Internal-external marks

Internal-external marks may be defined as characteristic to individuals or to groups (populations) which can be either on the outside or inside. These are either naturally occurring or artificially produced marks, that are characteristic to either individuals or, more often, to groups of fish. Int.- Ext.Marks can be produced by inducing a controlled growth pattern, leaving distinct 'checks' in the bony structures. Internal marks may also be entirely natural biological marks: morphological traits (number of gill rakes, vertebrae etc) distinguishing groups of fish from each other, or parasites only found in certain populations.

Chemical Marks in otoliths and bony structures -

Thermal marks-


Biological marks

Pros and cons

Meristics and Morphometrics

Chemical Marks in otoliths, scales and other bony structures .

Chemical (alizarin, strontium, Calcein and oxytetracykcline). A recognisable chemical mark is deposited in the otoliths or other bony structure of the fish. The marking is mostly done by submersion in a solution of the chemical or through the feed and is thus considered to have little impact on the fish. With this method, a large number of fish of any size may be marked within a short time. Detection is time consuming, requires special equipment; a microscope fitted with UV-light, and the extraction of an otolith or bony part of the fish. Applications involving the use of certain chemicals may be controversial to either later consumption of marked fish or to the entry of unwanted chemicals in the food chain. In most cases the final concentrations in the fish are very low and may be negligible. Brothers (1990) gives an overview of various otolith marking techniques, while e.g. Reinert et al (1998), Akinicheva & Rogatnykh (1996), Ruhle & Winecki-Kuehn (1992), Monaghan (1993), Ennevor (1994) provide information on various internal marking techniques.

Thermal marks

Thermal marks. By exposing the fish to different temperature regimes, distinct and recognisable patterns appear in the otolith structures. The method requires precise control of temperature and knowledge of growth patterns at different temperatures. Furthermore experimental facilities are required for prolonged periods, limiting this method for reared species. The induced mark is permanent and the method is considered to have little impact on fish health. An intrusive method in that the fish must be sacrificed in order to remove and examine the otolith. Generally a time-consuming method. Brothers (1990) gives an overview of various otolith marking techniques, while e.g. Reinert et al (1998), Akinicheva & Rogatnykh (1996), Ruhle & Winecki-Kuehn (1992), Monaghan (1993), Ennevor (1994) provide information on various internal marking techniques.



Elastomeres are internal marks. - externally and visibly detected. Marks made of elastomers may be in various colours, injected with a syringe sub-cutaneously in fish in places where they are visible from the outside.

 Applications of elastomers.

In an attempt to combine the advantages of external marks with those of the internal marks elastomers were developed. They are applied in studies where a minimal disturbance of the fish is required. These marks are made of coloured and/or fluorescent plastic paint and placed for example between the fin rays or at the base of the fins. Most often used in research work where the institute carrying out the project does recovery of marked specimens, since marks may easily be overlooked. The use of elastomers has been evaluated by Godin et al (1996) and Morgan & Paveley (1996).

Pros and cons of Elastomers



Biological marks-parasites

Parasites. Naturally occuring parasites have been used as indicators of varoous aspects of biology of fish and acuatic invertebrates. This way of separating populations has been used in sockeye salmon (Moles et al 1990)

Pros and Cons







Meristics and Morphometrics


Meristic as defined by MacGraw-Hill Dictionary of Bioscience: Biological meaning: Pertaining to a change in number or in geometric relation to parts of an organism. Zoological meaning: of pertaining to, or divided into segments.

If this definition is used Meristics can be used as heading for a large methodological category of methods used to identify or differentiate between genera, species groups, species, subspecies, populations or groups within species and individuals. In that case methods where variability in numbers are used (numbers of vertebrae, number of fin rays or gill-rakers), and methods where variability in shapes and measurements (otolith shape or truss-pattern) are used, would come under this heading.

The problem with this pooling of methods is that these methods are very unrelated by nature. Counting of fin-rays or vertebrae produces discontinuous or discrete variables where as the measurements of body parts produces continuous variables (Sokal & Rohlf, 1981).

Therefore the counting methods (Meristics) and measuring methods (Morphometrics) are frequently separated by definitions as below.

Meristic characters are enumerable morphological features (Anon. 1995). Examples of external meristic features of this sort which are commonly used are: fin-rays or spines, gill-rakers, scales. Examples of internal meristic features are: vertebrae, pyloric caeca, pterygiophores, branchiostegal rays.

Morhometric characters are those which are obtained by measurements of body parts. Examples of methods used in this category: Fourier shape analysis, Truss-patterns. The shape of otoliths or scales.

Application of Meristics: Meristcs have been used both to identify species and to delimit populations within species. Examples of species identification: the American eel (Anguilla rostrata) and the European eel (Anguilla anguilla) (Boetius, 1980), sandeels (***** ). Examples of stocks or populations: Herring populations ( Fridriksson, 1958), Cod populations (Johannes Smith 1930). Applicationof morphometrics: Morphometric methods are frequently used for identification of populations within species of fish (Messieh, 1972). Lately the use of Truss patterns, which is a system of measurements between many external landmarks, has become useful for this purpose (Schweigert, 1990). 

In treatment of meristic and morphometric data it is often necessary to use more than one character to discriminate between groups. In that case computer programs of multivariate methods are needed (Sneath & Sokal 1973).

References to literature on meristics and morphometrics: 

Anon, 1996, Report of the Study Group on Stock Identification Protocols for Finfish and Shellfish Stocks. ICES CM 1996/M:1. ref.: G,H,J,K, Assess.

Boetius, J. 1980. Atlantic Anguilla. A presentation of old and new data of total numbers of vertebrae with special reference to the occurrence of Anguilla rostrata in Europe. Dana 1:1-28.

Fridriksson, A. 1958. The tribes of North Coast herring of Iceland with special reference to the period 1948-1955. Rapp Cons. Explor. Mer. 143 (2):36-44.

Fournier, D. A., T.D. Beacham, B. E. Riddell, and C. A. Busack. 1984. Estimating stock composition in mixed stock fisheries using morphometric, meristic and electrophoretic characteristics. Can. J. Fish. and Aqat. Sciences. 41:400-408.

Messieh, S. N. 1972. Use of otoliths in identifying herring stocks in southern Gulf of Lawrence and adjacent waters. J. Fish. Res. Board Can. 29:1113-1118.

Schmidt, J. 1930. Raceundersogelser. X Den Atlantiske torsk (Gadus callarias L.) og dens Lokale Racer. Medelser fra Carslberg Laboratoriet, bind 18, Nr. 5.

Schweigert, J.F. 1990. Comparison of Morphometric and Meristic data against Truss Networks for describing Pacific Herring Stocks. American Fisheries Society Symposium 7:7:47-62.

Sokal, R. R. and F. J. Rohlf. 1981. Biometry (second edition). W. H. Freeman and Company, San Francisco.

Sneath, H. A. and R. R. Sokal. 1973. Numerical Taxonomy. W. H. Freeman and Company, San Francisco.


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