Mercury and stable isotope cycles in baleen plates are consistent with year-round feeding in two bowhead whale (Balaena mysticetus) populations

Food chain without giants: modelling the trophic impact of bowhead whaling on little auk populations in the Atlantic Arctic

In the Atlantic Arctic, bowhead whales (Balaena mysticetus) were nearly exterminated by European whalers between the seventeenth and nineteenth centuries. The collapse of the East Greenland-Svalbard-Barents Sea population, from an estimated 50 000 to a few hundred individuals, drastically reduced predation on mesozooplankton. Here, we tested the hypothesis that this event strongly favoured the demography of the little auk (Alle alle), a zooplanktivorous feeder competitor of bowhead whales and the most abundant seabird in the Arctic. To estimate the effect of bowhead whaling on little auk abundance, we modelled the trophic niche overlap between the two species using deterministic simulations of mesozooplankton spatial distribution. We estimated that bowhead whaling could have led to a 70% increase in northeast Atlantic Arctic little auk populations, from 2.8 to 4.8 million breeding pairs. While corresponding to a major population increase, this is far less than predicted by previous studies. Our study illustrates how a trophic shift can result from the near extirpation of a marine megafauna species, and the methodological framework we developed opens up new opportunities for marine trophic modelling.

A longitudinal study of endocrinology and foraging ecology of subadult gray whales prior to death based on baleen analysis

Individual-level assessments of wild animal health, vital rates, and foraging ecology are critical for understanding population-wide impacts of exposure to stressors. Large whales face multiple stressors, including, but not limited to, ocean noise, pollution, and ship strikes. Because baleen is a continuously growing keratinized structure, serial extraction, and quantification of hormones and stable isotopes along the length of baleen provide a historical record of whale physiology and foraging ecology. Furthermore, baleen analysis enables the investigation of dead specimens, even decades later, allowing comparisons between historic and modern populations. Here, we examined baleen of five sub-adult gray whales and observed distinct patterns of oscillations in δ15N values along the length of their baleen plates which enabled estimation of baleen growth rates and differentiation of isotopic niche widths of the whales during wintering and summer foraging. In contrast, no regular patterns were apparent in δ13C values. Prolonged elevation of cortisol in four individuals before death indicates that chronic stress may have impacted their health and survival. Triiodothyronine (T3) increased over months in the whales with unknown causes of death, simultaneous with elevations in cortisol, but both hormones remained stable in the one case of acute death attributed to killer whale predation. This parallel elevation of cortisol and T3 challenges the classic understanding of their interaction and might relate to increased energetic demands during exposure to stressors. Reproductive hormone profiles in subadults did not show cyclical trends, suggesting they had not yet reached sexual maturity. This study highlights the potential of baleen analysis to retrospectively assess gray whales' physiological status, exposure to stressors, reproductive status, and foraging ecology in the months or years leading up to their death, which can be a useful tool for conservation diagnostics to mitigate unusual mortality events.

Exploring the spatial variation of mercury in the Gulf of St. Lawrence using northern gannets as fish samplers

Mercury (Hg) is a ubiquitous and pervasive environmental contaminant with detrimental effects on wildlife, which originates from both natural and anthropogenic sources. Its distribution within ecosystems is influenced by various biogeochemical processes, making it crucial to elucidate the factors driving this variability. To explore these factors, we employed an innovative method to use northern gannets (Morus bassanus) as biological samplers of regurgitated fish in the Gulf of St. Lawrence. We assessed fish total Hg (THg) concentrations in relation to their geographical catch location as well as to pertinent biotic and anthropogenic factors. In small fish species, trophic position, calculated from compound-specific stable nitrogen isotopes in amino acids, emerged as the most influential predictor of THg concentrations. For large fish species, THg concentrations were best explained by δ13C, indicating higher concentrations in inshore habitats. No anthropogenic factors, such as pollution, shipping traffic, or coastal development, were significantly related to THg concentrations in fish. Moreover, previously published THg data in mussels sampled nearby were positively linked with THg concentrations in gannet prey, suggesting consistent mercury distribution across trophic levels in the Gulf of St. Lawrence. Our findings point to habitat-dependent variability in THg concentrations across multiple trophic levels. Our study could have many potential uses in the future, including the identification of vulnerability hotspots for fish populations and their predators, or assessing risk factors for seabirds themselves by using biologically relevant prey.

