Welcome to the Marine Mammal Research Unit



MMRU conducts research of the highest standards to enhance marine mammal conservation and reduce conflicts with human uses of our shared oceans.



The Killers of California and Oregon
Thirteen years of photo-identification data of killer whales observed in California and Oregon provide new insights into the distribution and population structure of mammal-eating killer whales in the eastern North Pacific Ocean.
Blue herons identified as a top juvenile salmon predator

It is more than just seals that are preying on the bounty of juvenile salmon exiting river mouths each spring.

How to Power A Walrus

New study shows loss of sea ice will require walruses to swim more and eat more to survive climate change

Harbour seals respond differently to pulses of out-migrating coho and Chinook salmon smolts 

Biologging data from foraging harbour seals shows less impact on outmigrating salmon than expected. A few seals in the study…



(in press)
New colony of Guadalupe fur seals (Arctocephalus townsendi) discovered on El Farallón de San Ignacio Island, Gulf of California.
del Carmen, G.-O.M., J.M. Díaz-Gaxiola, A.W. Trites and C.J. Hernández-Camacho. (in press).
Marine Mammal Science
Guadalupe fur seals (Arctocephalus townsendi) have established a second colony in the Gulf of California on El Farallón de San Ignacio Island, along the mainland coast of Mexico. They appear to have been coming to this island since 2014, and numbered as many as 771 in 2020 (mostly juveniles). The discovery of this new haulout brings the total number of sites where this species is known to rest and breed to just four. Guadalupe fur seals remain vulnerable and may require additional protection in the southern Gulf of California to secure their future.

keywords     Guadalupe, fur seal, endangered, Mexico, El Farallón de San Ignacio Island, Gulf of California, haulout
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Southern resident killer whales encounter higher prey densities than northern resident killer whales during summer.
Sato, M., A.W. Trites and S. Gauthier. (in press).
Canadian Journal of Fisheries and Aquatic Sciences
The decline of southern resident killer whales may be due to a shortage of prey, but there is little data to test this hypothesis. We compared the availability of prey (Chinook salmon) sought by southern residents in Juan de Fuca Strait during summer with the abundance and distribution of Chinook available to the much larger and growing population of northern resident killer whales feeding in Johnstone Strait. We used ship-based multifrequency echosounders to identify differences in prey fields that may explain the dynamics of these two killer whale populations. Contrary to expectations, we found that both killer whale habitats had patchy distributions of prey that did not differ in their frequencies of occurrence, nor in the size compositions of individual fish. However, the density of fish within each patch was 4-6 times higher in the southern resident killer whale habitat. These findings do not support the hypothesis that southern resident killer whales are experiencing a prey shortage in the Salish Sea during summer and suggest a combination of other factors is affecting overall foraging success.
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The First Records of Antarctic Type B and C Killer Whales (Orcinus orcas) in Australian Coastal Waters.
Donnelly, D.M., J.D. McInnes, K.C.S. Jenner, M.N.M. Jenner and M. Morrice. 2021.
Aquatic Mammals 47(3):292-302.
Five ecotypes of killer whales occur in the southern hemisphere: Types A, B (B1 and B2), C, and D. Antarctic Type A has a circumpolar distribution around Antarctica, and are often associated with their preferred prey (minke whales, seals and penguins) in ice-free waters. Ecotype B typically occurs near or within pack ice where they predomi­nantly consume seals, as well as fish, squid, and penguins. Type C killer whales have been predominantly sighted in East Antarctica, and relatively little is known about the Type D individuals. We report seven sightings of Antarctic Type B and C killer whales in Australian coastal waters—as well as a third morphological form, closely resembling the Antarctic Type A ecotype. These records confirm that at least two of the five Antarctic eco-types described from the Southern Hemisphere also occur in Australian coastal waters.
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Body growth of North Atlantic right whales (Eubalaena glacialis) revisited.
Fortune, S. M. E., M. J. Moore, W. L. Perryman and A. W. Trites. 2021.
Marine Mammal Science 37:433-447.
Knowing size‐at‐age is important for determining food requirements and making inferences about the nutritional status of individuals and their populations. Accurate growth curves are also needed to quantify drug dosages to treat wounded or entangled animals. However, body sizes are often based on small numbers of measured animals that must be improved as new data become available. We updated an existing body growth model for North Atlantic right whales (NARWs) using new data from dead animals and from older individuals. Our models indicate that NARWs attain mean lengths and weights of 4.3 m and 1.0 mt at birth, and 13.1 m and 31.7 mt when sexually mature. Calves more than double their length and attain nearly three‐quarters of their asymptotic adult size during their first year of life. Overall, our length estimates agreed well with previous estimates, but our mass‐at‐age values were considerably higher. These differences revealed that necropsy data used alone in allometric models underestimate mass due possibly to several of the stranded animals in the database having been chronically entangled and in poor body condition. Augmenting the database with healthier individuals, such as harvested North Pacific right whales, yielded mass predictions that reflect both healthy and unhealthy individuals.

