Growth in marine mammals: a review of growth patterns, composition and energy investment

Growth of structural mass and energy reserves influences individual survival, reproductive success, population and species life history. Metrics of structural growth and energy storage of individuals are often used to assess population health and reproductive potential, which can inform conservation. However, the energetic costs of tissue deposition for structural growth and energy stores and their prioritization within bioenergetic budgets are poorly documented. This is particularly true across marine mammal species as resources are accumulated at sea, limiting the ability to measure energy allocation and prioritization. We reviewed the literature on marine mammal growth to summarize growth patterns, explore their tissue compositions, assess the energetic costs of depositing these tissues and explore the tradeoffs associated with growth. Generally, marine mammals exhibit logarithmic growth. This means that the energetic costs related to growth and tissue deposition are high for early postnatal animals, but small compared to the total energy budget as animals get older. Growth patterns can also change in response to resource availability, habitat and other energy demands, such that they can serve as an indicator of individual and population health. Composition of tissues remained consistent with respect to protein and water content across species; however, there was a high degree of variability in the lipid content of both muscle (0.1-74.3%) and blubber (0.4-97.9%) due to the use of lipids as energy storage. We found that relatively few well-studied species dominate the literature, leaving data gaps for entire taxa, such as beaked whales. The purpose of this review was to identify such gaps, to inform future research priorities and to improve our understanding of how marine mammals grow and the associated energetic costs.

Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche

Many marine mammal populations are recovering after long eras of exploitation.^1,2 To what degree density-dependent body size declines in recovering species reflect a general response to increased resource competition is unknown. We examined skull size (as a proxy for body size), skull morphology, and foraging dynamics of the top marine predator, the California sea lion (Zalophus californianus), which have been steadily increasing over the last few decades and have approached or reached their carrying capacity in southern California.³ We show that, contrary to predictions, male California sea lions increased rather than decreased their average body size over a 46-year (1962-2008) recovery period. Larger males had proportionally longer oral cavities and more powerful bite strength, and their foraging niche expanded. Females between 1983 and 2007 maintained stable skull dimensions, but their isotopic niche was broader than contemporary males. Increased male body size is compatible with an intensification of density-dependent sexual selection for larger and more competitive individuals concurrent with an expanding foraging niche. High foraging variability among females would explain their body size stability during decades of population recovery. We demonstrate that body size reduction is not the universal response to population recovery in marine mammals and show that selective ecological dynamics could contribute to protecting populations against the increased density-dependent intraspecific competition. However, prey shifts associated with climate change will likely prevent California sea lions (and other marine mammals) from attaining these ecological dynamics, augmenting their vulnerability to resource competition and diminishing their capacity to overcome it.

Rapid re-establishment of top-down control at a no-take artificial reef

Establishment of artificial reefs and no-take areas are management measures available for restoring deteriorated marine ecosystems, compensating for habitat loss and strengthening harvested populations. Following the establishment of no-take artificial reefs in western Sweden to compensate for hard bottoms lost to a shipping lane, we detected rapid positive effects on crustaceans and demersal fish compared to fished reference areas. The relative abundance and size structure of European lobster (Homarus gammarus) increased strongly in the no-take area indicating more than doubled and tripled egg production in 5 and 10 years, respectively. For benthic fish and crustacean communities, the abundances of gadoids and wrasses increased and the abundances of small decapod crustaceans decreased in the no-take area, likely indicating cascading effects of increased predation. The study demonstrates that relatively small no-take areas, enhanced by artificial reefs, can rapidly invigorate populations of lobster and fish that in turn may re-initiate local top-down control.

Ten years of marine evolutionary biology-Challenges and achievements of a multidisciplinary research initiative

The Centre for Marine Evolutionary Biology (CeMEB) at the University of Gothenburg, Sweden, was established in 2008 through a 10-year research grant of 8.7 m€ to a team of senior researchers. Today, CeMEB members have contributed >500 scientific publications, 30 PhD theses and have organised 75 meetings and courses, including 18 three-day meetings and four conferences. What are the footprints of CeMEB, and how will the centre continue to play a national and international role as an important node of marine evolutionary research? In this perspective article, we first look back over the 10 years of CeMEB activities and briefly survey some of the many achievements of CeMEB. We furthermore compare the initial goals, as formulated in the grant application, with what has been achieved, and discuss challenges and milestones along the way. Finally, we bring forward some general lessons that can be learnt from a research funding of this type, and we also look ahead, discussing how CeMEB's achievements and lessons can be used as a springboard to the future of marine evolutionary biology.

