Threats to Cetaceans: There’s More than Meets the Eye

Paper: Díaz-Delgado J, Fernández A, Sierra E, Sacchini S, Andrada M, Vela AI, et al. (2018) Pathologic findings and causes of death of stranded cetaceans in the Canary Islands (2006- 2012). PLoS ONE 13(10): e0204444. https://doi. org/10.1371/journal.pone.0204444

Rapid global development paired with the critical nature of climate change has marine animals facing a variety of threats which impact their populations. One such group affected by multiple stressors are cetaceans: the order inclusive of whales, dolphins, and porpoises. These marine mammals face both anthropogenic (human influenced) and natural (such as bacteria influenced) causes of harm.

Humpback whales are classified as cetaceans. Source: Wikimedia Commons, Whit Welles.

All cetaceans are currently protected in the United States under the Marine Mammal Protection Act, while some species are additionally protected by international agreements designed to mediate anthropogenic causes of dwindling populations. While these policies have been successful in helping specific populations bounce back from a status of threatened or endangered, more work is needed to understand what puts these incredibly long-lived, migratory animals at risk. Cetaceans are a publicly revered order of marine mammal that has consistently fallen victim to many human influenced dangers, such as vessel strikes and entanglements. There are still, however, unknown factors causing declines in their population. Studying these animals over time is hard– especially because they constantly travel– so just how can we look at different stressors impacting the health and mortality rates of cetaceans?

A topographic view of the Canary Islands. Source: Wikimedia Commons, Mysid.

Díaz-Delgado et al. (2018) spent seven years specifically studying deceased, stranded cetaceans along the coastline of the Canary Islands in Spain to figure out what most likely caused their deaths. Within the Canary Islands lie the greatest diversity of cetaceans in the European coastline, housing 30 species (7 baileen whales and 23 toothed whales). Not only did these researchers pick a highly diverse site for their study, they also chose to examine a stressor not commonly discussed in conservation communications: natural, pathological causes of death. Disease in aquatic and marine animals is estimated to be a severe repercussion of climate change: as our bodies of water warm at alarming rates, they become breeding grounds for bacteria. Bacteria severely impacts environmental degradation and attempts at conservation, as it can directly contribute to the decline of a population, the imbalance of an environment, and vulnerability of already fragile ecosystems.

A stranded orca. Source: Wikimedia Commons, kalev kevad.

Between the seven year study period, Díaz-Delgado et al. (2018) found 320 deceased, stranded cetaceans representing 22 species. On average, about 46 cetaceans stranded each year of the study period, with strandings peaking between March and May. While researchers only examined deceased animals, 30 of these animals actually stranded alive and passed away shortly prior to their discovery. 46.9% of the sample was female, 48.7% male, and 4.5% unknown. Researchers also classified these animals by age: 22.8% were neonatal or calves, 26.3% were juveniles or subadults, and 50.4% were adults. Finally, the researchers categorized the animals into nutritional status. 25% of the sample were in “good” status, 26.8% in “moderate”, 28.6% in “poor”, and 7.1% in “cachectic” or wasting.

Each probable cause of death including intra and interspecific traumatic interactions and neonatal or perinatal pathology (both of which accounted for the fewest amount of deaths). Click for larger image. Source: Díaz-Delgado et al. (2018).

Anthropogenic threats are commonly cited in conservation communications, but Díaz-Delgado et al.’s (2018) study reveals that human activity may not be the biggest culprit. Researchers conducted postmortem exams deducing the most probable cause of death for 208 of the sampled animals and found that while human activity accounted for a large portion (33%) of cetacean deaths, diseases were responsible for nearly double that amount (62%). For cetaceans with both good and poor nutritional status, infectious diseases were the most responsible for death (60% and 69.4% respectively) followed by parasitic diseases (28.6% and 26.5% respectively). Interestingly enough, Díaz-Delgado et al. (2018) discovered significant central nervous system inflammation in the cetaceans which stranded alive, a symptom they were unable to attribute to natural or anthropogenic causes. Researchers divided anthropogenic activity into three subcategories: interaction with fishing activities (like bycatch), which was responsible for 4.8% of deaths, foreign body-associated pathology (the presence of a pollutant like plastics in the animal’s digestive system), which was responsible for 2.5% of deaths, and vessel collisions, which were responsible for 11.5% of death

If contemporary and traditional conservation knowledge and environmental science teaches us anything, it’s that we need to be holistic and understand the interconnectedness of ecosystems when devising management policies. Tackling tangible anthropogenic sources such as unsustainable fishing methods, pollutants which aquatic animals can ingest, vessel strikes, and excessive hunting are critical (indispensable, really), and perhaps the lowered rates of anthropogenically influenced deaths could be attributed to the effectiveness of current policy. However, we need to remember that the fight to conserve and protect isn’t over. While Díaz-Delgado et al. (2018) themselves don’t suggest conservation policy to tackle this surprising rate of pathologically caused death, it’s important that we understand its correlation to climate change and warmer waters to bolster research with further work and answer those unanswered questions. Each sector of environmental and biological science all need to collaborate so that we can effectively ideate and implement ways to supplement current policy with this novel knowledge.

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