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Effects of inconsistent reporting regulation changes and market demand on abundance indices of sharks caught by pelagic longliners off southern Africa

机译：报告不一致法规变化和市场需求对南部非洲中上层延绳钓捕捞鲨鱼的丰度指数的影响

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The assumption of a proportional relationship between catch-per-unit-effort (CPUE) and the abundance of sharks caught by pelagic longliners is tenuous when based on fisher logbooks that report only retained specimens. Nevertheless, commercial logbooks and landings statistics are often the only data available for stock status assessments. Logbook data collected from local and foreign pelagic longline vessels operating in four areas off southern Africa between 2000 and 2015 were used to construct standardized CPUE indices for blue sharks Prionace glauca and shortfin makos Isurus oxyrinchus. Generalized linear mixed models were used to explore the effects of year, month, vessel, fleet and presence of an observer on blue shark and shortfin mako variability. Landing statistics and auxiliary information on the history of the fishery, regulation changes, and market factors were superimposed on the CPUE indices, to test hypotheses that they would influence CPUE trends. Indices in the West and Southwest (Atlantic) areas were elevated for both species, compared to the South and East (Indian Ocean). The scale of year-on-year CPUE increments, up to an order of magnitude for blue sharks, reflected occasional targeting and retention, interspersed with periods where blue sharks were not caught, or discarded and not reported. Increments were smaller for higher value shortfin makos, suggesting that indices were less affected by unreported discarding. CPUE indices and landings of both shark species have increased in recent years, suggesting increased importance as target species. Analysis of logbook data resulted in unreliable indicators of shark abundance, but when trends were interpreted in conjunction with landings data, disaggregated by area and month, and with hindsight of market demand and regulation changes, anomalies could be explained.
机译：当基于仅记录保留标本的费舍尔日志时，单位捕捞努力量（CPUE）与中上层延绳钓捕捞的鲨鱼数量之间成比例关系的假设是微不足道的。但是，商业日志和着陆统计通常是唯一可用于库存状态评估的数据。从2000年至2015年从在南部非洲以外四个地区作业的本地和国外远洋延绳钓船收集的航海日志数据被用于构建蓝鲨Prionace glauca和短鳍马科斯Isurus oxyrinchus的标准化CPUE指数。使用广义线性混合模型来研究年，月，船，船队和观察者的存在对蓝鲨和短鳍马科变异性的影响。关于渔业历史，法规变化和市场因素的着陆统计数据和辅助信息被叠加在CPUE指数上，以检验可能影响CPUE趋势的假设。与南部和东部（印度洋）相比，西部和西南（大西洋）地区的两种物种的指数均升高。蓝鲨的CPUE同比增长规模达到一个数量级，反映了偶尔的目标锁定和保留，以及零星未捕获，丢弃或未报告蓝鲨的时期。较高价值的短鳍马科鱼的增幅较小，这表明未受报告的丢弃物对指数的影响较小。近年来，这两种鲨鱼的CPUE指数和着陆量都有所增加，表明作为目标物种的重要性越来越高。对日志数据的分析导致了不可靠的鲨鱼数量指标，但是如果将趋势与着陆数据结合起来（按面积和月份分列）以及对市场需求和法规变化的后见之明，则可以解释异常情况。

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