EUPHAUSIID SPATIAL DISTRIBUTION ACROSS A STEEP BATHYMETRIC FEATURE AND IMPLICATIONS FOR WHALE PREDATION (E)
Euphausiids are important prey for many marine organisms and often occur in patchy aggregations. Obligate euphausiid predators, such as blue whales, may be drawn into observable “hot spots” by the distribution of these aggregations. I investigated a blue whale hot spot called Nine Mile Bank near San Diego, CA, defined by an area of steep bathymetry, to determine whether the frequent whale sightings can be explained by the distribution of euphausiids across the bank. Surveys were conducted within the summer window of blue whale presence in July 2014 and June 2015. I derived a euphausiid index from EK60 multifrequency acoustic backscatter using dB differencing to determine the spatial patterns of euphausiid-like backscatterers. Consistently lower euphausiid-like backscatter was observed offshore of the bank, while the highest backscatter was over the bank more often than inshore of it. Species identification of mandibles from whale scat collected around the bank revealed Thysanoessa spinifera as primary blue whale prey. In a series of bongo net transects, T. spinifera was consistently less abundant offshore with its highest abundance inshore of the bank in 2014 and on the bank in 2015. Euphausia pacifica was also consumed by blue whales, though in much lower numbers, and was abundant throughout the study area. A whale visual survey in 2015 found a higher whale encounter rate associated with higher T. spinifera abundance, both peaking over the bank, suggesting that the whales may follow the movements of specific prey species, often in association with bathymetric features.
Nickels, C. F., Scripps Institution of Oceanography, USA, email@example.com
Location: 323 B
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