Decline in abundance and apparent survival rates of fin whales (Balaenoptera physalus) in the northern Gulf of St. Lawrence

Migratory movements of fin whales from the Gulf of St. Lawrence challenge our understanding of the Northwest Atlantic stock structure

Fin whales, Balenoptera physalus, are capital breeders, having the potential to separate breeding and feeding both spatially and temporally. Fin whales occur throughout the Northwest Atlantic, but stock structure and seasonal movements remain unclear. By deploying satellite transmitters on 28 individuals, we examine movement patterns within and beyond the Gulf of St. Lawrence (GSL), Canada, and challenge the current understanding of stock structure. Eight individuals left the GSL in autumn, with five tags persisting into January. Migration patterns of these whales showed considerable variation in timing and trajectory, with movements extending south to 24°N, and thus beyond the assumed distribution limit of the species in the Northwest Atlantic. A rapid return to the Scotian Shelf or Gulf of Maine was observed from several whales after incursions in southern waters, suggesting that fin whales in the Northwest Atlantic may not have a common winter destination that fits the definition of a breeding ground. Area-restricted search (ARS) behavior dominated fin whale activities during summer (92%) and fall (72%), with persistence into the winter (56%); ARS occurred at multiple locations in the GSL, Scotian Shelf and Shelf edge, and near seamounts of the North Atlantic, having characteristics consistent with foraging areas.

Long-term evolution of the structure of the St. Lawrence (Canada) marine ecosystem in the context of climate change and anthropogenic activities: An isotopic perceptive

Documenting long-term changes in the trophic structure of food webs and how species respond to these changes is essential to forecast their vulnerability and resilience to environmental stressors. Over the past decades, the St. Lawrence marine ecosystem (Canada) has experienced major changes in its physical, chemical, and biological conditions from overfishing, acoustic and chemical pollution, climate change, and the increased abundance of some top predators. These changes have likely affected the trophodynamics of the ecosystem, and are suspected to have deleterious effects on endangered species of mammals and other components of the ecosystem, such as blue whales (Balaenoptera musculus), fin whales (B. physalus), and beluga (Delphinapterus leucas). This study examined the trophic structure of the St. Lawrence marine ecosystem, including the isotopic niche of various species, over two periods of contrasting pressures from anthropogenic and climatic stressors (1995-2003 vs. 2019-2021). Stable isotope ratios were measured in 1240 samples of 21 species of marine invertebrates, fishes, and mammals sampled during both periods. A significant change in the isotopic value and niche position between periods is observed in most of the sampled species. While the direction of change and effect size were not uniform among species, these changes confirmed that substantial modifications in community structure have occurred over time. Niche overlap decreased considerably among some of the pelagic and demersal fishes, and among whale species during the most recent period. Combined with a concomitant reduction in niche breadth in several species, these observations suggested that resource sharing was limited among these species. This study highlighted some degree of dietary plasticity in several species, and a long-term change in the trophic structure of the St. Lawrence marine ecosystem, with likely effects on diet composition and energetics of several populations, including endangered species.

Hybridization in the absence of an ecotone favors hybrid success in woodrats (Neotoma spp.)

Hybridization is a common process that has broadly impacted the evolution of multicellular eukaryotes; however, how ecological factors influence this process remains poorly understood. Here, we report the findings of a 3-year recapture study of the Bryant's woodrat (Neotoma bryanti) and desert woodrat (Neotoma lepida), two species that hybridize within a creosote bush (Larrea tridentata) shrubland in Whitewater, CA, USA. We used a genotype-by-sequencing approach to characterize the ancestry distribution of individuals across this hybrid zone coupled with Cormack-Jolly-Seber modeling to describe demography. We identified a high frequency of hybridization at this site with ~40% of individuals possessing admixed ancestry, which is the result of multigenerational backcrossing and advanced hybrid-hybrid crossing. F1, F2, and advanced generation hybrids had apparent survival rates similar to parental N. bryanti, while parental and backcross N. lepida had lower apparent survival rates and were far less abundant. Compared to bimodal hybrid zones where hybrids are often rare and selected against, we find that hybrids at Whitewater are common and have comparable survival to the dominant parental species, N. bryanti. The frequency of hybridization at Whitewater is therefore likely limited by the abundance of the less common parental species, N. lepida, rather than selection against hybrids.

