Diel variation in migration rates of eastern Pacific gray whales measured with thermal imaging sensors

We recorded the blows of gray whales during their southbound migration past central California in January 1994, 1995, and 1996, using thermal imaging sensors. For our sampling purposes, we defined day (0730-1630) and night (1 630-0730) to coincide with the on/off effort periods of the visual counts b eing conducted concurrently We pooled data across the three years of sampli ng and tested for diel variation in surfacing interval, pod size, offshore distance, migration rate, and swimming speed by comparing paired day/night means for samples collected within the respective 24-h period. We performed these tests using data from the entire migration period and then repeated the tests for samples collected prior to and after the approximate median m igration date (15 January). Over the entire migration period we observed la rger diurnal pod sizes ((x) over bar(day) = 1.75 +/- 0.280, (x) over bar(ni ght) = 1.63 +/- 0.232) and greater diurnal offshore distances ((x) over bar (day) = 2.30 +/- 0.328 km, (x) over bar(night) = 2.03 +/- 0.356 km) but fou nd no diel variation in surfacing interval. For the entire migration period , the nocturnal migration rate (average number of whales passing per hour) was higher than the diurnal rate. During the first half of the migration we detected no diel variation in pod size or surfacing interval, but diurnal offshore distances were larger than at night ((x) over bar(day) = 2.28 +/- 0.273 km, (x) over bar(night) = 1.96 +/- 0.318 km). Diurnal and noctural mi gration rates prior to 15 January were not different. During the second hal f of the migration, there was no diel variation in surfacing interval, pod site, or distance offshore, but the nocturnal migration rate was higher (28 %, SE = 11.6%) than the diurnal rate. We found no diel variation in swimmin g speed in any comparison. We propose thar later migrants socialite more du ring the day, which effectively slows their diurnal rate of migration relat ive to nocturnal rates.