Reaction of Dolphins to a Survey Vessel: Effects on Census Data

A field experiment is described in which a helicopter was used to observe the efficiency of shipboard line-transect sampling of dolphin populations in the eastern tropical Pacific Ocean. Nineteen dolphin schools were tracked; 13 of these were detected by observers aboard the ship and 5 of these reacted to the approach of the ship by altering the direction and/or the speed of their movement; however, only 1 school reacted prior to shipboard detection. The results suggest that dolphin schools only occasionally react to the approach of a survey vessel prior to their detection by shipboard observers and that the use of a monotonically decreasing detection function is adequate to minimize bias. Aerial and shipboard estimates of school size and species composition for six schools compared favorably The Marine Mammal Protection Act of 1972 prohibits t h e harves t of mar ine mammals and specifies that the Federal Government may issue permits for their take only under special circumstances. One such circumstance involves the incidental kill of dolphins associated with the yellowfin tuna fishery in the eastern tropical Pacific Ocean. Before issuing the permits, the government must first certify the viability of the affected dolphin populations. To meet this requirement, scientists at the Southwest Fisheries Center define stocks and monitor their population demography, reproductive output, and abundance. The vital statistics are derived primarily from specimens obtained from the tuna fishery. However, to estimate abundance, surveys a re conducted using ships and aircraft independently of t he fishery. The surveys, using line-transect methods (Burnham et al. 1980), have yielded estimates of the density of dolphins in the eastern tropical Pacific Ocean (Holt and Powers 1982). A critical assumption in the application of t he method is that the animals do not move, in reaction to the observer, prior to their detection. In practice, a detection function, which is relatively insensitive to nonrandom movement, is used to describe the probability of observing a school of dolphins given its position relative to the observer's transect. A field experiment.was designed with the following objective: 1) Test the assumption tha t the animals do not alter their movement in reaction to the approach of a survey vessel prior to shipboard detection. During a survey the unit of observation is a school of dolphins. In addition, species composition and the number of individual animals in a school (school size) are estimated. Surveys routinely collect information to determine the precision of these estimates by recording independent observations of several observers; however, determining their accuracy is more difficult and attempted less often (Holt and Powers 1982). Six schools were closely approached and observed from both a n aircraft and a ship with the following objective: 2) Compare shipboard and aerial estimates of school size and species composition. Although not an absolute determination of accuracy, the comparison yielded estimates from two very different viewpoints (high-altitude plan view versus low-altitude profile view). A similar experiment was conducted using the NOAA Ship Surveyor and a ship-supported helicopter in 1977 (Au and Perryman 1982). They observed the reaction of eight dolphin schools to the approach of a ship; all eight schools swam away from the projected trackline of the ship. Au and Perryman also suggested that, in some cases, avoidance began beyond the visual range of shipboard observers. The present study was intended to collect additional data under a wider variety of conditions. 187 Manuscript accepted April 1984. FISHERY BULLETIN VOL. 83. NO. 2.1985. FISHERY BULLETIN: VOL. 83, NO. 2 were tracked for about an hour’s time until one of three events occurred: 1) the school passed abeam of the ship; 2) the school passed beyond the visual range of shipboard observers; or 3) the aerial observers lost sight of the school and had to terminate the track prematurely; in all of the latter cases the presence of the animals was obscured by deteriorating sea state. During a track, the helicopter was positioned over the school a t a minimum altitude of 1,200 ft (370 m); the radar range and bearing to the helicopter were determined from the approaching survey vessel about every 4 min (an interval sufficient to record the appropriate navigational data and still provide continuity in the track). A transponder, mounted on the aircraft, facilitated accurate radar measurements. In addition, OMEGA navigation positions were recorded from dual systems aboard the helicopter and the ship. As the track progressed, field notes were taken on visual observations of school behavior and associated birds and fish. The tracking altitude appeared to METHODS The experiment was designed to observe the efficiency of shipboard survey operations by using a helicopter to track dolphin schools before, during, and after shipboard detection. This approach was an enhancement of the design employed by Au and Perryman (1982) which focused only on the behavior of the dolphins. A simulated survey operation was included in the experiment for the