Seasonal Abundance of and Estimated Food Consumption by Cetaceans in Icelandic and Adjacent Waters

Estimating energetic intake for marine mammal bioenergetic models

Bioenergetics is the study of how animals achieve energetic balance. Energetic balance results from the energetic expenditure of an individual and the energy they extract from their environment. Ingested energy depends on several extrinsic (e.g prey species, nutritional value and composition, prey density and availability) and intrinsic factors (e.g. foraging effort, success at catching prey, digestive processes and associated energy losses, and digestive capacity). While the focus in bioenergetic modelling is often on the energetic costs an animal incurs, the robust estimation of an individual's energy intake is equally critical for producing meaningful predictions. Here, we review the components and processes that affect energy intake from ingested gross energy to biologically useful net energy (NE). The current state of knowledge of each parameter is reviewed, shedding light on research gaps to advance this field. The review highlighted that the foraging behaviour of many marine mammals is relatively well studied via biologging tags, with estimates of success rate typically assumed for most species. However, actual prey capture success rates are often only assumed, although we note studies that provide approaches for its estimation using current techniques. A comprehensive collation of the nutritional content of marine mammal prey species revealed a robust foundation from which prey quality (comprising prey species, size and energy density) can be assessed, though data remain unavailable for many prey species. Empirical information on various energy losses following ingestion of prey was unbalanced among marine mammal species, with considerably more literature available for pinnipeds. An increased understanding and accurate estimate of each of the components that comprise a species NE intake are an integral part of bioenergetics. Such models provide a key tool to investigate the effects of disturbance on marine mammals at an individual and population level and to support effective conservation and management.

A regime shift in the Southeast Greenland marine ecosystem

Two major oceanographic changes have recently propagated through several trophic levels in coastal areas of Southeast Greenland (SEG). Firstly, the amount of drift-ice exported from the Fram Strait and transported with the East Greenland Current (EGC) has decreased significantly over the past two decades, and a main tipping element (summer sea ice) has virtually disappeared since 2003 leading to a regime shift in oceanographic and ecological conditions in the region. The following 20-year period with low or no coastal sea ice is unique in the 200-year history of ice observations in the region, and the regime shift is also obvious in the volume of ice export through the Fram Strait after 2013. In the same period, the temperature of the EGC south of 73.5 N has increased significantly (>2°C) since 1980. Secondly, the warm Irminger Current, which advects warm, saline Atlantic Water into the region, has become warmer since 1990. The lack of pack ice in summer together with a warming ocean generated cascading effects on the ecosystem in SEG that are manifested in a changed fish fauna with an influx of boreal species in the south and the subarctic capelin further north. At higher trophic levels there has been an increase in the abundance of several boreal cetaceans (humpback, fin, killer, and pilot whales and dolphins) that are either new to this area or occur in historically large numbers. It is estimated that the new cetacean species in SEG are responsible for an annual predation level of 700,000 tons of fish. In addition, predation on krill species is estimated at >1,500,000 tons mainly consumed by fin whales. Simultaneously, there has been a reduction in the abundance and catches of narwhals and walruses in SEG and it is suggested that these species have been impacted by the habitat changes.

Baleen whale prey consumption based on high-resolution foraging measurements

Baleen whales influence their ecosystems through immense prey consumption and nutrient recycling^1-3. It is difficult to accurately gauge the magnitude of their current or historic ecosystem role without measuring feeding rates and prey consumed. To date, prey consumption of the largest species has been estimated using metabolic models^3-9 based on extrapolations that lack empirical validation. Here, we used tags deployed on seven baleen whale (Mysticeti) species (n = 321 tag deployments) in conjunction with acoustic measurements of prey density to calculate prey consumption at daily to annual scales from the Atlantic, Pacific, and Southern Oceans. Our results suggest that previous studies^3-9 have underestimated baleen whale prey consumption by threefold or more in some ecosystems. In the Southern Ocean alone, we calculate that pre-whaling populations of mysticetes annually consumed 430 million tonnes of Antarctic krill (Euphausia superba), twice the current estimated total biomass of E. superba¹⁰, and more than twice the global catch of marine fisheries today¹¹. Larger whale populations may have supported higher productivity in large marine regions through enhanced nutrient recycling: our findings suggest mysticetes recycled 1.2 × 10⁴ tonnes iron yr^-1 in the Southern Ocean before whaling compared to 1.2 × 10³ tonnes iron yr^-1 recycled by whales today. The recovery of baleen whales and their nutrient recycling services^2,3,7 could augment productivity and restore ecosystem function lost during 20th century whaling^12,13.

