Informe Anual de la Comisión Interamericana del Atún Tropical, 19

34 TUNA COMMISSION
Reports ofthe IATTC, only a brief outline is given here, with emphasis on new developments.
Abundance is estimated from information on sightings and searching effort collected by
IATTC and NMFS scientific technicians aboard tuna vessels. Line transect methodology is used to
obtain abundance estimates from these data. The line transect method, in theory, is straightforward,
but numerous problems are encountered in its application, so the staff must devote considerable
time and effort to overcoming these problems, to the extent that this is possible. Sorne of the
problems which the staff has had to deal with are described below.
Because neither the dolphins nor the fishing effort are uniformly distributed in the eastern
Pacific Ocean, it was stratified into geographic regions, each representing approximately the ranges
of the stocks of interest. Within each region, when there are sufficient data, further stratification is
carried out. A smoothed encounter rate is calculated for each 1-degree quadrangle, and then
quadrangles with similar estimates are combined in the same strata. Within each stratum the
distribution of dolphin herds is roughly uniform, thus making it possible to assume random
distribution of herds within the stratum. Ideally, further stratification would be carried out, for
example to separate the data by season or fishing mode. For most stocks, however, the data are
insufficient to do this, but it is believed that the most important stratification factors are those
designed to offset the effects ofnon-random searching. Some problems can still arise; for example, if
local concentrations of herds occur and the fishermen are successful in locating these, the density
will be overestimated.
Helicopter sightings arise from a very different searching process. The helicopter does not
stay on the trackline ofthe vessel, and if the perpendicular distances from the trackline of sightings
detected by the helicopter are examined, the shoulder ofthe detection curve is seen to extend many
miles beyond the trackline. These sightings cannot be ignored, since it is not possible to know which
herds would have been detected by the vessel ifthe helicopter had not found them first. Therefore the
helicopter and crew sightings are combined. Conceptually, the helicopter is considered by this
approach as a mechanism to increase the probability of detection of a herd away from the trackline.
Since the perpendicular distances are truncated at 5 nautical miles (nm), a distance at which the
target herds are likely to be detected from the vessel in the absence ofa helicopter, the few additional
herds that are detected by the helicopter that would have otherwise have escaped notice will have
little impact on the analyses. Analyses ofthe data tend to confirm this, for although helicopter use
increased from virtually zero in 1975 to most ofthe fleet in 1986, it is not possible to detect a trend
over time in the estimates of effective track width.
In poor sighting conditions, lower encounter rates and narrower effective track widths are
expected. Analyses indicate that there is a reduction in encounter rate when data collected during
periods when the wind exceeded Beaufort force 3 (7 to 10 knots) are included in the analyses, but no
effect ofwind on the effective track width is discernible. As the densitywould be underestimated ifall
the data are used, sightings and searching effort data when the wind exceeded Beaufort force 3 are
not included in the analyses.
There are periods during most days of a fishing trip when the vessel is searching but the
observer is not on duty, for example during lunch breaks or while he is collecting samples. Because
some sightings made during these periods are not reported to the technician by the crew, use ofthe
data for these periods would reduce the encounter rate. Accordingly, to prevent negative biases in the
density estimates, these data are not used.
If the perpendicular distance of a herd from the trackline is to be estimated accurately the
observer must accurately record both the sighting distance and the sighting angle before the herd
reacts to the vessel. Unfortunately, the distances and angles are sometimes not recorded until the
vessel has already turned túward the herd, resulting in an average sighting angle for the trip that is
less than expected. Alow average sighting angle suggests that either the herds were not reported to
the technician when first detected or that the technician failed to record accurately the angles and