Multiple-model stock assessment frameworks for precautionary management and conservation on fishery-targeted coastal dolphin populations off Japan

1. Stock assessment approaches are often oversimplified due to lack of biological knowledge and insufficient data. In spite of worldwide attention, fishery-targeted coastal dolphin species in Japan have lacked in-depth quantitative stock assessments because of limited time-series of population size and an absence of associated biological information. We consequently developed integrated population models that analyzed multiple sources of data simultaneously with published biological information within a Bayesian framework.

2. We estimate population status and trends for three main species targeted by fisheries, bottlenose dolphins Tursiops truncatus, Risso’s dolphins Grampus griseus, and short-finned pilot whales Globicephala macrorhynchus, using single-species age-aggregated and age-structured population dynamics models. Models were fit to absolute abundance estimates from systematic line-transect surveys, four series of abundance indices calculated from fisher’s logbooks, and historical catch records. Published biological information was used to develop prior distributions for the biological parameters. We assessed the possible effects of ecological disturbance and competition using state-space and multispecies models.

3. The multispecies model was selected by the model selection both for the age-aggregated and the age-structured approaches.

4. Single-species assessments found that population size declined 4.2% (Risso’s dolphin) to 8.0% (bottlenose dolphin) for three species since the late 1800s based on median posterior values, while the state-space and multispecies models found that environmental disturbance and an interaction among species could have reduced population size more substantially.

5. ‘Policy implications’ Simple single-species models are often used to provide conservation and management advice for wild animals but, in this case, results from such models are overly optimistic because they overlook important ecological process. In contrast, current population status could be less favorable if environmental disturbance and interspecific competition actually control population dynamics. Even if that is not the case, considering ecological process in the model will provide more precautionary ways. Our approach of combining multiple modelling frameworks is applicable to many other management systems, and offers increased confidence in estimated status and trends over assessments that consider only a single model.

Methods
All the methods are described in detail in the scientific paper associated.

1. 由于生物学知识匮乏与数据不足，种群资源评估（stock assessment）方法往往过于简化。尽管日本近海以渔业为目标的海豚类物种已受到全球关注，但由于种群规模时间序列有限且缺乏相关生物学信息，这类物种始终缺乏深入的定量资源评估。为此，我们开发了整合种群模型，在贝叶斯框架（Bayesian framework）内结合已发表的生物学信息，同时对多源数据进行分析。
2. 我们针对三种主要渔业目标物种——宽吻海豚（Tursiops truncatus）、里氏海豚（Grampus griseus）以及短肢领航鲸（Globicephala macrorhynchus），利用单物种年龄聚合与年龄结构种群动力学模型，估算其种群现状与变化趋势。模型拟合了系统线迹调查（systematic line-transect surveys）得到的绝对丰度估计值、基于渔民捕捞日志计算的四组丰度指数（abundance indices），以及历史捕捞记录。我们以已发表的生物学信息为基础，为生物学参数设定先验分布。此外，通过状态空间模型（state-space models）与多物种模型，评估了生态扰动与种间竞争的潜在影响。
3. 无论是年龄聚合模型还是年龄结构模型，多物种模型均通过了模型选择检验。
4. 单物种评估结果显示，基于后验中位数（median posterior values），自19世纪末以来，三种物种的种群规模均出现下降，降幅范围为4.2%（里氏海豚）至8.0%（宽吻海豚）；而状态空间模型与多物种模型则表明，环境扰动与物种间相互作用可能会使种群规模出现更大幅度的缩减。
5. 「政策启示」 简化的单物种模型常被用于为野生动物提供保护与管理建议，但在本研究中，这类模型的结果过于乐观，因为它们忽略了重要的生态过程。反之，如果环境扰动与种间竞争确实主导种群动态，那么当前的种群现状可能并不乐观。即便并非如此，在模型中纳入生态过程也能提供更具预防性的管理方案。我们结合多种建模框架的研究方法可推广至众多其他管理体系，相较于仅考虑单一模型的评估，该方法能提升种群现状与变化趋势估计的可信度。

研究方法
所有研究方法均详见关联的学术论文。