keywords     body size, Eubalaena glacialis, growth models, length, mass, morphometry, photogrammetry
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Foraging habitat of North Atlantic right whales has declined in the Gulf of St. Lawrence, Canada, and may be insufficient for successful reproduction.
Gavrilchuk, K., V. Lesage, S.M.E. Fortune, A.W. Trites, and S. Plourde. 2021.
Endangered Species Research. 44:113-136.
North Atlantic right whale (Eubalaena glacialis) population fed in the Gulf of St. Lawrence (Canada) in recent years. However, little is known about the distribution of copepods in the Gulf, and whether their abundance is sufficient to energetically sustain right whales. We used a mechanistic modeling approach to predict areas within the Gulf that have foraging potential for adult female right whales, based on the annual energetic needs of resting, pregnant, and lactating females, and their theoretical prey density requirements. We identified suitable foraging areas for right whales by coupling a foraging bioenergetics model with a 12-year data set (2006-2017) describing the abundance and three-dimensional distribution of late-stage Calanus spp. in the Gulf. Prey densities in the southern Gulf (from Shediac Valley to the Magdalen Islands) supported all three reproductive states in most (≥ 6) years. However, foraging habitat became progressively sparse in the southern Gulf over time, with noticeably less suitable habitat available after 2014. Few other potentially suitable foraging areas were identified elsewhere in the Gulf. Overall, the availability of foraging habitat in the Gulf varied considerably between years, and was higher for resting females than for pregnant and lactating females. Our findings are consistent with the recent low calving rates, and indicate that prey biomass in the Gulf of St. Lawrence may be insufficient in most years to support successful reproduction of North Atlantic right whales.

keywords     nutritional stress, bioenergetics, endangered, copepods, energy requirements, foraging, habitat, Calanus, right whale, Gulf of St Lawrence, prey density
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Transient Killer Whales of Central and Northern California and Oregon: A Catalog of Photo-Identified Individuals.
McInnes, J.D., C.R. Mathieson, P.J. West-Stap, S.L. Marcos, V.L. Wade, P.A. Olson and A.W. Trites. 2021.
NOAA Technical Memorandum NMFS-SWFSC-644:149 p.
Photo-identification studies of transient killer whales (Orcinus orcas) off western North America have primarily been conducted in the coastal inland waterways of Washington State, British Columbia, and southeastern Alaska. Less is known about transient killer whales along the outer coast and offshore waters of Oregon and central and northern California. We examined 13 years of photo-identification data to identify individuals and obtain a minimum census for this region, and to summarize information that could be useful for evaluating a hypothesis that whales using this area belong to a distinct assemblage. Data contributions came from opportunistic marine mammal surveys, whale watch ecotours, and dedicated line transect surveys. Transient killer whale photographs were obtained from 146 encounters between 2006 – 2018. These included 136 encounters in Monterey Bay, California, 5 encounters off central and northern California, and 5 encounters off Oregon. The number of unique individuals seen during this time totaled 155, of which 150 were considered to be alive (as of 2018). These included 34 adult males, 51 adult females, 24 sub-adults, and 41 juveniles. Through repeated observations of association patterns, a total of 30 matrilineal groups were identified. New whales were identified each year, including previously unidentified adults and new calves. Identification images of the dorsal fins, saddle patches and postocular patches were obtained. Details on sex, maternal ancestry, sighting history, and distribution are provided where known. These cataloged transient killer whales were predominantly encountered off the outer coast near the continental shelf break or in deep pelagic waters overlying the Monterey Submarine Canyon. The vast majority (>83 %) of whales identified in the study area could not be matched to transient killer whales in photo ID catalogs for coastal waters of the Pacific Northwest. These factors are consistent with there being a distinct “outer coast” assemblage within the west coast population of transient killer whales, but more research is needed to investigate this further.
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Resting and swimming metabolic rates in juvenile walruses (Odobenus rosmarus).
Rosen, D.A.S. 2021.
Marine Mammal Science 37:162-172.
Changes in Arctic ice conditions have raised concerns regarding potential impacts on energy expenditure and food requirements of walruses. Modelling the repercussions of environmental changes requires accurate species-specific measures of bioenergetic expenditures. This is particularly true for walruses, who have a unique anatomy and foraging ecology from other pinnipeds. This study measured resting metabolic rate (RMR) and subsurface swimming metabolism in two juvenile walruses over a 13-month period. The walruses had relatively low RMR compared to studies of other young pinnipeds. RMR was greater for the male than the female, as expected given his larger size; the reverse was true on a mass-specific basis. There was also considerable variability in RMR for each walrus during the year that could not be accounted for by changes in body mass. Metabolism while swimming was about twice RMR, and locomotor costs were higher than generally predicted for other marine mammals. The lower calculated swimming efficiency may reflect the fact that walruses are not “high velocity” pursuit predators. The estimates of metabolic expenditure obtained in this study for young walruses are invaluable for quantifying the energetic consequences of behavioral changes induced by environmental shifts in the wild.

