Online Version ISSN: 1314-412XVolume 5, Number 2

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2013

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English Editor

Yanka Ivanova (Bulgaria)

2013

ISSN 1313 - 8820 Volume 5, Number 2June 2013

Food spectrum of grey mullet (Mugil cephalus L.) along the Bulgarian Black Sea coast

1 2 1 1 1 1R. Bekova *, G. Raikova-Petrova , D. Gerdzhikov , E. Petrova , V. Vachkova , D. Klisarova

1Deptartment of Hydrobiology, Institute of Fishing Resources, 4 Primorski, 9000 Varna, Bulgaria2Department of General and Applied Hydrobiology, Faculty of Biology, Sofia University St. Kliment Ohridski, 8 Dragan Tsankov, 1164 Sofia, Bulgaria

Abstract. For the present paper 35 specimens of sea mullet (Mugil cephalus) from different age groups were investigated. The material was collected between May 2010 – September 2011 along the coast, the main points are two: Varna Bay and the mouth of the river Veleka. The main objective of the study was to establish the food spectrum of mullet, and thus a connection could be made between food components. The methods which were used and defined to study the food spectrum and the relationship between different food components are as follows: frequency of occurrence and dominance index of the nutritional components; the filling index and stomach emptiness index, as well as selectivity coefficient of a meal.Results showed that detritus, zoobenthos (Rissoa sp. and Trochammina inflate – 71.43%) and macrophytes (Cladophoraceae family – 63.64%) dominated in stomachs. Several potential toxic species were also found: Amphora coffeaeformis, Pseudo-nitzschia delicatissima, Pseudo-nitzschia seriata, Prorocentrum cordatum, E. pyriformis, P. minimum, Protoperidinium crassipes. The results for the frequency of occurrence (pF) showed that the most common in the stomach of a mullet of the macrophytes are the species of family Ulvaceae – 45.5% ; family Cladophoraceae – 45.5% ; of phytoplankton – Navicula spp (109,09%).; of zoobenthos: Nematoda g.sp. – 42.9% and Trochammina inflate – 28.6 %. The low emptiness index (0.0857) defines the grey mullet as an often eating fish. The small fish reaching up to 3–4 cm eat phytoplankton and zooplankton. Until now, research on the food spectrum of sea mullet by the Bulgarian sector of the Black Sea have not been conducted.

Keywords: sea mullet, food, frequency of occurrence, index of dominance, selectivity, Black Sea coast

AGRICULTURAL SCIENCE AND TECHNOLOGY, VOL. 5, No 2, pp , 2013173 - 178

Introduction walls in the highly modified gizzard-like pyloric stomach (Thompson, 1966). Juvenile mullet are primarily carnivorous until they reach a standard length of about 30 mm during this period feed on mosquito In the classical concept of aquatic food chains zooplankton are larvae, copepods and other zooplankton.considered the first important link in secondary production. What is

M. cephalus have uniquely equipped for its trophic feeding often ignored is that this concept, like so many other basic principles niche. Its highly modified stomach has been described by Al-in marine biology, was derived from observations and research Hussaini (1947) as consisting of two parts: the cardiac portion, a thin carried out either in the open ocean or at relatively high latitudes. The walled, saccular, blind caecum used for storage, and the pyloric zooplankton is not the most important herbivore link. Williams et al. portion, a thick, muscular gizzard-like structure used for pulverizing (1968) have demonstrated that in the vicinity of Cape Hatteras, the food. The intestine is extremely long and coiled, often being more North Carolina, zooplankton become steadily less important with than five times the length of the fish. Finally, there is a pharyngeal decreasing depth in areas shallower than 100 meters, in shallow filtering device, described in detail by Ebeling (1957). This inshore areas they were shown to be relatively unimportant in food mechanism aids in selecting very fine particles from the coarser chains since they grazed only two to nine per cent of available sediments which are expelled. This finer material has been shown to phytoplankton.be much richer in adsorbed micro-organisms than the particles Moreover, in shallow estuaries the phytoplankton itself has which are rejected (Wood, 1964; Odum, 1968 a,b).been found to be of secondary importance (Schelske and Odum,

