Diving patterns in shags and cormorants (Phalacrocorax): tests of an optimal breathing model

Four species of shags and cormorants were observed while diving for food in estuaries and coastal waters in southwest England and the South Island of New Zealand. Times spent on the surface and under water were recorded. The correlations between dive and surface times were examined between and within bouts of dives. All species showed a positive correlation between the mean dive time and the mean surface time for the bout. Analysis of within-bout correlations, however, showed different patterns between species. For the Shag Phalacrocorax aristotelis, there was a positive relationship within bouts between dive time and the preceding surface time, and correlations between dive time and the succeeding surface time were due only to the autocorrelation of surface times. This anticipatory breathing pattern was predicted by an optimal breathing model proposed by D. L. Kramer. For the Little Shag Phalacrocorax melanoleucus, dive times were significantly correlated within bouts with the following surface time, but correlations with the succeeding surface time were due only to the autocorrelation of surface times. This implies that breathing in this species is reactive rather than anticipatory. The Cormorant Phalacrocorax carbo and the Pied Shag Phalacrocorax varius showed independent within-bout correlations with both preceding and succeeding surface times, implying both anticipatory and reactive breathing. The within-bout dependence of dive time on surface times was weak in all species, suggesting that oxygen is stored from one dive to another to some extent.