Managing a Dynamic North Sea in the light of its ecological dynamics: Increasing occurrence of large baleen whales in the southern North Sea

Abstract

A recent increase in sightings and strandings of humpback and to a lesser extent, fin whales in the southern North Sea has been well documented. This positive trend in the marine mammal community is to be welcome. A similar increase has been documented off the south and southwest coasts of Ireland over the past 20 years. The first international match of a humpback whale outside of Ireland was to the Netherlands in 2007, demonstrating a link between these two feeding grounds. Data from Ireland, largely obtained through citizen science, is presented as an insight into what may occur in the southern North Sea in the near future. Between 1999 and 2014 the number of individual humpback whales recorded in Ireland rose slowly. A dramatic increase occurred in 2015, when the number of individually identified whales increased from 30 to 66 and, to a lesser extent, in 2017 and 2020 when numbers increased by 10 and 12. By the end of 2020, there were 109 individual humpback whales in the Irish Humpback Whale Photo-id Catalogue. Inter-annual re-sighting rates are high with a mean ± SD re-sighting rate of 63 ± 20 % in years when 10 or more individual whales were recorded. Minimum residency times within a season ranged from 2 to 186 days with a mean ± SD of 48.0 ± 10.3 days. Just over one fifth (21.3%) were recorded over a period of 10 days, or less, 45.3% over a period of 30 days or less and 13.5% over a period of at least 100 days. Thus humpback whales returning to Irish waters are remaining for an average of 7 weeks with 16% for over 3 months. A similar pattern may occur in the southern North Sea with continued monitoring. An increasing population of humpback whales will bring new management issues, which should be considered early in order to sustain this increase. Some of these issues are presented and discussed.