Dutch Caribbean: “home” of unique, and likely vulnerable, populations of Bryde’s and humpback whales
During the last decade several marine areas in the Wider Caribbean have been awarded extra protection as “marine mammal reserves”, such as the the Yarari Marine Mammal and Shark Sanctuary that was established in the Caribbean Netherlands on September 1, 2015. The Yarari Sanctuary comprises all the waters of Bonaire and Saba and since September 2018 also St. Eustatius. The name of the Sanctuary “Yarari”is a Taíno Indian word, meaning ‘a fine place’. It is intended to provide “a fine place” for marine mammals and sharks, where they will receive the necessary attention to ensure they are optimally protected.
Among the large marine mammals, the North Atlantic Humpback whales (Megaptera novaeangliae) are known to overwinter in the wider Caribbean as well as in the Cape Verde archipelago and other areas off western Africa. The available data suggests that the humpback whales that overwinter in the Dutch Caribbean are notpart of the western North Atlantic humpback whale population that breeds north of the Dominican Republic, as previously assumed, but instead more likely to be part of a relatively small “eastern” North Atlantic humpback population that also overwinter in the Cape Verde archipelago and which summers off Iceland and in the Barents Sea. The Dutch Caribbean is also home to Bryde’s whales, Balaenoptera edeni spp.,which is an all-year low latitude resident. However, data on Bryde’s whales are very scarce and our understanding of Bryde’s whales in the Dutch Caribbean and adjacent waters is near-zero.
Humpback migrations between feeding and breeding grounds
Humpback whales undertake some of the longest seasonal migrations known among animals (Stone et al.1990). Summers are spent on foraging grounds at high latitudes in Temperate and Sub-polar waters (Katona & Beard, 1990). During the fall and early winter most individuals migrate towards the equator to spend winter in Tropical waters on the breeding grounds (Dawbin, 1966; Katona & Beard, 1990). Despite the extensive fall migration humpbacks do not feed during the winter which is the mating and calving season (Dawbin, 1966). Females give birth to one calf during the winter on the breeding grounds, approximately one year after becoming pregnant (Robbins, 2007). The calf relies upon its mother for sustenance in the form of high-caloric milk. The lactation places very high energetic demands on the mother, which loses around 30 percent of her body weight during this period (Robbins, 2007). During the spring the calf migrates together with its mother to a high latitude summer foraging ground where it spends the summer, eventually separating from the mother during fall when it will migrate back to the winter breeding grounds on its own (Clapham et al.,1993; Robbins, 2007). With very few exceptions individual humpback whales keep returning to their maternal summer foraging ground their entire life (Clapham et al.,1993; Palsbøll et al.,1995; Robbins, 2007).
Recovering western North Atlantic humpback population
Humpback whales are relatively slow-moving whales which, along with a tendency to congregate in specific, predictable areas, made them a target for commercial whaling during the late 19thand early 20thCenturies in the North Atlantic (Punt et al.,2006). As a result, the population was decimated to very low numbers on both sides of the North Atlantic. After full protection was afforded in the early 1960s, the humpback whales in the western North Atlantic appear to have increased to approximately 10,000 (Smith et al.1999)and 12,000 (unpublished) individuals in 1992/93 and 2004/5, respectively.
30 years of trans-North Atlantic collaborative research projects
These estimates were based upon two large trans-North Atlantic collaborative research efforts, YoNAH (Years of the North Atlantic Humpback Whale) conducted in 1992 and 1993, and a second mainly US-based effort again in 2004 and 2005 (project MoNAH, More of the North Atlantic Humpback Whale). Two key kinds of data were collected during the Y/MoNAH projects; photographs of the underside (ventral) of the tail, also called the fluke, as well as small skin biopsies.
Tail (fluke) photographic identification
The pigmentation pattern on the ventral side of the fluke as well as the serrations along the trailing edge are unique to individuals and have been used to identify and map the movements of individual humpback whales globally since the late 1970s (Katona & Whitehead 1981). The large-scale collection of skin biopsies from free-ranging whales during the YoNAH project was something new at the time.
