Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
be ones in which a fishery has an adverse effect on one or more marine mammal populations without
necessarily overfishing the target species of the fishery.
Climatic fluctuations have produced a growing concern about the effects of climate change on marine
mammal populations (MacGarvin and Simmonds 1996; IWC 1997; Evans 2002; Würsig et al. 2002).
Large-scale climatic events and long-term temperature change may affect the distribution and abundance
of marine mammal species, either impacting them directly or indirectly through alterations of habitat
characteristics and distribution or prey availability (Kenney et al. 1996; IWC 1997; Harwood 2001; Greene
and Pershing 2004). In the Pacific Ocean, climate variability has been linked to the PDO and the ENSO.
The PDO is a long-term climatic pattern capable of altering SST, winds, and SLP over the Pacific Basin
(Mantua 2002; Mantua and Hare 2002) and has been linked to cycles in fishery production (e.g. Hare
1996). In years when the PDO Index is positive, the central North Pacific experiences anomalously cold
SSTs, while the Western coast of the United States experiences anomalously cold temperatures. When
the PDO index is negative, the opposite occurs. The PDO is weak in tropical areas, such as the Marianas
MRA study area, and thus climatic anomalies are most likely due to ENSO forcings (Mantua 2002;
Mantua and Hare 2002). The ENSO is the result of interannual swings in sea level pressures in the
tropical Pacific between the eastern and western hemispheres (Conlan and Service 2000). ENSO events
typically last 6 to 18 months, and can initiate large shifts in the global atmospheric circulation. In the
western equatorial Pacific, SST is lower than in non-El Niño years (Kubota 1987), and rainfall patterns
shift eastward across the Pacific as the strength of the tradewinds weakens, resulting in increased
(sometimes extreme) rainfall in the southern U.S. and Peru and drought conditions in the western Pacific
(Conlan and Service 2000). While the ENSO can have dramatic effects on rainfall, atmospheric
circulation, and SST, it appears to have little, if any, effect on primary production in the western tropical
Pacific (Mackey et al. 1997; Higgins and Mackey 2000). For further information regarding the PDO and
the ENSO, refer to Chapter 2. Gregr and Trites (2001) noted difficulties in interpreting climate change
impacts in working with habitat prediction models, including that using incorrect spatial scales might fail to
detect a shift in habitat usage.
3.1.2 Marine Mammals of Guam and the Commonwealth of the Northern Mariana Islands
Marine mammals are not well-documented in Micronesia. The first compilation of available information for
19 species of marine mammals from Micronesia was provided by Eldredge (1991) with additional records
in Eldredge (2003). Taking into consideration marine mammal distribution and habitat preferences, the list
is expanded to 32 marine mammal species with confirmed or with possible occurrence records in Guam
and the Commonwealth of the Northern Mariana Islands. The vast majority (29) are cetaceans (whales,
dolphins, and porpoises) (Table 3-1). The dugong very rarely strays into this area, and no pinniped
species is known to be a regular inhabitant of the Micronesia region (Reeves et al. 1999). There are rare
reports of seals at these islands; however, species identification could not be verified. Eldredge (1991)
called attention to the possibility that Hawaiian monk seals and northern elephant seals wander far
enough from their normal ranges to appear at the Marshall or Gilbert Islands in the Micronesia region.
The Marianas study area lies within an oceanic province with generally very low levels of primary
productivity (see Figure 2-6). Any local hotspots in production may attract all components of the local
food chain. Strong interactions between the local oceanography and topography exist in the study area.
The Marianas MRA study area is comprised of a series of seamounts. Seamount topography has been
previously correlated with enhanced production due to the formation of vortices capable of mixing
nutrients to the surface and entraining phytoplankton in the overlying waters (reviewed by Rogers 1994).
For an in depth examination of seamounts, refer to Chapter 2.
In addition, the passage of the NEC through the Mariana archipelago is capable of creating regions of
enhanced turbulence. Passage of the current of the NEC can initiate the formation of eddies on the lee
side of the islands (Wolanski et al. 2003); these are capable of entraining phytoplankton and creating
localized regions of enhanced primary production visible as increased concentrations of chl a in the
surface waters (see the lee side of Guam, Figure 2-6). In addition, passage of currents through a narrow
channel (as found between the islands of Tinian and Saipan) can create localized zones of
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