The correction factor for analysis of long-term data on the standard length of fish

Fish size is an important indicator in many ichthyological studies. Standard length is one of the most commonly used size measurements for fish. Until 1957 in Russia, standard length was defined not as the distance from the tip of the snout to the base of the middle rays of the caudal fin (the modern measurement method), but rather as the distance from the center of the eye to the posterior edge of the base or rays of the anal fin. This discrepancy renders direct comparisons between standard length data collected in the first half of the twentieth century and modern measurements inaccurate. The aim of this study was to establish correction coefficients to recalibrate pre-1957 standard length data of Cyprinidae according to contemporary standards, using the roach Rutilus rutilus (Linnaeus, 1758) and ide Leuciscus idus (Linnaeus, 1758) as examples from various water bodies in Western Siberia. We measured the standard length of roach and ide from Lake Chany (54°41'N, 77°38'E) and the River Ob (58°44'N, 81°30'E) using three methods: (A) the distance from the tip of the snout to the base of the middle rays of the caudal fin - modern standard; (B) the distance from the middle of the eye to the posterior edge of the anal fin base - one of the possible measurement methods used before 1957; and (C) the distance from the middle of the eye to the posterior edge of the anal fin rays - another measurement method used before 1957 (See Fig. 1). We measured 115 roach and 105 ide from the Ob, and 102 roach and 94 ide from Lake Chany. Correction coefficients were calculated as the ratio of the standard length according to the modern method (A) to the standard length according to the older methods (B and C), defined as Coefficient 1 = A/B and Coefficient 2 = A/C. The calculated coefficients for estimating the standard length of roach in the Ob River were 1.336 for the first option and 1.242 for the second (See Table 1); for roach in Lake Chany, the coefficients were 1.358 for the first option and 1.245 for the second (See Table 2). The calculated coefficients for estimating the standard length of ide in the Ob River were 1.364 for the first option and 1.270 for the second (See Table 4); for ide in Lake Chany, the coefficients were 1.367 for the first option and 1.266 for the second (See Table 5). We compared these calculated coefficients with data on the "standard length" and "fish length to the end of the scale cover" of roach and ide from 1934 (See Tables 3 and 6) and determined that, in Western Siberia, the "standard length" was measured from the middle of the eye to the end of the posterior edge of the anal fin rays. Thus, we established that, for analyzing the standard length of roach in Western Siberia, a correction coefficient of 1.24 should be applied to values reported in sources before 1957, and a correction coefficient of 1.27 should be applied to the standard length of ide. This coefficient does not show statistically significant differences for roach across different types of water bodies and size groups, whereas the coefficient for ide decreases in the largest size group. The article contains 1 Figure, 6 Tables and 21 References. The Authors declare no conflict of interest.

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