Growth pattern and natural mortality of elephant fishes (Mormyrus kannume, Mormyridae) in the Damietta branch of Nile, Egypt

The present study sheds light on the growth pattern of the elephant snout fish (Mormyrus kannume, Mormyridae) in the Damietta branch of the Nile during the period from January to December 2010. This is done through studying the biometric characters, length–weight relationship, condition factors, growth performance index as well as the natural mortality. The linear regression for 15 morphometric characters in total length and 4 morphometric characters in head length was applied. The higher coefficient of the characters’ determinations which fit best was also applied. Five meristic characters were determined (dorsal fin rays, 49–69; pectoral fin rays, 12–15; ventral fin rays, 6; anal fin rays, 17–21 and vertebrae, 49–52). Fish length varied between 14.0 cm and 43.0 cm TL with a modal length range from 19 to 22 cm representing 61.36% of the fish sample. A length–weight relationship using the total weight was found to be Log W = −2.2218 + 3.063 Log L (R2 = 0.997) and using the gutted weight was Log W = −2.1549 + 3.003 Log L (R2 = 0.995) hence exhibiting isometric growth. The average condition factors for different lengths were 0.76 ± 0.06. The chi-square of the overall sex ratio (male:female = 1:0.42) was significant at P < 0.01. The life span of this species is five years, with individuals of two years of age dominating the population. The different growth parameters were estimated at (K = 0.141, to = −0.271, L∞ = 80.65 cm, W∞ = 4151 g, ФL = 2.96 and ФW = 1.56). The natural mortality was 0.34, the M/K ratio was 2.38 and the variation of M by age model was also computed.