Trophic niche partitioning and diet composition of sympatric fin (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) in the Gulf of Alaska revealed through stable isotope analysis

Fin and humpback whales are large consumers that are often sympatric, effectively sharing or partitioning their use of habitat and prey resources. Stable carbon and nitrogen isotopes in the skin of fin and humpback whales from two regions in the western Gulf of Alaska, Kodiak, and Shumagin Islands, were analyzed to test the hypothesis that these sympatric baleen whales exhibit trophic niche partitioning within these regions. Standard ellipse areas, estimated using Bayesian inference, suggested that niche partitioning between species is occurring in the Kodiak region but not in the Shumagin Islands. Isotopic mixing models based on stable isotopes from whales and local prey samples, were used to estimate possible diet solutions for whales in the Kodiak region. Comparison of isotopic niches and diet models support niche partitioning, with fin whales foraging primarily on zooplankton and humpback whales foraging on zooplankton and small forage fish. The results of this study show that niche partitioning between sympatric species can vary by region and may be the result of prey availability, prey preferences, or both.     

The diets of humpback whales (Megaptera novaeangliae) on the shelf and oceanic feeding grounds in the western North Pacific inferred from stable isotope analysis

Humpback whales feed on a variety of prey, but significant differences likely occur between regional feeding grounds. In this study, the diets of humpback whales were analyzed by comparing stable isotope ratios in animal tissues at three humpback whale feeding grounds in the Russian Far East: Karaginsky Gulf, Anadyr Gulf, and the Commander Islands. Anadyr Gulf is a neritic zone far from a shelf break, Karaginsky Gulf is a neritic zone close to a shelf break, and the Commander Islands represent an open oceanic ecosystem where whales feed off the shelf break. Samples from the Commander Islands had the lowest mean δ¹³C and δ¹⁵N values (mean ± SE: δ¹³C = ?18.7 ± 0.1, δ¹⁵N = 10.4 ± 0.1) compared to the samples from Karaginsky Gulf (δ¹³C = ?17.2 ± 0.1, δ¹⁵N = 12.7 ± 0.2) and Anadyr Gulf (δ¹³C= ?17.8 ± 0.1, δ¹⁵N = 14.0 ± 0.4). The samples from Anadyr Gulf had the highest δ¹⁵N values, while the samples from Karaginsky Gulf had the highest δ¹³C values. Both δ¹³C and δ¹⁵N values differed significantly among all three areas. Our data support the hypothesis that humpback whales tend to feed on fish in neritic areas and on plankton in deep oceanic waters.     

The effect of close approaches for tagging activities by small research vessels on the behavior of humpback whales (Megaptera novaeangliae)

Small research vessels are often used as platforms for tagging activities to collect behavioral data on cetaceans and they have the potential to disturb that group or individual. If this disturbance is ignored, results and conclusions produced by that study could be inaccurate. Here land‐based behavioral data of migrating humpback whales (Megaptera novaeangliae) (n = 29) were used to determine the effect of close approaches for tagging by research vessels on their diving, movement and surface behaviors. Groups of whales were tagged, using digital recording tags, by small research vessels, as part of a behavioral response study. In groups that were approached for tagging, temporary changes in movement behaviors during close approaches were found, with subsequent recovery to “pre‐approach” levels. In female‐calf groups more long‐term changes in travel speed were found. Results suggest that, although close approaches for tagging by small vessels may cause behavioral changes in humpback whales, this change may be small and temporary. However, in female‐calf groups, the behavioral change may be greater and longer lasting. This study shows that when using small vessels for behavioral research, disturbance, and recovery should be measured to ensure integrity of data used for other analyses.     

