989; Resley et al., 2006). They travel alone or in small school and are often found near some kind of structure, whether floating or in the water column (Kaiser and Holt, 2005). 2.1.3 Food and Nutrition Cobia is a carnivorous species and widely distributed in tropical and subtropical waters (Ditty and Shaw, 1992). Excellent flesh quality, rapid growth, and adaptability to culture conditions, confer highly desirable characteristics for global commercial aquaculture on cobia (Holt et al., 2007). The nutritional value of several plant protein sources have been evaluated for potential use in cobia formulated feeds (Chou et al., 2004; Lunger et al., 2006). Dietary requirements for macronutrients including crude protein (Chou et al., 2001; Craig et al., 2006), lipid (Wang et al., 2005), methionine (Zhou et al., 2006), and lysine (Zhou et al., 2007). They are opportunistic carnivores that eat many 4 species of fish, crab, shrimp and squid. Stomach content data show that they prefer crustaceans, particularly portunids (swimming crabs). Cobia have elongated bodies and grow to 6.5 feet (2 m) and 135 pounds (61 kg). Females grow both larger and faster than males (Kaiser and Holt, 2005). Growth of cobia is rapid for the first two years, after which it slows gradually. Females attain a larger size at age than males (Thompson et al., 1991, Burns et al., 1998, Franks et al., 1999). One important aspect of larval nutrition is providing adequate levels of highly unsaturated fatty acids (HUFAs) including arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), and docosahexaenoic acid (DHA, 22:6n-3) (Sargent et al., 1999a). HUFAs play an important role in maintaining cell membrane structure and function, stress tolerance, and proper development and functioning of neural and visual systems (Kanazawa, 1997; Rainuzzo et al., 1997; Sargent et al., 1997). Several studies have shown that marine fishes are unable to convert shorter chain fatty acids such as linolenic acid (18:3n-3) and linoleic acid (18:2n-6) to longer chain HUFAs due to low activity of the necessary enzymes, thus making it necessary to provide these fatty acids through the diet (Mourente and Tocher, 1993; Ghioni et al., 1999). Recently, researchers have suggested that the biochemical composition of eggs and yolksac larvae reflect the basic nutritional requirements of first feeding larvae (Rainuzzo et al., 1997; Sargent et al., 1999b). Faulk and Holt (2003) examined the fatty acid composition of cobia Rachycentron canadum eggs and yolksac larvae and reported high levels of HUFAs with DHA, EPA, and ARA accounting for approximately 80% of the polyunsaturated fatty acids. This suggests that cobia larvae may require high levels of these fatty acids in their diets (Extracted from Faulk and Holt, 2005). 2.1.4 Reproduction Females spawn multiple times during the season. The exact size and age at which cobia are sexually mature varies with location; however, research has shown that males are generally 1 to 2 years old and females are 2 to 3 years old at first spawning (Kaiser and Holt, 2005). In the northwestern Gulf of Mexico, they arrive in the spring and can be caught into the early fall, spawning multiple times from April to September, with activity peaking in July. Spawning occurs in both nearshore and offshore waters where females 5 release several hundred thousand to several million eggs (1.4 mm diameter) which are then fertilized by the attending males. The viable eggs begin development, are heavily pigmented, buoyant, and hatch in approximately 24 hours. Cobia larvae grow rapidly and are large in comparison to most marine species at 3.5 mm TL at hatching. Juvenile fish are found in both nearshore and offshore waters, often among Sargassum patches or weedlines where they seek shelter from predators and can feed (FAO, 2013). According to Patricia et al., 1994 Protein was the major constituent of cobia ovaries and its contribution remained fairly constant (49 - 55% of the dry weight) throughout all stages of development. Lipid was the second most abundant component but the levels, ranging from 21 to 41%, changed depending on the stage of ovarian development. Lipid concentration increased from stage 1 through 3 and decreased slightly in stage 4. 2.2 Overview about the status of hatchery and farming production of cobia 2.2.1 Overview about the status of hatchery and farming production of Cobia in the world As early as 1975, researchers in North Carolina collected cobia eggs from the wild and reared them successfully. However, it was not until the early 1990s that Taiwan reported captive spawning of cobia. Successful efforts in the U.S. followed in 1996. Taiwan now has a commercial industry that produced nearly 5,000 tons in 2004, most of which was cultured. An offshore cage project in Puerto Rico has successfully grown cobia to market size since 2003. Research in Florida, Mississippi, South Carolina, Texas and Virginia has resulted in many successful spawns, both natural and hormone induced. Grow-out trials of juvenile