keywords     bioenergetics, metabolism, swimming, walrus
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The effect of food restriction on growth rates in Steller sea lions, Eumetopias jubatus.
Rosen, D.A.S. 2021.
Marine Mammal Science
This study quantified the effect of changes in prey intake on the growth of individual Steller sea lions. Data from 12 female sea lions subject to various experimental episodes of restricted food intake were used to produce an overall model predicting changes in growth rates from different levels of unpredicted reductions in energy intake. The resulting equation was robust across different types and levels of restriction, seasons, and age classes. This predictive relationship between changes in food intake and growth is invaluable for incorporating into bioenergetic models estimating the effects of environmental changes on wild Steller sea lions.

keywords     Steller sea lion, food intake, growth, body mass, energy intake, bioenergetics
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Pacific great blue herons consume thousands of juvenile salmon.
Sherker, Z.T., K. Pellett, J. Atkinson, J. Damborg and A.W. Trites. 2021.
Canadian Journal of Zoology 99:349-361.
An array of predators that consume juvenile salmon may account for the poor returns of adult salmon to the Salish Sea. However, the Pacific great blue heron (Ardea herodias fannini) is rarely listed among the known salmon predators, despite being regularly seen near salmon streams. Investigating heron predation by scanning nesting sites within 35 km of three British Columbia rivers for fecal remains containing Passive Integrated Transponder (PIT) tags implanted in >100,000 juvenile salmon from 2008-2018 yielded 1,205 tags, representing a minimum annual predation rate of 0.3-1.3% of all juvenile salmon. Most of this predation (99%) was caused by ~420 adult herons from three heronries. Correcting for tags defecated outside of the heronry raised the predation rates to 0.7-3.2%-and was as high as 6% during a year of low river flow. Predation occurs during chick-rearing in late spring, and accounts for 4.1-8.4% of the heron chick diet. Smaller salmon smolts were significantly more susceptible to heron predation than larger conspecifics. The proximity of heronries relative to salmon bearing rivers is likely a good predictor of heron predation on local salmon runs, and can be monitored to assess coast-wide effects of great blue herons on salmon recovery.

