Number of identified individuals and returns

In Ogasawara, a total of 4,452 humpback whale tail fins were photographed in 17 seasons from 1987 to 2003. As a result of matching these photos, we have identified about 1,100 individuals among them. Of these, about half have been confirmed as repeat visitors to Ogasawara for two or more seasons.

If you look at the top ranking of repeat visitors from 1987 to 2002, you will notice that there are many males. Males are said to move around more often than females in search of breeding opportunities, which may be why they encounter humans more often. The whale that has been identified for the most number of seasons is O-120, which has been identified almost every year in Ogasawara for 12 seasons since it was first identified in 1989 until 2002. O-120 is also thought to be a male, as it was identified as a singer and escort, and had a high encounter rate with our research vessel. However, there were some female whale repeaters in the top rankings, strongly indicating that the Ogasawara area is a specific breeding ground for these whales.

The percentage of repeat visitors in Ogasawara is much higher than that in Hawaii, another breeding ground for humpback whales in the North Pacific, where similar long-term research has been conducted. The reason for this may be that the humpback whales visiting Ogasawara tend to use a narrower area as a breeding ground than the Hawaiian group, and thus have a higher chance of being re-identified. However, it is also possible that the difference in the size of the islands and the survey methods may be a factor, so we must pay attention to these points in our future surveys and analyses.

Marked Release Method and Population Estimation

It is difficult to count the number of whales that live in the sea which have a wide range of behaviors. One method for estimating their numbers is the “mark-recapture” method. This method estimates the number of animals and their survival rates based on how many times they are caught after being released post-marking, and it’s often used in fish surveys. In the case of humpback whales, the pattern on the ventral side of the tail fin can be used as a natural marker without the need for placing a real marker on their bodies. We estimated the number of whales using the model equation for abundance analysis from records of whales re-identified in Ogasawara and Okinawa up to 1994, and we were able to estimate the number to be about 700. In addition, researchers in various parts of the North Pacific have used data from 1991 to 1993 to determine the total number of whales in the North Pacific, and have arrived at a figure of 6,000 to 8,000. However, each of these figures is limited by several factors, including the following:

(1) The records used for this calculation are already over 10 years old.

(2) Since accurate figures on the number of inhabitants were not available in the past, it is impossible to determine any trends.

(3) The data obtained for the estimation of the number of inhabitants is not used, so there is a possibility that the calculation is biased and the survey area is also biased.

 The data does not include all feeding areas (the data for Mexico has been missing for several years and Central America is not included, etc.).

(4) Genetic analysis is not included.

(5) Does not include song analysis.

Increasing trend in population

It is said that the number of humpback whales visiting the coastal waters of countries around the world has been drastically reduced due to commercial whaling. However, since the global ban on humpback whaling by the International Whaling Commission in 1966, they have escaped the direct threat to their population from humans. In the North Pacific, before modern whaling began, the population of humpback whales in the early 1900s was estimated at 10,000 or 15,000, with 2,500 in the western North Pacific alone. However, by 1965, the year before hunting was banned, this number was estimated to have dropped to 1,000.

Researchers in various parts of the world, including the western North Atlantic Ocean in the New England region of the United States, which is famous for whale-watching, and the eastern and western sides of Australia, have been trying to calculate the current population numbers using various methods. A typical method is the aforementioned whale marking and releasing method, which uses the pattern of the whale’s tail fin. There is also the line transect method, in which the number of whales seen by airplane or boat moving along a planned line is used to determine the number of inhabitants. Along the coast, where the north-south migration is clearly visible, there is also an attempt to count the number of whales from land and determine the variation in the number of whales seen per hour.

All of these methods show that the humpback whale population, while still far short of pre-whaling numbers, has at least recovered from the time of the whaling ban nearly 40 years ago. On Chichijima, humpback whales are frequently seen from various locations, such as Mikazuki-yama, Hatsuneyuura Observatory and Chihiroiwa, and on Hahajima, Shinyuhigaoka, Sameigasaki and Kofuji. However, a long time ago, humpback whales were not observed around the islands as frequently as they are now.

The number of humpback whales spotted per hour of survey time for the 15 seasons from 1988 to 2000 is shown in the graph below. 9,864 whales were spotted during the onshore survey and 4,443 during the at-sea survey. The number of animals found per hour of survey time for the 9,864 found in the land survey and the 4,443 found in the sea survey over the 15 seasons is shown in Fig. 1. However, if, for example, the number of whales that prefer to come to the west side of Chichijima repeatedly increased during the same season, and we counted them repeatedly without noticing it, we could not say that the number of visiting whales was increasing at all. To confirm this, we took out duplicates taken during the season from among the whales we were able to photograph and used 1,535 photographs to see how many whales came to the west side of Chichijima each year. As a result, we found that the number of whales coming to the west side of Chichijima has clearly increased over the past 15 years, although this is only an annual variation of the number of sightings and not an estimate of the population.

The west side of Chichijima is the area where whales and humans have the most contact in Ogasawara. So, are the whales preferring this area? Or is the population increasing in Ogasawara as a whole? The question remains. We know that the whales seen in the waters west of Chichijima have traveled to Mukojima, Hahajima, Iwo Jima, Okinawa, and even the Philippines. We would like to analyze this and other information such as overall movements with regard to habitat numbers.