Effect of variable oceanic productivity on the survival of an upper trophic predator, the Hawaiian monk seal Monachus schauinslandi

Abstract

The Hawaiian monk seal population is declining, and low juvenile survival due to prey limitation is believed to be a primary cause. The Transition Zone Chlorophyll Front (TZCF) is a large-scale oceanographic feature separating the vertically stratified, low surface chlorophyll subtropical waters and the vertically mixed, cool, high chlorophyll Transition Zone waters. The TZCF annually migrates over 1000 km in latitude, and its southern extent in winter varies. We hypothesize that when the front migrates southward, it brings colder, more productive waters into monk seal foraging habitat, thereby enhancing the prey base and consequently survival. We expect this effect will be strongest at seal populations situated farthest north and nearest the TZCF. To test this hypothesis, we explored relationships of the survival of more than 3000 monk seals during 1984-2004 to the southernmost latitude of the 18°C isotherm (a proxy for the TZCF). We found a statistically significant non-linear relationship between the winter position of the TZCF and survival of monk seals through 4 yr of age at the most northerly atolls. When the front remained farther north, survival was poorer. The relationship was strongest following a 1 or 2 yr lag, perhaps indicating the time required for enhanced productivity to influence the food web and improve the seals' prey base. No such relationship was found at subpopulations located farther south or among adult animals at any site. Variation in ocean productivity may mediate prey availability in monk seal foraging habitat and consequently influence juvenile survival in the northern portion of their range. © Inter-Research 2007.