Genetic differentiation between humpback whales (Megaptera novaeangliae) from Atlantic and Pacific breeding grounds of South America

Humpback whales wintering in tropical waters along the Atlantic and Pacific coasts of the South American continent are thought to represent distinct populations or “stocks.” Here we present the first analysis of genetic differentiation and estimates of gene flow between these breeding stocks, based on both mitochondrial DNA (mtDNA) control region sequences (465 bp) and 16 microsatellite loci from samples collected off Brazil (n = 277) and Colombia (n = 148), as well as feeding areas near the western Antarctic Peninsula (n = 86). We found significant differentiation between Brazilian and Colombian breeding grounds at both mtDNA (F_ST = 0.058) and microsatellite (F_ST = 0.011) markers and corroborated previous studies showing genetic similarity between humpbacks from Colombia and those from Antarctic Peninsula feeding areas. Estimates of long‐term gene flow between Brazil and Colombia were low to moderate, asymmetrical, and mostly mediated by males. Assignment procedures detected some cases of interchange and individuals of admixed ancestry between breeding grounds, indicating limited mixing of individuals between these stocks. Overall, results highlight the differentiation of humpback whale breeding populations with adjacent feeding grounds. This appears to be a remarkable example of fidelity to seasonal habitat in the absence of any contemporary barriers.     

Evidence for the functions of surface‐active behaviors in humpback whales (Megaptera novaeangliae)

As part of their social sound repertoire, migrating humpback whales (Megaptera novaeangliae) perform a large variety of surface‐active behaviors, such as breaching and repetitive slapping of the pectoral fins and tail flukes; however, little is known about what factors influence these behaviors and what their functions might be. We investigated the potential functions of surface‐active behaviors in humpback whale groups by examining the social and environmental contexts in which they occurred. Focal observations on 94 different groups of whales were collected in conjunction with continuous acoustic monitoring, and data on the social and environmental context of each group. We propose that breaching may play a role in communication between distant groups as the probability of observing this behavior decreased significantly when the nearest whale group was within 4,000 m compared to beyond 4,000 m. Involvement in group interactions, such as the splitting of a group or a group joining with other whales, was an important factor in predicting the occurrence of pectoral, fluke, and peduncle slapping, and we suggest that they play a role in close‐range or within‐group communication. This study highlights the potentially important and diverse roles of surface‐active behaviors in the communication of migrating humpback whales.     

The potential beginning of a postwhaling recovery in New Zealand humpback whales (Megaptera novaeangliae)

Between the 1940s and 1970s Southern Hemisphere populations of humpback whales (including eastern Australia and Oceania populations) were hunted to near extinction by extensive commercial whaling activities in Antarctica, with fewer whales taken in shore whaling operations in New Zealand, Australia (including Norfolk Island) and Tonga. Variable rates of recovery of these populations have been documented, ranging from recovery to prewhaling numbers in eastern Australian humpbacks to very little sign of recovery in many Oceania populations. Here we analyze recent sighting data collected over 12 yr, from annual surveys in Cook Strait, New Zealand. The data show an increase in sightings, at an estimated rate of 13% (95% CI of 4.9% and 21.7%) in 2015, of humpback whales migrating through Cook Strait. The wide confidence intervals preclude substantive conclusions about the rate of increase but suggest humpback whales are returning to this region in increasing numbers, indicating an influx of immigrants from neighboring areas, namely eastern Australia.     

Factors driving the variability in diving and movement behavior of migrating humpback whales (Megaptera novaeangliae): Implications for anthropogenic disturbance studies

Humpback whales (Megaptera novaeangliae) undertake one of the longest migrations of any animal and while on a broad‐scale this journey appears direct, on a fine‐scale, behaviors associated with socializing and breeding are regularly observed. However, little is known about which social and environmental factors influence behavior during this time. Here we examined the effect of multiple factors on the movement (speed and course) and diving behavior (dive and surfacing duration) of humpback whales during migration off the eastern coast of Australia. Focal data (202 h) were collected on 94 different whale groups with simultaneous social and environmental context data. The environmental factors water depth and wind speed were found to be important predictors of dive and movement behavior, whereas social factors were less influential at this site. Groups tended to dive for longer with increased water depth but traveled more slowly in increasing wind speeds. These baseline studies are crucial when examining the effect of anthropogenic disturbance. Determining which natural factors significantly affect behavior ensures any observed behavioral changes are correctly attributed to the disturbance and are not a result of other factors. In addition, any responses observed can be put into biological context and their relative magnitude determined.     