The Mugilidae family relates 8 genera with approximately 100 1961; Radotzkie, 1959; Teal, 1962; Pomeroy, 1960). The most species of fish. The Black Sea coast found six species of three important primary producers in such systems are marsh grasses genera (Karapetkova and Zhivkov, 2010) from which most catches (Spartina, etc.), sea grasses (Zostera, Thalassia, etc.), attached gray mullet (Mugil cephalus), followed by Golden grey mullet (Liza macroalgae, mangroves (Rhizophora, etc.) and the benthic aurata) and Leaping mullet (Liza saliens) (Bekova and Raikova-microflora (benthic diatoms, dinoflagellates, filamentous green and Petrova, 2011).blue-green algae). In most cases food chains are based on detritus

M. cephalus is one of the oldest objects of fish farming and one derived from marsh grasses, sea grasses, macro-algae and of the most popular objects for industrial aquaculture, but not good mangroves, or directly on the benthic and epiphytic microflora. knowledge on the nutritional demand makes its cultivation under Animals which are able to utilize such energy sources replace artificial conditions very difficult. Most difficult cultivation of evident is zooplankton as the critical herbivore link.after the switching to active feeding first week of age. The young Typically M. cephalus feeds either by sucking up the surface larvae caught from natural waters grow much easier (Georgiev, layer of the mud or by grazing on submerged rock and plant 2002; Boyadjiev, 2011).surfaces. An extensive literature reviewed by Thompson (1954) has

Being a detritofag fish the grey mullet (Mugil cephalus L.) finds established that the major contents of the stomach may be favorable conditions and food in the Black sea area. The newly categorized as: microalgae including epiphytic and benthic forms, hatched grey mullet fish feed on phytoplankton and zooplankton. As decaying plant detritus, and inorganic sediment particles. The latter they grow up in their menu are included macrophytes, appear to function as a grinding paste in the degradation of plant cell

173

* e-mail: radoslavabekova@gmail.com

174

macrozoobenthos, detritus etc. There are no contemporary data Altogether 35 grey mullet (aged 1 to 6 years) stomachs were about the grey mullet food in the available literature for Bulgarian investigated. Twenty five of these were originating from the Varna part of the Black Sea. There are a variety of contemporary literature Bay and 10 from the mouth of the river Veleka (Figure 1).The about the methodology and principles of the research food spectrum standard length (SL), to the nearest 1 mm and the weight (W) to the 1 in different species, but they are not methodologically and g were measured. Digestive tracts were excised and preserved in 10 operationally responsible for the operation of this work. The only % formalin. The length of each gut (to the nearest 1 mm) and the published data are about the composition of the grey mullet in the weight (to the nearest 0.0001 g) of its contents were measured. Age coastal lakes (Aleksandrova, 1964,1967,1973). was determined on scales at magnification of 17.5x with the aid of

The main objective of the research was to establish the food projector Dokumator, Lasergerat (manufactured by Carl Zeiss, spectrum of the grey mullet, and thus reveal relationships between Jena). The occurrence frequency (pF) (number of digestive tracts the different food components and the environment. with a certain food item against the total number of digestive tracts,

expressed as a proportion) of each element in the gut was calculated. The index of dominance (DF) (number of guts in which certain food item dominates, against the whole number of investigated guts, expressed as a proportion) was also calculated. To asses the activity and rhythm of feeding of the grey mullet vacuity

Fish were sampled during the period June – November 2010. index (Huslop, 1980) (mean total weight of gut content relative to fish

The material were collected by cast net fishing – size of the eye from weight, in %) and vacuity index (the total number of empty guts

0.5 cm to 1.5 cm, pound nets whit size of the eye – 6 cm, 6.5 cm and relative to the total number guts, in %) were determined. Selectivity

24 cm. GPS - coordinates of the pound nets “Trakata” are factor E is the definite by Iliev (1955). Selectivity coefficient is defined

43°13'04''N and 27°58'94''E, for the pound nets “Galata” are as E-r-p/r+p (r – type of food in the stomach; p – type of food in the

43°10'27''N and 27°56'73''E. A pound net is a visible passive environmentin).