An automated work-flow for pinniped surveys: A new tool for monitoring population dynamics

Detecting changes in population trends depends on the accuracy of estimated mean population growth rates and thus the quality of input data. However, monitoring wildlife populations poses economic and logistic challenges especially in complex and remote habitats. Declines in wildlife populations can remain undetected for years unless effective monitoring techniques are developed, guiding appropriate management actions. We developed an automated survey workflow using unmanned aerial vehicles (drones) to quantify the number and size of individual animals, using the well-studied Scandinavian harbour seal (Phoca vitulina) as a model species. We compared ground-based counts using telescopes with manual flights, using a zoom photo/video, and pre-programmed flights producing orthomosaic photo maps. We used machine learning to identify and count both pups and older seals and we present a new method for measuring body size automatically. We evaluate the population’s reproductive success using drone data, historical counts and predictions from a Leslie matrix population model. The most accurate and time-efficient results were achieved by performing pre-programmed flights where individual seals are identified by machine learning and their body sizes are measured automatically. The accuracy of the machine learning detector was 95–97% and the classification error was 4.6 ± 2.9 for pups and 3.1 ± 2.1 for older seals during good light conditions. There was a clear distinction between the body sizes of pups and older seals during breeding time. We estimated 320 pups in the breeding season 2021 with the drone, which is well beyond the expected number, based on historical data on pup production. The new high quality data from the drone survey confirms earlier indications of a deteriorating reproductive rate in this important harbour seal colony. We show that aerial drones and machine learning are powerful tools for monitoring wildlife in inaccessible areas which can be used to assess annual recruitment and seasonal variations in body condition.

Origin and expansion of the world's most widespread pinniped: range-wide population genomics of the harbour seal (Phoca vitulina)

The harbour seal (Phoca vitulina) is the most widely distributed pinniped, occupying a wide variety of habitats and climatic zones across the Northern Hemisphere. Intriguingly, the harbour seal is also one of the most philopatric seals, raising questions as to how it colonised virtually the whole of the Northern Hemisphere. To shed light on the origin, remarkable range expansion, population structure and genetic diversity of this species, we used genotyping-by-sequencing to analyse ~13,500 biallelic SNPs from 286 individuals sampled from 22 localities across the species' range. Our results point to a Northeast Pacific origin, colonisation of the North Atlantic via the Canadian Arctic, and subsequent stepping-stone range expansions across the North Atlantic from North America to Europe, accompanied by a successive loss of genetic diversity. Our analyses further revealed a deep divergence between modern North Pacific and North Atlantic harbour seals, with finer-scale genetic structure at regional and local scales consistent with strong philopatry. The study provides new insights into the harbour seal's remarkable ability to colonise and adapt to a wide range of habitats. Furthermore, it has implications for current harbour seal subspecies delineations and highlights the need for international and national red lists and management plans to ensure the protection of genetically and demographically isolated populations.

Life cycle bioenergetics of the gray seal (Halichoerus grypus) in the Baltic Sea: Population response to environmental stress

Wildlife population dynamics are shaped by multiple natural and anthropogenic factors, including predation, competition, stressful life history events, and external environmental stressors such as diseases and pollution. Marine mammals such as gray seals rely on extensive blubber layers for insulation and energy storage, making this tissue critical for survival and reproduction. This lipid rich blubber layer also accumulates hazardous fat soluble pollutants, such as polychlorinated biphenyls (PCBs), that can directly impact adipose function or be mobilized during periods of negative energy balance or transferred to offspring to exert further impacts on target tissues or vulnerable life stages. To predict how marine mammals will respond to ecological and anthropogenic stressors, it is necessary to use process-based modelling approaches that integrate environmental inputs, full species life history, and stressor impacts with individual dynamics of energy intake, storage, and utilization. The purpose of this study was to develop a full lifecycle dynamic energy budget and individual based model (DEB-IBM) that captured Baltic gray seal physiology and life history, and showcase potential applications of the model to predict population responses to select stressors known to threaten gray seals and other marine mammals around the world. We explore variations of three ecologically important stressors using phenomenological simulations: food limitation, endocrine disrupting chemicals that reduce fertility, and infectious disease. Using our calibrated DEB-IBM for Baltic gray seals, we found that continuous incremental food limitation can be more detrimental to population size than short random events of starvation, and further, that the effect of endocrine disruptors on population growth and structure is delayed due to bioaccumulation, and that communicable diseases significantly decrease population growth even when spillover events are relatively less frequent. One important finding is the delayed effect on population growth rate from some stressors, several years after the exposure period, resulting from a decline in somatic growth, increased age at maturation and decreased fecundity. Such delayed responses are ignored in current models of population viability and can be important in the correct assessment of population extinction risks. The model presented here provides a test bed on which effects of new hazardous substances and different scenarios of future environmental change affecting food availability and/or seal energetic demands can be investigated. Thus, the framework provides a tool for better understanding how diverse environmental stressors affect marine mammal populations and can be used to guide scientifically based management.

