FISHERY BULLETIN: VOL. 85, NO. 3 



mate (Table 1). The adjusted combined estimate 

 for the entire ETP was 2.81 schools/1,000 km^ 

 with a standard error of 0.152, a 4% increase from 

 the unadjusted estimate. 



Using the experimental results to adjust aerial 

 estimates for sun glare (and possibly sea state), 

 effects may be suspect because of differences in 

 procedures followed and observational conditions 

 encountered in the experiment and the surveys: 



1) The wings on the aircraft used during the ex- 

 periment were attached on the lower part of the 

 fuselage, whereas wings on the 1977 and 1979 

 aircraft were attached to the upper part of the 

 craft which allowed better lateral observation. 



2) Procedures used to adjust for presence of sun 

 glare during the surveys and the experiment dif- 

 fered. Observers during the surveys were in- 

 structed to stop searching if they believed condi- 

 tions prevented their detecting trackline schools, 

 but observers in the experiment searched during 

 all conditions. 3) More rough seas were encoun- 

 tered during the surveys (74%) than in the exper- 

 iment (62%). Also, more (46% as compared to 

 15%) of the surveys' total effort occurred at ex- 

 treme Beaufort 4 and 5 conditions. Because of 

 these uncertainties, I used the unadjusted density 

 estimate to determine school densities. 



Comparisons of the 1979 aerial observer teams' 

 estimates did not indicate observers of either 

 team missed dolphin schools on the trackline but 

 both teams may have been equally affected by 

 searching conditions. These results were consis- 

 tent with results of the aerial experiment (Holt 

 fn. 8) where comparisons of observer teams' per- 

 formance also indicated no significant differ- 

 ences. 



Ship Data 



The density estimates calculated from calm sea 

 data were larger than estimates calculated from 

 rough sea data (Table 1). The difference was prob- 

 ably not due to missed trackline schools during 

 rough seas. Schools on the trackline would proba- 

 bly be detected as the ship approached unless the 

 schools avoided the approaching ship. In a ship- 

 helicopter experiment Hewitt (1985) investigated 

 the reaction of dolphins to survey vessels and 

 found that dolphin schools only occasionally react 

 to the approach of a vessel before they are de- 

 tected by shipboard observers (1 of 12 schools). 



The differences between calm and rough sea 

 estimates may have resulted from actual differ- 

 ences in densities in areas surveyed during calm 



and rough sea states (Fig. 8). Another possibility 

 is that estimation errors resulted from observers 

 detecting schools at greater radial distances dur- 

 ing calm conditions (mean radial distance was 

 4.16 km) than during rough conditions (mean ra- 

 dial distance was 3.55 km). Estimation of sight- 

 ing angles and distances of schools at greater dis- 

 tances from the ship may have been less accurate 

 and may have increased the probability of schools 

 being erroneously recorded near or on the track- 

 line. 



Although sun glare was not shown to affect the 

 shipboard density estimates, Cologne and Holt 

 (fn. 7) found that shipboard observers tended to 

 avoid searching areas with sun glare. However, 

 because of the relatively slow speed of the ship 

 and the dolphins and because sun glare at any 

 specific time is usually concentrated in a small 

 region of the observers' field of view, all regions 

 may be observed without glare. 



The occurrence of errors in angle and distance 

 estimations may have positively biased shipboard 

 estimates. An inordinate proportion of dolphin 

 schools (25% of all schools) was recorded as being 

 on the trackline. Smearing the perpendicular dis- 

 tance distributions helped alleviate the bias but 

 may not have eliminated it. 



Comparison of Aerial and 

 Ship Estimates 



The estimates of dolphin densities in the in- 

 shore and the total areas using only ship data 

 were slightly larger than estimates which used 

 aerial inshore data (Table 1). This is logical be- 

 cause ship surveys were designed to overlap with 

 aerial coverage in the inshore area and to provide 

 systematic coverage of the offshore area. There- 

 fore, they spent disproportionately more of their 

 effort in the inshore area compared to its relative 

 size and, within the inshore area, they spent dis- 

 proportionately more effort in the northern 

 nearshore region (Fig. 1), which has relatively 

 high dolphin density. Although the inshore area 

 represented 31% of the total area, 37% of the 

 ship's effort was in the inshore area. In addition, 

 61% of the inshore effort was in the northern in- 

 shore region which represented approximately 

 44% of the inshore area. During the aerial sur- 

 veys a systematic survey of the inshore area was 

 conducted. Therefore, the best estimates of densi- 

 ties in the inshore and total areas are estimates 

 calculated using the unadjusted aerial inshore 

 data. 



432 