(stationary) gear type used for the live-entrapment of fish species. Microalgae are defined in nanoplankton counting chambers

This gear is constructed using an arrangement of fiber netting "Sedgwick Rafter" with volume 0.05 ml of a microscope Olympus

supported upon a series of stakes or pilings. A pound net usually BX41. Taxonomic identification of zoobenthos species was

consists of three main sections: a “crib/pound” which is the enclosed performed by Morduhay-Boltovskoy (1968, 1969, 1972) and

end where the entrapment takes place; a “heart” that aids in Marinov (1977).

funneling fish into the crib and “hedging/leader” which is a long straight net that leads the fish off shore towards the crib. Fish swimming along shore are turned towards the crib by the hedging,

Results and discussionguided into the heart, and then into the crib where they are removed periodically using various techniques.

In the grey mullet's stomach content microalgae, macrophytes and zoobenthos are present. Several potential toxic species were also found: Amphora coffeaeformis, Pseudo-nitzschia delicatissima (syn. Nitzschia delicatissima), Pseudo-nitzschia seriata (syn. Nitzschia seriata), Prorocentrum cordatum (syn. E. cordata), E. pyriformis, P. minimum and Protoperidinium crassipes.

Very little of the food ingested by mullet originates from the water column; the possible exception would be settled planktonic forms on the benthos.

The food ingredients have the following frequency of occurrence (pF): Navicula sp. – 109.09%; Prorocentrum cordatum – 27.27%, Prorocentrum micans E. – 45.5%, Peridinium sp. E. – 45.5 %. These kinds of microalgae are a common component of phytoplankton in the Black Sea (Petrova and Gerdzhikov, 2012).

The species of family Ulvaceae are 45.5%; family Cladophoraceae – 45.5%, Nematoda g.sp. – 42.9% and Trochammina inflate – 28.6 % (Figure 2).

Hartley (1947) has suggested that the organization of the alimentary system of a particular species, as for example in the relative concentrations of its digestive enzymes, may be such as to obtain maximum advantage for only a limited part of the range of material which the animal is actually capable of ingesting. If this idea is true, M. cephalus should show a preference for either plant detritus or live plant material when both are present in abundance.

Figure 3 shows the index of dominance (DF) of the food ingredients. From the microalgae species the highest percentage is for Prorocentrum micans – 72.7%. From the zoobenthos species with equal percentage are Rissoa sp. and Trochammina inflate – 71.43%. Macrophytobenthos species are dominated by the Cladophoraceae family – 63.64%.

The grey mullet's species preferences can be evaluated by the

Material and methods

Figure 1. Stations of catches

175

different quantities in the stomach content and the environment. The “Diversa “ – 12%. From the Polychaetes some typical species can be dominating species phytoplankton in the open sea areas and the identified, e.g. Prionospio cirrifera,Wirén; from the molluscs Varna Bay during the September period are: small flagellates Cerastoderma edule Linnaeus and Chamelea gallina Linnaeus.

6 6 6 Frequently occurring within the Crustaceans group are the ubiquist (Cryptophyceae) – 0.40.10 cells/ltr; 0.36.10 cells/ltr; 0.57.10 6 species Ampelisca diadema Cost and Balanus improvisus, Darwin. cells/ltr; diatomeus (Chaetoceros insignis Pr.-Lavr., 1955) – 0.17.10 6 The spieces dominating in the stomach content are very cells/ltr; peridineus (Gymnodinium sp. Stein, 1878) – 0.03.10

different from the spices dominating in the environment. That clearly cells/ltr; green-blue algae (Meristomedia sp. Meyen, 1839 ) – 6 shows the grey mullet's selectivity towards different food types 0.31.10 cells/ltr; Oscillatoria sp. Vaucher ex Gomont, 1892 – 6 microalgae and zoobenthos (Table 1). Most of the stomachs 0.17.10 cells/ltr and Oscillatoria bulgarica Komárek, 1956 – 6 examined are showing predominant algae particles which doesn't 0.08.10 cells/ltr.

