Reservoir Patchiness Influences Fish Condition: Khor-Scale Heterogeneity in Lake Nasser (Egypt) and Implications for Global Inland Fisheries

Abstract

Global inland fisheries depend on large reservoirs, yet many assessments assume spatial homogeneity despite complex embayed morphometry. We tested whether khor-scale structure within Lake Nasser, Egypt, explains variation in two commercially important catfishes. Six distinct khors spanning the north–south axis (Ghazal, Sheniara, Garf Hussein, Amada, Toshka, and Hamedo) were sampled with standardized effort (72 net-nights; 12 per khor, 3 representative areas ≥ 500 m apart × 4 campaigns, March–June 2021), yielding 398 fish (268 Chrysichthysauratus, 130 C. rueppelli). Abundance was expressed as standardized CPUE (fish·net-night−1). C. auratus CPUE ranged from 2.83 (Ghazal) to 5.92 (Garf Hussein), while C. rueppelli CPUE was relatively uniform (1.50–1.92). C. auratus attained the largest TL in Ghazal (25.51 ± 2.9 cm) and the highest condition K in Garf Hussein (1.46 ± 0.18). C. rueppelli attained the greatest TL and weight in southern khors, especially Hamedo (23.3 ± 2.33 cm, 146.7 ± 30.5 g), while its highest condition was in Toshka (1.30 ± 0.15). Sex ratio was significantly female-biased for C. auratus (92 M:176 F, 1:1.91, p < 0.001) but not for C. rueppelli (55 M:75 F, 1:1.36, p = 0.079). Results show embayment-scale CPUE and condition, not whole-lake means, capture key population patterns, improving precision of mega-reservoir fisheries management.

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