Genetic “tagging” from skin biopsies
The genetic analyses of the skin biopsies were originally not intended for identification of individuals, but aimed at assessing large-scale population structure. However, recent advances in “genetic fingerprinting” (i.e., CSI-style DNA identification) made it possible to identify individuals and their sex in all ~3,000 skin biopsies collected from North Atlantic humpback whales at the time (Palsbøll et al., 1997). The subsequent MoNAH project (in 2004 & 5) collected an additional 3,700 skin biopsy samples and the resulting abundance estimate was based mainly upon genetic (rather than photographic) identification of individuals, although fluke photos were also collected. All in all, the current collection of skin biopsy samples from North Atlantic humpback whales now counts ~8,000 skin biopsy samples, representing 5,700 unique genetic fingerprints (i.e., individuals) and is curated by Drs. Per Palsbøll and Martine Berube at University of Groningen (the Netherlands).
Separate eastern North Atlantic humpback whale population
During both the YoNAH/MoNAH project all efforts on the breeding grounds were directed towards the western North Atlantic. The vast majority of skin samples (and fluke photographs) were collected from humpback whales on the main banks and bays north of the Dominican Republic; such as Silver Bank, Navidad Bank and Samana Bay. In contrast, only few skin samples and fluke photos have been collected in the eastern Caribbean and eastern North Atlantic breeding grounds. So far 50 and 13 samples have been collected in the Cape Verde Archipelago and St. Martin in the eastern Caribbean, respectively. One key question is if there is/was a separate eastern North Atlantic humpback whale population. The Yankee whalers from the US East Coast caught many humpback whales in the Cape Verde Archipelago during the early 20thCentury (Punt et al.,2006), as did the Norwegians off northern Norway during the summer and winter (Ingebrigtsen, 1929). Re-identifications of individual humpback whales from their fluke photographs as well as genetic fingerprints have identified individuals that were “observed” both in the Cape Verde Archipelago/eastern Caribbean and northern Norway (Stevick et al., 2016). Some individuals were even sighted both in the Cape Verde Archipelago and the eastern Caribbean (Stevick et al., 2016). The abundance estimates based upon these photo matches suggest a very low population size in the Cape Verde Archipelago at 2-300 individuals (Ryan & Wenzel 2014)in contrast to ~12,000 in the western North Atlantic. Humpback whales satellite tagged in the eastern Caribbean showed some latitudinal movements across the Caribbean although most tracks are of too short duration to make long-term deductions (Kennedy et al.2013; 2014).The genetic analyses also found the genetic diversity in samples from the eastern Caribbean and Cape Verde Archipelago to be much lower compared to the western North Atlantic, which was consistent with the very low abundance estimate for the Cape Verde Archipelago (Palsbøll, unpublished data).
Humpback “breeding” populations
Genetic data, of the kind collected from the North Atlantic humpback whale skin biopsies, can, amongst many other uses, be employed to assess how many “breeding” populations the sampled individuals possibly originate from, as well as which individuals originate from the same breeding population. It is also possible to identify individuals of “mixed” ancestry, i.e., individuals that are offspring of parents from two different “populations”. The result of this kind of analysis in 200 western Caribbean humpback whale samples and all individual humpback whales sampled in the Cape Verde Archipelago/eastern Caribbean revealed an unexpected pattern. All individuals, but one individual, sampled in the western Caribbean were inferred as originating from one population. In contrast, the samples from the eastern Caribbean and Cape Verde Archipelago contained individuals from two differentbreeding populations, one of which was that which the western Caribbean individuals belonged to (Palsbøll et al., unpublished data). The other “population” was only identified among the eastern North Atlantic individuals (and a single western Caribbean individual). In addition, a number of eastern North Atlantic individuals were of mixed ancestry, i.e., they appeared to be offspring of parents of which one parent belonged to an “eastern” and the other to a “western” North Atlantic population.
A preliminary genetic analysis of 14 humpback whale skin biopsy samples collected on Saba Bank during 2014 confirmed this hypothesis i.e., the humpback whales sampled on Saba Bank were genetically more similar to the humpback whales sampled in the Cape Verde archipelago than they were to samples collected from humpback whales in the “western” Caribbean (Palsbøll et al., unpublished data).
