Pinnipeds, commonly known as seals, are marine mammals distributed globally across various habitats. Providing a single, fixed number for annual mortality is impossible because seals die across vast ocean areas, often sinking or being consumed before observation. Scientists rely on regional monitoring, population modeling, and data from specific threats to estimate annual deaths. These estimates show that mortality rates are highly variable, influenced by localized factors, species, and the significant impact of both natural processes and human activity.
Estimating Global and Regional Mortality
The primary challenge in estimating global seal deaths is that most mortalities go unobserved, occurring far from shore. Researchers calculate population-level mortality indirectly, using models that incorporate adult and juvenile survival rates derived from long-term monitoring studies.
When focusing on specific, quantifiable causes, the numbers become clearer. Accidental entanglement in commercial fishing gear, known as bycatch, is one of the most quantifiable human-caused threats. Conservative global estimates suggest that at least 345,000 seals and sea lions are killed annually due to this single factor. This figure highlights that total annual mortality, encompassing all natural and human-related causes, is substantially higher than any single reported estimate. Mortality rates also differ markedly between age groups, with the youngest seals facing the highest risk of death.
Leading Causes of Natural Seal Deaths
Natural mortality is highest among seal pups in their first year of life, with rates often ranging between 10% and 35% before they are weaned, depending on the species and location. The leading cause of death in stranded pups is nutritional failure, or starvation, which accounts for up to 70% of deaths in some monitored populations.
Starvation is frequently compounded by infectious agents, as poor nutrition makes young animals more susceptible to bacterial or viral diseases. Predation is another natural regulator, primarily by large sharks and killer whales, which target all age classes. Non-anthropogenic trauma, such as being washed away by high surf or succumbing to crushing injuries from adult seals, also contributes significantly to natural losses.
Major Human-Related Mortality Factors
Beyond bycatch, human activities generate other pervasive mortality factors. Disease outbreaks are increasingly linked to human impacts, particularly the rise of zoonotic viruses that spread through global wildlife populations. Recent years have seen devastating mass mortality events (MMEs) from the H5N1 Avian Influenza, which killed over 17,000 elephant seals in one colony alone, including nearly 97% of the pups.
Historically, disease events like the Phocine Distemper Virus (PDV) caused massive spikes in death, such as outbreaks in 1988 and 2002 that killed approximately 23,000 and 30,000 harbor seals in Europe. Chronic exposure to chemical pollutants, such as polychlorinated biphenyls (PCBs), may weaken the seals’ immune systems, making them more vulnerable to these diseases. Furthermore, the accumulation of marine debris and plastic poses a constant threat, causing entanglement that leads to severe injury or drowning.
Vessel strikes represent a direct form of traumatic mortality, particularly in regions with high recreational or commercial boat traffic. Climate change acts as an indirect, population-level threat, as species like the hooded seal face a loss of sea ice platforms necessary for pupping, resting, and accessing foraging areas.
Tracking Mortality: Data Collection and Limitations
Scientists arrive at mortality estimates through dedicated monitoring programs that track deaths and population health. Stranding networks play a primary role by documenting and necropsying deceased seals that wash ashore, providing direct evidence of death causes like disease, vessel strike, or starvation. This method is geographically limited, however, and only samples a fraction of all animals that die.
Population dynamics modeling is the backbone of broader mortality estimation, using data on birth rates, reproductive capacity, and survival rates to calculate how many animals must be dying to explain observed population trends. Researchers also utilize aerial surveys to count seals on haul-out sites and satellite telemetry tags to track the movements and survival of individual animals. The primary limitation across all methods is that most seals die at sea and sink, meaning observed counts and stranding data represent only a conservative minimum of the true annual mortality.