A standard weight equation to assess the body condition of pejerrey Odontesthes bonariensis.

We developed a standard weight equation Ws to aid in the analysis of pejerrey Odonthestes bonariensis body condition over time and across populations using the relative weight index Wr. Weight--length data were compiled from 73 populations of pejerrey (N=16.022) from the Argentine pampas region. We used the regression-line-percentile technique, which provides a 75th-percentile standard by length intervals of 10 mm, to develop the Ws equation. The proposed equation is log 10 Ws=-5,267+3,163 log10 Lst; Ws is weight in grams and Lst is standard length in millimeters. This equation is proposed for use with pejerrey between 120 and 520 mm of Lst. Values for Wr calculated with the Ws equation did not consistently increase or decrease as function of fish length, indicating absence of length bias. We analyze the values and distribution of Wr for pejerrey and suggest how to interpret its results. The equation of Ws that intends to calculate the index of Wr, represents a useful tool of analysis, because not only it allows to statistically compare the physical condition of the pejerrey, independently of its size, capture moment or the individual origin, but also it facilitates to relate it with other variables.