The Hawaiian monk seal (Neomonachus schauinslandi) is a critically endangered marine mammal endemic to the Hawaiian archipelago, with a population numbering approximately 1,600 individuals. Understanding the feeding habits of this species is crucial, as their ability to find food is directly linked to their survival and recovery. To understand the ecology of this species, it is necessary to examine the specific prey that compose its diet and the methods it uses to hunt in the complex waters of Hawaiʻi.
Primary Prey Items
Hawaiian monk seals are classified as generalist feeders, consuming a wide variety of prey depending on availability. Their diet is heavily focused on organisms living on or near the seafloor, making them primarily benthic foragers. The three main categories of prey are bottom-dwelling fish, cephalopods, and crustaceans.
The fish they consume often hide in the sand or under rocks, including moray eels, flounders, and wrasses. Other common fish families include triggerfishes, surgeonfishes, and groupers. Cephalopods, such as octopus and squid, are also regularly consumed.
Crustaceans, including crabs, spiny lobsters, and shrimp, form a major component of their meals. Monk seals do not typically hunt pelagic, or open-ocean, gamefish like tuna or mahi-mahi, preferring prey that is easier to catch.
Hunting and Foraging Behavior
The foraging behavior of the Hawaiian monk seal is solitary and specialized for the benthic zone. While they can dive to impressive depths exceeding 1,800 feet, they typically forage at much shallower depths. Most dives are less than 200 feet deep and last for an average of six minutes, focusing on the seafloor.
The seals use an effective technique to capture prey that hides in crevices and under substrate. They often use their noses to flip over rocks or stir up sand, flushing out hidden eels, fish, and crustaceans. This method of active searching is crucial for accessing the cryptic prey that forms the bulk of their diet.
Monk seals exhibit behavioral flexibility in their feeding, utilizing both suction feeding for smaller prey and biting for larger items. Their foraging often occurs in deep water coral beds and near the edge of the continental shelf.
Dietary Variation and Specialization
The diet of Hawaiian monk seals shows variations depending on the age of the animal and its geographic location within the archipelago. Seals in the Main Hawaiian Islands (MHI) appear to be in better physical condition than those in the Northwestern Hawaiian Islands (NWHI), suggesting differences in resource availability or quality. While the overall composition of prey taxa remains similar between the two regions, MHI seals may experience less competition for food, which allows for greater foraging success.
Younger seals and pups must develop their foraging skills, meaning their initial diet is restricted to smaller, shallower-water prey. After a five-to-seven-week nursing period, weaned pups begin to forage independently in shallow reefs near their pupping site. They require time to build the necessary endurance and diving capabilities to access the deeper-water resources exploited by adult seals.
The flexibility in the monk seal’s diet is a significant adaptation that allows them to thrive in varied habitats ranging from shallow coral reefs to deep slopes. This opportunistic feeding strategy is important for the species, but seals in the MHI sometimes accidentally consume human-related debris, including discarded fishing gear and hooks, which pose a threat.