The yellow-eyed penguin, also known as Hoiho in Māori, is a unique seabird found exclusively in New Zealand. Understanding their diet is important for comprehending their ecological role and the challenges they face. The availability and quality of their food directly influence their health, breeding success, and survival.
Main Dietary Components
The diet of yellow-eyed penguins consists primarily of fish, making up about 90% of their prey by weight. This is supplemented by cephalopods and some crustaceans.
Historically, their diet included opalfish, sprat, and red cod, particularly in the 1980s. Opalfish, a bottom-dwelling species, has consistently been a major component due to its abundance and nutritional value. Other important fish include silversides and ahuru. Arrow squid is also a regular and important part of their diet.
Most prey items are relatively small, typically less than 200 millimeters, allowing for easy ingestion.
A notable shift has occurred in recent decades, with blue cod increasingly replacing red cod in many areas. This change is concerning because blue cod are larger, ranging from 160 to 220 millimeters, making them more difficult for penguin chicks to swallow. Blue cod also offers fewer nutrients than the red cod they traditionally consumed, impacting their nutrition.
Foraging Habits and Hunting Techniques
Yellow-eyed penguins are adept underwater hunters. They typically forage in the mid-shelf region, 2 to 25 kilometers offshore, though some may venture up to 50 kilometers from the coast. They are pursuit divers, actively chasing prey underwater.
These penguins can dive to depths between 40 and 120 meters, with some dives reaching 160 meters. A typical dive lasts 3.5 to 4 minutes as they search for bottom-dwelling fish and cephalopods.
Their foraging is predominantly benthic, meaning they hunt near or on the seafloor. They utilize their agility and excellent underwater vision to locate prey, which is generally swallowed whole while still underwater.
Factors Influencing Diet
Several external factors influence the diet of yellow-eyed penguins, leading to variations in prey composition. Seasonal changes directly impact the availability and abundance of different fish species, requiring penguins to adapt their foraging strategies. Geographical differences also play a role, with local fish populations dictating the specific prey consumed in various breeding areas.
Environmental changes, such as shifts in ocean temperature, can alter the distribution and health of prey species, affecting the penguins’ access to food. Human fishing pressure has also had a substantial impact, contributing to the decline of preferred prey like red cod. This pressure has led to increased reliance on less nutritious alternatives like blue cod.
Fisheries-related disturbances to their benthic foraging habitats may also favor certain species more tolerant to such disruptions. Fluctuations in food supply, influenced by these environmental and anthropogenic factors, can result in poor foraging years, often linked to reduced breeding success and higher mortality rates for chicks and adults.