and market-size cobia in ponds, recirculating systems and cages will likely be conducted in the future (Kaiser and Holt, 2005). The artificial breeding of cobia in Taiwan was first recorded in 1992. Mass seed production technique was developed in 1997. It has fast became one of the most favorable species in the domestic offshore cage culture. Later, sea farmers in Japan imported cobia seed and started to culture in sea cages off Okinawa. The seed production of cobia was 3 million in 1999 from 4 hatcheries, as compared to 1.4 million in 1998. About 2 million seed were exported to Japan, Peoples Republic of China (PRC), and Viet Nam. The rest, 1 million seed were stocked locally by 28 cage farmers. The present market price is US$ 0.5 6 per seed (10 cm) and US$ 6.0 per kg of adult (6 – 8 kg) (Yel et al., 2004). In the case of cobia, brood stocks were usually collected from the wild before artificial propagation was developed. Currently, cobia intended for broodstock are produced from hatcheries and reared in open cages until they attain sexual maturity (about 1.5 – 2 years when fish weighs about 10 kg). Handling and spawning of mature brooders have been described in detail (Su et al., 2000; Liao et al., 2001; Liao, 2003). Maturing brooders are selected from sea cages and transferred to land-based spawning ponds (400 – 600 m2 area; 1.5 m depth) with flow – through seawater, at a density of 100 fish per pond and a sex ratio of about 1:1 (male/female). Fish are fed to satiation with raw fish (e.g. sardines, mackerels, squids) once or twice a day. Brooders spawn spontaneously year around, with a peak in spring and autumn when water temperature is maintained at 23 – 27oC. The fertilized eggs are collected using a seine net installed against the current created by paddlewheels. The eggs are then transferred to outdoor larval rearing ponds (earthen ponds; < 5000 m2 area ; 1 – 1.2 m water depth) with well – maintained ‘‘green water’’ (Chlorella sp.) and abundant number of copepods. Water exchange is minimal or unnecessary in the early stage as long as the ‘‘green water’’ is maintained. Eggs hatch 21 – 37 h after fertilization at temperature of 31 – 22oC. Cobia larvae are vigorous and more resistant to some stressors compared to other tropical marine fish (e.g. grouper). They open their mouth and starts feeding at day 3 after hatching. Rotifers and copepod nauplii are provided at this stage, with higher preference to copepods during the first feeding stage. Larvae are reared up to day 20 with survival rate of 5 – 10% (Liao et al., 2004). Human chorionic gonadotropin (HCG) has been used successfully to induce ovulation in a variety of marine fish (Lam, 1982; Donaldson and Hunter, 1983; Zohar, 1989), including Nassau grouper (Epinephelus striutus) (Tucker et al., 1991) and gag grouper (Mycteroperca microlepis) (C. Koenig, personal communication, 1992). Researchers in the U.S. have also used hormones to induce adult cobia caught during their natural spawning season to produce eggs. Both HCG (human chorionic gonadotropin) injected at 275 IU/kg and a slow – release pellet containing salmon GnRHa (gonadotropin – releasing hormone analog) implanted in fish have resulted in spawns. Both of these spawning methods have advantages and disadvantages, but the goal is the same – 7 consistent production of high – quality eggs and larvae for use in the aquaculture industry and research. Cobia eggs are 1.20 to 1.40 mm in diameter, heavily pigmented, and hatch in about 24 hours at 80 to 84°F (27 to 29°C). The fertilized eggs are buoyant and can be gathered easily in a recirculating culture system using a side – looped egg collector with an 800 – micrometer mesh bag. After collection, the eggs are counted (approximately 420 eggs/ml) volumetrically using graduated cylinders, which also allow separation of viable (floating) and nonviable eggs. Eggs are usually stocked into rearing tanks at a density of 5 to 10 per L, although this phase of cobia production is still being researched in order to optimize yield per tank (Kaiser and Holt, 2005). Global aquaculture production of cobia has been increasing rapidly since 2002, reaching approximately 30,000 metric tonnes (at a value of USD 60 million) in 2007 (Son, 2010). The three main producers of cobia in 2007 were China, Taiwan and Vietnam, where annual production was approximately 26,000; 4,000 and 1,500 tonnes, respectively. Production in Vietnam was estimated to be 2,600 tonnes in 2009 (Nhu et al., 2010). Of the production reported to FAO in 2004, 80.6 percent was produced in China and all the rest in Taiwan Province of China (FAO, 2013). Figure 2.2 Global aquaculture production of Rachycentron canadum (FAO Fishery Statistic, 2013) 2.2.2 Overview about the status of hatchery and farming production of Cobia in Vietnam Cobia culture in Vietnam began with the first successful intensive mass production of fingerlings in 1999 (Nguyen, 2002; Nguyen et al., 2003). The industry has expanded 8 from protected areas into more exposed areas in the ocean