Seasonal and ontogenetic changes in subtropical krill lipids: implications for temporary blue whale and resident fin whale stocks that inhabit the Gulf of California

The subtropical-neritic euphausiid Nyctiphanes simplex is a key food source for several resident and migratory organisms in the Gulf of California and the West Coast of the Baja California peninsula. Possibly, their main predators are two large mammals: a resident population of fin whales (Balaenoptera physalus), and blue whales (B. musculus) during their winter migration. Evidence shows that both baleen whale species breed and feed in the Gulf of California. In order to estimate the relevance of the region in terms of biomass intake by predators, it is necessary to understand and estimate the effect of development, reproduction, season and location on the energy content of krill. The development and maturity processes of this euphausiid were studied along with changes in size, weight and biochemical composition of the different life stages. Ovigerous females carrying eggs in pouches provide significantly higher energy than other life stages. Contribution of total lipids to dry weight was higher during the most productive months (February and May) in clear association with the spring reproductive peak previously reported for the species. Nonetheless, total energy content and body condition were higher during warmer months as a consequence of an increase in protein content, suggesting an increase in carnivory during warm months. Fasting experiments revealed that it was possible to detect unfavourable feeding conditions between 2.5 to 5 days as lipid content and the hepatosomatic index had a large negative significant correlation with fasting time. Wild euphausiids collected during spring 2010 never fasted >2.5 days and fatty acid analysis revealed that diatoms were the main food source in upwelling areas. However, cyanobacteria, Chlorophyceae or even copepods seemed to be a relevant food source for N. simplex in warmer areas. Energy content of euphausiids, together with seasonal patterns, metabolic rates and estimated abundances of blue and fin whales (Balaenoptera musculus and B. physalus) were used to calculate prey biomass demands of both top predators. Individual estimates of daily energy requirements during winter were 10–19% higher for blue whales than for fin whales. However, individual estimates of prey biomass demand during spring were ~6% higher for fin whales. In fact, the density of krill required to satisfy the estimated biomass demand by female fin whales at the end of spring is very close to the maximum estimated density of euphausiids in the region. This suggests that alternative food sources are needed to sustain the resident population of fin whales in the area. On the other hand, the density of krill required to satisfy the estimated biomass demands by blue whales is within the limits of the estimated density of winter swarms. This is important as the North Pacific population of blue whales appears to be clearly recovering from exploitation. However, the closeness between the densities of euphausiids required in spring and those estimated in the field suggest that a decrease on euphausiid population density might be key in the migratory movement of blue whales to more profitable feeding areas.