Biological reference points such as F
max, F
25%, and F
40% have often been used to develop fishery management strategies. Several authors have advocated designating F
40% as a target reference point and F
25% as a threshold reference point in order to obtain near optimal yields while insuring against stock collapse (Hildén 1993; Leaman 1993; Mace and Sissenwine 1993; Rivard and Maguire 1993; Mace 1994; Griffiths 1997; Kirchner 2001; Sun et al. 2005). Our results based on per-recruit analyses suggest that the current fishing mortalities for each sex population are constantly higher than the corresponding threshold reference point F
25% at the lower and intermedia values of M, while the current fishing mortalities are lower than the corresponding threshold reference point F
25% but higher than the corresponding target reference point F
40% at the higher values of M, which indicates that the stock status of O. stewartii in the Yarlung Zangbo River is sensitive to the estimated values of M. However, the evaluation of various empirical estimators, implemented by Then et al. (2014), shows clearly that Eq. (7) is both the best and a sufficient predictor of M, indicating that the estimated higher value of M may be more reliable than the others. In addition, Smith et al. (2012) reported that the weighted regression estimator of fishing mortality (derived from Z) was always negatively biased, indicating that the estimated F
current might be underestimated. Therefore, the O. stewartii population may be in near full exploitation in the Yarlung Zangbo River.
Imposition of a closed season during the breeding period is only effective under three circumstances: (1) if fishing disturbs and reduces reproduction of individuals that are not captured, (2) if the target species aggregates to breed and becomes vulnerable to capture, and (3) if the closed season achieves a reduction of annual fishing effort (Arendse et al. 2007). Our results indicated that imposition of the closed season during the spawning season (February to May) was not effective, while that during September to December was effective. Our field survey found that O. stewartii aggregated to lay eggs and became vulnerable to capture during the spawning season. However, there were no spawning sites in our sampling locations. Moreover, O. stewartii is intensively captured during September to December every year, which corresponded to the gonad development period. Thus, the imposition of the closed season during September to December could reduce the annual fishing effort and help to conserve the O. stewartii stock.
Although increasing the age at first capture and introducing the closed season could be effective measures to conserve the O. stewartii stock, we prefer the imposition of the closed season to manage its population based on three reasons: (1) Compared with increasing the age at first capture, the closed season could be easily implemented and monitored, especially for the vast territory of Tibet. (2) The field survey found that the local people had an important custom that they would buy a lot of all kinds of small fish, including juveniles of O. stewartii, to release back to the river. Thus, the income of local fishermen is derived from both the adult fish and small fish. If the measure of increasing the age at first capture was carried out, part of the income from the sales of small fish would be reduced. If the closed season was implemented, the income from the sales of adult fish would be decreased. Therefore, regardless of which measures are taken, they have similar impact on the income of local fishermen. (3) By increasing the age at first capture, the catch of endemic small fish would likely decline dramatically, which likely will cause local people to switch to releasing the alien small fish back to the river. Invasive species usually threaten the survival of the native species due to competition for habitat and dietary resources.
