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

AUGUST 2005 FINAL REPORT
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1.4.3 Inherent Problems with Marine Survey Data
When attempting to use aerial and shipboard survey data as a major indicator of a species’ occurrence, it
is necessary to first recognize the inherent problems associated with each survey type. One of the main
drawbacks of surveys in the marine environment is that aerial and shipboard surveys count the number of
animals at the water’s surface, where species such as cetaceans and sea turtles spend relatively little
time. Since sea turtles spend over 90% of the time underwater, it has been estimated that marine surveys
undersample (underestimate) the total number of sea turtles in a given area by as much as an order of
magnitude (Shoop and Kenney 1992; Renaud and Carpenter 1994). Although scientists have devised
mathematical formulas to account for animals not seen at the surface, the diving behavior of one
individual may be different from that of other members of the same species. Even though marine
mammals and sea turtles are obligated to come to the surface to breathe, many individuals will not
surface within an observer’s field of view. This is of particular concern when attempting to sight species
that dive for extended periods of time, do not possess a dorsal fin, and are known to exhibit cryptic
behavior, such as beaked whales and sperm whales (Würsig et al. 1998; Barlow 1999). Beaked whales
are often solitary individuals, which makes their sightability much different from a species that regularly
occurs in large groups, such as dolphins in the genus Stenella (Scott and Gilbert 1982).
Sighting conditions also affect the sightability of marine mammals and sea turtles. Sighting frequencies
vary due to the amount of sun glare on the water’s surface, the sea state, and the water clarity. Both sea
state and glare have statistically significant effects on sighting frequency (Scott and Gilbert 1982;
Thompson 1984). When water clarity is poor, animals are difficult to see below the water’s surface, and
only those animals at the water’s surface that are extremely close to the observer are usually identifiable.
Problems also arise when attempting to select an optimal and efficient survey method for sampling
marine mammals and sea turtles. Since most surveys are multi-species surveys, the sampling design,
although likely cost- and labor-efficient, cannot be considered optimal for each species (Scott and Gilbert
1982). The altitude at which marine mammal aerial surveys are flown is much higher than is desirable to
sight sea turtles (which are typically much smaller than cetaceans). Shipboard surveys designed for
sighting marine mammals are adequate for detecting large sea turtles but usually not the smaller-sized
turtles. Their relatively small size, diving behavior, and startle responses to vessels and aircraft make
smaller sea turtles difficult to sight or visually observe from a ship. The youngest age-classes, which often
inhabit waters far from land, are extremely difficult to spot and identify to species.
In addition, marine surveys are unable to assist scientists in accurately describing the seasonal
occurrence of marine mammals and sea turtles in extremely large areas, such as the Pacific Ocean. The
occurrence of marine mammals and sea turtles in an area often changes on a seasonal basis in response
to changes in water temperature, the movement and availability of prey, or an individual’s life history
requirements, such as reproduction; therefore, the number of sightings on a specific date over a specific
trackline may not be representative of the number of individuals occurring in the entire area over the
course of an entire season or year. As a result, sighting frequency is often a direct result of the level of
survey effort expended in a given area. Other difficulties with marine surveys include weather, time, and
logistical constraints. For example, the operating cost for a large research vessel is approximately
$10,000 per day (Forney 2002).
As a result of these problems, there have been no large-scale surveys in the western North Pacific Ocean
designed to specifically address information needs relative to marine mammals and sea turtles. Scientists
have, however, conducted smaller-scale aerial, marine tow, SCUBA, and beach surveys designed
specifically to sight sea turtles in the nearshore waters around Guam and the CNMI. These surveys have
not only allowed scientists to determine high-use foraging and nesting areas, they have also provided
scientists with sufficient information in order to estimate trends in sea turtle abundance over time.
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