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On 18 June 2005, an environmental organization in Hong Kong conducted a census on Hong Kong Chinese white dolphins, together with other corporations. In the census 150 volunteers on 30 boats altogether sighted 25 Chinese white dolphins. This figure is 40% lower than that collected on the first dolphin census, which had been carried out 11 years ago. Thus, the organization attributed the declined number of dolphins to over-fishing in Hong Kong. They pointed out that over-fishing leads to depleting fish abundance in Hong Kong, which has been an important food source for the dolphins, and thus number of Chinese white dolphins has been dwindling. You must be wondering, are Chinese white dolphins in Hong Kong really seriously threatened?
Based on the press information released by this organization, and the survey information published by the Agriculture, Fisheries and Conservation Department (AFCD), we believe most probably the ‘dolphin census' cannot reflect the real status of Chinese white dolphins in Hong Kong. Here are the reasons:
• Hong Kong waters are not a confined water body. On the contrary, situated at Pearl River Estuary, Hong Kong waters are very much connected with those of China. Therefore, it is very common for Chinese white dolphins to traverse between Hong Kong and Chinese waters, leading to greatly variable dolphin abundance in Hong Kong. We believe only two hours of census (including one hour conducted on 18 June 05 plus the other one conducted 11 years ago) are not enough for estimating dolphin abundance in Hong Kong accurately.
• Environmental condition is a critical factor affecting dolphin encounter rate on sea. Different kinds of environmental factors do affect dolphin distribution and mode of activity. They include water current, visibility on sea, sea state, wind direction and wind velocity. On 18 June, Hong Kong was faced with strong southwestern wind, which created countless white caps on the sea. Rough sea conditions do make sighting dolphins difficult, and probably make researchers underestimate number of dolphin sightings. As a matter of fact, dolphin researchers worldwide usually abandon dolphin data collected under adverse weather conditions (exclude the data from analysis).
• Observers involved in the dolphin census were mostly volunteers recruited by the organizations. This might affect the accuracy and quality of the data collected. Dolphin observing is not a difficult task though, researchers conducting dolphin survey should possess a related qualification, experiences, and should have received related training. Collecting data for dolphin research is not a task ready for everyone.
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This is how dolphin field staff carry out line transect surveys |
• One of the most recognized and reliable method in the world to estimate dolphin abundance in an area (or estimate size of a dolphin population) is line transect method. Since 1995, the AFCD of the Hong Kong government has been continuously estimating and monitoring abundance of Chinese white dolphins in Hong Kong waters. In the past decade, dolphin researchers have been going for dolphin field surveys two to three times a week, following specified transect line; and the data collected have been used to establish a systematic database. The latest figure released by AFCD in 2005 indicate that number of Chinese white dolphins utilizing Hong Kong waters ranges from 91 (Spring) to 207 (Autumn). When compared to data in the past years, no significant increase and decline of number has been detected, so we may conclude that dolphin abundance in Hong Kong is rather stable. Since dolphin survey conducted by the AFCD has larger sampling size, and it has covered a much longer time period and much more extensive survey area, so we can see that dolphin abundance estimated by AFCD is more accurate and reliable.
Now you may be more relieved after learning that Hong Kong Chinese white dolphins are not in very serious trouble. We should always be cautious when someone or some environmental groups claim that "Chinese white dolphins are now endangered ". But of course, we should not ever slacken our effort on conserving them. For the most up-dated and accurate information on Chinese white dolphins, you may go to the marine conservation website of AFCD and HKDCS website. |
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