Foraging Research Program

The Foraging Research Program (FRP), part of the Hawaiian Monk Seal Research Program (HMSRP), aims to characterize the foraging ecology of monk seals throughout the Northwestern (NWHI) and main Hawaiian Islands (MHI). Research is directed towards evaluating monk seal diet, foraging behavior and habitat use, and understanding linkages between foraging success and changing oceanographic conditions.

The Hawaiian Monk Seal Foraging Research Program is led by Charles Littnan, PhD.

As with all programs of the HMSRP, the goal of the FRP is to provide a scientific basis to management options promoting the recovery of the Hawaiian monk seal. Current FRP research emphasis is placed on the foraging success and survival of juvenile monk seals. Assimilation of data from all research areas of the FRP will enable managers to focus protection on important prey species and associated habitats to increase monk seal foraging success, promote juvenile survival and overall population growth.

Hawaiian Monk Seal Diet

Monk seals generally forage within demersal (near sea floor) and benthic (sea floor) habitats near breeding colonies. Some individuals are known to travel to submerged seamounts and reefs along the Hawaiian submarine ridge to depths of 500 meters (1,640 feet) or more. Monk seal diet includes a variety of fishes (e.g., eels, wrasses, squirrelfish, soldierfish, triggerfish, parrotfish), cephalopods (e.g., octopus and squid), and crustaceans (e.g., lobster). The proportion and type of prey consumed varies significantly by island, year, age and sex. It is unknown how much a monk seal eats daily, however, most large carnivores consume about 4-10% of their body weight. One study of a captive monk seal estimated daily consumption to be about 5% of its weight. The seals at the Waikiki Aquarium are fed about 2-3% of their body mass per day.

Limited prey availability appears to be a central factor in the continued decline of Hawaiian monk seal population in the NWHI. The ongoing lack of foraging success has negatively affected body condition, growth and age of maturation and of particular concern, the survival rates of juvenile seals. Fewer young animals, particularly females, mean fewer reproductive individuals for the future and likely a continued decline in population growth over the next decade. In contrast, the MHI population is steadily increasing with higher juvenile survival rates and most seals in a robust body condition, indicating greater food availability or foraging success.

Fish Remains tool.
Click to view Fish ID Database.

To better understand the source of variable foraging success throughout the monk seal range and to provide managers with informed mitigation strategies, the FRP in collaboration with Ken Longeneker, PhD from Bishop Museum are conducting a prey morphology-based dietary analysis of the Hawaiian monk seal. This project identifies the diet of the Hawaiian monk seal based on the morphology (shape or appearance) of prey hard parts (bones, otoliths – or ear stones, and scales) recovered from spew (regurgitate) and scat (feces). Prey identification is done by comparing hard parts with a reference collection of fish skeletons and by looking for osteological synapomorphies (features of bones unique to a particular taxonomic group). Current efforts are focused on describing the relationship between the size of fishes and their hard parts. These relationships will be used to determine what portion, by weight, each prey type contributes to the seal’s diet. The information gained from this project can be used by managers to better understand which prey resources are most important to the Hawaiian monk seal, identify important foraging habitat (based on the known habitat association of prey), determine locations where seals are likely to thrive, and understand the link between large-scale climactic fluctuation and changes in population demography. A large portion of this project is geared toward developing web-based tools to allow researchers without formal training in ichthyology (the study of fishes) to conduct similar analyses.

Another technique used to determine foraging habits of monk seals is called quantitative fatty acid signature analysis or QFASA. Each prey item has a unique fatty acid composition or signature, which is largely unchanged when eaten by a predator. By comparing the fatty acid signatures in the seal’s blubber to its potential prey, the seal’s diet can be inferred. To augment these diet studies, the Genetics Program is conducting research on Hawaiian monk seal diet by identifying prey DNA found in seal scats (feces).

Further reading:

Monk Seal Foraging Habitat and Behavior

An instrumented weaned pup. Track of an instrumented seal in the MHI.
Left: an instrumented weaned pup. Right: track of an instrumented seal in the MHI.

Research activities of the FRP on the movements and dive patterns of Hawaiian monk seals have led to a better understanding of the types of habitats utilized by foraging seals. Data is gathered via attachment of GPS/GSM dive recorders which record dive behavior and GPS locations of monk seals. In the NWHI, foraging dives are generally to depths of 150 meters (492 feet) or less; however, some individuals dive as deep as 500 meters (1,640 feet). Although the depth of foraging dives and distance a seal travels from a haul out location may vary by sex and age, foraging effort differs substantially between the NWHI and MHI seals. In the MHI, recorded foraging depths are substantially shallower and in shorter distance from haul out sites than monk seal foraging effort in the NWHI. These foraging patterns, correlated with body condition, suggest differences in prey abundance among sites. In the NWHI, limited food resources may be limiting population recovery versus MHI seals where robust body condition and increasing population growth suggest sufficient prey resources.

Data from GPS/GSM dive recorders will provide information on the potential overlap or separation between seal foraging areas and commercial/recreational fishing areas in the MHI.

Lastly, with this high resolution data, the FRP will develop an energetics model for monk seals, which will potentially illustrate how changes in ocean temperatures and prey availability among other variables will affect monk seal energetic demands and therefore survival.

Further reading:

Impacts On Foraging Success: Oceanographic Variability

The FRP examines a variety of factors that may influence monk seal foraging. The physical dynamics of the Hawaiian Archipelago and Central Pacific marine ecosystems influence monk seal biology and ecology. Therefore understanding the dynamics of the system is necessary to mitigate factors negatively influencing the recovery of monk seals.

In temperate and polar regions, where the vast majority of pinniped species are found, research has indicated seal and sea lion populations have crashed following El Niño events due to increased sea-surface temperatures and the resulting decline in available prey. However, El Niño events are associated with cooler waters in Hawaii, and studies have shown improved body condition and survival of Hawaiian monk seal pups and juveniles following El Niño years. These cooler waters can lead to improved diet or foraging efficiency of Hawaiian monk seals through changes in prey availability, distribution, or quality. In an effort to determine the effects of El Niño (cool) and La Niña (warm) years, the FRP is evaluating the number and types of prey eaten by Hawaiian monk seals and how large scale changes in sea-surface temperatures affect their diet.

Further reading: