Ecology and Vulnerability

North Atlantic Right Whale

Marine mammals, particularly the large migratory whales and porpoises that comprise the Northeast RSGCN, are experiencing high rates of ocean warming, acidification, and other climate impacts to affect their distribution, phenology, and population dynamics (Letrick et al., 2023). Relatively few studies of marine mammal responses to climate change exist in part due to the difficulties in detecting and tracking their populations over the broad spatial scales that constitute their geographic ranges. Several of the large migratory whale RSGCN have shown shifts in range and phenology over recent decades; however, the rate, magnitude, and direction of these shifts vary widely across sub-regions of their range and studies (Ramp et al., 2015; Charif et al., 2020; Pendleton et al., 2022). These conflicting patterns of earlier or later arrival may be due to the different methods used to monitor populations (e.g., acoustics vs aerial visual observations) with different detection abilities and spatial coverages.

Shifts in the distribution of marine mammals are largely attributed to the indirect effects of climate change, as prey availability, abundance, and quality are altered due to ocean warming and acidification. Planktivorous species like North Atlantic Right Whales, which depend on seasonally reliable concentrations of highly nutritious and lipid-rich Calanoid copepods, are more sensitive to bottom-up changes in primary and secondary production compared to piscivorous whales (Pendleton et al., 2022). However, rapid warming in the Gulf of Maine is having broad impacts on regional food webs with ramifications not only for whales but many other marine fish and seabird RSGCN that consume zooplankton or planktivorous forage fish such as sand lance and herring (Staudinger et al., 2019; Pershing et al., 2021). Most notably, increasing SSTs are negatively affecting cold-adapted subpolar copepod species, including Calanus finmarchicus through decreased growth, productivity, and shifts in overwintering phenology schedules (Staudinger et al., 2019; Staudinger et al., 2020; Pershing et al., 2021).

As food resources become increasingly unpredictable, migratory whales are occurring in unexpected locations and at unexpected times or showing elevated mortality and stranding rates, as indicated by the declaration of Unusual Mortality Events in the region (NOAA, 2024)1 . This increases the potential for conflict and the incidence of injury and mortality of these already atrisk and endangered species with human activities such as shipping, fishing, and wind energy development.

Shifts in Range, Elevation, or Depth

From 2004 – 2014, North Atlantic Right Whales appeared less prevalent in the Gulf of Maine but more common in the Mid-Atlantic and Southern New England, though the study did not evaluate the causation of the shift (Davis et al., 2017; O’Brien et al., 2022). Models using climate change projections (specifically RCP 4.5 and 8.5) show that much of the Gulf of Maine will be unsuitable habitat for North Atlantic Right Whales in late summer and fall, except for some areas along the Scotian Shelf (Ross et al., 2021).

Shifts in Phenology

North Atlantic Right Whales appear to be leaving Massachusetts Bay to feed in Cape Cod Bay 40 days earlier each late-winter / early spring between 2008 – 2011, though results are variable (the trend reversed the last two years of the study), which may be due to warming conditions in the northwestern Atlantic Ocean (Charif et al., 2020). Moreover, another study found a similar pattern; earlier spring transition dates have driven earlier movements between habitats from 1998 - 2017, indicating that North Atlantic Right Whales may respond to temperature to start migration to Cape Cod Bay (Ganley et al., 2022). Peak habitat use of North Atlantic Right Whales in Cape Cod Bay shifted 18.1 days earlier from 1998 – 2018 and was negatively related to the earlier onset of spring in the Gulf of Maine (Pendleton et al., 2022). North Atlantic Right Whales are also staying longer in Cape Cod Bay later in the spring season, albeit with high interannual variability (Pendleton et al., 2022). Shifts in annual and seasonal migration patterns of North Atlantic Right Whales have been linked to changes in the distribution and availability of their primary copepod prey, Calanus finmarchicus (Pershing et al. 2021). Further, recent population changes and decreased calving rates have been linked to the higher energetic costs associated with longer search times for optional foraging grounds and lower quality food resources associated with warming waters in the region (Staudinger et al., 2020; Pershing et al., 2021; Pendelton et al., 2022).