Stock assessment of the dusky spine foot Siganus luridus from the Gulf of Aqaba, Egypt, using a Length-based Bayesian Biomass (LBB) method

The dusky spinefoot, Siganus luridus (Rüppel, 1829), stands as one of the most economically significant herbivorous species inhabiting the coral reefs of the Gulf of Aqaba, Egypt. Despite its prominence, our understanding of its biology and population structure remains limited. Thus, this study marks the first attempt to shed light on the demographic structure and stock assessment of S. luridus in the Aqaba Gulf. Over the span of January to December 2018, a total of 649 specimens were meticulously collected on a monthly basis from three landing sites (Nuwebaa, Dahab, and Abu Galum) within the Gulf of Aqaba, primarily utilizing trammel nets as the main fishing method. Two distinct length-based stock assessment methodologies were employed. Firstly, the routines and subroutines integrated within the FiSAT program were applied. Secondly, the length-based Bayesian biomass (LBB) estimator method was adopted, representing a novel approach in evaluating fishery status using length frequency (LF) data. Population structure evolution suggests a lifespan of approximately 5 years with a maximum age (tmax) of around 6.12 years. The estimated von Bertalanffy growth function (VBGF) parameters were calculated as L∞ = 29.71, K = 0.49, and tо = -0.334. The estimated total, natural, and fishing mortalities (1.531, 0.632, and 0.899, respectively) revealed an overexploitation of this species, with the exploitation level surpassing the ideal threshold (E= 0.587/year). Furthermore, the Y/R model analysis and the estimation of biological reference points (Fmax, F0.1, and F0.5) indicated that the current rate of fishing mortality (Fcurr = 0.899 year-1) exceeds the target, limit and precautionary reference points (0.853, 0.789, 0.588 respectively), confirming the situation of overfishing. The outputs of the LBB model revealed that Lmean/Lopt parameters fall below unity, alongside similar findings from Lc/Lc_opt ratios, indicating a truncated length structure and the capture of very young individuals.