Analysis of Sea Bird Distribution in the Northwest Pacific Ocean 
Nagahisa Kuroda 1 
This PAPER is based on data obtained by the 
author on board the fur seal research ship, 
"Geizan Maru,” No. 8 (capacity, 78 tons), of 
the Japanese Fisheries Bureau, between June 4 
and July 14, 1954, on a trip from Japan to the 
west Bering Sea. This paper presents the relative 
abundance of sea birds by divisions of sea zones, 
and discusses their correlation with air and water 
temperatures. Specific records on birds and sea 
mammals have been reported in other places 
(Kuroda, 1955, 1956) and a diary of the cruise 
has been prepared (in MS). 
METHOD 
All of the sea birds observed were recorded, 
together with pertinent data upon the time, air 
and water temperatures (these were regularly 
measured every hour by the crew ) , and the loca- 
tion in longitude and latitude. When flocks of 
birds were seen, their number was estimated. 
The author was the observer for an average of 
seven hours a day, with two or three rest periods. 
The records were arranged in a table under each 
observation time. The numbers of individuals 
observed per day and per hour were calculated 
(Table 1). Specimens of almost all of the species 
recorded were obtained (Kuroda, 1955). 
GENERAL REMARKS 
Although it was the breeding season, observa- 
tions were concerned chiefly with nonbreeding 
populations of sea birds, since the ship took a 
pelagic course. 
The density of sea birds can be given by 
linear and dimensional estimates. The linear esti- 
mate may be either per unit of time or per unit 
distance of travel. In this paper, the number of 
birds observed per hour, at a ship’s speed of 
seven miles per hour, was used for various 
analyses. For the absolute population density of 
^^Yamashina Museum of Birds, 49 Nanpeidai- 
machi, Shibuya, Tokyo, Japan. Manuscript received 
March 5, 1958. 
sea birds, the following formula, which was de- 
veloped in research upon the fur seal (Austin 
& Wilke, 1950: 35; also in 1954) might be 
applied: 
Number of 
birds per 
square mile 
Total number of birds 
observed 
Distance passed dur- 
ing observation ( in 
miles) X 0.114 
The fur seal frequently rests on the water, ex- 
posing only small parts of its head and flippers. 
The figure 0.114 mile is the distance to each 
side of the ship at which the seal can be visually 
detected when in this position. To collect ade- 
quate data for a certain sea area, several ships 
should work, as in the fur seal census, or a 
single ship should take a zigzag or irregular 
course to cover a greater part of the sea area by 
that "sight range belt.” In our research, the latter 
type of course was taken in the northern sea 
and dimensional bird density was calculated 
tentatively, applying the above formula for sub- 
divisions of this sea zone (Table 2). But the 
significance of the sight range of 0.114 mile in 
the study of birds needs future study, and, in 
actuality, all of the identifiable species of birds 
were recorded, irrespective of distance. It has 
been our experience, however, in many cruises 
off Japan, that floating sea birds were often 
identified at about the same range as the fur 
seals were detected, although a closer range was 
necessary to identify such similar species as 
Synthliboramphus antiquus and S. wumiztisume, 
because binoculars are not usable, owing to 
vibration on a small research ship. On the other 
hand, flying birds, especially when in flocks, can 
be seen and identified at much greater distances, 
but some closely similar species, such as Puffinus 
tenuirostris and P. griseus, must often be iden- 
tified at a closer range. Moreover, weather, direc- 
tion of sunlight, and wave conditions influence 
very much the visibility and the success of the 
observer’s identifications. 
55 