Ingestion of synthetic particles by fin whales feeding off western Iceland in summer

In this study we aim to assess the daily ingestion rates of synthetic particles by the fin whales (Balaenoptera physalus) that feed off the western coast of Iceland. To do so, we collected and analysed samples from the stomach content of 25 fin whales, consisting solely of northern krill (Meganyctiphanes norvegica). The particles found consisted of fibres and fragments, mainly blue, black and red, with an average size of 1.2 ± 1.3 mm. To confirm the synthetic nature of these particles, we used Micro-Fourier Transform Infrared Spectroscopy and comparison with a polymer library. The mean concentration of synthetic particles in the krill samples found in the stomachs of whales was 0.057 particles per gram, a value much lower than that previously reported for particle uptake by krill. From this concentration in krill, we estimated that the daily intake of synthetic particles for the North Atlantic fin whale would be ranging from 38,646 ± 43,392 to 77,292 ± 86,784 particles per day. Although at this level it is not possible to assess the impact of synthetic particles and their associated chemicals on the North Atlantic fin whale population, concentrations of these contaminants are likely to increase in the future, potentially causing adverse effects on whales and other marine mammals.

Fission-fusion dynamics of a pelagic delphinid in the arctic: the white-beaked dolphin (Lagenorhynchus albirostris)

Delphinids exhibit great variability in their social structures. It is therefore important to document lesser known species, in extreme changing habitats, to compare and contrast mechanisms driving sociality. Here, we describe the first long-term assessment of social structure of white-beaked dolphins (Lagenorhynchus albirostris) using a compiled version of SOCPROG 2.8 and an 11-year photo-identification dataset (2002-2013) collected from whale-watching vessels at 2 sites (Faxaflói and Skjálfandi bays) off Iceland. We identified a total of 487 dolphins which are suggested to be part of an open population as shown by the discovery curve obtained analyzing photo-id data. The social analyses were restricted to 35 adults which were sighted on ≥5 different days. The mean residency time of white-beaked dolphins in our 2 study areas was of 95 days (SE = 35.63; 95% CI: 23-171), with the "migration-full interchange" model best describing movements of dolphins in and out of our 2 study sites. Social differentiation in this population was high (Likelihood: S = 0.87, SE = 0.04; r = 0.31, SE = 0.03) indicating diverse, non-random social relationships. Temporal associations best fit the model of "casual acquaintances" against the standardized lagged association rates with the majority of white-beaked dolphin associations being short-term, but with a few long-term across years. Testing for preferred companionship, long-term associations are favored over short-term. In this study, associations among white-beaked dolphins are short-term but with desirably long-term associations fitting into a society with fission-fusion dynamics. This information expands the latitudinal range for which social structure has been described for oceanic dolphin species.

Multi-year occurrence of sei whale calls in North Atlantic polar waters

In 2009-2014, autonomous hydrophones were deployed on established long-term moorings in the Fram Strait and Greenland Sea to record multi-year, seasonal occurrence of vocalizing cetaceans. Sei whales have rarely been observed north of ∼72°N, yet there was acoustic evidence of sei whale presence in the Fram Strait for several months during all five years of the study. More sei whale calls were recorded at the easternmost moorings in the Fram Strait, likely because of the presence of warm Atlantic water and a strong front concentrating prey in this area. Sei whale vocalizations were not recorded at the Greenland Sea 2009-2010 mooring, either because this area is not part of the northward migratory path of sei whales or because oceanographic conditions were not suitable for foraging. No clear relationship between whale presence and water temperature data collected coincident with acoustic data was observed, but decadal time series of water temperature data collected in the eastern Fram Strait by others exhibit a warming trend, which may make conditions suitable for sei whales. Continued monitoring of the region will be required to determine if the presence of sei whales in these polar waters is ephemeral or a common occurrence.

