Marine Mammal Research Unit, Institute for the Oceans and Fisheries, The University of British Columbia, Vancouver, BC, Canada.
PLoS One. 2024 Oct 10;19(10):e0311388. doi: 10.1371/journal.pone.0311388. eCollection 2024.
Differences in the availability of prey may explain the low numbers of southern resident killer whales and the increase in northern resident killer whales in British Columbia and Washington State. However, in-situ data on the availability of their preferred prey (Chinook salmon, Oncorhynchus tshawytscha) in the core feeding areas used by these two populations of fish-eating killer whales have been lacking to test this hypothesis. We used multi-frequency echosounders (38, 70, 120, and 200 kHz) to estimate densities of adult Chinook (age-4+, > 81 cm) within 16 hot-spot feeding areas used by resident killer whales during summer 2020 in the Salish Sea and North Island Waters. We found Chinook were generally concentrated within 50 m from the bottom in the deep waters, and tended to be absent near the surface in the shallow waters (< 50 m). In general, the densities of Chinook we encountered were highest as the fish entered the Salish Sea (from Swiftsure Bank in the south) and Johnstone Strait (from Queen Charlotte Strait to the north)-and declined as fish migrated eastward along the shoreline of Vancouver Island. Median densities of Chinook for all sampled areas combined were 0.4 ind.·1000 m-2 in northern resident foraging areas, and 0.9 ind.·1000 m-2 in southern resident killer whale areas (p < 0.05, Mann-Whitney U test). Thus, Chinook salmon were twice as prevalent within the hot-spot feeding areas of southern versus northern resident killer whales. This implies that southern resident killer whales have greater access to Chinook salmon compared to northern residents during summer-and that any food shortage southern residents may be encountering is occurring at other times of year, or elsewhere in their range.
猎物的可获得性差异可能解释了南居留地虎鲸数量较少和北居留地虎鲸在不列颠哥伦比亚省和华盛顿州数量增加的原因。然而，在这两个以鱼为食的虎鲸种群使用的核心觅食区，关于其首选猎物（Chinook 三文鱼，Oncorhynchus tshawytscha）的可获得性的现场数据一直缺乏，无法验证这一假设。我们使用多频回声测深仪（38、70、120 和 200 kHz）来估计 2020 年夏季在 Salish 海和北岛水域，居留地虎鲸在 16 个热点觅食区使用期间，成年 Chinook（年龄 4+，>81 cm）的密度。我们发现 Chinook 通常集中在深水区的底部 50 米以内，而在浅水区（<50 米）的水面附近则很少见。一般来说，我们遇到的 Chinook 密度在鱼进入 Salish 海（从南部的 Swiftsure 银行）和约翰斯顿海峡（从夏洛特皇后海峡到北部）时最高，并在鱼沿着温哥华岛的海岸线向东迁移时下降。所有采样区域的 Chinook 中位数密度在北居留地觅食区为 0.4 ind·1000 m-2，在南居留地虎鲸区为 0.9 ind·1000 m-2（p<0.05，Mann-Whitney U 检验）。因此，在南居留地虎鲸的热点觅食区，Chinook 三文鱼的出现频率是北居留地虎鲸的两倍。这意味着与北居留地虎鲸相比，南居留地虎鲸在夏季获得 Chinook 三文鱼的机会更大——南居留地虎鲸可能遇到的任何食物短缺都发生在一年中的其他时间，或者在其活动范围的其他地方。
