Somers Isles and Adjacent Seamounts IMMA

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

Boat-based surveys since 2007 revealed the Somers Isles to be a humpback whale (Megaptera novaeangliae) migratory stopover ground (Grove et al., 2023), with whales observed passing through Bermuda waters on their way south to the Caribbean breeding/calving grounds from mid-December into February as well as on their way north from the breeding/calving grounds to their northern feeding grounds March through to mid-May. Peak aggregations occur from mid-March to late April. An abundance survey indicated some 1,000-2,000 humpbacks migrate through Bermuda, with reconstructed abundances showing exponential growth since 2012 (Grove et al. 2022, 2023). Numerous important habitats have been identified including areas for feeding, resting, using sand holes to remove parasites, and singing (Stevenson and Werth, 2024).  It is believed that the complex topography of the canyon enhances song transmission (Payne and Payne, 1985). On their way north, large numbers of humpbacks aggregate in the IMMA, with layovers up to 21 days. Groups with over a dozen individuals can be observed milling around for many hours. Their behaviour differs from earlier in the season when competitive groups are frequently observed. These large groups will then suddenly move at a consistent speed and heading (north) with coordinated breathing in what appears to be a convoy.

Criterion C: Key Life Cycle Activities

Sub-criterion C1: Reproductive Areas

From 2016 onwards, female humpback whales have been observed with very small calves from late December to early January (Stevenson et al. 2017; Stevenson 2019; 25 minutes 22 seconds). Notably in the early 20^th century, calves 4.5 to 9.1 m in length were sighted until early June; from 1980-1985 and over 37 days of sighting effort, 6% of sightings were calves (Stone et al. 1987). These small calves could not have migrated to Bermuda from the Caribbean in December or early January, suggesting that some female humpbacks use Bermuda and the adjacent seamounts as birthing grounds on their southward migration (Kettemer 2023). Mothers with 3–4-month-old calves are also the last humpbacks to enter Bermuda’s waters in the latter part of April or early May. These calves can be observed nursing in shallow waters (Stevenson 2019; 19 minutes 07 seconds). From early to late March, male competitive groups are observed chasing females, with males engaged in fierce fighting such as swimming on top of another male to prevent it from surfacing, and slashing pectoral fins at one another (Stevenson 2019; 22 minutes, 48 seconds to 25 minutes 20 seconds). Aerial footage reveals competitive groups of as many as 25 whales chasing and fighting over a single female (Stevenson 2019; 22 minutes 48 seconds to 25 minutes 20 seconds).

Sub-criterion C2: Feeding Areas

Systematic boat-based surveys conducted from 2007 onward by WhalesBermuda have observed humpback whales foraging on the edges of the Bermuda Platform and adjacent seamounts, particularly Sally Tuckers and the windward edge of Challenger Bank (Stevenson 2011). Humpback whales are thought to be diving at dusk and overnight to feed on the deep scattering layer in this area where there are also signs of defecation (Stone et al. 1987; Hamilton et al. 1997). Recent boat-based photos of defecation episodes while tail lobbing, together with aerial footage and collected faecal samples provide additional evidence of regular feeding in the IMMA. Bermuda and its adjacent seamounts are thought to provide an important stopover ground (Grove et al. 2023) and feeding station, providing opportunistic feeding opportunities to whales before they migrate north (Stone et al. 1987; Katona and Beard 1990). DNA sequencing of solid faecal specimens indicate krill (Thysanopoda) and bristlemouth fish (Cyclothone) are important to humpback diets in the area (A. Stevensen and L. Blanco Bercial, unpublished data).

Sub-criterion C3: Migration Routes

Bermuda is one of the only areas in the North Atlantic where humpback whales are regularly observed during migration (Stone et al. 1987), with some individuals exhibiting strong fidelity to Bermuda (Beaudette et al. 2009). Photo identification and passive acoustic monitoring indicate that humpback whales are present in from mid-December to mid-May (Narganes Homfeldt et al. 2022), and densities in the IMMA are highest when whales are on their southerly route mid-December and again in early March as they migrate northwards. Modelled annual abundance estimated between 1,000-2,000 humpbacks migrate over the Bermuda Platform and adjacent seamounts (Grove et al. 2022, 2023), with 13% of humpbacks recorded across multiple years (Grove et al. 2023). Residency times based on photo-identification usually do not exceed three weeks, and updated photo identifications of 2,327 humpback whales suggest Bermuda and its adjacent seamounts are migratory stopover grounds (Grove et al. 2023) before continuing their journey north to the feeding grounds. The layovers of multiple days in Bermuda waters on similar dates year-to-year emphasises the hypothesis that Bermuda and its adjacent seamounts are not simply transitory migratory corridors (Grove et al. 2023). WhalesBermuda and others have matched whales in Bermuda to whales in the Caribbean, the eastern United States and Canada, and as far northeast as Franz Josef Land in Russia, Iceland, Ireland, Greenland, and the Azores (Stone et al. 1987).