Swain Douglas P, Jonsen Ian D, Simon James E, Myers Ransom A
Fisheries and Oceans Canada, Gulf Fisheries Centre, P.O. Box 5030, 343 University Avenue, Moncton, New Brunswick EIC9B6, Canada.
Ecol Appl. 2009 Jul;19(5):1347-64. doi: 10.1890/08-1699.1.
Population models are needed to assess the threats to species at risk and to evaluate alternative management actions. Data to support modeling is limited for many species at risk, and commonly used approaches generally assume stationary vital rates, a questionable assumption given widespread ecosystem change. We describe a modeling approach that can be applied to time series of length composition data to estimate vital rates and test for changes in these rates. Our approach uses stage-structured population models fit within a Bayesian state-space model. This approach simultaneously allows for both process and observation uncertainty, and it facilitates incorporating prior information on population dynamics and on the monitoring process. We apply these models to populations of winter skate (Leucoraja ocellata) that have been designated as "endangered" or "threatened." These models indicate that natural mortality has decreased for juveniles and increased for adults in these populations. The declines observed in these populations had been attributed to unsustainable rates of bycatch in fisheries for other groundfishes; our analyses indicate that increased natural mortality of adults is also an important factor contributing to these declines. Adult natural mortality was positively related to grey seal (Halichoerus grypus) abundance, suggesting the hypothesis that increased adult mortality reflected increased predation by expanding grey seal herds. Population projections indicated that the threatened population would be expected to stabilize at a low level of abundance if all fishery removals were eliminated, but that the endangered population would likely continue to decline even in the absence of fishery removals. We note that time series of size distributions are available for most marine fish populations monitored by research surveys, and we suggest that a similar approach could be used to extract information from these time series in order to estimate mortality rates and changes in these rates.
需要种群模型来评估濒危物种所面临的威胁，并评估替代管理行动。对于许多濒危物种来说，支持建模的数据有限，而且常用方法通常假定生命率是固定不变的，鉴于生态系统普遍变化，这一假设值得怀疑。我们描述了一种建模方法，可应用于长度组成数据的时间序列，以估计生命率并检验这些率的变化。我们的方法使用适合贝叶斯状态空间模型的阶段结构种群模型。这种方法同时考虑了过程和观测的不确定性，并且便于纳入有关种群动态和监测过程的先验信息。我们将这些模型应用于被指定为“濒危”或“受威胁”的冬鳐（Leucoraja ocellata）种群。这些模型表明，这些种群中幼鱼的自然死亡率下降，而成鱼的自然死亡率上升。这些种群数量的下降曾被归因于捕捞其他底栖鱼类时不可持续的兼捕率；我们的分析表明，成鱼自然死亡率的增加也是导致这些下降的一个重要因素。成鱼自然死亡率与灰海豹（Halichoerus grypus）数量呈正相关，这表明了一个假设，即成鱼死亡率增加反映了不断扩大的灰海豹种群捕食压力的增加。种群预测表明，如果消除所有渔业捕捞，受威胁种群预计将在低丰度水平上稳定下来，但濒危种群即使在没有渔业捕捞的情况下可能仍会继续下降。我们注意到，大多数由研究调查监测的海洋鱼类种群都有大小分布的时间序列，我们建议可以使用类似的方法从这些时间序列中提取信息，以估计死亡率及其变化。