Investigating local population dynamics of bottlenose dolphins in the northern Bahamas and the impact of hurricanes on survival

Little Bahama Bank in the northern Bahamas supports several populations of bottlenose dolphins (Tursiops truncatus). We provide the first estimates of birth rate and age-class-specific apparent survival rates for the local South Abaco population using data from a long-term (1997–2014) photo-identification (photo-ID) study and use the estimated life history parameters in a population viability analysis (PVA) to predict future population trends. Hurricane events are predicted to become more intense due to climate change but knowledge of how hurricanes may impact cetacean populations is limited. Little Bahama Bank is subject to hurricane activity, so we also investigate the potential impact of hurricanes on calf, juvenile and adult survival. Photo-ID data confirmed the existence of a core adult population with relatively high site fidelity in South Abaco, but also evidence of transient animals. Estimated annual birth rate was 0.278 (95% CI: 0.241–0.337). We found strong support for a decline in apparent survival for all age-classes. Estimated survival declined by 9% in adults (0.941 in 1998, to 0.855 in 2013), 5% in juveniles (0.820 in 2000, to 0.767 in 2013) and 36% in calves (0.970 in 1997, to 0.606 in 2013). Evidence that survival was influenced by repeated hurricane activity leading to increased mortality and/or emigration was stronger for calves and juveniles than for adults. PVA simulations of an assumed isolated South Abaco population showed that declines would lead to extinction within decades, even under the most optimistic scenario. Future work should focus on establishing if South Abaco is part of natural source–sink metapopulation dynamics on Little Bahama Bank by assessing trends in abundance in local populations and establishing how they interact; this will be important for assessing their conservation status in a potentially increasingly changing environment.

Climate change and cetacean health: impacts and future directions

Climate change directly impacts the foraging opportunities of cetaceans (e.g. lower prey availability), leads to habitat loss, and forces cetaceans to move to other feeding grounds. The rise in ocean temperature, low prey availability and loss of habitat can have severe consequences for cetacean survival, particularly those species that are already threatened or those with a limited habitat range. In addition, it is predicted that the concentration of contaminants in aquatic environments will increase owing to Arctic meltwater and increased rainfall events leading to higher rates of land-based runoff in downstream coastal areas. These persistent and mobile contaminants can bioaccumulate in the ecosystem, and lead to ecotoxicity with potentially severe consequences on the reproductive organs, immune system and metabolism of marine mammals. There is a need to measure and assess the cumulative impact of multiple stressors, given that climate change, habitat alteration, low prey availability and contaminants do not act in isolation. Human-caused perturbations to cetacean foraging abilities are becoming a pervasive and prevalent threat to many cetacean species on top of climate change-associated stressors. We need to move to a greater understanding of how multiple stressors impact the metabolism of cetaceans and ultimately their population trajectory. This article is part of the theme issue 'Nurturing resilient marine ecosystems'.

Detection of humpback whale (Megaptera novaeangliae) non-song vocalizations around the Vema Seamount, southeast Atlantic Ocean

Humpback whales are a cosmopolitan, highly vocal species. Investigated here are their vocalizations recorded at the Vema Seamount (31°38'S, 08°20'E) from moored hydrophones in the austral spring of 2019. During the 11-d recording period over 600 non-song calls were detected. Calls were predominantly detected at night over three consecutive days. The most common calls were low, frequency-modulated sounds (whups). An impulsive sound (gunshot) previously unknown in humpback whales was also detected. The location and timing of the calls suggests that humpback whales may be using the Vema Seamount as a temporary stop on their migration to their polar feeding grounds.

In plane sight: a mark-recapture analysis of North Atlantic right whales in the Gulf of St. Lawrence

North Atlantic right whales Eubalaena glacialis are most commonly observed along the eastern seaboard of North America; however, their distribution and occupancy patterns have become less predictable in the last decade. This study explored the individual right whales captured photographically from both dedicated and opportunistic sources from 2015 to 2019 in the Gulf of St. Lawrence (GSL), an area previously understudied for right whale presence. A total of 187 individuals, including reproductive females, were identified from all sources over this period. In years when more substantial survey effort occurred (2017-2019), similar numbers of individuals were sighted (mean = 133, SD = 1.5), and dedicated mark-recapture aerial surveys were highly effective at capturing almost all of the whales estimated in the region (2019: N = 137, 95% CI = 135-147). A high rate of inter-annual return was observed between all 5 study years, with 95% of the animals seen in 2019 sighted previously. Capture rates indicated potential residencies as long as 5 mo, and observed behaviors included feeding and socializing. Individuals were observed in the northern and southern GSL, regions divided by a major shipping corridor. Analyses suggest that individuals mostly moved less than 9.1 km d-1, although rates of up to 79.8 km d-1 were also calculated. The GSL is currently an important habitat for 40% of this Critically Endangered species, which underscores how crucial protection measures are in this area.

