western approaches of the English Channel and the effect of responsive movement
A survey using line-transect techniques was conducted during two winters providing the first estimates of common dolphin, Delphinus delphis, abundance (number of dolphins) on winter pelagic trawl fishing grounds in the English Channel. Independent teams of observers searched with binoculars or naked...
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Text |
| Language: | English |
| Subjects: | |
| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.172.1959 http://www.oers.ca/journal/Volume1/Issue1_5deboer2008.pdf |
| Summary: | A survey using line-transect techniques was conducted during two winters providing the first estimates of common dolphin, Delphinus delphis, abundance (number of dolphins) on winter pelagic trawl fishing grounds in the English Channel. Independent teams of observers searched with binoculars or naked eye. These methods were intended to allow for the calculation of a correction factor for both animals missed on the trackline and for responsive movement. Results indicated that the naked eye observers missed 7 % of the dolphins on the trackline, but that there was a strong responsive movement towards the vessel. Comparing initial locations of animals detected by the two independent teams showed that just using naked eye observations would result in apparent densities that were 1.5 times larger than the dual platform analysis. Using these factors the mean corrected winter density of common dolphins in the study area across both years was 0.74 dolphins/km 2 (CV = 0.39) giving a mean abundance of 3,055 dolphins (95 % CI = 1,425-6,544). However, these estimates are most likely positively biased due to responsive movement not being fully accounted for. Nevertheless, the relative index for abundance (number of schools per 100km effort, mean school size 5.1) was the highest recorded from comparable surveys in the North Atlantic and shows that the Channel is a very important winter habitat for common dolphins. [JMATE. 2008;1(1):15-21] Key Words: Delphinus delphis; line-transect survey, mark recapture distance sampling |
|---|