Assessing cetacean populations in the Mariana Archipelago: A summary of data and analyses arising from Pacific Islands Fisheries Science Center surveys from 2010-2019

The Pacific Islands Fisheries Science Center (PIFSC) Cetacean Research Program has conducted research on cetaceans in the Mariana Archipelago since 2010. A cooperative effort with funding from U.S. Navy Pacific Feet and PIFSC has included summer and winter small-boat surveys off the southernmost islands (Saipan, Tinian, Aguijan, Rota, and Guam); shipboard visual and passive acoustic surveys in portions of the EEZ in 2010, 2015, and 2018; development of photo-identification catalogs; and analyses of collected tissue samples and satellite telemetry tag data. PIFSC has also carried out long-term passive acoustic monitoring on moored recorders off Saipan and Tinian (since 2010) and off Pagan (since 2015) as part of the Pacific Islands Passive Acoustic Network (PIPAN) and deployed drifting acoustic recorders for examination of beaked whale and other cetacean distribution during the 2018 shipboard survey. The goal of these efforts has been to collect the data necessary to conduct the first population assessments for cetaceans within the Mariana Archipelago, including the determination of their occurrence, population structure and abundance, movements, distribution, and habitat use. In addition, these data may be used to evaluate the potential exposure of cetaceans to human-caused stressors within the waters surrounding the Mariana Archipelago including U.S. Navy operations (e.g., sonar, use of explosives), fisheries interactions, and dolphin tourism.

This report summarizes the surveys, data collection, and analyses conducted by PIFSC for cetaceans within the Mariana Archipelago to evaluate the current state of the data with respect to the overall goal of cetacean population assessment. Identification photos and encounter data from surveys conducted by other researchers were contributed by the U.S. Navy for incorporation into the PIFSC data sets for a variety of species and are therefore also represented in the summary reported here. The report is not intended to be an exhaustive review of all cetacean effort conducted in the archipelago, though all significant survey efforts, including those not conducted by PIFSC are referenced and, on occasion, discussed in greater detail.

A total of 20 cetacean species have been observed or acoustically detected by PIFSC within the Mariana Archipelago. During small-boat surveys in the southern islands from 2010-2019, 14 species of cetaceans were seen, including spinner dolphin (Stenella longirostris), pantropical spotted dolphin (Stenella attenuata), bottlenose dolphin (Tursiops truncatus), rough-toothed dolphin (Steno bredanensis), short-finned pilot whale (Globicephala macrorhynchus), sperm whale (Physeter macrocephalus), false killer whale (Pseudorca crassidens), melon-headed whale (Peponocephala electra), pygmy killer whale (Feresa attenuata), dwarf sperm whale (Kogia sima), Blainville's beaked whale (Mesoplodon densirostris), Cuvier's beaked whale (Ziphius cavirostris), Bryde's whale (Balaenoptera edeni), and humpback whale (Megaptera novaeangliae). Risso's dolphins (Grampus griseus) and striped dolphins (Stenella coeruleoalba) were observed during PIFSC shipboard visual surveys in 2015 and 2018. Passive acoustic monitoring has provided occurrence data on additional species not yet sighted, including Longman's beaked whale (Indopacetus pacificus), an unknown species of beaked whale (referred to as the Cross Seamount beaked whale, or BWC), blue whales (Balaenoptera musculus), fin whales (B. physalus), and minke whales (B. acutorostrata).

Although data are sparse for many species, the aggregate of all data collected to date reveal insights into the distribution and population structure for several species, and adequate data are available to assess abundance and movement patterns for others. The bulk of available data come from surveys near the southern islands, limiting the geographic extent of the conclusions to date. Shipboard visual survey data collected by PIFSC are not currently adequate to conduct abundance analyses for the broader archipelago, though do provide important data on encounter rate and can inform survey design for a dedicated line-transect abundance survey in the future.

Several high-priority analyses could be conducted with the data currently available or with a modest amount of additional data collection. Analyses and activities are considered high priority if they will directly inform National Marine Fisheries Service (NMFS) assessments under the Marine Mammal Protection Act (MMPA) or Endangered Species Act (ESA), they are relatively low cost and may provide focus or direction to future analyses or data collection efforts, or they will inform current Navy monitoring plan questions or consideration of future monitoring efforts in the region. The recommended high-priority analyses or activities include:

- A large-scale visual and passive acoustic shipboard line-transect abundance survey, which is currently planned for 2021 as part of the Pacific Marine Assessment Program for Protected Species (PacMAPPS), a multi-agency rotational cetacean survey plan for the North Pacific.

- Mark-recapture abundance estimation for spinner dolphins, bottlenose dolphins, and short-finned pilot whales within the southern archipelago.

- Genetic analyses of collected samples from spinner dolphins, pantropical spotted dolphins, bottlenose dolphins, rough-toothed dolphins, short-finned pilot whales, false killer whales, and melon-headed whales.

- Examination of habitat preferences of spinner dolphins from environmental and physical features.

- Examination of movements and habitat association of false killer whales and bottlenose dolphins using satellite telemetry data.

- Analysis of the full passive acoustic record for baleen whales, including Bryde's whales, to examine seasonality and distribution within the archipelago and possible migratory connections to other parts of the Pacific.

Hill MC, Oleson EM, Bradford AL, Martien KK, Steel D, Baker S. 2020. Assessing cetacean populations in the Mariana Archipelago: A summary of data and analyses arising from Pacific Islands Fisheries Science Center surveys from 2010 to 2019. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-PIFSC-108, 98 p. doi:10.25923/wrye-6h14

Copies of this report are available from

Science Operations Division

Pacific Islands Fisheries Science Center

National Marine Fisheries Service

National Oceanic and Atmospheric Administration

1845 Wasp Boulevard, Building #176

Honolulu, Hawaii 96818

Or online at

https://repository.library.noaa.gov/