Arctic methylmercury cycling

Anthropogenic mercury (Hg) undergoes long-range transport to the Arctic where some of it is transformed into methylmercury (MeHg), potentially leading to high exposure in some Arctic inhabitants and wildlife. The environmental exposure of Hg is determined not just by the amount of Hg entering the Arctic, but also by biogeochemical and ecological processes occurring in the Arctic. These processes affect MeHg uptake in biota by regulating the bioavailability, methylation and demethylation, bioaccumulation and biomagnification of MeHg in Arctic ecosystems. Here, we present a new budget for pools and fluxes of MeHg in the Arctic and review the scientific advances made in the last decade on processes leading to environmental exposure to Hg. Methylation and demethylation are key processes controlling the pool of MeHg available for bioaccumulation. Methylation of Hg occurs in diverse Arctic environments including permafrost, sediments and the ocean water column, and is primarily a process carried out by microorganisms. While microorganisms carrying the hgcAB gene pair (responsible for Hg methylation) have been identified in Arctic soils and thawing permafrost, the formation pathway of MeHg in oxic marine waters remains less clear. Hotspots for methylation of Hg in terrestrial environments include thermokarst wetlands, ponds and lakes. The shallow sub-surface enrichment of MeHg in the Arctic Ocean, in comparison to other marine systems, is a possible explanation for high MeHg concentrations in some Arctic biota. Bioconcentration of aqueous MeHg in bacteria and algae is a critical step in the transfer of Hg to top predators, which may be dampened or enhanced by the presence of organic matter. Variable trophic position has an important influence on MeHg concentrations among populations of top predator species such as ringed seal and polar bears distributed across the circumpolar Arctic. These scientific advances highlight key processes that affect the fate of anthropogenic Hg deposited to Arctic environments.

Temporal Baseline of Essesntial and Non-essential Elements Recorded in Baleen of Western Arctic Bowhead Whale (Balaena mysticetus)

This study established the first baseline of changing elemental concentrations in bowhead whale baleen over time (1958-1999). From previously published stable isotope data, year, season (summer or winter), and location (Beaufort or Bering/Chukchi seas) were attributed to each sample. Thirteen elements (Al, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, V, Zn) in baleen from nine subsistence-harvested bowhead whales (n = 138) were detected. Al, Cu, and Fe were the highest concentrations while Cd and V were among the lowest. Our data supports absorption as the main route of exposure to environmental elements rather than biomagnification due to bowhead whales' low trophic position. A linear mixed-effects model confirmed most elements' concentrations increased with time, while location and sex were insignificant explanatory factors. These temporal fluctuations were most likely a product of environmental changes due to a warming climate and human activities.

Case studies on longitudinal mercury content in humpback whale (Megaptera novaeangliae) baleen

Quantification of contaminant concentrations in baleen whales is important for individual and population level health assessments but is difficult due to large migrations and infrequent resighings. The use of baleen allows for a multiyear retrospective analysis of contaminant concentrations without having to collect repeated samples from the same individual. Here we provide case studies of mercury analysis using cold vapor atomic absorption spectroscopy in three individual humpback whales (Megaptera novaeangliae), a 44.5-year-old female and two males aged ≥35 and 66 years, over approximately three years of baleen growth. Mercury concentrations in the female's baleen were consistently 2-3 times higher than in either male. Age did not affect mercury concentrations in baleen; the younger male had comparable levels to the older male. In the female, mercury concentrations in the baleen did not change markedly during pregnancy but mercury did spike during the first half of lactation. Stable isotope profiles suggest that diet likely drove the female's high mercury concentrations. In conclusion, variations in baleen mercury content can be highly individualistic. Future studies should compare sexes as well as different populations and species to determine how the concentrations of mercury and other contaminants vary by life history parameters and geography.

