Data-Limited Population-Status Evaluation of Two Coastal Fishes in Southern Angola Using Recreational Catch Length-Frequency Data

Excessive truncation of a population’s size structure is often identified as an important deleterious effect of exploitation, yet the effect on population persistence of size-structure truncation caused by exploitation is often not quantified due to data limitations. In this study, we estimate changes in eggs per recruit (EPR) using annual length-frequency samples over a 9 year period to assess persistence of the two most important recreational fishes in southern Angola: west coast dusky kob (Argyrosomus coronus) and leerfish (Lichia amia). Using a length- and age-structured model, we improve on an existing method to fit this type of model to length-frequency data and estimate EPR. The objectives of the methodological changes are to add flexibility and robustness to the approach for assessing population status in data-limited situations. Results indicate that dusky kob presents very low levels of EPR (5%-10% of the per recruit reproductive capacity in the absence of fishing) in 2013, whereas large inter-annual variability in leerfish estimates suggest caution must be applied when drawing conclusions about its exploitation status. Using simulated length frequency data with known parameter values, we demonstrate that recruitment decline due to overexploitation leads to overestimation of EPR values. Considering the low levels of EPR estimated for the study species, recruitment limitation is not impossible and true EPR values may be even lower than our estimates. It is, therefore, likely that management action, such as the creation of Marine Protected Areas, is needed to reconstitute the west coast dusky kob population.

种群体型结构的过度截除常被视为捕捞产生的重要有害效应，但因数据限制，捕捞引发的体型结构截除对种群存续的影响往往难以量化。本研究基于安哥拉南部9年间的年度体长频率样本，估算单位补充量产卵量（eggs per recruit, EPR）的变化，以评估当地两种最重要的休闲渔业鱼种的种群存续状况：西岸暗色石首鱼（Argyrosomus coronus）与利氏鱼（Lichia amia）。本研究借助体长年龄结构模型，对现有将此类模型适配于体长频率数据并估算EPR的方法进行了改进。此次方法改进的目标，是为数据匮乏情境下的种群现状评估方法提升灵活性与鲁棒性。结果显示，2013年西岸暗色石首鱼的EPR水平极低，仅为无捕捞状态下单位补充量生殖能力的5%~10%；而利氏鱼的估算结果存在较大的年际波动，这表明在评估其捕捞现状时需谨慎行事。本研究通过参数已知的模拟体长频率数据，证实过度捕捞引发的补充量下降会导致EPR估值被高估。鉴于本研究中受试鱼种的EPR估算值偏低，补充量限制并非不可能，真实的EPR值甚至可能低于本次估算结果。因此，或许需要通过设立海洋保护区（Marine Protected Areas）等管理措施，来恢复西岸暗色石首鱼的种群数量。