Use of Relative Weight in the study of European Freshwater Fish Species: a review

Objective: This study aims to summarize the multiple applications of Relative weight (Wr) to the study of biology and ecology of freshwater fish species and to provide a review on the current use of this index in the study of European freshwater fish species. Method: Relative weight (Wr) is an index of condition based on the comparison between the actual weight of a fish and the ideal weight of a specimen of the same species in good physiological condition (standard weight, Ws). Over other condition indices, Wr has the great advantage to enable comparison of the condition of fish of different lengths and from different populations, since it is not influenced by changes in body shape. Thus, variations in Wr values will be primarily due to extant ecological factors. The use of Wr is not limited only to the evaluation of fish “plumpness’’. It can serve as a surrogate to estimate body composition of fish, assess the abundance of prey, fish stockings and management actions. Moreover, Wr represent an easy and powerful tool to recognize ecological changes, such as inter-species or intra-species competition. Wr is commonly used in USA whereas the use of Wr to assess the condition of European species is still fairly marginal because of the lack of species-specific Ws equation that need to be developed by using a large dataset of data collected throughout the distribution range of each species. Results and Discussion: Thank to the collaboration with numerous universities and research centers, species-specific Ws equations were already developed for several European native species (Barbus tyberinus, Salmo trutta, Perca fluviatilis, Squalius squalus, Squalius lucumonis, Telestes muticellus and Salmo marmoratus) and some endemic Turkish species (S. fellowesii, S. pursakensis, B. pergamonensis, Capoeta bergamae and Ladigesocypris irideus). Further studies are encouraged to extend the use of this methodology to other species. To this aim, the first step is a strong collaboration among institutions in order to collect and share length and weight data for different freshwater fish species from different watercourses in order to develop species-specific standard weight equations for other several European species and to promote the use of Wr as a standard tool in the study of freshwater populations