allow for the macrophytobenthos and detritus to be separated. 42 taxa of macrozoobenthos were identified in 2010. The group Because of that they can be identified generally, only as family type. with the highest percentage is the Polychaetes – 38% followed by The sand presence into the stomach content is most likely due to the Crustaceans – 29%; Molluscs – 21% and the mixed group

PunctariaceaeCystoseiraceae

LomentariaceaeCeramiaceae

CladophoraceaeUlvaceae

Nematoda g sp.Foraminifera (Trochammina inflata)

OstracodaGammaraus sp.Crangon (chast)

Pseudacuma sp.Microdeutopus gryllotalpa

Rissoa sp.Bittium reticulatum

Capitomastus minimusTalassiosira parya

Thalassionema nitzschioidesScrippsiella trochoidea

Pseudosolenia calcar-avisPseudo-nitzschia delicatissima

ProtozoaProtoperidinium crassipes

Prorocentrum sp.Prorocentrum micans

Prorocentrum cordatumProboscia alataPleurosigma sp.

Peridinium sp.Protoperidinium divergens

Nitzschia sp.Nitzschia sigmoidea

Pseudo-nitzschia seriataPseudo nitzschia delicatissima

Navicula sp.Navicula cansellata

Licmophora sp.Heterocapsa triquetra

Gymnodinium sp.Gonyaulax polvedra

Fragilaria sp.Diploneis sp.

Cyclotella caspiaCopepoda

Cocconeis sscutelumCiclotella caspiaCeratium fususCeracium furca

Amphuprora sp.Amphora sp.

Amphora hyalinaAmphora coffeaeformis

Achnanthes sp.Achnanthes brevipes

200 40 60

Percentage ratio (%)

80 100 120

Figure 2. Occurance frequency of food components in the digestive tract of M. cephalus, %

176

accidental intake together with the food. bottom as they grow older. Adult striped mullet have been described In this present study, M. cephalus fed mainly on algae, diatoms, as herbivorous, detritivorous and “interface” feeders. The diet and

plant materials, desmids annelids, crustaceans, bivalves, detritus feeding behaviour of mullet may vary by location, but their major and sand grains. Odum (1970) cited by Wells (1984) hypothesized foods are epiphytic and benthic microalgae, macrophyte detritus, or that grey mullet show a distinct preference for live plant material inorganic sediment particles (Collins, 1985). Wells (1984) noted that (algae) over plant detritus when both are plentiful. The almost the grey mullet M. cephalus fed all year despite temperatures constant high percentage of bowels fullness during the different ranging from 7 – 26ºC on a wide range of algal species, some times of the day and year is a proof of the fact that the grey mullet macrophyte detritus, inorganic particles and occasionally the snail. feed habits is intensive. From the total number of analyzed stomachs Under normal conditions mullet appear to feed almost continuously, (35), three of them were completely empty. The low emptiness index however the intensity of feeding is not always the same. Since (0.0857) defines the grey mullet as an often eating fish. ingested material is continuously transferred from the cardiac

Thompson (1966) cited by Wells (1984) agreed that sand grains stomach to the pyloric stomach, the feeding intensity is reflected by were found throughout the alimentary canal of mullets, and those the amount of material present in the cardiac stomach. If feeding is helped in the trituration of food in the gizzard-like pyloric stomach. intensive the cardiac stomach will remain filled, but if feeding is The proportion of sand and detritus in the gut of juveniles increased sporadic the cardiac stomach will be only partially filled. with length, indicating that they tend to take more food from the For a “broad spectrum” herbivore such as Mugil cephalus food

CystoseiraceaeCeramiaceae

CladophoraceaeUlvaceae

Nematoda g. sp.Foraminifera

OstracodaRissoa sp.

Pseudosolenia calcar-avisProrocentrum micans

Prorocentrum cordatumPeridinium sp.

Nitzschia delicatissimaNavicula sp.

Fragilaria sp.Diploneis sp.

CopepodaAmphora sp.