Wait your turn, North Atlantic fin whales share a common feeding ground sequentially

Highly migratory marine species pose a challenge for the identification of management units due to the absence of clear oceanographic barriers. The population structure of North Atlantic fin whales has been investigated since the start of whaling operations but is still the subject of an ongoing scientific debate. Here we measured stable isotopes of carbon, nitrogen and oxygen in skin samples collected from 151 individuals from western Iceland, Galicia (NW Spain), the Azores archipelago and the Strait of Gibraltar (SoG). We found spatiotemporal differences in stable isotope ratios suggesting that fin whales sampled in these four areas may share a common feeding ground within the Northeast Atlantic at different times during the year. Our results also suggest that SoG whales use this common feeding ground in summer but exploit Mediterranean resources during the winter months, further supporting the existence of a limited but current exchange of individuals between these two basins.

Microplastic ingestion by fish: Body size, condition factor and gut fullness are not related to the amount of plastics consumed

This study investigates the frequency of microplastic (MP) ingestion and the relationship between microplastics in the guts of two commercial fish species in Iceland (cod; Gadus morhua and saithe; Pollachius virens) and the weight, length, gut fullness, and condition index (CI) of the fish. MPs were found in 20.5% of the cod (n = 39) and 17.4% of the saithe (n = 46). There was no significant correlation between gut fullness nor CI and findings of MPs, indicating that, especially in large individuals, MPs are not retained to a large extent, and if so, the CI is most likely not affected. A difference was found in fish length between fish containing plastic and fish without plastics. Further studies such as this must be conducted in all water ecosystems if we are to fully understand the impact that MP's are having at the individual, population, species, and ecosystem levels.

Assessing the recovery of an Antarctic predator from historical exploitation

The recovery of whale populations from centuries of exploitation will have important management and ecological implications due to greater exposure to anthropogenic activities and increasing prey consumption. Here, a Bayesian population model integrates catch data, estimates of abundance, and information on genetics and biology to assess the recovery of western South Atlantic (WSA) humpback whales (Megaptera novaeangliae). Modelling scenarios evaluated the sensitivity of model outputs resulting from the use of different data, different model assumptions and uncertainty in catch allocation and in accounting for whales killed but not landed. A long period of exploitation drove WSA humpback whales to the brink of extinction. They declined from nearly 27 000 (95% PI = 22 800-33 000) individuals in 1830 to only 450 (95% PI = 200-1400) whales in the mid-1950s. Protection led to a strong recovery and the current population is estimated to be at 93% (95% PI = 73-100%) of its pre-exploitation size. The recovery of WSA humpback whales may result in large removals of their primary prey, the Antarctic krill (Euphausia superba), and has the potential to modify the community structure in their feeding grounds. Continued monitoring is needed to understand how these whales will respond to modern threats and to climate-driven changes to their habitats.

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.

Subarctic singers: Humpback whale (Megaptera novaeangliae) song structure and progression from an Icelandic feeding ground during winter

Humpback whale songs associated with breeding behaviors are increasingly reported outside of traditional low latitude breeding grounds. Songs from a subarctic feeding ground during the winter were quantitatively characterized to investigate the structure and temporal changes of the songs at such an atypical location. Recordings were collected from 26. January to 12. March, 2011, using bottom mounted recorders. Humpback songs were detected on 91% of the recording days with peak singing activities during 9.–26. February. The majority of the recordings included multiple chorusing singers. The songs were characterized by a) common static themes which transitioned consistently to predictable themes, b) shifting themes which occurred less predictably and c) rare themes. A set median sequence was found for four different periods (sets) of recordings (approximately 1 week each). The set medians were highly similar and formed a single cluster indicating that the sequences of themes sung in this area belonged to a single cluster of songs despite of the variation caused by the shifting themes. These subarctic winter songs could, thus, represent a characteristic song type for this area which is comparable to extensively studied songs from traditional low latitude breeding grounds. An increase in the number of themes per sequence was observed throughout the recording period including minor changes in the application of themes in the songs; indicating a gradual song progression. The results confirm that continual singing of sophisticated songs occur during the breeding season in the subarctic. In addition to being a well-established summer feeding ground the study area appears to be an important overwintering site for humpback whales delaying or canceling their migration where males engage in active sexual displays, i.e. singing. Importantly, such singing activity on a shared feeding ground likely aids the cultural transmission of songs in the North Atlantic.

