Marine Mammal Monitoring on Guam

Robert Weybright McNulty


Data on marine mammals, specifically information on their stock numbers, seasonal migrations, population structure, habitat use, and behaviors is limited. This study of island associated resident pods of the long nosed spinner dolphin (Stenella longirostris) was conducted near the island of Guam, part of the Mariana Archipelago, south of Japan. These spinner dolphins may be negatively impacted by an increased United States military presence on Guam which leads to expanded training activity, population growth, coastal development and associated tourism. This study compiles historical and current data on marine mammal sightings and behaviors to study possible impact of US military presence. The long nosed spinner dolphins (S. longirostris) are of particular interest because of their proximity to shorelines, their habitat use, and their daily activity schedule. Current data was collected from interviews and sighting events. Standard photo documentation and field survey protocols developed by the National Marine Fisheries Service (NMFS) Pacific Islands Fisheries Science Center (PIFSC) were used. This study created a preliminary marine mammal database for Guam and has expanded the limited information available. In addition, sighting information was used along with geographical information system (GIS) software to combine NOAA benthic habitat maps with multibeam bathymetry to gain a better understanding of the habitats occupied by documented species. Preliminary results suggest that S. longirostris populations in Guam have similar circadian and seasonal rhythms as Hawaiian S. longirostris populations. The results also demonstrate that Guam spinner dolphins show a preference for clear, relatively shallow open water bays with underlying, unconsolidated sediment (sandy bottoms). An increase in military use of shallow bays and dolphin-associated ecotourism could therefore negatively impact resident populations of the Guam spinner dolphins.

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