Immature euphausiids do not appear to be prey for humpback whales (Megaptera novaeangliae) during spring and summer in Southeast Alaska

Humpbacks whales (Megaptera novaeangliae) have shown a remarkable recovery in the North Pacific, raising concerns regarding their impact on marine communities. In Southeast Alaska, humpbacks feed heavily on euphausiids; however, it remains unclear whether they target immature individuals despite evidence that they do so elsewhere. I evaluate the hypothesis that humpbacks target immature euphausiids in late spring‐summer in Southeast Alaska. Plankton samples were collected at random sites (n = 44) and near whales (n = 53) between 8 June and 9 September 2008 in Frederick Sound and Stephens Passage. The proportion of samples containing immature euphausiids, and immature euphausiid abundance within those samples, were compared between the two sample types. Similar analyses were conducted for adult euphausiids (prey) and calanoid copepods (nonprey) for comparison. I found no statistical difference between the whale and random samples with respect to the occurrence or numerical density of immature euphausiids, which is consistent with the hypothesis that whales did not target them in 2008. Smaller size, insufficient numerical densities and lower energy density of immature euphausiids are suggested as possible reasons. These findings can assist in resolving regional humpback abundance and distribution patterns, and can contribute to an understanding of the trophic interactions characterizing the local ecosystem.     

Sex‐related site fidelity of humpback whales (Megaptera novaeangliae) to the Fueguian Archipelago feeding area,Chile

We investigated sex‐related site fidelity by humpback whales to the Fueguian Archipelago, a new feeding area in the eastern South Pacific, by examining the resighting histories of 45 males and 39 females recorded from 2003 to 2012. Results indicated an overall annual return to the feeding area of 74.8%, and annual sex ratio is roughly equal in the population. The probability of an individual being resighted across years and in subsequent years was not significantly different for both males and females, however, the proportion of resighting within a year was significantly higher for individual males compared to females. Potential sources of sex‐related bias were analyzed, but none were found to be significant. Greater intraannual resighting frequency for males may reflect sex‐based differences in spatial occupation and short‐range movements due to potential differences in energy budgets.     

Global diversity and oceanic divergence of humpback whales (Megaptera novaeangliae)

Humpback whales (Megaptera novaeangliae) annually undertake the longest migrations between seasonal feeding and breeding grounds of any mammal. Despite this dispersal potential, discontinuous seasonal distributions and migratory patterns suggest that humpbacks form discrete regional populations within each ocean. To better understand the worldwide population history of humpbacks, and the interplay of this species with the oceanic environment through geological time, we assembled mitochondrial DNA control region sequences representing approximately 2700 individuals (465 bp, 219 haplotypes) and eight nuclear intronic sequences representing approximately 70 individuals (3700 bp, 140 alleles) from the North Pacific, North Atlantic and Southern Hemisphere. Bayesian divergence time reconstructions date the origin of humpback mtDNA lineages to the Pleistocene (880 ka, 95% posterior intervals 550–1320 ka) and estimate radiation of current Northern Hemisphere lineages between 50 and 200 ka, indicating colonization of the northern oceans prior to the Last Glacial Maximum. Coalescent analyses reveal restricted gene flow between ocean basins, with long-term migration rates (individual migrants per generation) of less than 3.3 for mtDNA and less than 2 for nuclear genomic DNA. Genetic evidence suggests that humpbacks in the North Pacific, North Atlantic and Southern Hemisphere are on independent evolutionary trajectories, supporting taxonomic revision of M. novaeangliae to three subspecies.     

