Foraging and Fisheries Interactions Research Initiative

The Foraging and Fisheries Interactions Research Initiative aims to characterize the foraging ecology of monk seals throughout the Northwestern (NWHI) and main Hawaiian Islands (MHI) by evaluating monk seal diet, foraging behavior and habitat use, and understanding linkages between foraging success and changing oceanographic conditions. Interactions, both direct and ecological, between monk seals and fisheries in the MHI are also examined.

Limited prey availability is a central factor in the continued decline of the NWHI subpopulation, which resides in the protected Papahānaumokuākea Marine National Monument. 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. In contrast, the MHI subpopulation is steadily increasing with higher juvenile survival rates and most seals in a robust body condition, indicating greater food availability or foraging success. Foraging studies indicate that MHI and NWHI seals eat similar diets; however, the disparity in body conditions of seals between regions indicates a possible difference in intra- or inter-specific competition, prey availability, and quality.

Assimilation of foraging data can enable managers to focus protection and enhancement of important prey species and associated habitats to increase monk seal foraging success, promote juvenile survival and overall population growth. This information can also be used to identify areas where seals are likely to thrive and to understand the link between large-scale climatic fluctuation and changes in population demography. Additionally, understanding the overlap with local fisheries, will allow the development of strategies to minimize these interactions to benefit both seals and fishermen.

The Hawaiian Monk Seal Foraging and Fisheries Interactions Research Initiative is led by Charles Littnan, PhD.

Hawaiian Monk Seal Diet and Consumption Rates

Monk seals generally forage within demersal (near sea floor) and benthic (sea floor) habitats near breeding colonies. They are generalist feeders and their diet includes a variety of fishes (eels, wrasses, squirrelfish, soldierfish, triggerfish, parrotfish), cephalopods (octopus and squid), and crustaceans (crab, shrimp, and lobster). Diet studies indicate that they prefer prey that hide in the sand or under rocks, unlike most of the locally popular game fish, e.g. ulua, papio and ʻoʻio, and the proportion and type of prey consumed varies significantly by island, year, age and sex. However, lobster has not been identified as a primary monk seal prey item.

Techniques to Identify Monk Seal Prey

Fish Remains tool.
Click to view Fish ID Database.

Traditional diet analyses include identifying prey remains in the feces (scat) and regurgitate (spew) of monk seals using the morphology (shape or appearance) of prey hard parts (bones, otoliths – or ear stones, and scales) and comparing them to a reference collection of fish skeletons and by looking for osteological synapomorphies (features of bones unique to a particular taxonomic group). A web-based Hawaiian Fish Remains (bones, otoliths, and scales) ID key was developed with Ken Longeneker, PhD of Bishop Museum primarily for monk seal diet analyses. This key is now freely available and allows anyone (even without formal training in ichthyology – study of fishes) to conduct similar analyses. Building on this foundation, the relationship between the size of fishes and their hard parts can be used to determine what portion, by weight, each prey type contributes to the seal's diet.

Only prey whose hard parts survive the digestive process relatively intact can be identified via traditional diet analyses. And for some fish, identification can only be determined to the family, instead of the finer scale species level. To address these limitations, other methods have been pursued to supplement these data in determining monk seal dietary habits. One technique 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 those found in tissues from potential prey, the seal's diet can be inferred. Additionally, under the Genetics Research Initiative, methods are being developed to determine diet by identifying prey DNA in monk seal scats (feces). Lastly, some limited prey identification has been gleaned from monk seal foraging footage recorded using National Geographic Crittercams (seal born video cameras).

Monk Seal Consumption Rates

Monk seals eat about 4-8% of their body weight daily, depending on the age class of the seal. With an increasing population of seals in the MHI there is concern that seals may be negatively impacting the local ecosystem. A MHI seal consumes about 6.8 kilograms (15 pounds) of fish per day, which is only 0.009% of estimated available prey biomass. With 7,622 square kilometers (2,943 square miles) of foraging habitat in the MHI and an estimated monk seal population size of 200, this translates to a total consumption rate of about 0.45 kilograms (1 pound) of fish per square mile.

