Análises demográficas estruturadas indicam estratégias de manejo alternativas para pequenos tubarões costeiros

Abstract

Sharks are important fishery resources around the world that have been an object of concern among managers because of specific life-history traits that makes them particularly vulnerable to high fishing pressures. Along all the Brazilian coastline, the fast-growing sharpnose sharks Rhizoprionodon porosus and R. lalandii are widely exploited, but no management measures have been implemented to date because of the absence of studies addressing their level of exploitation. Then, this study aimed to determine crucial demographic parameters of those species through matrix-based demographic analyses with the incorporation of stochasticity with Monte Carlo simulations, which allowed finding how population growth rates behaveunder different exploitation scenarios. Then, potential management measures such as gauntlet fisheries were tested by incorporating age-specific hypothetical levels of fishing mortality to both species. The results demonstrated that both populations exhibit positive rates of population growth when unfished, but have different demographic parameters such as generation time and reproductive values. The Monte Carlo simulations for age-independent levels of fishing mortality (no size restriction measure) demonstrated that critical levels of fishing pressure are equal to 0.13 and 0.15 for R. porosus and R. lalandii, respectively. Conversely, when the application of gauntlet management scenarios is simulated for both species, it is clear that excluding older sharks from the fisheries would have a positive effect for both fisheries and conservation, allowing increases in fishing effort of up to 0.3. Thisstudy indicates that such fast-growing sharks may also benefit from gauntlet fisheries measures, which may be considered for these species in the Brazilian coastline, particularly in the absence of comprehensive stock assessments for local stocks.