Information on genetic resources is important for aquaculture, management and conservation

Information based on electrophoresis has been arranged in three tables: the ELECSTUDIES table gives an overview of the studies that have been conducted on different populations of a certain species; the ELECDAT table shows the loci that have been investigated in a certain study; and the ELECSUB table contains the alleles that have been detected at a certain locus.

Together, the tables provide information on the genetic structure and variability of both natural and cultured fish populations. This is important for species/strain selection for aquaculture and will help the management and conservation programs for natural stocks.

As more data are entered in this table, it will become possible to identify research gaps (i.e., important species that have been little studied) and the most appropriate methods and reporting formats for the genetic characterization of various species.

The tables contain allele frequencies from electrophoretic studies of fish populations, both wild and cultured. They also contain information on the enzymes, the total number of loci studied, the tissues and the buffer systems used, heterozygosity values and proportions of polymorphic loci. The fields of the tables are:


Locality and Country: Refer to the site where the specimens were collected.

Sample source: Refers to whether specimen came from captivity or open waters.

Total loci: States the number of loci examined.

Heterozygosity indicates the potential for selective breeding

Observed heterozygosity is the proportion of individuals in a population that are heterozygous at a given number of loci. An individual with two different alleles at a particular locus is called a heterozygote. An individual is called a homozygote when two alleles at a particular locus are the same.

The Expected heterozygosity, on the other hand, is the proportion of individuals which are prospective heterozygotes based on the allele frequencies and assuming Hardy-Weinberg equilibrium. These are computed for every locus, population and species and help to indicate, for example, the potential for selective breeding (see Fig. 59).

Gel electrophoresis is the most common method

Polymorphic loci: refer to the number of loci in a sample found to be polymorphic divided by the total number of loci examined (see Fig. 59). To standardize the data, the 95% criterion is used here, wherein a locus is considered polymorphic if the frequency of the most common allele does not exceed 0.95. If the data refer to the 99% criterion, this is indicated in the comment field.

Enzyme: Includes names, abbreviations, and numbers recommended for enzymes and other proteins commonly analyzed in fish genetics work. The names and numbers used are based on the nomenclature recommended by the International Union of Biochemistry’s Nomenclature Committee (Shaklee et al. 1990).

Locus: Refers to the specific position or location of a gene on the chromosome. A gene is a specific length of DNA occupying a locus. A locus is called monomorphic if only one allele is known, and polymorphic when different alleles can occur in a locus. Where two or more loci are involved in producing different forms of a protein (isozymes), the most anodal locus is designated as 1, the next 2, and so on. Sometimes the locus is designated by letters, the most anodal is designated as A, the next B, etc.

Tissue: The type of tissue sample used for electrophoresis. The available choices are: skeletal muscle; visceral muscle; heart; kidney; liver; blood; mucus; eye lens; whole body; others. The last choice refers to tissues that are specified in the Comment field.

Method used: Refers to the type of electrophoretic method used. Gel electrophoresis is one of the most common methods for studying the genetic variation of individuals at both strain and species levels. Four choices are given: starch gel; polyacrylamide gel; sodium dodecyl sulfate; other methods.

Buffer system: Refers to the electrophoretic buffer system used for clear resolution of specific proteins and enzymes. The fifteen buffer systems most commonly used are described by Boyer et al. (1963), Ridgway et al. (1970), Shaw and Prasad (1970), Selander et al. (1971), and Clayton and Tretiak (1972).

pH: Refers to the acidity of the buffer system used.

Samples: Gives the number of samples per site or per population screened.

An allele is one of several alternative forms of a specific gene

An Allele is one of several alternative forms of a specific gene. Alleles are distinguished by their protein products (enzymes) during electrophoresis. The relative electrophoretic mobility of enzymes in a zymogram is expressed in terms of numbers. Relative mobilities are calculated based on the most common allele, which is considered 100 (or -100 for a cathodal locus). A minus sign is assigned to any allele exhibiting cathodal mobility.

Fig. 59. Expected vs. observed heterozygosity of Oreochromis niloticus niloticus (black dots) and miscellaneous fishes. The line represents 1 : 1 ratios. Values well above the line may be the result of inbreeding. Values well below the line may result from crossing of strains


The Allele frequency at a given locus is calculated using the following formula: frequency of allele A = 2 (frequency of genotype AA) + (frequency of genotype Aa) / 2n, where n = number of individuals screened.


The tables currently hold over 11,000 records (one record represents alleles at a single locus) of allele frequencies for over 900 studies of over 800 fish populations/strains of over 200 species. The updating of this table in collaboration with and using the references identified by Skibinski et al. (1991) has made it the largest repository of data on the genetic variability of fishes.


Several graphs can be generated from this table showing:

  • the correspondence line between expected and observed heterozygosity (see Fig. 59); examines whether genetic variability (H and P) has been reduced in captive populations relative to populations from open waters;

  • the relationship between DNA content and phylogenetic order following Nelson’s Fishes of the World (1994) (see Fig. 57);

  • the relationship between chromosome number and DNA content; and

  • the relationship between DNA content and aspect ratio of the caudal fin (see Fig. 58 and Box 34 for an explanation of this graph).

All these graphs can be accessed through the GENETICS table, by highlighting the specific species. Alternatively, you can select them from the Graphs menus under Reports.