Multi-scale habitat preference analyses for Azorean blue whales

Blue whales are sighted every year around the Azores islands, which apparently provide an important seasonal foraging area. In this paper we aim to characterize habitat preferences and analyze the temporal distribution of blue whales around São Miguel Island. To do so, we applied Generalized Additive Models to an opportunistic cetacean occurrence dataset and remotely sensed environmental data on bathymetry, sea surface temperature, chlorophyll concentration and altimetry. We provide a brief description of the oceanography of the area, emphasizing its high spatio-temporal variability. In order to capture this dynamism, we used environmental data with two different spatial resolutions (low and high) and three different temporal resolutions (daily, weekly and monthly), thus accounting for both long-term oceanographic events such as the spring bloom, and shorter-term features such as eddies or fronts. Our results show that blue whales have a well-defined ecological niche around the Azores. They usually cross the archipelago from March to June and habitat suitability is highest in dynamic areas (with high Eddy Kinetic Energy) characterized by convergence or aggregation zones where productivity is enhanced. Multi-scale studies are useful to understand the ecological niche and habitat requirements of highly mobile species that can easily react to short-term changes in the environment.

Distribution, maturity and population structure of Meganyctiphanes norvegica and Thysanoessa inermis around Iceland in spring

This study aims to explain the distribution, maturity and population structure of Meganyctiphanes norvegica and Thysanoessa inermis in springtime in relation to main hydrographic regions around Iceland: Atlantic in the southwest, Atlantic-Arctic mixture in the north and Arctic in the east. Krill were collected 14–29 May 2013 using a macrozooplankton trawl. Biomass of both species combined was significantly higher in the southwest than in north and east. M. norvegica clearly dominated in Atlantic waters, whereas T. inermis was more evenly distributed around the island, while the highest values were also observed in the southwest for this species. Simple linear regressions showed that the abundance of M. norvegica was positively related to temperature, salinity and phytoplankton concentration, while the abundance of T. inermis was negatively related to bathymetry. Multiple linear regression analyses did not add to this information of a positive relationship between abundance and temperature for M. norvegica, while T. inermis was shown to be negatively related to both temperature and bathymetry. During the latter half of May, the main spawning of both species was confined to the regions off the southwest coast. Sex ratio (males/females) of M. norvegica was higher in the southwest than in the north and east, whereas T. inermis showed a similar sex ratio all around the island. In all regions, M. norvegica appears to have a lifespan of 2 years while T. inermis of 1 year in the southwest and possibly 2 years in north and east.

The fin whale, a marine top consumer, exposes strengths and weaknesses of the use of fluoride as ecological tracer

Fluoride is retained in bone tissues of animals and its availability in the environment varies between regions according to natural and anthropogenic sources. These properties suggest this element as a suitable tracer of origin, distribution or movements of animals. In marine environments, krill builds-up fluoride concentrations that are transferred to its predators. In this study we examine the ability of bone fluoride concentrations to discriminate two separate populations of a krill consumer, the fin whale. Background levels of the sampling areas (Western Iceland and North-Western Spain) were determined through the analysis of krill samples. As expected, due to the high load of volcanic-derived fluoride in Icelandic waters, krill from W Iceland showed much higher fluoride concentrations than that from NW Spain. Concentrations in whales' bone were correlated with sex and age, increasing linearly with age in females and showing significantly lower values and a different age-related pattern of accumulation in males. Fluoride concentrations in whales' bone were much higher than in krill, indicating accumulation of the element but, rather unexpectedly, the area of origin had no influence on concentrations. This apparent contradiction may be explained either by the integration in bone of food consumed in other areas, or by the activation of homeostatic responses at very high levels of fluoride exposure. It is concluded that fluoride can be a useful tracer only if age and sex data are integrated into the analysis, year-round information on diet is available and/or the investigated population is exposed to mild levels of this element.