Achnanthes brevipes

0 10 20 30 40

Percentage ratio (%)

50 60 70 80

Figure 3. Index of dominance (DF) of the food ingredients.

Table 1. Species composition of nutrients in the environment and stomach contents of grey mullet.

The species dominating in the environment

Microalgae

Zoobenthos

Chaetoceros insignis

Pseudo-nitzschia delicatissima

Cyclotella caspia

Gymnodinium sp.

Merismopedia sp.

Oscilatoria sp

Oscilatoria bulgarica

Prionospio cirrifera

Cerastoderma edule

Chamelea gallina

Ampelisca diadema

Balanus improvisus

Rissoa sp.

Ostracoda

Foraminifera (Trochammina inflata)

Nematoda g. sp.

Achnanthes brevipes

Amphora sp.

Copepoda

Diploneis sp.

Fragilaria sp.

Navicula sp.

Nitzschia delicatissima

Peridinium sp.

Prorocentrum cordatum

Prorocentrum micans

Pseudosolenia calcar-avis

The species dominating in the stomach

177

does not appear to be a limiting factor. The ability to ingest and Project № 56/05.04.2012apparently utilize benthic and epiphytic diatoms, dinoflagellates, We would like to express our special thanks to the Associate Protozoa, green and blue-green algae along with macroplant Professor Stoyko Stoykov for his help in determination of the detritus and its associated micro-organisms ensures a constant stomach contents of Mugil cephalus.energy source. In all the environmental systems examined at least two of these components were present in copious amounts. In the event of severre depletion or competition for any of these food Referencesmaterials, the mullet possesses the ability to switch to a different type of feeding. Al-Hussani H, 1947. The feeding habits and the morphology of the

alimentary track of some teleosts living in the neighbourhood of the Marine biological Station, Ghardaqa, Red Sea. Publications of the Marine Biological Station, Ghardaqa, 5, 61 pp.ConclusionAlexandrova K, 1957. The growth of the grey mullet (Mugil cephalus L.) of the Bulgarian Black Sea shores. Bulletin de l'Institut It is suggested that in shallow estuaries most food chains are de Pisciculture et de Pecherie, Varna, 1, 61-76.based not on phytoplankton, but on macroplant detritus and Alexandrova K, 1964. The growth of Mugil saliens Risso along the benthic and epiphytic microalgae. It is the animals capable of Bulgarian coast of the Black Sea. Bulletin de l'Institut de Pisciculture utilizing such energy sources which replace zooplankton as the et de Pecherie, Varna, 5, 117-128.critical herbivore link.Alexandrova K, 1967. Mullets in the lakes along the Bulgarian Black From analyses of the diet it appears that M. cephalus is able to Sea coast. Proceeding of Researcher Institute of Fisheries and utilize either the direct grazing or plant detritus food chains as an Oceanography. Varna, 8, 263-293. energy source depending upon which is the easiest to exploit. When Bekova R and Raikova-Petrova G, 2011. The catch of mullets fish both food source are present in abundance, the mullet exhibits a along the Bulgarian Black sea coast. Youth scientific conference preference for living microalgae. Such an algal diet has a higher “Kliment's days”, November 22-23, Sofia, Sofia University caloric content than a macroplant detritus diet. Some of the “St.Kliment Ohridski”, 77-80.microalgae that are part of its ration are potentially toxic species.Boyadzhiev N, Hadzhinikolova L, Zaykov A, Hubenova T, Fine particles of decaying plant detritus were present in virtually Klisarova D, Stoykov S, Petrova-Pavlova E, Uzunova E, all mullet stomachs which were examined. Bacteria and Protozoa Gerdzhikov D, Bekova R, Borisov D and Piralkov V, 2011. 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Food chains and the food cycle in Hawalian fish Protozoa, green and blue-green algae along with macroplant ponds. Part I. the food and feeding habits of mullet (Mugil cephalus), detritus ensures a constant energy source. From zooplankton milk fish (Chanos chanos) and the tenpounder (Elops machnata). species in the mullet stomach the copepods forms dominate and Transactions of the American Fisheries Society, 74, 2, 61-250.from zoobenthos species – Nematoda and Foraminifera. Mullet Ivanov L and Ivanova G, 1967. Fish productivity in the largest populations adjust morphologically to the type of food available in Bulgarian lakes along the Black Sea coast and the efficiency of the environment by lengthening the intestine for coarser diets. The fishing in them. Proceeding of the Researcher Institute of Fisheries maximum power output for Mugil cephalus results from a large, and Oceanography. 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Influence of environments on the amount and stability of grain yield in modern winter wheat cultivars I. Interaction and degree of variabilityN. Tsenov, D. Atanasova

