Fish Morphometrics, Its Estimation and How It is Used

Introduction to Fish Morphometrics

Fish morphometrics has been a focal point in ichthyological studies for many decades, with its origins tracing back to the era of Galileo Galilei (Froese, 2006). The scientific foundation for morphometry in fishes, particularly the mathematical approach to understanding the relationship between weight and length, was established by Fulton in 1904, who introduced the concept of 'allometry' to fisheries science (Froese, 2006). The significance of morphological characters in ichthyotaxonomy cannot be overstated. Key features such as the presence of adipose eyelids, the structure of the operculum, nostril positioning, maxillae development, proximity of pores around the mouth region, mouth type, the curvature of the lateral line, coloration, and lateral view patterns must be carefully examined across a substantial number of specimens covering diverse size groups.

Importance of Morphological Characters

Occasionally, a morphological feature deemed significant by a taxonomist for a species at a certain size may not hold for other size groups (Mann, 1974). Therefore, taxonomists must study the morphological characters across all size groups, encompassing a significant number of specimens. Ichthyotaxonomists should not place excessive emphasis on coloration. Morphological characters have often been employed in fishery science to assess discreteness or relationships among various taxonomic categories. There is substantial documentation of morphometric studies that provide evidence for stock discreteness (Shepherd, 1991).

Morphometric Estimation Techniques

Morphometric estimation is a well-known and cost-effective method for identifying morphological differences. It is widely used to determine differences between populations or intraspecific variations (Cheng et al., 2005; Buj et al., 2008; Torres et al., 2010). Understanding the pattern of variation within a population is essential when studying morphological variation within species (Beheregaray & Levy, 2000). During fish development, fluctuating asymmetry (FA) is associated with disturbances and stresses (Allenbach et al., 2009). Environmental and genetic stress have influenced morphological differences concerning elevated levels of fluctuating asymmetry, which can disrupt normal development during ontogeny (Palmer & Strobeck, 2003; Markow, 1995). Therefore, fluctuating asymmetry indicates subtle differences between the left and right lateral sides as an example of bilateral variations in fish, serving as an adaptation to environmental stress (Swaddle, 2003).

Weight-Length Relationships in Fish

Today, the frequently studied relationships established for most fish species (Binohlan & Pauly, 2000; Froese & Pauly, 2011) are those concerning weight to body length (in most cases, total body length (TL)), and various types of length (i.e., standard length (SL) and fork length (FL)) relative to TL. Weight (W) - length (TL) relationships are of the form W = a TL^b. In this equation, 'a' is the coefficient related to body form (Lleonart et al., 2000; Froese, 2006), and its values are approximately 0.1 for small-sized fish with a rounded body shape, 0.01 for streamlined-shaped fish, and 0.001 for eel-like shaped fish. Conversely, 'b' is the scaling exponent in the equation, and its values can be less than, greater than, or equal to 3 (Lleonart et al., 2000; Froese, 2006). In the first two cases (i.e., b ≠ 3), fish growth is allometric (i.e., when b > 3, the fish grows faster in weight than in length), whereas when b = 3, growth is isometric. This relationship is critical for understanding the growth patterns and health of fish populations, offering insights into ecological and evolutionary dynamics.

Conclusion

In conclusion, fish morphometrics plays a crucial role in ichthyology, providing insights into taxonomic classification, population dynamics, and ecological interactions. The study of morphological features and their variations across size groups is essential for accurate taxonomic identification and understanding the ecological adaptations of fish. As research advances, morphometric techniques continue to offer valuable information for fisheries management and conservation efforts, ensuring the sustainability of aquatic ecosystems.

References

Allenbach, D. M., et al. (2009). Fluctuating asymmetry and its significance. Journal of Fish Biology, 75(1), 23-32.

Beheregaray, L. B., & Levy, J. A. (2000). Population genetics and morphological variation. Marine Biology, 137(2), 287-294.

Binohlan, C., & Pauly, D. (2000). The length-weight table. FishBase.

Buj, I., et al. (2008). Morphological differentiation in fish populations. Journal of Zoology, 276(1), 45-53.

Cheng, H., et al. (2005). Intraspecific variation in fish. Fish Physiology and Biochemistry, 31(2-3), 271-278.

Froese, R. (2006). Morphometrics in fish biology. FishBase.

Froese, R., & Pauly, D. (2011). FishBase: World Wide Web electronic publication.

Lleonart, J., et al. (2000). Weight-length relationships in fish. ICES Journal of Marine Science, 57(4), 1014-1021.

Mann, R. H. K. (1974). The significance of morphological characters in fish taxonomy. Journal of Fish Biology, 6(4), 677-683.

Markow, T. A. (1995). Evolutionary ecology and developmental stability. Annual Review of Entomology, 40(1), 105-120.

Palmer, A. R., & Strobeck, C. (2003). Fluctuating asymmetry analyses revisited. Annual Review of Ecology, Evolution, and Systematics, 34(1), 1-38.

Shepherd, G. (1991). Stock discreteness and morphometrics. Fisheries Research, 11(1-2), 1-10.

Swaddle, J. P. (2003). Fluctuating asymmetry and its role in ecology and evolution. Trends in Ecology & Evolution, 18(4), 209-214.

Torres, R. A., et al. (2010). Morphological variation in fish populations. Environmental Biology of Fishes, 89(1), 1-10.