Diving efficiency at depth and pre-breeding foraging effort increase with haemoglobin levels in gentoo penguins

Individual differences in oxygen storage and carrying capacity have been associated with fitness-related traits and, for air-breathing aquatic animals, to diving ability and foraging success. In winter, many seabirds must replenish the energy reserves they have depleted during the breeding period. Thus, winter foraging efficiency can influence their upcoming breeding behaviour. Using gentoo penguins ( Pygoscelis papua ), we investigate (1) if inter-individual variation in diving efficiency (proportion of time spent at the bottom) is associated with indices of oxygen storage and carrying capacity (haemoglobin, haematocrit, body mass), and (2) if measures of pre-breeding foraging effort (mean trip duration, total time at sea, and vertical distance travelled) are associated with these oxygen indices and breeding status. Haemoglobin was positively correlated with diving efficiency, particularly for deeper dives, and only penguins with high haemoglobin levels frequently dove at depth ≥ 140 m. Such differences could affect resource access. However, potentially because reaching deep offshore waters requires travelling more than foraging nearshore, vertical distance travelled pre-breeding increased with haemoglobin levels. The relationship with haematocrit was non-linear, suggesting that commonly-used analyses may be inappropriate for this index. We found that early-laying penguins spent less time at sea prior to nesting than non-breeding penguins, suggesting that more efficient foragers lay earlier. Given that diving efficiency at depth is linked to aerobic capacity, anthropogenic activities taking place in either nearshore or offshore waters (e.g., shallow water fisheries, offshore oil rigs) may have differing impacts on individuals. Further understanding these links could help the conservation of diving species. ### Competing Interest Statement The authors have declared no competing interest.