Jounela Pekka, Auttila Miina, Alakoski Riikka, Niemi Marja, Kunnasranta Mervi
Natural Resources Institute Finland, Turku, Finland.
Metsähallitus, Parks & Wildlife Finland, Savonlinna, Finland.
PLoS One. 2024 Dec 5;19(12):e0311255. doi: 10.1371/journal.pone.0311255. eCollection 2024.
Over the past three decades, incidental bycatch has been the single most frequent verified cause of death of the endangered Saimaa ringed seal (Pusa hispida saimensis). Spatial and temporal fishing closures have been enforced to mitigate bycatch, which is mainly caused by the gillnets of recreational fishers. In this study, we employed an array of statistical machine learning methods to recognize patterns of death and to evaluate the impacts of annual fishing closures (15th April-30th June) on the recovery of the Saimaa ringed seal population during 1991-2021. We additionally used the potential biological removal (PBR) procedure to assess bycatch sustainability. The study shows that gillnet restriction areas are reflected in the timing of juvenile bycatch mortality of the Saimaa ringed seal. In the 1990s, peak mortality occurred at the beginning of June, but as the restrictions expanded regionally in the 2000s, the peak shifted to the beginning of July. Longer temporal coverage of annual closures would have improved juvenile survival. The study also shows that estimated bycatch mortality is higher than observed: the estimated bycatch averaged approximately two unobserved bycatches per one observed bycatch. Despite the continuing bycatch mortality, a larger number of juveniles nowadays survive to the age of 15 months due to fishing closures, and the population (some 420 individuals) has increased an average 4% per year between 2017 and 2021. However, human-caused mortality limits (PBR) were exceeded by observed bycatch only, which could lead to population depletion in the long run.
在过去三十年里，意外兼捕一直是濒危的塞马环斑海豹（Pusa hispida saimensis）最常见的经证实的死亡原因。为了减轻兼捕，已经实施了空间和时间上的捕鱼限制，兼捕主要是由休闲渔民的刺网造成的。在本研究中，我们采用了一系列统计机器学习方法来识别死亡模式，并评估年度捕鱼限制（4月15日至6月30日）对1991年至2021年期间塞马环斑海豹种群恢复的影响。我们还使用了潜在生物移除（PBR）程序来评估兼捕的可持续性。研究表明，刺网限制区域反映在塞马环斑海豹幼崽兼捕死亡的时间上。在20世纪90年代，死亡高峰期出现在6月初，但随着21世纪区域限制的扩大，高峰期转移到了7月初。年度禁渔期更长的时间覆盖范围本可以提高幼崽的存活率。研究还表明，估计的兼捕死亡率高于观察到的死亡率：估计的兼捕平均每观察到一次兼捕就有大约两次未观察到的兼捕。尽管兼捕死亡率仍在持续，但由于捕鱼限制，如今有更多的幼崽存活到15个月大，并且在2017年至2021年期间，种群数量（约420只个体）平均每年增长4%。然而，仅观察到的兼捕就超过了人为造成的死亡率限制（PBR），从长远来看，这可能导致种群数量减少。
