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Abstract: Constant harvest rate as a management strategy for the Pacific sardine (Sardinops caeruleus) fishery in the Gulf of California is supported by an analysis of variations in the catchability coefficient (q), stock abundance and commercial catch. Catchability was analyzed based on population length-structured data standard length (SL), expressed as CPUE, for 26 fishing seasons (1972–1973 to 1997–1998). We used a deterministic model of catchability to estimate the catchability-at-length assuming catchability depends on length and sardine behavior. Results demonstrated that catchability increases with sardine length, with low values for older sardines (sizes greater than 20.5cm SL), which are poorly represented in catches. Most changes in q-at-length values can be observed within the range of 12.5–15.0cm SL, which corresponded to first maturity and first capture lengths, respectively. Catchability-at-length pattern was represented for an exponential model with adjusted R 2 =0.94. Moreover, we observed an inverse power relationship between catchability and stock abundance with a slope value of β d =−0.58, which indicates the existence of hyperstability or density-dependent catchability. The strong fluctuations in biomass of the Pacific sardine, the pattern of hyperstability, and the current state of the sardine fishery defined to be harvested at the maximum allowable catch, are the rationale for recommending a constant harvest rate as management strategy, with a value of HR=0.29 and maintain the current minimum legal size of capture. [Copyright &y& Elsevier]

Abstract: Lake Taihu is the third largest freshwater lake in China and has provided local communities with valuable fisheries for centuries. However, we have only a limited knowledge of its ecosystem. In this study, a trophic model was constructed for the Lake Taihu ecosystem. This model was used to evaluate and analyze the food web structure and other properties of this ecosystem using data covering the period from 1991 to 1995. Using the model, we evaluated the impacts on local fisheries of various management scenarios comprising two basic management regimes: (1) setting fishing mortality for the top predator (large culters, Erythroculter mongolicus and Erythroculter ilishaeformis) to 0, 0.3, 0.6, 0.9 and 1.2, and (2) adjusting overall fishing effort to 0.25, 0.5, 0.75 and 1.25 times the current level. For both scenarios, fishery profit and cost were evaluated to provide an understanding of how components of the ecosystem interact. We identified possible causes of fishery overexploitation in the lake ecosystem and described the necessity of developing ecosystem-based management. The results showed that Lake Taihu had six theoretical trophic levels (TLs), with the trophic flows primarily occurring through the first five TLs. System properties such as transfer efficiency, Finn''s index, Finn''s mean length, connectance index, system omnivory index, primary production/respiration ratio, and net primary production all indicated that Lake Taihu was an immature, fairly simple ecosystem in which a relatively low fraction of total primary production was utilized. At the same time, the ecosystem was also experiencing high fishing pressure. Yet despite this, the low ascendency index (25.9%) and high system overhead ratio (74.1%) indicated that the system was highly developed and relatively stable, a condition that might result from the high degree of recycling in the system. Among the harvesting strategies considered, a strategy of either decreasing the fishing mortality of the top predator (large culters) to 0.3 or, alternatively, reducing the overall effort on the system by a factor of 0.75 appeared to be most effective at increasing the efficiency of the fisheries. [Copyright &y& Elsevier]

Many fisheries have alternative target stocks and selectively exploit the one with the highest expected income. Although target switching is very common in practice, few attempts have thus far been made to study target switching. In this paper, we investigated the potential effects of target switching on the yield and sustainability of fish stocks by equilibrium analysis and stochastic simulation. The equilibrium analysis showed that we can increase Fextinction by switching. The stochastic simulation revealed that well-planned target switching increases yields and simultaneously decreases the risk of the stocks collapsing. Target switching decreases fishing pressure on the less-abundant stock and helps the declined stock to recover. Therefore, the minimum stock level is increased by switching. As switching keeps both stocks at productive levels, the total yield is increased by switching. Target switching is effective, especially when the catchability increases with the depletion of a stock population. Target switching depends on the availability of information on stock abundance. Thus, we examined the vulnerability of switching to stock assessment errors. If the stock assessment is very uncertain, then little or no switching is recommended. Target switching can have substantial effects on fisheries. Therefore, we must investigate the mechanisms of switching and incorporate switching into management plans. [Copyright &y& Elsevier]