Embracing uncertainty, continual spawning, estimation of the stock–recruit steepness, and size-limit designs with length–based per-recruit analyses for African tropical fisheries

Abstract

Per-recruit analysts of African tropical fish mostly applied deterministic and sex-aggregated length–based Beverton–Holt per-recruit models, but rarely examined the spawning biomass per-recruit (SBPR), spawning potential ratio (SPR), size-limit regulations, and stock–recruit steepness (h). To relax the restrictive assumptions of the previous models, this study outlines a framework of using length–based per-recruit models accounting for continual spawning and incorporating uncertainty in sex-specific and total yield per-recruit (YPR), SPR, h, equilibrium yield (Ye), and biological reference points, especially those based on fishing mortality, F (FBRPs). Continual spawning is incorporated through weighting SBPR with monthly proportions of the spawning-capable stage. Uncertainty is introduced into life-history parameters through the resampling with replacement and with bivariate normal distributions, and into various metrics through Monte Carlo simulations. The Beverton–Holt and Ricker stock–recruit relationships are assumed in yield functions. The approach is illustrated with the Bukabuka (Lates stappersii Boulenger, 1914) fishery in southern Lake Tanganyika, for which the potential effects of alternative minimum length limits (Lmin) and catch-and-release values on various metrics are examined. Females Bukabuka represent 63% of the combined YPR and SBPR because they predominate in large sizes; h is greater if recruitment were dependent on both sexes than on females; the most imprecise metrics are the YPR, Ye, and MSY benchmarks; cryptic mortalities of sublegal individuals would exacerbate growth-overfishing; higher Lmin and discard survivals would lead to higher FBRPs, but can prevent recruitment-overfishing; in the F–Lmin planes, the contours maximizing YPR and Ye can be the basis for optimal Lmin designs. Strategies most likely to benefit the Bukabuka fishery would apply Lmin=250–300mm fork length and promote full compliance with releases and ways to reduce post-discard mortality.