Important references used so far are Winans (1980), McAndrew and Majumdar (1983), Macaranas et al. (1986, 1995), van der Bank et al. (1989), Carvalho et al. (1991) and Pouyaud and Agnèse (1995).

To achieve a complete coverage of the allele frequencies and related information on fish so far published is a rather daunting challenge and will involve resolving the problems posed by lack of standardization among publications, which still precludes pooling of data (Agustin et al. 1993, 1994).

How to get there

Clicking on the Biology button in the SPECIES window, the Genetics button in the BIOLOGY window, then the Allele Frequency button in the following window will give a list of populations studied and by clicking on the specific locality, you get to the details of the specific study.

From this point, you get to the ELECDAT table by clicking on the Electrophoretic data button. A list of enzymes used is then presented and clicking on a particular enzyme gives you details of the enzyme involved.

You get to the ELECSUB table by clicking on the Allele Frequencies button at the bottom of this window.


On the Internet, you get to electrophoretic data by clicking the Allele frequencies link in the ‘More information’ section of the ‘Species Summary’ page. You can create a list of all species with available data by selecting the Allele frequency radio button in the ‘Information by Topic’ section of the ‘Search FishBase’ page.


We thank R.E. Brummett, A.E. Eknath, G.C. Mair, J.G. McGlade, D. Pauly, R.S.V. Pullin and D.O. Skibinski for their advice on the structure and contents of this table.


Agustin, L.Q., R. Froese, A.E. Eknath and R.S.V. Pullin. 1993. Documentation of genetic resources for aquaculture - the role of FishBase, p. 63-68. In D. Penman, N. Roongratri and B. McAndrew (eds.) International Workshop on Genetics in Aquaculture and Fisheries Management. ASEAN-EEC Aquaculture Development and Coordination Programme, Bangkok, Thailand.

Agustin, L.Q., M.L.D. Palomares and G.C. Mair. 1994. FishBase: a repository of genetic information on fish. Poster presented at the Fifth International Symposium on Genetics in Aquaculture, 19-25 June 1994, Dalhousie University, Halifax, Nova Scotia, Canada.

Boyer, S.H., D.C. Fainer and E.J. Watson-Williams. 1963. Lactate dehydrogenase variant from human blood: evidence for molecular subunits. Science 141:642-643.

Carvalho, G.R., P.W. Shaw, A.E. Magurran and B.H. Seghers. 1991. Marked genetic divergence revealed by allozymes among populations of the guppy Poecilia reticulata (Poeciliidae), in Trinidad. Biol. J. Linn. Soc. 42:389-405.

Clayton, J.W. and D.N. Tretiak. 1972. Amine-citrate buffers for pH control in starch gel electrophoresis. J. Fish. Res. Board Can. 29:1169-1172.

Macaranas, J.M., N. Taniguchi, M.J.R. Pante, J.B. Capili and R.S.V. Pullin. 1986. Electrophoretic evidence for extensive hybrid gene introgression into commercial Oreochromis niloticus (L.) stocks in the Philippines. Aquacult. Fish. Manage. 17:249-258.

Macaranas, J.M., L.Q. Agustin, M.C.A. Ablan, M.J.R. Pante, A.E. Eknath and R.S.V. Pullin. 1995. Genetic improvement of farmed tilapias: biochemical characterization of strain differences in Oreochromis niloticus. Aquaculture International 3:43-54.

McAndrew, B.J. and K.C. Majumdar. 1983. Tilapia stock identification using electrophoretic markers. Aquaculture 30:249-261.

Nelson, J.S. 1994. Fishes of the world. 3rd edition. John Wiley and Sons, New York. 600 p.

Pouyaud, L. and J.-F. Agnèse. 1995. Phylogenetic relationships between 21 species of three tilapiine genera Tilapia, Sarotherodon and Oreochromis using allozyme data. J. Fish Biol. 47(1):26-38.

Ridgway, G.J., S.W. Sherburne and R.D. Lewis. 1970. Polymorphism in the esterases of Atlantic herring. Trans. Am. Fish. Soc. 99:147-151.

Selander, R.K., M.H. Smith, S.Y. Yang, W.E. Johnson and J.B. Gentry. 1971. Biochemical polymorphism and systematics in the genus Peromyscus. I. Variation in the old field mouse (Peromyscus polionotus). Studies in Genetics VI. Univ. Texas Publ. 7103:49-90.

Shaklee, J.B., F.W. Allendorf, D.C. Morizot and G.S. Whitt. 1990. Gene nomenclature for protein-coding loci in fish. Trans. Am. Fish. Soc. 119:2-15.

Shaw, C.R. and R. Prasad. 1970. Starch gel electrophoresis of enzymes - a compilation of recipes. Biochem. Genet. 4:297-320.

Skibinski, D.O.F., M. Woodwark and R.D. Ward. 1991. The protein diversity database. University College of Swansea, Singleton Park, Swansea, Wales and CSIRO Division of Fisheries, Tasmania, Australia. 16 p.

van der Bank, F.H., W.S. Grant and J.T. Ferreira. 1989. Electrophoretically detectable genetic data for fifteen southern African cichlids. J. Fish Biol. 34:465-483.

Winans, G.A. 1980. Geographic variation in the milkfish Chanos chanos. I. Biochemical evidence. Evolution 34(3):558-574.

Christine Casal and Liza Agustin