Stock assessment and management implications of an endemic fish, Oxygymnocypris stewartii, in the Yarlung Zangbo River in Tibet, China

Abstract

Background: Oxygymnocypris stewartii is an endemic species only inhabiting the middle and upper reaches of the Yarlung Zangbo River and its tributaries at altitudes above 3600 m in the Qinghai-Tibet Plateau and has suffered a continuous decline of population size owing to anthropogenic disturbance. However, little is known about the stock status of this species and how to protect it. In this study, we evaluated the population status of O. stewartii and investigated two management policies including the minimum age limits and seasonal closure in the Yarlung Zangbo River. Results: Estimates of the total mortality rate were 0.35 year −1 for females and 0.65 year −1 for males. The natural mortality rates were estimated as 0.10 and 0.22 year −1 for females and 0.12 and 0.29 year −1 for males using two empirical methods. The O. stewartii population may be in near full exploitation under the current harvesting strategy in the Yarlung Zangbo River, with a high chance of recruitment failure in the future. Although both minimum age limits and seasonal closure could be effective measures to conserve the O. stewartii stock, it was proposed that a seasonal closure was implemented each year from September to December according to the complexity of the application and the effects on the income of local fishermen and on the biological invasion. Conclusions: The stock of O. stewartii may be in near full exploitation under the current harvesting strategy, with a high chance of recruitment failure in the future. Our results indicated that the imposition of the seasonal closure from September to December might be the effective measure to conserve the O. stewartii population.

An assessment of European bream Abramis brama (Linnaeus, 1758) fishery in the downstream of the Irtysh River in China

“…The estimates of YPR, SSB/R and corresponding BRPs fluctuated widely with M, which indicated that the current stock status of A. brama in the downstream of the Irtysh River was sensitive to the levels of M. The sensitivity of per-recruit analysis to the level of M has been reported in a large number of documents (e.g. Sun et al 2002;Huo et al 2015). The situations where both the YPR and SSB/R decreased with increasing M, and the BRPs increased with increasing M might be attributed to higher M leaving fewer fish to be captured, reducing the yield and mitigating the impact of fishing on the survival of spawners (Sun et al 2002).…”

“…Hence, yield per recruit (YPR) and spawning stock biomass per recruit (SSB/R) models were developed for the pooled data. The YPR and SSB/R were calculated using the following models (Huo et al 2015):…”

“…The sensitivity of the YPR and SSB/R computations to the variation of M were examined in consideration of the difficulty in obtaining reliable estimates of M for exploited fish populations (Sun et al 2002;Huo et al 2015). To evaluate the impact of uncertainty of growth and maturity parameters on per-recruit analysis, new parameters were randomly selected from a normal distribution defined by the mean and standard error for each parameter (Zischke and Griffiths 2015).…”

“…Changing YPR and SPR were also investigated under various combinations of t c and F using isopleth plot (mean parameters were used), to obtain the best compromise between the conservation of A. brama stock and maximizing YPR and SPR (Huo et al 2015).…”

“…Several fishing mortality rate (F)-based biological reference points (BRPs) derived from per-recruit analysis have been used to evaluate whether the YPR is optimum or the SSB/R is sufficient for the population to persist. It also provides fisheries managers with the ability to develop harvest policies (Aprahamian et al 2006;Lin et al 2015;Huo et al 2015). The BRPs usually appear in three forms: targets (target reference points, TRPs), thresholds (threshold reference points, ThRPs) and limits (limit reference points, LRPs).…”