Bowhead whales use two foraging strategies in response to fine-scale differences in zooplankton vertical distribution

As zooplanktivorous predators, bowhead whales (Balaena mysticetus) must routinely locate patches of prey that are energy-rich enough to meet their metabolic needs. However, little is known about how the quality and quantity of prey might influence their feeding behaviours. We addressed this question using a new approach that included: (1) multi-scale biologging and unmanned aerial system observations of bowhead whales in Cumberland Sound, Nunavut (Canada), and (2) an optical plankton counter (OPC) and net collections to identify and enumerate copepod prey species through the water column. The OPC data revealed two prey layers comprised almost exclusively of lipid-rich calanoid copepods. The deep layer contained fewer, but larger, particles (10% greater overall biomass) than the shallow prey layer. Dive data indicated that the whales conducted long deep Square-shaped dives (80% of dives; averaging depth of 260.4 m) and short shallow Square-shaped dives (16%; averaging depth of 22.5 m) to feed. The whales tended to dive proportionally more to the greater biomass of zooplankton that occurred at depth. Combining behavioural recordings with prey sampling showed a more complex feeding ecology than previously understood, and provides a means to evaluate the energetic balance of individuals under current environmental conditions.

Spatial and temporal variability in ringed seal (Pusa hispida) stable isotopes in the Beaufort Sea

Arctic ecosystem dynamics are shifting in response to warming temperatures and sea ice loss. Such ecosystems may be monitored by examining the diet of upper trophic level species, which varies with prey availability. To assess interannual variation in the Beaufort Sea ecosystem, we examined spatial and temporal trends in ringed seal (Pusa hispida) δ13C and δ15N in claw growth layers grown from 1964 to 2011. Stable isotopes were correlated with climate indices, environmental conditions, seal population productivity, and geographic location. Sex and age did not influence stable isotopes. Enriched 13C was linked to cyclonic circulation regimes, seal productivity, and westward sampling locations. Higher δ15N was linked to lower sea surface temperatures, a higher percentage of pups in the subsistence harvest, and sample locations that were eastward and further from shore. From the 1960s to 2000s, ringed seal niche width expanded, suggesting a diversification of diet due to expansion of prey and/or seal space use. Overall, trends in ringed seal stable isotopes indicate changes within the Beaufort Sea ecosystem affected by water temperatures and circulation regimes. We suggest that continued monitoring of upper trophic level species will yield insights into changing ecosystem structure with climate change.

Methylmercury exposure in wildlife: A review of the ecological and physiological processes affecting contaminant concentrations and their interpretation

Exposure to methylmercury (MeHg) can result in detrimental health effects in wildlife. With advances in ecological indicators and analytical techniques for measurement of MeHg in a variety of tissues, numerous processes have been identified that can influence MeHg concentrations in wildlife. This review presents a synthesis of theoretical principals and applied information for measuring MeHg exposure and interpreting MeHg concentrations in wildlife. Mercury concentrations in wildlife are the net result of ecological processes influencing dietary exposure combined with physiological processes that regulate assimilation, transformation, and elimination. Therefore, consideration of both physiological and ecological processes should be integrated when formulating biomonitoring strategies. Ecological indicators, particularly stable isotopes of carbon, nitrogen, and sulfur, compound-specific stable isotopes, and fatty acids, can be effective tools to evaluate dietary MeHg exposure. Animal species differ in their physiological capacity for MeHg elimination, and animal tissues can be inert or physiologically active, act as sites of storage, transformation, or excretion of MeHg, and vary in the timing of MeHg exposure they represent. Biological influences such as age, sex, maternal transfer, and growth or fasting are also relevant for interpretation of tissue MeHg concentrations. Wildlife tissues that represent current or near-term bioaccumulation and in which MeHg is the predominant mercury species (such as blood and eggs) are most effective for biomonitoring ecosystems and understanding landscape drivers of MeHg exposure. Further research is suggested to critically evaluate the use of keratinized external tissues to measure MeHg bioaccumulation, particularly for less-well studied wildlife such as reptiles and terrestrial mammals. Suggested methods are provided to effectively use wildlife for quantifying patterns and drivers of MeHg bioaccumulation over time and space, as well as for assessing the potential risk and toxicological effects of MeHg on wildlife.

