- Scientific name: Balaenoptera borealis
- Species of Greatest Conservation Need (MA State Wildlife Action Plan)
- Endangered (MA Endangered Species Act)
- Endangered (US Endangered Species Act)
Description
Sei whales grow to lengths of 12-18 m (40-60 feet) and weigh up to 45 tons. Their colors closely resemble those of the fin whale and the more tropical Bryde’s whale, with a long sleek body that is dark bluish-gray to black with paler coloration to white underneath. The dorsal fin is about 25-60 cm (10-24 inches) high, located roughly two-thirds of the way back along the body. Its tip has a sharp angle. To differentiate sei whales from fin whales, look for a uniform dark color on both sides of the lower jaw and on the baleen all the way around the mouth, a more sharply angled dorsal fin, and a generally smaller and sleeker body. Sei whale baleen is dark in color, with grayish-white fine inner fringes. Sei whales are about 4.5 m (15 feet) long at birth and reach 9-10 m (30-33 feet) by one year of age. Female sei whales are somewhat larger than males, which is true of baleen whales in general. In toothed whales, the males are larger.
Life cycle and behavior
Sei whales live between 50 and 70 years, reaching sexual maturity at 6 to 12 years and around 13 m (43 feet) in length. Females breed every 2 to 3 years and have a gestation period of approximately one year, with breeding and calving occurring during late fall and winter. Mating peaks in December and January. Sei whales usually travel singly or in small groups of 2 to 5 animals. Occasionally, they are found in larger loose aggregations of 30 to 50 individuals. The species’ diet mainly consists of zooplankton (such as copepods and krill), small schooling fish, and squid. They prefer to feed at dawn and can dive for 5 to 20 minutes to retrieve prey. Calves nurse for 6 to 9 months and are weaned on the feeding grounds. The sei whale has few predators that are capable of killing them. These include the killer whale and larger sharks. Sea lampreys, “cookie-cutter” sharks, or parasitic copepods may be the cause of the various small white scars seen on many sei whales.
Population status
The sei whale is classified as an Endangered species in Massachusetts and under the Federal Endangered Species Act. Commercial whaling was a major historic cause of population decline. Although sei whales yield relatively low quantities of oil, 825 were taken by the Blandford, Nova Scotia whaling station from 1965 to 1972. Whales were taken in June and July as they moved north, and again in September and October as they moved south. In the entire North Atlantic, an estimated 16,933 sei whales were harvested from the late 1800s and the 1900s. The current worldwide population is estimated to have grown to about 80,000 since whaling was outlawed. Although sei whales are considered to be relatively abundant in the North Atlantic and North Pacific oceans, populations in the Southern Oceans remain severely depleted. Only two have been reported dead in Massachusetts over the past few decades, both killed by ship strikes.
Distribution and abundance
The sei whale can be found in all the world’s oceans, excluding tropical and extreme polar seas. The species prefers subtropical and subpolar waters. In the western North Atlantic, it seasonally ranges from Greenland and Iceland south to the Caribbean and Gulf of Mexico. Sei whales move north in the early spring to feed in the Gulf of Maine and Canadian waters. The southern edge of the summer feeding range extends south to New Jersey. In the winter (November-February), they are found from Georges Bank off New England, south to the West Indies. They can be found on Georges Bank year-round. The location of the breeding grounds is unknown. Sei Whales seldom use inshore waters, but 40 were documented in Cape Cod Bay in April 2017, and others have been noted close to shore in MA. The distribution of sei whales is dependent on where concentrations of zooplankton have formed. These whales may unexpectedly show up in large numbers to feed on these concentrations. They are often joined by basking sharks and other species of baleen whales.
Shifts in Range, Elevation, or Depth: From 2010 to 2017, the centroid of the sei whale’s core range shifted 70 km (43 miles) in the spring, 97 km (60 miles) in the summer, 134 km (83 miles) in the fall, and 179 km (111 miles) in the winter, all in a southwestern direction along the northeastern U.S. Atlantic Coast (Chavez-Rosales et al., 2022).
Habitat
Sei whales are most abundant off Massachusetts in the spring when they concentrate along the eastern and southwestern edges of Georges Bank, into the Northeast Channel, and south of Nantucket. They occur in the vicinity of Georges Bank year-round. They generally stay offshore in deeper water, so are seldom seen, but as many as 40 were in Cape Cod Bay in April 2017, along with North Atlantic right whales and other species. The species is considered to be relatively abundant in the North Atlantic.
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Threats
Marine mammals, particularly the large migratory whales and porpoises in the Northeast, 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 whales 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 seabirds 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 at-risk and endangered species with human activities such as shipping, fishing, and wind energy development.
Conservation
Consistent and expanded distribution monitoring of whale and porpoise populations is needed to better track populations over time and space and predict occurrence, especially as wind energy development areas are pursued and a better understanding of the distribution and occurrence of key prey populations and their responses to climate change. The region's most important and least understood populations that are shared prey across whales and other RSGCN include calanus copepods, C. finmarchicus in particular, and forage fishes, including sand lance, Atlantic herring, and river herring species. Studies on the physiological responses and energetic demands associated with climate-induced shifts in distribution, phenology, and foraging behaviors in marine mammals will help fill key knowledge gaps and increase understanding of changes in population changes due to altered growth and survival rates. Model projections of climate-induced distribution and phenology changes paired with current and future fishing areas, shipping corridors, and wind energy development areas can aid marine mammal conservation efforts by identifying intersecting spatiotemporal risk areas under different closure and operation schedules scenarios.
Stranded Whales
For all live or dead marine mammal strandings, please call one of the following hot lines:
For Cape Cod: 508-743-9548 (IFAW Marine Mammal Rescue and Research)
For all other areas: 866-755-NOAA (Northeast Marine Mammal and Sea Turtle Stranding and Entanglement Hotline)
Be prepared to provide the following information:
- Name and contact number
- Location of animal with detailed description and nearby landmarks, if possible
- Number, size, and type of animal
- Any other helpful information such as behavior or tidal cycle
- If the carcass is marked with grease pen or tagged, note the tag’s color, number, and position on the animal
- Keep a safe distance away from the animal.
References
Sei whale (Balaenoptera borealis). Office of Protected Resources , NOAA Fisheries. URL: https://www.fisheries.noaa.gov/species/sei-whale
National Marine Fisheries Service. 2011. Final Recovery Plan for the Sei Whale (Balaenoptera borealis). National Marine Fisheries Service, Office of Protected Resources, Silver Spring, MD. 108 pp.
National Marine Fisheries Service. 2021. Sei whale (Balaenoptera borealis) 5-year Review. National Marine Fisheries Service, Office of Protected Resources, Silver Spring, MD. 57 pp.
Staudinger, M.D., A.V. Karmalkar, K. Terwilliger, K. Burgio, A. Lubeck, H. Higgins, T. Rice, T.L. Morelli, A. D'Amato. 2024. A regional synthesis of climate data to inform the 2025 State Wildlife Action Plans in the Northeast U.S. DOI Northeast Climate Adaptation Science Center Cooperator Report. 406 p. https://doi.org/10.21429/t352-9q86.
Contact
| Date published: | February 21, 2025 |
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