AbstractEstimations of gray whale abundance have generally assumed that shore‐based observers record all whales migrating through the viewing area during periods uncompromised by visibility. We tested the repeatability of data collected at the standard gray whale census site at Granite Canyon Marine Laboratory in central California by using pairs of observers maintaining independent sighting records. Proximal shore sites were occupied 6 d (60 h) in January 1986 where one team counted 845 whales in 427 groups while the other team counted 990 whales in 477 groups. A comparison of the records showed that the first team missed 290 whales seen by the second team, and the second team missed 204 whales seen by the first team. The total number of whales in the viewing area was calculated for each team by the Petersen estimate, using mutually sighted whale groups as “recaptures”. On average, observers recorded only 79 of the whales. More whales (68) were missed when entire groups of whales were not seen rather than when groups were undercounted (32). Visibility did not appear to affect observed rates of missed whales. Whales migrating at intermediate distances from the shore were less often missed than were those>6 km or<1 km offshore. This count discrepancy test confirms that an uncorrected calculation of population size for gray whales based on sighting records from solitary observers will be underesti
展开▼