Variation of yield components in coriander (Coriandrum Sativum L.)N. Dyulgerov, B. Dyulgerova

Plant cell walls fiber component analysis and digestibility of birdsfoot trefoil (Lotus corniculatus L) in the vegetationY. Naydenova, A. Kyuchukova, D. Pavlov

Functional properties of maltitolV. Dobreva, M. Hadjikinova, A. Slavov, D.Hadjikinov, G. Dobrev, B. Zhekova

Food spectrum of grey mullet (Mugil cephalus L.) along the Bulgarian Black Sea coastR. Bekova, G. Raikova-Petrova, D. Gerdzhikov, E. Petrova, V. Vachkova, D. Klisarova

Metabolic and enzymatic profile of sheep fed on forage treated with the synthetic pyrethroid Supersect 10 ECR. Ivanova

Cultivation of Scenedesmus dimorphus strain for biofuel productionK. Velichkova, I. Sirakov, G. Georgiev

Study of the effect of soil trampling on the structural elements of yield and productivity of soybeanV. Sabev, S. Raykov, V. Arnaudov

Stability of herbicides and herbicide tank-mixtures at winter oilseed canola by influence of different meteorological conditionsG. Delchev

Screening of plant protection products against downy mildew on cucumbers (Pseudoperonospora Cubensis (Berkeley & M. A. Curtis) Rostovzev) in cultivation facilitiesS. Masheva, N. Velkov, N. Valchev, V. Yankova

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Efficacy and selectivity of vegetation-applied herbicides and their mixtures with growth stimulator Amalgerol premium at oil-bearing sunflower grown by conventional, Clearfield and ExpressSun technologiesG. Delchev

V. Atanasov, E. Valkova, G. Kostadinova, G. Petkov, Ts. Yablanski, P. Valkova, D. Dermendjieva

Seasonal and vertical dynamics of the water temperature and oxygen content in Kardzhali reservoir, BulgariaI. Iliev, L. Hadjinikolova

Condition and changes in types of natural pasture swards in the Sakar mountain under the influence of climatic and geographic factorsV. Vateva, K Stoеva, D. Pavlov

Comparative studies on the gross composition of White brined cheese and its imitations, marketed in the town of Stara ZagoraN. Naydenova, T. Iliev, G. Mihaylova, S. Atanasova

Effect of the environment on the quality of flour from common winter wheat cultivarsI. Stoeva, E. Penchev

Agriculture and Environment

Product Quality and Safety

Manganese levels in water, sediment and algae from waterbodies with high anthropogenic impact

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Instruction for authors

Preparation of papersPapers shall be submitted at the editorial office typed on standard typing pages (A4, 30 lines per page, 62 characters per line). The editors recommend up to 15 pages for full research paper ( including abstract references, tables, figures and other appendices)

The manuscript should be structured as follows: Title, Names of authors and affiliation address, Abstract, List of keywords, Introduction, Material and methods,Results, Discussion, Conclusion, Acknowledgements (if any), References, Tables, Figures.The title needs to be as concise and informative about the nature of research. It should be written with small letter /bold, 14/ without any abbreviations. Names and affiliation of authorsThe names of the authors should be presented from the initials of first names followed by the family names. The complete address and name of the institution should be stated next. The affiliation of authors are designated by different signs. For the author who is going to be corresponding by the editorial board and readers, an E-mail address and telephone number should be presented as footnote on the first page. Corresponding author is indicated with *. Abstract should be not more than 350 words. It should be clearly stated what new findings have been made in the course of research. Abbreviations and references to authors are inadmissible in the summary. It should be understandable without having read the paper and should be in one paragraph. Keywords: Up to maximum of 5 keywords should be selected not repeating the title but giving the essence of study. The introduction must answer the following questions: What is known and what is new on the studied issue? What necessitated the research problem, described in the paper? What is your hypothesis and goal ?Material and methods: The objects of research, organization of experiments, chemical analyses, statistical and other methods and conditions applied for the experiments should be described in detail. A criterion of sufficient information is to be