Our results based on the per-recruit analysis can be misleading if the assumptions of the analysis and its various inputs, namely growth parameters, sampling bias, fishing, and natural mortality, are not made explicit. First of all, the per-recruit analysis is heavily affected by von Bertalanffy growth parameters. A meaningful estimate of growth parameters is related to age and size inputs. Compared with other age materials, pre-analyses showed that lapillus, which provided the age input for the von Bertalanffy growth model, is the most precise and accurate structure for age estimation. In addition, growth model estimates are greatly influenced by the lack of very young or old individuals (Cailliet and Goldman 2004). Huo et al. (2012) reported that the size composition including enough juveniles (<100 mm SL) and old individuals (>500 mm SL) provided a reliable estimate of growth parameters. Secondly, we acknowledge that the gill net has its own sampling bias. However, to reduce the sampling bias, the gill nets with multiple mesh sizes were deployed to collect O. stewartii, and the ranges in standard length and age extended broadly indicating that sampling bias did not drastically appear. Thirdly, our results are only valid if natural and fishing mortality rates used in our analysis are representative of the long-term situation. Our field investigations find that the catches of this species fluctuate little in recent years and the possibility for this trend may be continuous in the future. Thus, it is possible that the age structure of the population may remain the same in the future as we have documented here. Fourthly, the natural mortality is generally difficult to be estimated reliably and directly. By direct, the natural mortality prefers to be estimated using information strictly pertaining to the stock of interest. However, direct estimation methods of M are often data intensive, thus limiting their application to relatively data-rich stocks (Then et al. 2014). For data-poor stocks, indirect or empirical equations are often used to estimate M. Although these empirical methods are less reliable than their data-rich counterparts, a consensus is that empirical methods are useful and very important particularly for the data-poor stocks. In our study, we evaluated the sensitivity of the per-recruit models to various estimates of M within the range of plausible values for M for the O. stewartii stock by applying two empirical methods, which represent both the best and sufficient predictors of M. Fifthly, catch curve analysis can be used to estimate the current fishing mortality (derived from Z) under the assumption that all fish are equally vulnerable to the fishing gear above some determined age. This assumption is often not satisfied in many fisheries, particularly gill net fisheries, due to size selectivity effects of the mesh size. Due to lack of fishing gear selectivity data, the gear selectivity is assumed to be a knife-edge selection in our per-recruit analysis. However, it is clear from the catch curve analysis that size selectivity does exist in the old age groups of O. stewartii fishery, indicating that the estimated F
current may be negatively biased. Moreover, the estimated F
current may also be negatively biased using the weighted regression method proposed by Smith et al. (2012). Overall, our estimate of F
current may therefore be underestimated and should be viewed with caution. Finally, although the stock status of O. stewartii is sensitive to the estimated values of M, the effects of increasing the age at first capture and introducing the closed season are insensitive to the values of M, indicating that both of measures can effectively prevent the O. stewartii population collapse.
Our results showed that the estimated F
current for males was two times higher than that for females. Generally, females may prefer to be captured, given that females actually grow to a larger size. However, for the fisheries using gill nets, small fish can pass through the meshes as is the case for trawl nets, while large fish may also avoid being caught in a gill net, because their heads are so large that they cannot be gilled (Sparre and Venema 1998). Thus, the F
current for males was estimated using higher age groups than that for females, indicating that the phenomenon that our estimation of F
current for males was higher than that for females may be reasonable.
O. stewartii belongs to the subfamily Schizothoracinae, which originated from primitive barbine fishes distributed in Tibet during the late Tertiary and subsequently evolved into the Qinghai-Tibet plateau fish fauna owing to the vicariance induced by the uplift of Tibet (Cao et al. 1981; Wu and Tan 1991; He and Chen 2006; He and Chen 2007). The isolation made this specialized fish fauna, including O. stewartii, more vulnerable to anthropogenic activities. Moreover, O. stewartii is the top predator inhabiting the Yarlung Zangbo River (Huo et al. 2014) and changes in its abundance can alter ecosystem processes and community structure in aquatic ecosystems (McQueen et al. 1989; Carpenter and Kitchell 1993; Schindler et al. 1997). Thus, knowledge of the population dynamics of this species is of fundamental importance to conserving this stock. Our findings indicate that the stock of O. stewartii is likely to be in the near full exploitation under the current harvesting strategy, and both measures of increasing the age at first capture and introducing the closed season can be effective to protect the O. stewartii population. Our study represents an important investigation in better understanding the population dynamics of this species and its possible management policies within the Yarlung Zangbo aquatic ecosystem. Although our study characterized the likely stock status of O. stewartii over a wide range of harvesting strategies using the per-recruit model, we were unable to estimate the age-specific abundance and biomass of the O. stewartii population. More research work is needed to fill this information gap for O. stewartii.