Population Dynamics of Nile Tilapia (Oreochromis niloticus, Linnaeus, 1758) from Roseries Reservoir, Sudan

Abstract

Growth parameters are essential data points that can indicate the status of fisheries management and the extent of their exploitation. Fish growth parameters can be determined using two primary methods: direct readings from hard structures such as otoliths, spines, or vertebrae, and indirect estimates derived from length distribution data over time. This study investigated the population dynamics of Nile tilapia (Oreochromis niloticus) in the Roseries Reservoir, analyzing 636 specimens collected monthly from four sites, during January - December 2022. The total length of each fish was measured to the nearest 1.0 mm from the tip of the snout to the end of the upper lobe of the caudal fin (left side) utilizing a standard measuring board.  The von Bertalanffy growth model was applied to analyze growth patterns. The total annual instantaneous mortality rate Z was estimated using length-converted catch curves. The relative yield per recruit (Y'/R) and relative biomass per recruit (B'/R) were calculated as exploitation functions to determine biological reference points.  The age at first capture (tc) was determined from the estimated growth parameters (L∞, K, and t0) using the ELEFAN I method. The peak distribution occurred in September. Specimens ranged from 7 to 41.5 cm (TL), averaging 19.428 ± 6.581 cm. The length-weight relationship demonstrated a strong correlation (r = 0.945), indicating a negative allometric growth with a b-value of 2.804. Growth parameters were determined using the von Bertalanffy growth model, with an asymptotic length (L∞) of 45.15 cm, a growth coefficient (K) of 0.310 yr.⁻¹, and a theoretical age at length zero (t0) of -0.321 yr. -1. The growth performance index (Φ') was estimated at 2.801, with a maximum longevity (Tmax) of 9.36 years. Total mortality (Z) was calculated at 1.170 yr.⁻¹, natural mortality (M) at 0.70 yr.⁻¹, and fishing mortality (F) at 0.47 yr.⁻¹, resulting in an exploitation rate (E) of 0.41. O. niloticus exhibited one round of recruitment, peaking from April to August, coinciding with the rainy season, while the length at first capture (Lc) was determined to be 7 cm. The maximum relative yield per recruit (Y/R) was achieved at an exploitation rate (Emax) of 0.499. The estimated total mortality and fishing mortality rates indicate the current fishing pressure faced by O. niloticus, emphasizing the need for effective management practices to ensure sustainability. The achievement of maximum relative yield per recruit at a specific exploitation rate underscores the importance of aligning fishing practices with biological data to optimize yields while preserving the fish population. Overall, this research serves as a foundational reference for future studies on Nile tilapia in Sudan and informs local and federal fisheries management strategies.