Low levels of genetic differentiation characterize Australian humpback whale (Megaptera novaeangliae) populations

Humpback whales undertake long‐distance seasonal migrations between low latitude winter breeding grounds and high latitude summer feeding grounds. We report the first in‐depth population genetic study of the humpback whales that migrate to separate winter breeding grounds along the northwestern and northeastern coasts of Australia, but overlap on summer feeding grounds around Antarctica. Weak but significant differentiation between eastern and western Australia was detected across ten microsatellite loci (F_ST = 0.005, P = 0.001; D_EST = 0.031, P = 0.001, n = 364) and mitochondrial control region sequences (F_ST = 0.017 and Φ_ST = 0.069, P = 0.001, n = 364). Bayesian clustering analyses using microsatellite data could not resolve any population structure unless sampling location was provided as a prior. This study supports the emerging evidence that weak genetic differentiation is characteristic among neighboring Southern Hemisphere humpback whale breeding populations. This may be a consequence of relatively high gene flow facilitated by overlapping summer feeding areas in Antarctic waters.     

Evidence of two subaggregations of humpback whales on the Kodiak,Alaska, feeding ground revealed from stable isotope analysis

Knowledge of humpback whale (Megaptera novaeangliae) foraging on feeding grounds is becoming increasingly important as the growing North Pacific population recovers from commercial whaling and consumes more prey, including economically important fishes. We explored spatial and temporal (interannual, within‐season) variability in summer foraging by humpback whales along the eastern side of the Kodiak Archipelago as described by stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios of humpback whale skin (n = 118; 2004–2013). The trophic level (TL) of individual whales was calculated using basal food web δ¹⁵N values collected within the study area. We found evidence for the existence of two subaggregations of humpback whales (“North,” “South”) on the feeding ground that fed at different TLs throughout the study period. Linear mixed models suggest that within an average year, Kodiak humpback whales forage at a consistent TL during the feeding season. TL estimates support mixed consumption of fish and zooplankton species in the “North” (mean ± SE; 3.3 ± 0.1) and predominant foraging on zooplankton in the “South” (3.0 ± 0.1). This trend appears to reflect spatial differences in prey availability, and thus, our results suggest North Pacific humpback whales may segregate on feeding aggregations and target discrete prey species.     

Southeastern Pacific humpback whales (Megaptera novaeangliae) and their breeding grounds: Distribution and habitat preference of singers and social groups off the coast of Ecuador

Understanding the distribution, habitat preference, and social structure of highly migratory species at important life history stages (e.g., breeding and calving) is essential for conservation efforts. We investigated the spatial distribution and habitat preference of humpback whale social groups and singers, in relation to depth categories (<20 m, 20–50 m, and >50 m) and substrate type (muddy and mixed) on a coastal southeastern Pacific breeding ground. One hundred and forty‐three acoustic stations and 304 visual sightings were made at the breeding ground off the coast of Esmeraldas, Ecuador. Spatial autocorrelation analysis suggested singers were not randomly distributed, and Neu's method and Monte Carlo simulations indicated that singers frequented depths of <20 m and mixed substrate. Singletons, and groups with a calf displayed a preference for shallower waters (0–20 m), while pairs and groups with a calf primarily inhabited mixed bottom substrates. In contrast, competitive groups showed no clear habitat preference and exhibited social segregation from other whales. Understanding the habitat preference and distribution of humpback whales on breeding and calving grounds vulnerable to anthropogenic disturbance provides important baseline information that should be incorporated into conservation efforts at a regional scale.     

Photo‐identification matches of humpback whales (Megaptera novaeangliae) from feeding areas in Russian Far East seas and breeding grounds in the North Pacific

Humpback whales migrate seasonally from high latitude feeding areas to lower latitude breeding areas for mating and calving. In 2004–2006, a North Pacific basin‐wide study called SPLASH was conducted as an international collaboration among various groups of researchers. The Russian Far East consists of multiple high latitude feeding areas and during SPLASH, 102 whales were identified and compared to catalogs from breeding areas. Our goal in this study was to further investigate the migratory destinations of whales from the Russian Far East using a total of 1,459 photographs of whales identified from 2004 to 2014. We compared the latest Russian catalog with the SPLASH catalog from wintering areas (2004–2006) and with two additional regional catalogs from Okinawa (1989–2006) and the northern Philippines (2000–2006). We found a total of 152 matches: 106 with Asian, 35 with Hawaiian, and 11 with Mexican breeding grounds. The match rate was higher in mainland Kamchatka and consisted mostly of whales from the Asian breeding ground. In the Commander Islands, the proportion of whales from Asia was twice that of Hawaii and six times higher than Mexico. The total match rate was low, supporting the hypothesis of some undiscovered humpback whale breeding location in the North Pacific.     

