Major Pacific study reveals top predators homing in on ocean's sweet-spots

By Martin Leggett - 22 Jun 2011 17:11:0 GMT
Major Pacific study reveals top predators homing in on ocean's sweet-spots

(Image: Schematic of dominant ocean current features in the north Pacific Ocean. The California Current Large Marine Ecosystem is outlined (dashed line). The north Pacific Transition Zone is delineated by the dotted lines.)

The vast expanse of the Pacific Ocean is anything but a featureless blue plain - especially to the myriad of sharks, whales, seals and turtles that trek its endless waters. A decade-long study, published in today's Nature, has revealed in massive detail that shifting world - one of nutrient-poor deserts, girded by writhing currents that are rich in life. The ocean's top predators are found to be chasing between these vibrant biological hotspots, as the seasons and oceanic currents push them around the Pacific basin.

This ambitious study of the Pacific's top predators - knowns as TOPP, for the Tagging of Pacific Predators - is unprecedented in scope and detail. Twenty-three of the ocean's signature species were tracked using 4,300 electronic tags over 10 years. The welter of information on the position and migration of these creatures -  ranging from blue whales to mako sharks to blue-fin tuna to black-footed albatrosses - was combined with satellite data covering temperature, salinity and chlorophyll fluctuations.

Quarterly median sea surface temperatures (a-d) and chlorophyll-a from 2002- 2009 (e-h). Contour lines represent the 95th percentile of relative density for the entire northeast Pacific Ocean shown in Figure 4a,b (the highest 5% of relative densities fall within the contour lines). The area of highest relative density moved northward as waters warmed and chlorophyll-a increased (quarter 3) and southward (quarter 4) as waters cooled.

That enabled the 75 scientists involved - participating is what is being hailed as the largest ever 'biologging' study - to come to new conclusions on what drives the migration of the ocean's top predators. 'This is the first publication that pulls all of the pieces together in one place,' said Dr. Daniel Costa, from the University of California. 'We brought together a large team of investigators to study diverse species and look at how these organisms use the ocean. It is an unprecedented examination of so many species over such a large scale.'

It seems that predators across the species boundaries share an uncanny knack to sense where the rich-pickings are, and to home in on them unfailingly - across thousands of miles, year after year. One of the Pacific's real sweet spots for attracting predators is the California Current, which flows south along the US west coast. Like the North Pacific Transition Zone, found between Hawaii and Alaska, this zone of upwelling cool waters, loaded with plankton, is perfect for supporting vast plumes of krill.

Track duration and fidelity. Multi-year track displaying fidelity in consecutive years (a) Pacific bluefin tuna (903 days), (b) Salmon shark (862 days) and (c) Mako shark (707 days). (d) Female northern elephant seals tagged in consecutive years show fidelity to rookery and migration paths. Lower track: female tracked during post-moult migrations of 2004 and 2005. Upper track: female tracked during post-breeding migrations in 2005 and 2006.

As the founding layer of the marine food chain, krill swarms then bring in many other predators - from blue whales, that feed on them directly, to blue-fin tuna, which are after the fish that also feast on the krill. Whilst some predators are happy to follow the California Current throughout their lives - such as California sea lions - others home in from the other side of the Pacific, like the maturing blue-fin tuna coming from Japan.

The ability to map how the ocean's main predators respond to shifting bio-hotspots has left scientists excited - such information is seen as essential in helping the preservation of vulnerable oceanic ecosystems. "Using satellite observations of temperature and chlorophyll concentrations alone, we can now predict when and where individual species are likely to be in a given ocean region and begin to understand factors that control their movements. This is fundamental to the concept of ecosystem-based management," said Dr. Costa.