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首页> 外文期刊>Conservation Biology >Depressed resilience of bluefin tuna in the western atlantic and age truncation
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Depressed resilience of bluefin tuna in the western atlantic and age truncation

机译：大西洋西部蓝鳍金枪鱼的复原力下降和年龄截断

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摘要

Following intense overfishing in the 1970s, the western stock of Atlantic bluefin tuna (Thunnus thynnus) experienced a long period of depressed abundance, which has been attributed to failure of the population to periodically produce large numbers of juveniles, the western stock mixing with the more highly exploited eastern stock (fisheries in the Northeast Atlantic Ocean and Mediterranean Sea), and regime shift in the population's ecosystem resulting in lower replacement rates. To evaluate the presence of relatively strong years of juvenile production, we analyzed age structure from a recent sample of otoliths (ear stones) collected from the western stock (2011-2013, North Carolina, U.S.A., winter fishery). Mixing levels for the recent sample were analyzed using otolith stable isotopes to test whether age structure might be biased through immigration of eastern stock bluefin tuna. Age structure from historical samples collected from United States and Canadian fisheries (1975-1981) was compared with more recent samples (1996-2007) to examine whether demographic changes had occurred to the western stock that might have disrupted juvenile production. Relatively high juvenile production occurred in 2003, 2005, and 2006. Otolith stable isotope analysis showed that these recruitments were mostly of western stock origin. However, these high recruitments were >2-fold less than historical recruitment. We found substantial age truncation in the sampled fisheries. Half the historical sample was >20 years old (mean age = 20.1 [SD 3.7]; skewness = -0.3), whereas <5% of the recent sample was >20 years old (mean age = 13.4 [SD 3.8]; skewness = 1.3). Loss of age structure is consistent with changes in fishing selectivity and trends in the stock assessment used for management. We propose that fishing, as a forcing variable, brought about a threshold shift in the western stock toward lower biomass and production, a shift that emulates the regime shift hypothesis. An abbreviated reproductive life span compromised resilience by reducing the period over which adults spawn and thereby curtailing the stock's ability to sample year-to-year variability in conditions that favor offspring survival (i.e., storage effect). Because recruitment dynamics by the western stock exhibit threshold dynamics, returning it to a higher production state will entail greater reductions in exploitation rates.
机译：在1970年代严重的过度捕捞之后，大西洋蓝鳍金枪鱼（Thunnus thynnus）的西部种群经历了长期的低迷丰度，这归因于人口无法定期生产大量的幼鱼，西部种群更多。东部种群的高度开发（东北大西洋和地中海的渔业），人口生态系统的政权转移导致替代率降低。为了评估幼年相对较强的年纪，我们从最近一次从西部种群（2011-2013年，美国北卡罗来纳州，冬季捕捞）收集的耳石（耳石）样本中分析了年龄结构。使用耳石稳定同位素分析了最近样本的混合水平，以测试年龄结构是否可能因东部原种蓝鳍金枪鱼的迁移而发生偏差。比较了从美国和加拿大渔业部门（1975-1981年）收集的历史样本与最近的样本（1996-2007年）的年龄结构，以检查西部种群是否发生了人口变化，从而可能破坏了幼鱼的生产。幼稚鱼的产量相对较高，分别发生在2003年，2005年和2006年。耳石稳定同位素分析表明，这些新兵大多来自西方种群。但是，这些高招募比历史招募少2倍多。我们在抽样的渔业中发现了明显的年龄截断。历史样本的一半是> 20岁（平均年龄= 20.1 [SD 3.7];偏度= -0.3），而最近样本的<5％是> 20岁（平均年龄= 13.4 [SD 3.8];偏度= 1.3）。年龄结构的丧失与捕捞选择性的变化以及用于管理的种群评估的趋势一致。我们建议捕捞作为强迫变量，导致西部种群向较低生物量和产量的阈值转移，该转移模拟了政权转移假说。缩短的生殖寿命会缩短成年期，从而降低种群在有利于后代生存的条件下（即储存效应）采样年变化量的能力，从而降低了恢复能力。由于西方储备的招聘动态具有阈值动态，因此将其恢复到较高的生产状态将需要更大程度地降低开采率。

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