Killer whale (Orcinus orca) population dynamics in response to a period of rapid ecosystem change in the eastern North Atlantic

This study investigates survival and abundance of killer whales (Orcinus orca) in Norway in 1988-2019 using capture-recapture models of photo-identification data. We merged two datasets collected in a restricted fjord system in 1988-2008 (Period 1) with a third, collected after their preferred herring prey shifted its wintering grounds to more exposed coastal waters in 2012-2019 (Period 2), and investigated any differences between these two periods. The resulting dataset, spanning 32 years, comprised 3284 captures of 1236 whales, including 148 individuals seen in both periods. The best-supported models of survival included the effects of sex and time period, and the presence of transients (whales seen only once). Period 2 had a much larger percentage of transients compared to Period 1 (mean = 30% vs. 5%) and the identification of two groups of whales with different residency patterns revealed heterogeneity in recapture probabilities. This caused estimates of survival rates to be biased downward (females: 0.955 ± 0.027 SE, males: 0.864 ± 0.038 SE) compared to Period 1 (females: 0.998 ± 0.002 SE, males: 0.985 ± 0.009 SE). Accounting for this heterogeneity resulted in estimates of apparent survival close to unity for regularly seen whales in Period 2. A robust design model for Period 2 further supported random temporary emigration at an estimated annual probability of 0.148 (± 0.095 SE). This same model estimated a peak in annual abundance in 2015 at 1061 individuals (95% CI 999-1127), compared to a maximum of 731 (95% CI 505-1059) previously estimated in Period 1, and dropped to 513 (95% CI 488-540) in 2018. Our results indicate variations in the proportion of killer whales present of an undefined population (or populations) in a larger geographical region. Killer whales have adjusted their distribution to shifts in key prey resources, indicating potential to adapt to rapidly changing marine ecosystems.

Individual and population dietary specialization decline in fin whales during a period of ecosystem shift

This study sought to estimate the effect of an anthropogenic and climate-driven change in prey availability on the degree of individual and population specialization of a large marine predator, the fin whale (Balaenoptera physalus). We examined skin biopsies from 99 fin whales sampled in the St. Lawrence Estuary (Canada) over a nine year period (1998–2006) during which environmental change was documented. We analyzed stable isotope ratios in skin and fatty acid signatures in blubber samples of whales, as well as in seven potential prey species, and diet was quantitatively assessed using Bayesian isotopic models. An abrupt change in fin whale dietary niche coincided with a decrease in biomass of their predominant prey, Arctic krill (Thysanoessa spp.). This dietary niche widening toward generalist diets occurred in nearly 60% of sampled individuals. The fin whale population, typically composed of specialists of either krill or lipid-rich pelagic fishes, shifted toward one composed either of krill specialists or true generalists feeding on various zooplankton and fish prey. This change likely reduced intraspecific competition. In the context of the current “Atlantification” of northern water masses, our findings emphasize the importance of considering individual-specific foraging tactics and not only population or group average responses when assessing population resilience or when implementing conservation measures.

Up in the air: drone images reveal underestimation of entanglement rates in large rorqual whales

Entanglement in fishing gear is a significant threat to many cetaceans. For the 2 largest species, the blue whale Balaenoptera musculus and the fin whale B. physalus, reports of entangled individuals are rare, leading to the assumption that entanglements are not common. Studies of interaction with fisheries in other species often rely on the presence of scars from previous entanglements. Here, scar detection rates were first examined in humpback Megaptera novaeangliae, fin and blue whales using standard vessel-based photo-identification photographs collected between 2009 and 2016 in the Gulf of St. Lawrence, Canada. We then examined aerial images of fin whales collected with a drone in 2018 and 2019 and compared both methods. Entanglement rates were 6.5% for fin and 13.1% for blue whales using photo-identification images of individuals. Prominent scarring was observed around the tail and caudal peduncle, visible only when animals lifted those body sections above water when diving. For the small subset of pictures which captured the entire caudal peduncle, entanglement rates ranged between 60% for blue and 80% for fin whales. This result was similar to the 85% entanglement rate estimated in humpback whales. The assessment of aerial-based photography yielded an entanglement rate of 44.1 to 54.7% in fin whales. Scars were always around the peduncle, often the tail, rarely the dorsal fin and never around the pectoral fins, while the mouth cannot be examined from above. Thus, in species that do not regularly expose their tail or peduncle, aerial imagery is the preferred method to quantify entanglement rates by assessment of scars.