Disentangling the cause of a catastrophic population decline in a large marine mammal.

Considerable uncertainties often surround the causes of long-term changes in population abundance. One striking example is the precipitous decline of southern sea lions (SSL; Otariaflavescens) at the Falkland Islands, from 80 555 pups in the mid 1930s to just 5506 pups in 1965. Despite an increase in SSL abundance over the past two decades, the population has not recovered, with the number of pups born in 2014 (minimum 4443 pups) less than 6% of the 1930s estimate. The order-of-magnitude decline is primarily attributed to commercial sealing in Argentina. Here, we test this established paradigm and alternative hypotheses by assessing (1) commercial sealing at the Falkland Islands, (2) winter migration of SSL from the Falkland Islands to Argentina, (3) whether the number of SSL in Argentina could have sustained the reported level of exploitation, and (4) environmental change. The most parsimonious hypothesis explaining the SSL population decline was environmental change. Specifically, analysis of 160 years of winter sea surface temperatures revealed marked changes, including a period of warming between 1930 and 1950 that was consistent with the period of SSL decline. Sea surface temperature changes likely influenced the distribution or availability of SSL prey and impacted its population dynamics. We suggest that historical harvesting may not always be the "smoking gun" as is often purported. Rather, our conclusions support the growing evidence for bottom-up forcing on the abundance of species at lower trophic levels (e.g., plankton and fish) and resulting impacts on higher trophic levels across a broad range of ecosystems.

Whales from space: counting southern right whales by satellite.

We describe a method of identifying and counting whales using very high resolution satellite imagery through the example of southern right whales breeding in part of the Golfo Nuevo, Península Valdés in Argentina. Southern right whales have been extensively hunted over the last 300 years and although numbers have recovered from near extinction in the early 20(th) century, current populations are fragmented and are estimated at only a small fraction of pre-hunting total. Recent extreme right whale calf mortality events at Península Valdés, which constitutes the largest single population, have raised fresh concern for the future of the species. The WorldView2 satellite has a maximum 50 cm resolution and a water penetrating coastal band in the far-blue part of the spectrum that allows it to see deeper into the water column. Using an image covering 113 km², we identified 55 probable whales and 23 other features that are possibly whales, with a further 13 objects that are only detected by the coastal band. Comparison of a number of classification techniques, to automatically detect whale-like objects, showed that a simple thresholding technique of the panchromatic and coastal band delivered the best results. This is the first successful study using satellite imagery to count whales; a pragmatic, transferable method using this rapidly advancing technology that has major implications for future surveys of cetacean populations.