Stock Assessment and Virtual Population Analysis of Carassius auratus (Linnaeus, 1758) in the Al-Diwaniya River, Middle of Iraq

Abstract

A total of 1128 individuals of C. auratus were collected from the Diwaniyah River, Iraq. Samples were collected between November 2016 and October 2017 using different fishing gears aiming to assess the stock assessment and virtual population analysis. Fish length ranged from 8.0 to 33.0 cm, with a length-weight relationship exhibiting positive allometric growth. Asymptotic length (L∞) was estimated at 33.0 cm, the growth coefficient (K) was 0.16, and the growth performance index (Ǿ) was 2.241. The values of total (Z), natural (M) and fishing (F) mortality rates were 0.89, 0.47, and 0.42, respectively. The exploitation rate (E) was 0.48. Length with a 50% probability of capture (LC50) was estimated at 18.7 cm and was higher than the length at first maturity (Lm50). The recruitment consisted of one seasonal pulse from April to July. The exploitation rate (E) was below the biological target reference points (E0.1 and Emax), indicating the stock was under exploitation state. The result of the virtual population analysis revealed that most of the fishing mortality happened in mid-lengths (18-26 cm) with a maximum rate at a length of 25 cm. Natural losses were highest among individuals within the length range of 8.0 to 15 cm and then decreased gradually to the length group of 30. For management purposes, this study suggests that more yields can be obtained by increasing the fishing activities on this invasive species, such as increasing the number of fishing boats and decreasing the mesh size to decline its abundance in the long term.