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Perryman2, and William G. Clark3 ABSTRACT The accuracy and precision of dolphin school size estimates based on aerial photograph counts were examined using data collected on recent aerial and ship surveys. These estimates were found to be accurate during a research cruise aboard a tuna purse-seiner; dolphin schools were photographed from the ship s helicopter, encircled with the purse-seine, and then counted as the dolphins were released from the net.
A comparison of photographic estimates with these counts indicated that the relationship was fairly close and gave no indication of significantly differing from 1: Photographic estimates were also compared with estimates made by aerial observers. Most observers tended to underestimate school size, particularly for large schools. The variability among observers was high, indicating that observers should be individually calibrated.
For dolphin species in the eastern tropical Pacific Editab,eaerial surveys have been used to provide data on the densities of schools and on average school sizes. School size data rditable typically based on counts or estimates by observers aboard the aircraft.
Observer estimates of large aggregations of animals, however, can be greatly in error.
Chattin has described such estimates fairly accurately as a wild guess plus ten per cent. Lavigne, gritsland, and Falconer presented observers with slides of large fr-01 of dots and aerial slides of pinniped groups hauled out on ice and demonstrated a tendency for these observers to overestimate. Ray and Wartzok showed similar results for observer estimates of photographs of pinnipeds and cetaceans.
Brownhowever, showed that observers underestimated large numbers of objects presented on photographs. Heyland compared aerial observer estimates of geese with aerial photographs taken at the same time, and found that the observers underestimated the number of geese present. Such disparity in results makes it difficult to generalize about observer estimates. Aerial photography has been increasingly used to collect cetacean group size data Heyland, ; Ray and Wartzok, ; R.
Not every school encountered, however, can be successfully photographed. Factors such as sea state, sun glare, water clarity, or behavior of the animals can obscure portions of the school, making the photographs unsuitable for analysis. Thus observer estimates, although often unsatisfactory for large aggregations, must often be relied upon for school size data. Aerial photographs were taken of dolphin schools during four surveys in the ETP Table 1. One of these surveys was designed to determine the accuracy of estimates derived from the photographs.
The precision of the photographic estimates was determined from the results of another editqble in which estimates made from photographs taken by cameras with different formats, on different passes, and by different readers were compared.
Data from all four surveys allowed the comparison of both aerial observer and photographic estimates. This study editale with those questions that can be answered by statistical analysis of the above data, such as: Observers on board the Gina Anne independently estimated the sizes of dolphin schools which were captured by the purse seiner.
The captured dolphins were then counted by two to four observers as the dolphins were released over the net during the backdown procedure Figure 1. These observers counted from small boats stationed along the backdown area. An estimate of the total school size was obtained by adding the backdown count, the estimated number of dolphins which escaped capture, and the number of dolphins killed, if any, during the set. Whenever possible, the helicopter made multiple photographic passes over the schools at altitudes of m and at speeds of knots.
Estimates of school size based on counts made from the photographs were compared to the estimates based rgp backdown counts so that the accuracy of the aerial photographic estimates could be assessed. Aerial estimates of school size were also made by one to three observers and these estimates editanle compared with both the photographic estimates and the backdown counts. Photographs were taken with a vertically mounted KC mm 9-inch camera and a handheld Hasselblad mm camera.
Several photographic passes were made over schools at altitudes of m and at speeds of knots.
Observers on board recorded notes on school behavior and formations. The schools were later circled so that the observers could estimate school size and identify species. Estimates were usually based on some type of count. Individual animals could be counted in very small schools, but usually subgroupings which contained approximately equal numbers of animals were counted in larger schools.
Thus, the larger the school, the larger fgp subgroupings had to be. For large schools containing thousands of animals, an observer may have counted forato increments of or more.
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These estimates by up to six observers per school were made independently and not discussed until after the field study had been completed. Not all of the schools for which school size estimates were made could be successfully photographed.
An aerial photogrammetric study was also conducted by the IATTC, with personnel and equipment support from the NMFS, in The study was designed primarily to test the feasibility of measuring dolphin lengths from aerial photographs Anonymous, In addition, one to four observers made independent school size estimates which were compared with school size estimates made from the photographs. The photographs taken with the KAA camera were not used, however, due to a camera malfunction. Photographic passes were made at altitudes of m at speeds of knots.
The schools were circled at an altitude of m to get observer estimates of school size. Photographs were also taken during a NMFS aerial survey designed to. Photographs were taken of dolphin schools off the coast of Costa Rica with a KAA mm camera and four Hasselblad mm cameras vertically-mounted aboard a Beechcraft AT airplane.
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The methods for obtaining photographs and observer estimates were the same as for the IATTC study. Photographic counts Processed photographs from all surveys were reviewed on a light table to eliminate imagery which was of poor quality or which did not contain all the boundaries of the school.
