Age and growth of elephant-snout fish Mormyrus kannume (Forsskål, 1775) (Family: Mormyridae) from Lake Nasser, Egypt

The elephant-snout fish Mormyrus kannume is one of the known interesting fish in the most interesting man-made lake in Egypt (Lake Nasser). Due to the global conservatory objectives and lake management, this species won scientific and commercial interest as there were no sufficient or recent studies on this species. The present work aimed to study the growth pattern of the elephant-snout fish (M. kannume) from Lake Nasser, Egypt via length-weight relationship, condition factor, growth in age and growth parameters. A total of 645 specimens were monthly collected from the commercial landing sites (Aswan, Garf Hussein and Abu Simbel harbours) from November 2017 to December 2018. The investigations illustarted a wide range of length from 15.3 to 77.5 cm (TL), with an average of 36.3±7.0 cm and a total weight (W) ranging from 25.2 to 2882.5 g, with an average of 360.3±278.6. The length-weight relationship showed negative allometric growth for males, females, and combined sexes. The absolute condition  (Kc)  showed a higher value during February (0.74±0.06) for males and during April (0.72±0.06), May (0.72±0.09) and July (0.72±0.06) for females. For C. sexes, the highest value was recorded during February (0.72±0.06). The lower Kc was during August for males, females and C. sex. In general, the conditions were located under “1” along the year and recorded an annual average of 0.68±0.072, 0.69±0.70 and  0.68±0.075 for males, females and C. sexes respectively. By using opercula reading, the life span/ growth rings of M. kannume were calculated as ten years with a mean length of 17.3, 26.64, 35.11, 42.98, 49.56, 55.91, 61.5, 65.82, 69.66 and 72.54 cm TL, for each age groups from one to ten respectively. The von Bertalanffy growth constants were computed as L_∞= 89.91 cm TL, K = 0.16 year^-1, and t_o = −0.91. The growth performance index (ϕ) was 3.11 and the longevity of age (t max) was 18.56 years. In spite of the presence of negative allometric growth combined with the lower value of conditions than “1” which may be affected by food and spawning, the others such as aging, growth performance, and age longevity have reflected a satisfied growth pattern in this lake and are valuable for creating a monitoring and management system for M. kannumein Lake Nasser recommending its conservation with require to more studies