Increased productivity in the subantarctic ocean during Heinrich events

Massive iceberg discharges from the Northern Hemisphere ice sheets, 'Heinrich events', coincided with the coldest periods of the last ice age. There is widespread evidence for Heinrich events and their profound impact on the climate and circulation of the North Atlantic Ocean, but their influence beyond that region remains uncertain. Here we use a combination of molecular fingerprints of algal productivity and radioisotope tracers of sedimentation to document eight periods of increased productivity in the subpolar Southern Ocean during the past 70,000 years that occurred within 1,000-2,000 years of a Northern Hemisphere Heinrich event. We discuss possible causes for such a link, including increased supply of iron from upwelling and increased stratification during the growing season, which imply an alteration of the global ocean circulation during Heinrich events. The mechanisms linking North Atlantic iceberg discharges with subantarctic productivity remain unclear at this point. We suggest that understanding how the Southern Ocean was altered during these extreme climate perturbations is critical to understanding the role of the ocean in climate change.     

Intercepting the first rat ashore

A single Norway rat released on to a rat-free island was not caught for more than four months, despite intensive efforts to trap it. The rat first explored the 9.5-hectare island and then swam 400 metres across open water to another rat-free island, evading capture for 18 weeks until an aggressive combination of detection and trapping methods were deployed simultaneously. The exceptional difficulty of this capture indicates that methods normally used to eradicate rats in dense populations are unlikely to be effective on small numbers, a finding that could have global implications for conservation on protected islands.     

Why fishing magnifies fluctuations in fish abundance

It is now clear that fished populations can fluctuate more than unharvested stocks. However, it is not clear why. Here we distinguish among three major competing mechanisms for this phenomenon, by using the 50-year California Cooperative Oceanic Fisheries Investigations (CalCOFI) larval fish record. First, variable fishing pressure directly increases variability in exploited populations. Second, commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly. Third, age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates. We find no evidence for the first hypothesis, limited evidence for the second and strong evidence for the third. Therefore, in California Current fisheries, increased temporal variability in the population does not arise from variable exploitation, nor does it reflect direct environmental tracking. More fundamentally, it arises from increased instability in dynamics. This finding has implications for resource management as an empirical example of how selective harvesting can alter the basic dynamics of exploited populations, and lead to unstable booms and busts that can precede systematic declines in stock levels.