possible for others to repeat the experi-ment in order to verify results.Results are presented in understandable tables and figures, accompanied by the statistical parameters needed for the evaluation. Data from tables and figures should not be repeated in the text.Tables should be as simple and as few as possible. Each table should have its own explanatory title and to be typed on a separate page. They should be outside the main body of the text and an indication should be given where it should be inserted.Figures should be sharp with good contrast and rendition. Graphic materials should be preferred. Photographs to be appropriate for printing. Illustrations are supplied in colour as an exception after special agreement with the editorial board and possible payment of extra costs. The figures are to be each in a single file and their location should be given within the text. Discussion: The objective of this section is to indicate the scientific significance of the study. By comparing the results and conclusions of other scientists the contribution of the study for expanding or modifying existing knowledge is pointed out clearly and convincingly to the reader.Conclusion: The most important conse- quences for the science and practice resulting from the conducted research should be summarized in a few sentences. The conclusions shouldn't be numbered and no new paragraphs be used. Contributions are the core of conclusions. References:In the text, references should be cited as follows: single author: Sandberg (2002); two authors: Andersson and Georges (2004); more than two authors: Andersson et al.(2003). When several references are cited simultaneously, they should be ranked by chronological order e.g.: (Sandberg, 2002; Andersson et al., 2003; Andersson and Georges, 2004).References are arranged alphabetically by the name of the first author. If an author is cited more than once, first his individual publications are given ranked by year, then come publications with one co-author, two co-authors, etc. The names of authors, article and journal titles in the Cyrillic or alphabet different from Latin, should be transliterated into Latin and article titles should be translated into English. The original language of articles and books translated into English is indicated in

parenthesis after the bibliographic reference (Bulgarian = Bg, Russian = Ru, Serbian = Sr, if in the Cyrillic, Mongolian = Мо, Greek = Gr, Georgian = Geor., Japanese = Jа, Chinese = Ch, Arabic = Аr, etc.)The following order in the reference list is recommended:Journal articles: Author(s) surname and initials, year. Title. Full title of the journal, volume, pages. Example:Simm G, Lewis RM, Grundy B and Dingwall WS, 2002. Responses to selection for lean growth in sheep. Animal Science, 74, 39-50Books: Author(s) surname and initials, year. Title. Edition, name of publisher, place of publication. Example: Oldenbroek JK, 1999. Genebanks and the conservation of farm animal genetic resources, Second edition. DLO Institute for Animal Science and Heal th, Netherlands.Book chapter or conference proceedings: Author(s) surname and initials, year. Title. In: Title of the book or of the proceedings followed by the editor(s), volume, pages. Name of publisher, place of publication. Example: Mauff G, Pulverer G, Operkuch W, Hummel K and Hidden C, 1995. C3-variants and diverse phenotypes of unconverted and converted C3. In: Provides of the Biological Fluids (ed. H. Peters), vol. 22, 143-165, Pergamon Press. Oxford, UK.Todorov N and Mitev J, 1995. Effect of level of feeding during dry period, and body condition score on reproductive perfor-

thmance in dairy cows,IX International Conference on Production Diseases in Farm Animals, Sept.11 – 14, Berlin, Germany, p. 302 (Abstr.).Thesis:Penkov D, 2008. Estimation of metabolic energy and true digestibility of amino acids of some feeds in experiments with muscus duck (Carina moshata, L). Thesis for DSc. Agrarian University, Plovdiv, 314 pp.

The Editorial Board of the Journal is not responsible for incorrect quotes of reference sources and the relevant violations of copyrights.

Volume 5, Number 2June 2013