Consumption by marine mammals on the Northeast U.S. continental shelf

The economic and ecological impacts of fish consumption by marine mammals, the associated interactions with commercial fish stocks, and the forage demands of these marine mammal populations are largely unknown. Consumption estimates are often either data deficient or not fully evaluated in a rigorous, quantitative manner. Although consumption estimates exist for the Northeast United States (NEUS) Large Marine Ecosystem, there is considerable uncertainty in those estimates. We examined consumption estimates for 12 marine mammal species inhabiting the regional ecosystem. We used sensitivity analyses to examine metabolically driven daily individual consumption rates, resulting in a suite of feasible parameter-pair ranges for each of three taxonomic groups: mysticetes, odontocetes, and pinnipeds. We expanded daily individual consumption to annual consumption based on abundance estimates of marine mammals found on the NEUS continental shelf coupled with estimates of annual residence time for each species. To examine consumptive removals for specific prey, diet compositions were summarized into major prey categories, and predatory removals by marine mammal species as well as for total marine mammal consumption were estimated for each prey taxa. Bounds on consumption estimates for each marine mammal species were determined using Monte Carlo resampling simulations. Our results suggest that consumption for these 12 marine mammal species combined may be similar in magnitude to commercial fishery landings for small pelagic and groundfish prey groups. Consumption by marine mammals warrants consideration both as a source of mortality in assessments of prey-stocks, and to determine marine mammal forage demands in ecosystem assessment models. The approach that we present represents a rigorous, quantitative method to scope the bounds of the biomass that marine mammals are expected to consume, and is appropriate for use in other ecosystems where the interaction between marine mammals and commercial fisheries is thought to be prominent.

Acoustically invisible feeding blue whales in Northern Icelandic waters

Fixed passive acoustic monitoring can be used for long-term recording of vocalizing cetaceans. Both presence monitoring and animal density estimation requires the call rates and sound source levels of vocalizations produced by single animals. In this study, blue whale calls were recorded using acoustic bio-logging systems in Skjálfandi Bay off Húsavík, Northeast Iceland, in June 2012. An accelerometer was attached to individual whales to monitor diving behavior. During 21 h recording two individuals, 8 h 45 min and 13 h 2 min, respectively, 105 and 104 lunge feeding events and four calls were recorded. All recorded calls were down-sweep calls ranging from 105 to 48 Hz. The sound duration was 1-2 s. The source level was estimated to be between 158 and 169 dB re 1μPa rms, assuming spherical sound propagation from the possible sound source location to the tag. The observed sound production rates and source levels of individual blue whales during feeding were extremely small compared with those observed previously in breeding grounds. The feeding whales were nearly acoustically invisible. The function of calls during feeding remains unknown.

Minke whales maximise energy storage on their feeding grounds

Seasonal trends in energy storage of the minke whale (Balaenoptera acutorostrata), a capital breeder, were investigated in Iceland, a North Atlantic feeding ground. The aim was to better understand the energy acquisition strategies of minke whales and the energetic costs that different reproductive classes face during the breeding season. We modelled total blubber volume, using blubber thickness and morphometric measurements of individual whales. Blubber volume was influenced by body length, and was higher for pregnant females than mature whales. Blubber volume increased linearly through the feeding season at the same rate for mature (mean ± s.e.m.=0.0028 ± 0.00103 m(3) day(-1); N=61 male, 5 female) and pregnant whales (0.0024 ± 0.00100 m(3) day(-1); N=49), suggesting that minke whales aim to maximise energy storage while on the feeding grounds. The total amount of blubber accumulated over the feeding season (0.51 ± 0.119 m(3) for mature and 0.43 ± 0.112 m(3) for pregnant whales), together with energy stored as muscle and intra-abdominal fats, constitutes the total amount of energy available for reproduction (fetus development and lactation) on the breeding grounds, as well as migration, daily field metabolic rates, growth and body maintenance. No seasonal variation was observed for immature whales (N=4 male, 12 female), suggesting that they are investing most of their excess energy into growth rather than reproduction, in order to reach the length of sexual maturity faster and start reproducing earlier. Our novel modelling approach provides insight into large whale bioenergetics and life history strategies, as well as the relationship between single-site measurement of blubber thickness and total blubber volume.

Epibiotic macrofauna on common minke whales, Balaenoptera acutorostrata Lacépède, 1804, in Icelandic waters

BACKGROUND

Whilst there is a body of scientific literature relating to the epibiotic macrofauna on large whales, there is little information on the cetaceans in Icelandic waters. Common minke whales, Balaenoptera acutorostrata Lacépède, 1804, are a common sighting between the months of April to November, however, the migration and distribution of the population in winter requires establishing. The present study provides baseline information on the species composition, geographic distribution and abundance of the epibiotic macrofauna on minke whales landed in Icelandic waters and comments on their acquisition.

