HOLT: DENSITY OF DOLPHIN SCHOOLS 



Comparisons with 

 Previous Density Estimates 



Density of ETP dolphin stocks have been esti- 

 mated previously (SWFC 1976' '; Holt and 

 Powers 1982). The methods I used to calculate 

 estimates were similar to those used by Holt and 

 Powers. Therefore, differences that they noted be- 

 tween their assessment in 1979 and the SWFC 

 1976 assessment are also applicable to compari- 

 sons between the SWFC 1976 assessment and 

 this study. My estimates differ from the 1979 esti- 

 mates in that mine include 



1) schools where either the observers' "best" or 

 "lowest" estimate of mean school size was more 

 than 14 animals (the 1979 assessment included 

 only schools with "best" estimates), 



2) use of the 1977 aerial data in the inshore den- 

 sity estimate, 



3) ship data collected in 1977, 1979, 1980, 1982, 

 and 1983 (the 1979 assessment included only 

 1979 ship data), 



4) investigation of aerial and ship data for effects 

 of sun, sea state, and observer performance, 



5) application of LT methods to ship data to calcu- 

 late density estimates. 



Density estimates calculated in this study were 

 similar to those presented in the 1979 assessment 

 (Holt and Powers 1982). My inshore and offshore 

 estimates were 4.18 and 2.04 schools/1,000 km^, 

 respectively, with standard errors of 0.902 and 

 0.263. Holt and Power's estimates were 3.51 and 

 1.89 schools/1,000 km^, respectively, with stand- 

 ard errors of 0.590 and 0.766. 



CONCLUSIONS 



LT methods were used on 1977 and 1979 aerial 

 survey data to estimate dolphin density in the 

 inshore area at 4.18 schools/1,000 km^. LT meth- 

 ods applied to 1977-83 ship data yielded an esti- 

 mate of offshore dolphin density of 2.04 schools/ 

 1,000 km^. By weighting aerial inshore and ship 

 offshore data by the respective size of the two 

 areas, the total dolphin density was estimated at 

 2.71 schools/1,000 km^. 



1 ISWFC (Southwest Fisheries Center). 1976. Report of the 

 workshop on stock assessment of porpoises involved in the east- 

 ern tropical Pacific vellowfin tuna fishery. Southwest Fish. 

 Cent. Adm. Rep. No" LJ-76-29, 60 p. National Marine Fish- 

 eries Service, NOAA, P.O. Box 271, La Jolla, CA 92038. 



I investigated differences among densities at 

 different visibility conditions for aerial data, but 

 results were inconclusive owing to confounding of 

 the factors with density gradient (area from 

 shore). Adjusting the data for sea state and sun 

 conditions increased the inshore aerial density 

 estimate 8% and the total density estimate by 4%. 



ACKNOWLEDGMENTS 



I thank P. Hammond for providing the data 

 smearing program. B. Vorndam provided exten- 

 sive computer assistance and J. Cologne provided 

 statistical consulting. K. Raymond and R. Allen 

 drew the figures. I thank the many observers who 

 participated in the surveys. J. Barlow, S. Buck- 

 land, J. Michalski, W. Parks, S. Reilly, and S. 

 Sexton provided many constructive comments for 

 improvement of the paper. Members of SWFC 

 Pre-Sops Panel C, including G. Broadhead, K. 

 Burnham, D. Chapman, and P. Hammond, pro- 

 vided many useful technical suggestions (i.e., re- 

 view of procedures used to fit the Fourier series 

 model to the data sets). 



LITERATURE CITED 



AU, D., AND W. Perryman. 



1982. Movement and speed of dolphin schools responding 

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 BOWDITCH, N. 



1966. American practical navigator, an epitome of navi- 

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 BUCKLAND, S. T. 



1985. Perpendicular distance models for line transect 

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 BuRNHAM, K , D Anderson, and J Laake 



1979. Robust estimation from line transect data. J. 

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1980. Estimation of density from line transect sampling 

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BUTTERWORTH, D. 



1982. On the functional form used for g(y) for Minke 

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 32:883-888. 



Crain. B., K. Burnham, D Anderson, and J Laake. 



1979. A Fourier series estimator of population density for 

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 25 p. 



Hammond, P. 



1984. An investigation into the effects of different tech- 

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Hewitt, R 



1985. Reaction of dolphins to a survey vessel: effects on 



433 