Delayed effects of prey fish quality and winter temperature during the birth year on adult size and reproductive rate of Baltic grey seals

Environmental conditions of mammalian juveniles may have delayed effects on their life histories and fitness, such as body size or reproductive rate later in their lives. In the present study, we tested this hypothesis on Baltic grey seals (Halichoerus grypus) and examined (1) the possible effects of prey fish quality and winter temperature on body condition of grey seal pups of both sexes and (2) the possible delayed impacts of pup environment on the body size and birth rate of adult grey seals. Body condition (blubber thickness) of especially female pups in April–May correlated negatively with winter temperatures, and body condition of male pups correlated positively with prey fish quality (herring Clupea harengus and sprat Sprattus sprattus weight). Males reached the asymptotic length at the age of 10.3 years, and body length of adult males (≥ 10 years old) was positively related to herring and sprat weight in their birth year. Females reached the asymptotic length at the age of 5.9 years. Birth rate of females (age 7–24 years) was negatively related to winter temperature in their birth year. We conclude that both changes in prey fish quality and climate may affect body condition of pups and thus also cause delayed effects on adult fitness: body size and reproductive rate of Baltic grey seals.

Human exposure to PFOS and mercury through meat from baltic harbour seals (Phoca vitulina)

The overall aim of the present study was to assess human exposure to environmental contaminants from consumption of harbour seal (Phoca vitulina) meat in the southwestern Baltic Sea. For this purpose, muscle tissue from harbour seals (n = 27) was sampled from Danish locations in the period 2005-2015 and analysed for concentrations of total mercury (Hg), organochlorine contaminants such as polychlorinated biphenyls (PCBs) and organochlorine pesticides as well as perfluoroalkyl substances (PFAS) with particular focus on perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). Hg, ∑PCB, PFOS and PFOA concentrations in the muscle tissue ranged between 0.27 and 4.76 μg g^-1 wet weight (ww; mean: 1.38 μg g^-1 ww, n = 27), 12.2-137 ng g^-1 ww (mean: 47.5 ng g^-1 ww, n = 10), 6.95-33.6 ng g^-1 ww (mean: 15.8 ng g^-1 ww, n = 10) and 0.16-0.55 ng g^-1 ww (mean: 0.28 ng g^-1 ww, n = 10), respectively. We compared the concentrations with literature-derived human tolerable weekly intake (TWI) values for mercury (1.3 μg kg^-1 week^-1), ∑PCB (2.1 μg kg^-1 week^-1), PFOS (0.013 μg kg^-1 week^-1) and PFOA (0.006 μg kg^-1 week^-1). The comparisons showed that the weekly consumption of harbour seal meat by children (weighing 30 kg), women (weighing 60 kg) and men (weighing 80 kg) should not exceed 28, 57 and 76 g (for Hg), 1.3, 2.7 and 3.5 kg (for ∑PCB), 25, 50 and 67 g (for PFOS) and 640, 1290 and 1720 g (for PFOA). In conclusion, Hg and PFOS are the contaminants of most importance in seal meat from this area with respect to existing tolerable intake rates and risks of adverse human health effects.

The necessity of a holistic approach when managing marine mammal-fisheries interactions: Environment and fisheries impact are stronger than seal predation

Seal populations are recovering in many regions around the world and, consequently, they are increasingly interacting with fisheries. We used an Ecopath with Ecosim model for the offshore Central Baltic Sea to investigate the interactions between the changes in fish stocks and grey seal (Halichoerus grypus) population under different fishing and environmental scenarios for the twenty-first century. The assumed climate, eutrophication and cod (Gadus morhua) fisheries scenarios modified seal predation impacts on fish. Fish biomass and catches are more affected by fishing mortality and the environment than by seal predation. Our results highlight that the impacts of the increasing seal population on lower trophic levels are complex; thus, we emphasize the need to consider a range of possible ecosystem contexts when evaluating potential impacts of top predators. Finally, we suggest that an increasing seal population is not likely to hinder the preservation of the main Baltic fish stocks.