Further reading:

Cahoon MK, Littnan CL, Longenecker K, Carpenter JR
2013. Dietary comparison of two Hawaiian monk seal populations: the role of diet as a driver of divergent population trends. Endang Species Res 20: 137–146. DOI: 10.3354/esr00491
Dye TS, Longenecker KR
2004. Manual of Hawaiian Fish Remains Identification Based on the Skeletal Reference Collection of Alan C. Ziegler and Including Otoliths. Society for Hawaiian Archaeology Special Publication 1. 134 pp.
Goodman-Lowe GD
1998. Diet of the Hawaiian monk seal (Monachus schauinslandi) from the northwestern Hawaiian islands during 1991 to 1994. Mar Biol 132:535−546. DOI: 10.1007/s002270050419
Iverson S, Piche J, Blanchard W
2011. Hawaiian monk seals and their prey: assessing characteristics of prey species fatty acid signatures and consequences for estimating monk seal diets using fatty acid signature analysis. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-PIFSC-23, 114 p. + appendices
Iverson SJ, Stewart BS, Yochem PK
2010. Report on validation and calibration of fatty acid signatures in blubber as indicators of prey in Hawaiian monk seal diet (A report submitted under Contract No. AB133F-030SE-1195, September 2003). Pacific Islands Fisheries Science Center Administrative Report H-10-05, 19 p
Longenecker K
2008. Relationships Between Otolith Size and Body Size for Hawaiian Reef Fishes. Pacific Science 62(4):533-539. DOI: 10.2984/1534-6188(2008)62[533:RBOSAB]2.0.CO;2
Longenecker K
2010. Fishes in the Hawaiian monk seal diet, based on regurgitate samples collected in the Northwestern Hawaiian Islands. Marine Mammal Science 26(2):420-429. DOI: 10.1111/j.1748-7692.2009.00332.x
Longenecker KR
2009. Fish Remains: A tool for identifying Hawaiian fishes from bones, otoliths & scales.
Longenecker KR, Dollar RA, Cahoon M
2006. Increasing taxonomic resolution in dietary analysis of the Hawaiian monk seal. Atoll Research Bulletin 543:103-113.
Parrish FA, Marshall GJ, Littnan CL, Heithaus M, Canja S, Becker B, Braun R, Antonelis GA
2005. Foraging of juvenile monk seals at French Frigate Shoals, Hawaii. Marine Mammal Science 21(1): 93-107. DOI: 10.1111/j.1748-7692.2005.tb01210.x
Piche J, Iverson SJ, Parrish FA, Dollar R
2010. Characterization of forage fish and invertebrates in Northwestern Hawaiian Islands using fatty acid signatures: species and ecological groups. Marine Ecology Progress Series 418: 1-15. DOI: 10.3354/meps08814
Sprague R, Littnan C, Walters J
2013. Estimation of Hawaiian monk seal consumption in relation to ecosystem biomass and overlap with fisheries in the main Hawaiian Islands. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-PIFSC-37, 42 p. + Appendices
Williams TM, Richter B, Kendall T, and Dunkin R
2011. Metabolic Demands of a Tropical Marine Carnivore, the Hawaiian Monk Seal (Monachus schauinslandi): Implications for Fisheries Competition. Aquatic Mammals 37(3), 372-376. DOI: 10.1578/AM.37.3.2011.372

Monk Seal Foraging Behavior and Habitat Use

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 on the movements and dive patterns of Hawaiian monk seals has led to a better understanding of the types of habitats utilized by foraging seals. Data are gathered via attachment of various types of instruments, often multiple instrument types attached simultaneously, that record seal location (using satellites or cell phone towers), dive patterns (time depth recorders), and underwater behavior (using National Geographic Crittercams - seal born video cameras and 3-axis accelerometers which measure the seal's body orientation and movement).

Seals feed both during the day and night, although this varies depending on age/sex class. During foraging, seals sometimes lift or turn over rocks and root around in coral rubble and sand on the bottom to find prey. Crittercam video footage has shown that prey are flushed during the seals' foraging activities, and jacks (trevally or ulua), sharks, and snappers are able to capitalize on the seals' efforts.

In the NWHI, foraging dives are generally to depths of 150 meters (492 feet) or less; however, some individuals dive over 3 times this depth. 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 growth rates and body condition, suggest differences in prey abundance among sites. Additionally, the higher abundance of seals and top predators, such as sharks and jacks (trevally or ulua) in the NWHI may also lead to direct intra- and inter-species competition for prey resources. 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.

Further reading:

Littnan CL, Baker JD, Parrish FA, Marshall GJ
2004. Effects of video camera attachment on the foraging behavior of immature Hawaiian monk seals. Marine Mammal Science 20(2): 345-352. DOI: 10.1111/j.1748-7692.2004.tb01164.x
Parrish FA
2009. Do monk seals exert top-down pressure in subphotic ecosystems? Marine Mammal Science 25(1): 91-106. DOI: 10.1111/j.1748-7692.2008.00245.x
Parrish FA, Abernathy K
2006. Movements of monk seals relative to ecological depth zones in the lower Northwestern Hawaiian Islands. Atoll Research Bulletin 543: 115-130.
Parrish FA, Abernathy K, Marshall GJ, Buhleier BM
2002. Hawaiian monk seals (Monachus schauinslandi) foraging in deep-water coral beds. Marine Mammal Science 18(1): 244-258. DOI: 10.1111/j.1748-7692.2002.tb01031.x
Parrish FA, Craig MP, Ragen TJ, Marshall GJ, Buhleier BM
2000. Identifying diurnal foraging habitat of endangered Hawaiian monk seals using a seal-mounted video camera. Marine Mammal Science 16(2):392-412. DOI: 10.1111/j.1748-7692.2000.tb00932.x
Parrish FA, Littnan CL
2008. Changing perspectives in Hawaiian monk seal research using animal-borne imaging. Marine Technology Society Journal 41(4): 30-34. DOI: 10.4031/002533207787441944
Parrish FA, Marshall GJ, Buhleier B, Antonelis GA
2008. Foraging interaction between monk seals and large predatory fish in the Northwestern Hawaiian Islands. Endangered Species Research 4(3): 299-308. DOI: 10.3354/esr00090
Parrish FA, Marshall GJ, Littnan CL, Heithaus M, Canja S, Becker B, Braun R, Antonelis GA
2005. Foraging of juvenile monk seals at French Frigate Shoals, Hawaii. Marine Mammal Science 21(1): 93-107. DOI: 10.1111/j.1748-7692.2005.tb01210.x
Stewart BS, Antonelis GA, Baker JD, Yochem PK
2006. Foraging biogeography of Hawaiian monk seals in the Northwestern Hawaiian Islands. Atoll Research Bulletin 543: 131-145.

Impacts of Climate-Oceanographic Variability on Monk Seal Foraging

Understanding the physical dynamics of the Hawaiian Archipelago and Central Pacific marine ecosystems, which influence monk seal biology and ecology. is necessary to mitigate factors negatively impacting 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. Impacts of oceanographic variability on monk seal foraging continue to be evaluated.

Further reading:

Antonelis GA, Baker JD, Polovina JJ
2003. Improved body condition of weaned Hawaiian monk seal pups associated with El Nino events: potential benefits to an endangered species. Marine Mammal Science 19(3): 590-598. DOI: 10.1111/j.1748-7692.2003.tb01323.x
Baker JD, Howell EA, Polovina JJ
2012. Relative influence of climate variability and direct anthropogenic impact on a sub-tropical Pacific top predator, the Hawaiian monk seal. Marine Ecology Progress Series 469: 175-189. DOI: 10.3354/meps09987
Baker JD, Johanos TC, Wurth TA, Littnan CL
2013. Body growth in Hawaiian monk seals. Marine Mammal Science. DOI: 10.1111/mms.12035
Baker JD, Polovina JJ, Howell EA
2007. Effect of variable oceanic productivity on the survival of an upper trophic predator, the Hawaiian monk seal Monachus schauinslandi. Marine Ecology Progress Series 346: 277-283. DOI: 10.3354/meps06968
Parrish FA, Howell EA, Antonelis GA, Iverson SJ, Littnan CL, Parrish JD, Polovina JJ
2011. Estimating the Carrying Capacity of French Frigate Shoals for the Endangered Hawaiian Monk Seal Using Ecopath with Ecosim. Marine Mammal Science. DOI: 10.1111/j.1748-7692.2011.00502.x

Seal-Fisheries Interactions

Because monk seals have such a diverse diet (including many prey items rarely targeted by fishers such as small wrasses, flatfishes, and shrimps) and forage over large areas (including habitats and deeper depths not targeted by fishers) the overlap with fishers is relatively small and seals are not likely to impact fish stocks. However, there could be impacts to individual fishermen when seals interact with their gear or catch. These direct fishing interactions occur when the seals are transiting to and from shore. Though seals are most often seen in shallow, nearshore waters, they are not necessarily foraging or spending most of their time there. This is the "driveway effect" where seals must pass through the shallows to transit between their resting and foraging areas, similar to people using their driveway to get to and from their home.

Fishing guidelines have been developed to minimize seal-fisheries interactions, which include using barbless hooks, not feeding seals, and avoiding fishing when seals are in the area. To better track fisheries interactions, NOAA Fisheries is working with fishers to report seal interactions, including interaction types (type of gear, bait, location, seal information, and other factors), frequencies and trends. Solutions can then be developed so that monk seals and fishermen can both use the marine resources with minimal conflict.

Further reading:

Boggs CH, Gonzales DP, Kokubun RM
2015. Marine mammals reported under catch lost to predators on fishermen's commercial catch reports to the State of Hawaii, 2003-2014 Pacific Islands Fisheries Science Center, PIFSC Data Report, DR-15-006, 14 p. DOI: 10.7289/v5pr7szm
Gobush KS, Wurth TA, Henderson JR, Becker BL, Littnan CL
2016. Prevalence of interactions between Hawaiian monk seals (Nemonachus schauinslandi) and nearshore fisheries in the main Hawaiian Islands. Pacific Conservation Biology. doi:10.1071/PC15029