Immigration from western North Atlantic population into eastern North Atlantic population
These outcomes of these genetic analyses led to the hypothesis of a recent relative increase of immigration of humpback whales from the rapidly recovering western North Atlantic population into the essentially non-recovering eastern North Atlantic population. The genetic analyses suggested that approximately ~50-60 humpback whales per generation migrated from the western Caribbean into the Cape Verde Archipelago/eastern Caribbean. The immigrant western Caribbean individuals appeared to have mated with eastern North Atlantic individuals, resulting in the high proportion of individuals among the eastern North Atlantic individuals with a mixed ancestry (Palsbøll et al., unpublished data).
Before whaling, the abundance in the western and eastern North Atlantic “populations” was estimated at ~5,000 and ~25,000 individuals (Punt et al.2006). However, now, a century after whaling, the difference in abundance (at 300 and 12,000 in the eastern and western Caribbean, respectively) is an order of magnitude larger due to the very different recovery rates. Hence, even if the emigration rate per “capita” has remained constant in both populations, the now much larger western Caribbean population is contributing a proportionally larger number of immigrants to the eastern North Atlantic population per generation. Immigrant individuals that inter-breed with “eastern North Atlantic” humpback whales. The result of this high immigration rate and subsequent mating is an ongoing decline of the eastern North Atlantic “gene pool” (Palsbøll et al., unpublished data). These recent findings sadly showed that even a century after its cessation, whaling continues to endanger and may perhaps result in the extinction of local whale populations. In this specific case, a “concealed” genetic extinction due to differential rates of post-whaling recovery. Modelling is ongoing to determine how long it will be before the humpback whales in the Cape Verde Archipelago and eastern Caribbean will be completely supplemented by “western” North Atlantic humpback whales (Palsbøll et al., unpublished data).
Several key questions have arisen from this work; how far west in the Caribbean does the distribution of humpback whales from eastern North Atlantic “population” stretch? Are there other breeding grounds in the eastern North Atlantic, such as off Mauritania where humpback whales have been sighted during the winter, as well as other areas within the eastern Caribbean. The recently initiated EU-funded project CARI’MAM will likely contribute some new data and potential new insights towards these aspects. However, in order to truly understand the current and past processes that determines the presence and abundance in the Dutch Caribbean a wide-ranging assessment of low latitude areas that host humpback whales during the winter is badly needed. The seasonal migrations also imply that connections between winter and summer areas are key as endangerment (e.g., entanglement in fishing gear on summer areas) may affect humpback abundance in parts of the winter breeding range, such as in the Yarari Sanctuary.
The Dutch Caribbean is also home to Bryde’s whales, a unique baleen whale that (contrary to most baleen whales) do not migrate to high latitudes during the summer to forage, but is an all-year low latitude resident. Only two genetic studies aimed at North Atlantic Bryde’s whales have been conducted to date (Rosel & Wilcox 2014; Luksenburg et al.2015). The little-studied Bryde’s whale is likely comprised of multiple genetically divergent populations, possibly representing different species. These two studies by Luksenburg et al.(2015)and Rosel and Wilcox (2014)strongly suggests that the “Bryde’s” whales in the Gulf of Mexico and Dutch Caribbean are unique and distinct forms of Bryde’s whales. Rosel and Wilcox (2014)analysis found that the Gulf of Mexico Bryde’s whale population was (i) evolutionary distinct (Luksenburg et al.2015)and (ii) contained very low levels of genetic variation. The latter could imply that the current population size is very low, or the low genetic diversity could be due to a low historical population size. However, the published genetic data from four Bryde’s whale samples collected in Aruba were from another sub-species, B. e. brydei,(Luksenburg et al.2015). At present the temporal densities and range of Bryde’s whales in the Dutch Caribbean as well as their abundance is unknown.
In conclusion, the waters of the Dutch Caribbean islands appear to be the “home” of unique, and likely vulnerable, populations of Bryde’s and humpback whales. However, the state of our current knowledge of these species in this area is poor and future research will hopefully fill our knowledge-gaps.