Density dependence only affects increase rates in baleen whale populations at high abundance levels

Most baleen whale populations are increasing after the end of industrial
whaling, but their recovery patterns challenge long-standing assumptions
about density dependence. It has long been assumed that
population growth rates will decline with recovery, until reaching
equilibrium (“carrying capacity”, K). Indeed, the International
Whaling Commission assumes that growth rates will slow long before K is
reached, with maximum productivity at 0.6K. This 0.6K population level is
used as an international benchmark that forms the basis of whaling
regulations and decisions about whether baleen whale populations are
declared depleted. We fit models to four long-term datasets for
baleen whales with multiple abundance estimates that span the range from
low to high abundance, finding strong evidence that increase rates remain
at near-maximal levels across a wide range of abundance levels, and only
decline as the population nears K. As a result, maximum
productivity occurs at 0.69–0.87 of K across these populations, which
predicts more rapid recovery for baleen whale populations than currently
assumed. The overall mean of these values (0.8K) would be a more sensible
default choice than the 0.6K currently assumed. Synthesis and
applications: Estimated recovery rates imply that management thresholds
currently used are lower than actual maximum productivity, and that
populations can increase rapidly even at high abundance. However, if
population models continue to assume that maximum productivity is at 0.6K,
they will estimate abundance relative to K to be lower than it is,
providing conservative assessment results. Our results should stimulate
further discussion about the role of maximum sustainable yield (MSY) as a
fundamental concept in fisheries and wildlife management.

工业捕鲸结束后，多数须鲸（baleen whale）种群数量正逐步恢复，但其恢复模式却对长期以来有关密度依赖的生态学假设提出了挑战。
长期以来学界普遍认为，种群增长率会随着恢复进程逐渐放缓，直至达到平衡状态——即环境容纳量（carrying capacity，K）。事实上，国际捕鲸委员会（International Whaling Commission）假定，种群增长率会在达到环境容纳量K之前很久就开始放缓，且种群的最大生产力出现在0.6K的水平。这一0.6K的种群丰度阈值被用作国际基准，以此制定捕鲸相关法规，并作为判定须鲸种群是否被列为枯竭种群的决策依据。
我们对四组涵盖从低到高丰度区间、且包含多次丰度估算的须鲸长期数据集开展模型拟合，结果发现强有力的实证证据表明：在极宽的丰度范围内，种群增长速率始终维持在接近最大值的水平，仅当种群接近环境容纳量K时，增长速率才会出现显著下降。
据此，上述种群的最大生产力均出现在0.69K至0.87K的区间内，这意味着须鲸种群的实际恢复速度要比当前学界假定的更快。上述数值的整体均值（0.8K）相比当前采用的0.6K，是更为合理的默认基准选择。
综合与应用：本次研究估算的恢复速率表明，当前采用的管理阈值实则低于种群实际的最大生产力，且即便种群处于较高丰度水平，仍能实现快速增长。但若后续种群模型仍沿用最大生产力出现在0.6K的假设，则会低估种群丰度相对于环境容纳量K的实际比例，最终得出偏保守的评估结果。
本研究结果有望推动学界就最大可持续产量（maximum sustainable yield，MSY）作为渔业与野生动物管理核心概念的地位展开进一步讨论。