with better water exchange. Cobia is cultured in Vietnam by small to medium – scale family farms (approximately 1,000 tonnes, mostly for local consumption) as well as cooperative farms (approximately 1,600 tonnes/year, mostly for export) (Huy, 2008; Nhu et al., 2010). Production in Vietnam is clustered in the north (from Ha Long Bay and Bai Tu Long Bay), north central region (Nghe An), centre (Van Phong Bay, Khanh Hoa) and south (Ba Ria – Vung Tau, Kien Giang); (Extracted from Petersen et al., 2011). The main factor constraining development of cobia culture in Vietnam is a shortage of quality fingerlings, although hatchery production in Vietnam is increasing at a rapid rate (Nhu et al., 2010). For example, the Research Institute for Aquaculture No 1 in Vietnam produced 400,000 fingerlings in 2007 and 900,000 in 2008 (Nhu et al., 2010). The industry still relies on fingerling imports from Taiwan and China (Hainan) (Huy, 2008). Other constraints include disease outbreaks and lack of locally extruded feeds (Nhu et al., 2010). Cobia are generally fed low – value fish (trash fish), although a small amount of pelleted diets are used in Vietnam, by small to medium - sized farmers (Son, 2010). The larger – scale cooperatives exclusively use pelleted feeds (Nhu et al., 2010). Problems associated with low – value fish feed include a short storage life, rapid decline of nutritional quality if stored for too long, unstable supply (depending on the season), relatively low growth rates (compared with pelleted diets (although data is still scare)), localised pollution and water quality degradation, and transmission of parasites and diseases. The relative benefits of pelleted diets include faster growth rates (feed to biomass conversion ratios are generally less than 2:1, compared with ratios of 8:1 and higher for low – value fish diets), fewer parasites and diseases, fewer environmental problems, and more stable water quality (Son, 2010); (Extracted from Petersen et al., 2011). 2.3 Food selection of fish Most fish larvae are visual particulate feeders (Greene, 1985), able to feed selectively on prey. Although factors such as prey colour and swimming behaviour can be important in determining prey perception and recognition (Checkley, 1982; Govoni et al., 1986), prey size is probably the major determinant of selectivity, and this is intimately related to the mouth size of fish larvae (Shirota, 1970; Hunter, 1981). Several studies have 9 reported prey size dependent patterns of food selection in muskellunge (Esox masquinongy) (Applegate, 1981); walleyes (Stizostedion vitreum vitreum); yellow perch (Perca flavescens) ( Raisanen, 1982; Raisanen and Applegate, 1983) and other fish species (e.g. Wong ang Ward, 1972). Fish food consumption might be influenced by many environmental factors such as water temperature, food concentration, stocking density, fish size and fish behaviour (Houlihan et al. 2001). The feeding rate relative to the body weight decreases as fish size increases; however, the rate of food consumed increases per individual (Wang et al. 1989). Smelt Osmerus eperlanus (L.) is the main zooplanktivorous fish species in Lake Peipsi and is so an important predator on zooplankton and large invertebrates in this lake. Smelt is zooplanktivorous at younger ages, gradually shifting to larger invertebrates during growth, and the oldest and largest smelts are piscivorous (Karjalainen et al., 1997; Vinni et al., 2004). When smelt start feeding, the number of suitable food organisms available is critical for fish survival. Rotifers and crustacean zooplankton (cladocerans and copepods) are important food items during the first summer because the mouth width seems to be the critical determinant of the ability of smelt to handle large food items (Strelnikova & Ivanova, 1983; Næsje et al., 1987). Earlier research on smelt feeding in Lake Peipsi and in Lake Pihkva (Tikhomirova, 1974) showed that smelt larvae feed mainly on cladocerans (Bosmina, Chydorus, Daphnia cucullata, and Diaphanosoma brachyurum, occasionally Leptodora and Sida) and copepods (Diaptomus, Cyclops, Mesocyclops) during the spring.summer period (Extracted from Salujoe et al., 2008). Both green sunfish fry (Lepomis cyanellus) and bluegill fry (Lepomis macrochirus) selected for Cyclops vernalis and consistently selected against cladoceran egg cases and Potamocypris spp. Moina brachiata was consistently selected for by bluegills but was initially consumed by green sunfish in approximately the same proportion as they were available and later preferentially selected for by larger green sunfish fry (Barkoh, 1984). Among brachionid rotifers, Brachionus plicatilis Miiller, 1786 (Monogononta) is probably one of the beststudied taxa because of its suitability as an initial live feed for various finfish and shellfish larvae (Lubzens, 1987). In B. plicatilis, the size and shape of lorica vary greatly according to the strain (Snell and Canillo, 1984). Rotifers are ideal as a 10
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