Amino acid δ15N underestimation of cetacean trophic positions highlights limited understanding of isotopic fractionation in higher marine consumers

Compound-specific stable isotope analysis (CSIA) of amino acids (AAs) has been rapidly incorporated in ecological studies to resolve consumer trophic position (TP). Differential 15N fractionation of "trophic" AAs, which undergo trophic 15N enrichment, and "source" AAs, which undergo minimal trophic 15N enrichment and serve as a proxy for primary producer δ15N values, allows for internal calibration of TP. Recent studies, however, have shown the difference between source and trophic AA δ15N values in higher marine consumers is less than predicted from empirical studies of invertebrates and fish. To evaluate CSIA-AA for estimating TP of cetaceans, we compared source and trophic AA δ15N values of multiple tissues (skin, baleen, and dentine collagen) from five species representing a range of TPs: bowhead whales, beluga whales, short-beaked common dolphins, sperm whales, and fish-eating (FE) and marine mammal-eating (MME) killer whale ecotypes. TP estimates (TPCSIA) using several empirically derived equations and trophic discrimination factors (TDFs) were 1-2.5 trophic steps lower than stomach content-derived estimates (TPSC) for all species. Although TPCSIA estimates using dual TDF equations were in better agreement with TPSC estimates, our data do not support the application of universal or currently available dual TDFs to estimate cetacean TPs. Discrepancies were not simply due to inaccurate TDFs, however, because the difference between consumer glutamic acid/glutamine (Glx) and phenylalanine (Phe) δ15N values (δ15NGlx-Phe) did not follow expected TP order. In contrast to pioneering studies on invertebrates and fish, our data suggest trophic 15N enrichment of Phe is not negligible and should be examined among the potential mechanisms driving "compressed" and variable δ15NGlx-Phe values at high TPs. We emphasize the need for controlled diet studies to understand mechanisms driving AA-specific isotopic fractionation before widespread application of CSIA-AA in ecological studies of cetaceans and other marine consumers.

Stable isotopes reveal winter feeding in different habitats in blue, fin and sei whales migrating through the Azores

Knowing the migratory movements and behaviour of baleen whales is fundamental to understanding their ecology. We compared δ¹⁵N and δ¹³C values in the skin of blue (Balaenoptera musculus), fin (Balaenoptera physalus) and sei (Balaenoptera borealis) whales sighted in the Azores in spring with the values of potential prey from different regions within the North Atlantic using Bayesian mixing models to investigate their trophic ecology and migration patterns. Fin whale δ¹⁵N values were higher than those recorded in blue and sei whales, reflecting feeding at higher trophic levels. Whales' skin δ¹⁵N and δ¹³C values did not reflect prey from high-latitude summer foraging grounds; instead mixing models identified tropical or subtropical regions as the most likely feeding areas for all species during winter and spring. Yet, differences in δ¹³C values among whale species suggest use of different regions within this range. Blue and sei whales primarily used resources from the Northwest African upwelling and pelagic tropical/subtropical regions, while fin whales fed off Iberia. However, determining feeding habitats from stable isotope values remains difficult. In conclusion, winter feeding appears common among North Atlantic blue, fin and sei whales, and may play a crucial role in determining their winter distribution. A better understanding of winter feeding behaviour is therefore fundamental for the effective conservation of these species.

How do baleen whales stow their filter? A comparative biomechanical analysis of baleen bending

Bowhead and right whale (balaenid) baleen filtering plates, longer in vertical dimension (≥3-4 m) than the closed mouth, presumably bend during gape closure. This has not been observed in live whales, even with scrutiny of video-recorded feeding sequences. To determine what happens to the baleen during gape closure, we conducted an integrative, multifactorial study including materials testing, functional (flow tank and kinematic) testing and histological examination. We measured baleen bending properties along the dorsoventral length of plates and anteroposterior location within a rack of plates via mechanical (axial bending, composite flexure, compression and tension) tests of hydrated and air-dried tissue samples from balaenid and other whale baleen. Balaenid baleen is remarkably strong yet pliable, with ductile fringes, and low stiffness and high elasticity when wet; it likely bends in the closed mouth when not used for filtration. Calculation of flexural modulus from stress/strain experiments shows that the balaenid baleen is slightly more flexible where it emerges from the gums and at its ventral terminus, but kinematic analysis indicates plates bend evenly along their whole length. Fin and humpback whale baleen has similar material properties but less flexibility, with no dorsoventral variation. The internal horn tubes have greater external and hollow luminal diameter but lower density in the lateral relative to medial baleen of bowhead and fin whales, suggesting a greater capacity for lateral bending. Baleen bending has major consequences not only for feeding morphology and energetics but also for conservation given that entanglement in fishing gear is a leading cause of whale mortality.