keywords     predation, salmon, smolts, herons, chicks, diet
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Cardiac examinations of anesthetized Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus), and a walrus (Odobenus rosmarus).
Storlund, R.L., D.A.S. Rosen, M. Margiocco, M. Haulena and A.W. Trites. 2021.
Journal of Zoo and Wildlife Medicine 52(2):507-519.
Pinniped hearts have been well described via dissection, but in vivo measurements of cardiac structure, function, and electrophysiology are lacking. Electrocardiograms (ECGs) were recorded under anesthesia from 8 Steller sea lions (Eumetopias jubatus), 5 northern fur seals (Callorhinus ursinus), and 1 walrus (Odobenus rosmarus) to investigate cardiac electrophysiology in pinnipeds. In addition, echocardiograms were performed on all 8 anesthetized Steller sea lions to evaluate in vivo cardiac structure and function. Measured and calculated ECG parameters included P‑wave, PQ, QRS, and QT interval durations, P‑, R‑, and T‑wave amplitudes, P‑ and T‑wave polarities, and the mean electrical axis (MEA). Measured and calculated echocardiographic parameters included left ventricular internal diameter, interventricular septum thickness, and left ventricular posterior wall thickness in systole and diastole (using M-mode), left atrium and aortic root dimensions (using 2D), and maximum aortic and pulmonary flow velocities (using pulsed wave spectral Doppler). ECG measurements were similar to those reported for other pinniped species, but there was considerable variation in the MEAs of Steller sea lions and northern fur seals. Echocardiographic measurements were similar to those reported for southern sea lions (Otaria flavenscens), including 5 out of 8 Steller sea lions having a left atrial to aortic root ratio < 1, which may indicate that they have an enlarged aortic root compared to awake terrestrial mammals. Isoflurane anesthesia likely affected some of the measurements as evidenced by the reduced fractional shortening found in Steller sea lions compared to awake terrestrial mammals. The values reported are useful reference points for assessing cardiac health in pinnipeds under human care.
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Molting strategies of Arctic seals drive annual patterns in metabolism.
Thometz, N.M., H. Hermann-Sorensen, B. Russell, D.A.S. Rosen and C. Reichmuth. 2021.
Conservation Physiology coaa112
Arctic seals, including spotted (Phoca largha), ringed (Pusa hispida), and bearded (Erignathus barbatus) seals, are directly affected by sea ice loss. These species use sea ice as a haul-out substrate for various critical functions, including their annual molt. Continued environmental warming will inevitably alter the routine behavior and overall energy budgets of Arctic seals, but it is difficult to quantify these impacts as their metabolic requirements are not well known—due in part to the difficulty of studying wild individuals. Thus, data pertaining to species-specific energy demands is urgently needed to better understand the physiological consequences of rapid environmental change. We used open-flow respirometry over a four-year period to track fine-scale, longitudinal changes in the resting metabolic rate (RMR) of four spotted, three ringed, and one bearded seal trained to participate in research. Simultaneously, we collected complementary physiological and environmental data. Species-specific metabolic demands followed expected patterns based on body size, with the largest species, the bearded seal, exhibiting the highest absolute RMR (0.48±0.04 L O2 min-1) and the lowest mass-specific RMR (4.10±0.47 ml O2 min-1 kg-1), followed by spotted (absolute: 0.33±0.07 L O2 min-1; mass-specific: 6.13±0.73 ml O2 min-1 kg-1) and ringed (absolute: 0.20±0.04 L O2 min-1; mass-specific: 7.01±1.38 ml O2 min-1 kg-1) seals. Further, we observed clear and consistent annual patterns in RMR that related to the distinct molting strategies of each species. For species that molted over relatively short intervals—spotted (33±4 days) and ringed (28±6 days) seals—metabolic demands increased markedly in association with molt. In contrast, the bearded seal exhibited a prolonged molting strategy (119±2 days), which appeared to limit the overall cost of molting as indicated by a relatively stable annual RMR. These findings highlight energetic trade-offs associated with different molting strategies and provide quantitative data that can be used to assess species-specific vulnerabilities to changing conditions.

keywords     Arctic seals, spotted seal, ringed seal, bearded seal, sea ice, molt, climate change, respirometry, resting metabolic rate, energetic trade-offs
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Behavioral insights into the decline and natural history of Steller sea lions.
Trites, A.W. 2021.
In C. Campagna and R.G. Harcourt (eds), Ethology and behavioral ecology of otariids and the odobenid. Springer, Cham, Switzerland. pp. 489-419.
Two competing hypotheses were proposed to explain why Steller sea lions had declined in the Gulf of Alaska, Bering Sea and Aleutian Islands. One of the theories was that young sea lions were starving because fisheries had reduced the abundance of groundfish-the overfishing hypothesis. The other was that these low-fat species of fish had increased in abundance as the sea lion population declined following the 1976-1977 oceanic regime shift, and were compromising sea lion reproductive and survival rates-the junk-food hypothesis. Behavioral ecologists tested these hypotheses by comparing sea lion behaviors in the declining region (Gulf of Alaska and Aleutian Islands) with sea lion behaviors in an increasing region (Southeast Alaska) to determine whether the populations exhibited behavioral differences consistent with food shortages. These studies involved comparing dive depths, dive durations, time spent foraging, and time spent nursing by regions and seasons. Research also focused on weaning-a critical life-history stage-to determine when and how it occurs. Collectively, these observations and measures of behavioral responses revealed that most dependent young begin supplementing their milk diet with fish between April and May, and wean just before the start of the upcoming June breeding season. However, the proportion of young sea lions that wean at 1, 2 or 3 years of age appears to vary by year due to regional and temporal differences in the quantity and quality of prey available to them once weaned. None of the behavioral studies of adult and juvenile Steller sea lions supported the overfishing hypothesis-but were, instead, consistent with the junk-food hypothesis. It appears that lactating females that consume large amounts of low-energy fish (such as walleye pollock and Pacific cod) have a high probability of miscarriage, and will keep their dependent young for an extra one or two years-thereby causing birth rates and population size to decline. In contrast, lactating females that consume larger amounts of fattier fish (such as sand lance and Pacific herring) can successfully wean a pup every year. Plasticity in age at weaning appears to be an evolutionary adaptation to natural shifts in community prey structure in the North Pacific Ocean-and is an adaptation that successfully slows population declines of Steller sea lions until the ocean shifts to an alternative state containing greater portions of energy-rich fish that allows sea lion numbers to increase again.

keywords     Eumetopias jubatus, Life history, Population decline,Endangered, Maternal strategies, Phenology,Weaning, Abortions, Breeding, Junk food, Overfishing,Regime shifts
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