Apex Predators: Hawaiian Monk Seals and Sharks

By: Madison Kearsey

Apex predators, such as the Hawaiian monk seal and sharks serve as indicators for the marine community due to their regulation of fish populations and biomagnification of pollutant chemicals (Bergmann, 2015; Stevenson et al. 2007). Biomagnification is the concentration of toxins that are ingested by plants and animals. In simple terms, they provide us with information about the health of the ecosystem. Large, top predators respond to fishing pressure consistently in a marine ecosystem as well as climate change. When large fisheries eliminate the majority of a local population of fish, predators such as the Hawaiian monk seal and sharks do not have a substantial food source. Apex predators are important determinants of healthy fish populations on reef ecosystems (Holzwarth et al. 2003). Their removal can have a detrimental impact on the surrounding ecosystem.

The Hawaiian monk seal is a marine mammal, and they are a highly important indicator of  marine pollution. Mammals can also be indicators for marine pollution due to being high on the food chain (high trophic level position). Large fat storage of lipid-rich blubber which is insulated fat in sea mammals, can be an indicator to pollutants as well as long lifespans (Cossaboon et al. 2019). Biomagnification of chemicals up the food chain occurs from organic chemicals from plastic that can transfer into animals lower on the food chain, then by ingestion, these chemicals build up in animals such as sharks and Hawaiian monk seals (Bergmann, 2015).

The Hawaiian monk seal range can extend more than 2500 km throughout the Hawaiian archipelago, but its largest population is in the atolls of the northwestern Hawaiian Islands. During the past 4 decades, the number of Hawaiian monk seals have increased in the main Hawaiian Islands, which also host over one million residents, plus tourists.  Hawaiian monk seals are generalist predators, feeding on a wide variety of fish,
cephalopods (squid, etc.) and crustaceans (lobster, etc.). A study was done to determine the shift of animals very low on the food chain such as phytoplankton (which can be as small as marine algae) in the northwestern Hawaiian Islands. They were associated with a decline in numbers of higher trophic level animals such as Hawaiian monk seals (Baker et al. 2012). This shift is represented by the top down effect, the apex predators at the top are the drivers, influencing the animals and plants at the bottom of the food chain (Kay, 2020). This means that without something small like phytoplankton, apex predators at the top of the food chain can be very affected.

Impact of Apex Predators

The removal of apex predators highly influences the intricate structure of the coral reef fish assemblage and influences a healthy ecosystem function. As desirable fish decline, like tuna, fisheries will switch to smaller fish lower on the food chain, like sardines, therefore influencing the predator population with a limited food source supply. This is called “fishing down the food chain” (Friedlander et al. 2002).

Similar to Hawaiian monk seals, sharks are another key apex predator facing threats and population decline. They serve as a high priority for conservation and management. The population of sharks overall have been decreasing in numbers rapidly due to fishing pressure, therefore invoking a community-wide detrimental effect on marine and human ecosystems. For example, fisheries tend to target larger fish species such as tuna, which is a large predator itself affecting the smaller fish species down the food chain and increasing their number for an unbalanced ecosystem (Frisch et al. 2016). True reef sharks as well as other sharks with a larger range occupy the nearshore coastal ecosystem of Hawaii. Similar to Hawaiian monk seals, these sharks directly implicate the health and structure of coral reefs. Sharks diets are made up of teleosts, cephalopods and crustaceans (Frisch et al. 2016).

Similar to the Hawaiian monk seal population density, history has recorded a higher density of sharks accumulated at the atolls of the northwestern Hawaiian Islands compared to the main Hawaiian Islands, which still stands to be relevant today. It is thought that the remote northwestern Hawaiian Islands create a more natural state for these animals, compared to the highly human populated main Hawaiian Islands. This does contrast the recent acknowledgment that Hawaiian monk seals have been able to tolerate an increased human population unlike the shark population, that seem to keep to the remote locations (Holzwarth et al. 2003).

Understanding the role of apex predators in our marine ecosystem is essential to conserving the ocean. Both Hawaiian monk seal and shark population changes can have cascading effects on the marine food chain. You can be an ocean conservation steward by sharing this information with family and friends. In doing so you are supporting the management of apex predators. You can also support sustainable seafoods by using the Monterey Bay Aquarium Seafood Watch app or by reducing your seafood intake overall.

References and further reading:

  • Baker, J., Howell, E. & Polovina, J. Relative influence of climate variability and direct anthropogenic impact on a sub-tropical Pacific top predator, the Hawaiian monk seal. Mar. Ecol. Prog. Ser.469, 175–189 (2012).

  • Cossaboon, J. M. et al.Apex marine predators and ocean health: Proactive screening of halogenated organic contaminants reveals ecosystem indicator species. Chemosphere221, 656–664 (2019).
  • Friedlander, A. & DeMartini, E. Contrasts in density, size, and biomass of reef fishes between the northwestern and the main Hawaiian Islands: the effects of fishing down apex predators. Mar. Ecol. Prog. Ser.230, 253–264 (2002).
  • Frisch, A. J. et al.Reassessing the trophic role of reef sharks as apex predators on coral reefs. Coral Reefs35, 459–472 (2016).
  • Stevenson, C. et al.High apex predator biomass on remote Pacific islands. Coral Reefs26, 47–51 (2007).
  • Kay, C. E. Are Ecosystems Structured from the Top-Down or Bottom-Up: A New Look at an Old Debate. 16 (2020).
  • Frisch, A. J. et al.Reassessing the trophic role of reef sharks as apex predators on coral reefs. Coral Reefs35, 459–472 (2016).
  • Holzwarth, Stephani, Schroeder, Robert, Zgliczynski, Brian, Laughlin, Joseph & Demartini, Edward. Sharks and Jacks in the Northwestern Hawaiian Islands From Towed-Diver Surveys. Isles of Refugedoi:10.1515/9780824846268-003.