Diet of harbor porpoises along the Dutch coast: A combined stable isotope and stomach contents approach

High stranding frequency of porpoises, Phocoena phocoena, along the Dutch coast since 2006 has led to increased interest in the ecology of porpoises in the North Sea. Stranded porpoises were collected along the Dutch coast (2006–2008) and their diet was assessed through stomach content and stable isotope analysis (δ¹³C and δ¹⁵N) of porpoise muscle and prey. Stable isotope analysis (SIAR) was used to estimate the contribution of prey species to the porpoises' diet. This was compared to prey composition from stomach contents, to analyze differences between long‐ and short‐term diet. According to stomach contents, 90.5% of the diet consisted of gobies, whiting, lesser sandeel, herring, cod, and sprat. Stable isotope analysis revealed that 70‐83% of the diet consisted of poor cod, mackerel, greater sandeel, lesser sandeel, sprat, and gobies, highlighting a higher importance of pelagic, schooling species in the porpoises' diet compared to stomach contents. This could be due to prey distribution as well as differences in behavior of porpoises and prey between the coastal zone and offshore waters. This study supports the need for multi‐method approaches. Future ecological and fishery impact assessment studies and management decisions for porpoise conservation should acknowledge this difference between the long‐ and short‐term diet.     

'Megapclicks': acoustic click trains and buzzes produced during night-time foraging of humpback whales (Megaptera novaeangliae)

Humpback whales (Megaptera novaeangliae) exhibit a variety of foraging behaviours, but neither they nor any baleen whale are known to produce broadband clicks in association with feeding, as do many odontocetes. We recorded underwater behaviour of humpback whales in a northwest Atlantic feeding area using suction-cup attached, multi-sensor, acoustic tags (DTAGs). Here we describe the first recordings of click production associated with underwater lunges from baleen whales. Recordings of over 34000 'megapclicks' from two whales indicated relatively low received levels at the tag (between 143 and 154dB re 1 microPa pp), most energy below 2kHz, and interclick intervals often decreasing towards the end of click trains to form a buzz. All clicks were recorded during night-time hours. Sharp body rolls also occurred at the end of click bouts containing buzzes, suggesting feeding events. This acoustic behaviour seems to form part of a night-time feeding tactic for humpbacks and also expands the known acoustic repertoire of baleen whales in general.     

Population trends for humpback whales (Megaptera novaeangliae) foraging in the Francisco Coloane Coastal‐Marine Protected Area,Magellan Strait,Chile

In 2003 a feeding aggregation of southeastern Pacific humpback whales (Megaptera novaeangliae) was reported in the Magellan Strait. While Chile established its first marine national park in the Strait to protect humpback whale habitat, fatal ship strikes remain a concern because of overlap with a busy shipping lane. To better understand population risk, we estimated abundance and survival for this population using Bayesian robust‐design mark‐recapture models fit to photographic data from 2004 to 2016. Overall, the model estimated a total of 204 whales (95% CI: 199–210) during the last 12 yr, and 93 (95% CI: 86–100) in the 2016/2017 austral summer. The population grew at 2.3% (CI: 2.1%–3.1%), an annual increase of two whales. Annual survival (including calves) was estimated at 0.892 (CI: 0.871–0.910). Our results corroborate a persistent feeding population, but one that is increasing relatively slowly. Owing to its vulnerability stemming from its small size, coupled with significant overlap with a busy shipping lane, we argue this subpopulation is at significant risk from ship strikes and may be one of the few populations where anthropogenic mortalities could regulate population dynamics. We therefore encourage continued monitoring via photographic mark‐resighting surveys, and analyses explicitly investigating potential population‐level ship strike effects.