Images of individual animals were independently marked by two to five readers on acetate mounted on the screen.
Slides taken on evitable Gina Anne study with the Hasselblad camera were viewed with a mm slide projector and marked on a large pad of drawing paper. The pass was the unit on which the estimate was based, and usually the school was contained on two to three frames from a single pass. The central frame which contained all or most of the school was counted first. Adjoining frames were then overlaid on the plots of the central frame and dolphins not previously counted were added to the plot.
The overlap between frames thus allowed questionable images to be re-examined on adjoining frames and allowed counts of animals which may have been too deep to be counted on other frames. Counts of outlying subgroups which appeared in some passes, but not on others, were included in ddr-01 school size estimates of the passes in which the subgroups were missing. Two types of data were recorded by the photograph readers.
One was the actual number of dolphins counted in each pass the sum of all dolphins plotted ; the other was an estimate based on the actual count, but including an additional percentage, usually small, to incorporate dolphins formatl were not observed.
A number of factors were considered in deriving the added percentage. These included the quality of the photograph, the amount of sun glare or splashes that might obscure dolphins, and the number of dolphins that were diving or swimming well below the surface as observed in the photographs or recorded in the field notes. The counts were therefore usually viewed as underestimates of the school size, particularly as the detectability of the dolphins decreased.
Reader variability is present in both the counts and the estimates. The photographic estimates and the backdown counts are presented in Table 2. The slope of the regression constrained to pass. It should be noted that the backdown counts are not an absolute ground truth. It was possible that dolphins were under- or over-counted when a large, tight group of animals was backed out through a single counting sector.
The error due to this, however, is likely to be small. Another source of error is that dolphins sometimes evaded capture after the photographs were taken.
The number of dclphins in these groups usually small groups of animals were estimated by the shipboard observers Table 2. While the photographic estimates of school sizes sampled during the Gina Anne study ranged from toinferential evidence indicates that the photographic estimates may be accurate for larger school sizes as well. The two schools were each photographed individually prior to their merging, and then photographed again after they had merged.
The sum of the average photographic estimates for the smaller, separate schools and was fairly close to the average estimate for the larger schoolindicating that the photographic dditable were likely to be consistent among large and small schools. The average counts formatp the average best estimates from the photographs are presented in Table 3.
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Analyses of variance were conducted to measure the effect of several factors on photographic estimates of school size Clark, ediitable Since the variance among readings of the same photograph was greater for large schools than small ones, a sditable transformation was used to standardize the within-cell variances. Bartlett s test did not show significant heterogeneity of variances in the logarithms of the photographic estimates, so these were used in all the analyses.
Roughly speaking, a difference between cells in the logarithms of the estimates can be regarded as a proportional difference. The sources of variation among the photographic estimates which were examined included: Different readers usually obtained different estimates from a single pass. These differences among readings of the same photograph measure the minimum uncertainty inherent in the method.
For eitable of variance analysis, they provided the within-cell variance with which other effects were compared. Tests edihable conducted to determine whether there were significant differences, on average, among estimates based on photographs taken with the same.
Tests were conducted to determine whether there were significant differences, on average, among estimates based on photographs taken by cameras with different formats or frame sizes 70 mm, d-r01, or mm on the same passes over the same school.
There were obviously large differences among schools in estimates of school size, and these were quite consistent among passes and formats. While there was no doubt that the effect of school was large and significant, this factor necessarily appeared in all the variance analyses so that it could be estimated and then removed to test those effects in question-pass and camera format. In terms of the analysis of variance, a mixed model with nesting was used: Only for schools photographed during the IATTC study in which multiple passes were made and all three camera formats were used was there an opportunity to treat the effects of school, pass, format, and reading simultaneously.
Effect of pass – In order to use as much data as possible, an analysis of variance was done for vr-01 camera format separately Table 4. Treating the formats separately for this purpose also simplified the model, since only school, pass, and reading remained as sources of variance. The analysis for each camera format used the dgl of all schools with at least two readings from each of at least two passes when the camera in question was operated.
The 9-inch camera used in could not be treated formatl all because it was not operated successfully on more than one pass over any school. Since an aircraft making repeated passes over a school affects the behavior of ediable animals, pass could be treated as a covariate instead as a variance component Table 5. When it was, the slope was positive and significant within each camera format, which meant there was a significant tendency for photographic estimates to increase from one pass to the next: The way in which it was handled thereafter, however, depended on the nature of the effect.
There were two possibilities at the extremes: In this rr-01 it would not be possible to compare estimates obtained with different formats on different passes, since the effects of pass and format would be confounded.
In this case it would be possible to compare estimates obtained with different formats on different passes since, for purposes of testing the effect of format, the effect of pass would simply add to the error variance.