METHODS

The epibiotic macrofauna and skin lesions on 185 and 188 common minke whales respectively, landed in Icelandic waters between April to September 2003-2007 were determined. For each whale, the fluke and one lateral side was examined.

RESULTS

A total of seven epibiotic species were found: the caligid copepod Caligus elongatus (prevalence (P) = 11.9%, mean intensity (M.I) = 95.5); the pennellid copepod Pennella balaenopterae (P = 10.3%, M.I = 1.6); the cyamid amphipod Cyamus balaenopterae (P = 6.5%, M.I = 37.0); the lepadid cirripedes Conchoderma virgatum (P = 0.5%, M.I = 4.0) and Conchoderma auritum (P = 0.5%, M.I = 1.0), the balanid cirriped Xenobalanus globicipitis (P = 1.6%, M.I = 5.3) and the sea lamprey Petromyzon marinus (P = 2.7%, M.I = 1.0). In addition, the hyperparasitic monogenean Udonella caligorum was found on C. elongatus (P = 6.6%) on 8 of the 22 whales infected with the copepod. No significant relationship was observed between parasite intensity and host body length for either C. balaenopterae or C. elongatus, while the proportion of infected hosts was higher in August-September than earlier in the summer for C. balaenopterae (χ2 = 13.69; p<0.01: d.f.=1) and C. elongatus (χ2 = 28.88; p<0.01: d.f.=1).

CONCLUSIONS

The higher prevalence of C. balaenopterae on male whales (χ2 = 5.08; p<0.05: d.f.=1), suggests possible different migration routes by the sexes. A likely explanation of the occurrence of P. marinus attached to the minke whales may be due to the gradually rising sea temperature in the area in recent years. This study represents the first known record of C. elongatus on a cetacean host.

Predation on Northern krill (Meganyctiphanes norvegica Sars)

We consider predation as a function of prey concentration with a focus on how this interaction is influenced by biological-physical interactions, and wider oceanographic processes. In particular, we examine how the anti-predation behaviour of Northern krill interacts with ocean-circulation process to influence its vulnerability to predation. We describe how three-dimensional (3D) circulation interacts with in situ light levels to modulate predator-prey interactions from small to large scales, and illustrate how the stability of the predator-prey system is sometimes perturbed as a consequence. Northern krill predators include a wide range of species from the pelagic and benthic strata, as well as birds. Many exhibit adaptations in their feeding strategy to take advantage of the dynamic physical-biological processes that determine the distribution, concentration and vulnerability of Northern krill. Among them, baleen whales appear to have developed particularly efficient predation strategies. A literature search indicates that Northern krill are a major contributor to ecosystem function throughout its distributional range, and a key species with respect to the flow of energy to upper trophic levels. A list of future research needed to fill gaps in our understanding of Northern krill predator-prey interaction is provided.

Salt and water balance of modern baleen whales: rate of urine production and food intake

Whales, as pelagic marine mammals, are thought to have evolved from fresh-water-dependent terrestrial mammals. Baleen whales feed primarily on salty euphausiids (krill) and have no access to fresh water. How have these mammals adapted to lifelong habitation in a hyperosmotic medium? A new approach is proposed for studying this by using allometry (scaling) of endogenous creatinine clearance in mammals together with determinations of creatinine concentration in fresh postmortem blood and urine of fin whales (Balaenoptera physalus) and sei whales (Balaenoptera borealis). From the predicted mean creatinine-clearance values and the measured mean creatinine concentrations, a urine-production rates of 974 and 627 L/day for the fin and sei whales, respectively, were computed. Average daily krill ingestion of about 1300 and 835 L is predicted for the fin and sei whales, respectively. The whales seem to ingest about 30% more than earlier reported of a prey, which has about 50% of the salt concentration of seawater, thus maintaining the salt and water balance with a minimum of 1–2% seawater ingestion. The method used to estimate the above volumes could be a valuable tool in further studies of the water and salt balance of the large baleen whales, which may not have the same osmoregulatory control mechanisms as the smaller Odontoceti.