{"id":582,"date":"2016-02-26T20:26:34","date_gmt":"2016-02-26T20:26:34","guid":{"rendered":"http:\/\/blogs.oregonstate.edu\/gemmlab\/?p=582"},"modified":"2016-02-26T20:26:34","modified_gmt":"2016-02-26T20:26:34","slug":"scratching-the-surface","status":"publish","type":"post","link":"https:\/\/blogs.oregonstate.edu\/gemmlab\/2016\/02\/26\/scratching-the-surface\/","title":{"rendered":"Scratching the Surface"},"content":{"rendered":"

By Dr. Leigh Torres, Assistant Professor, Oregon State University, Geospatial Ecology of Marine Megafauna Lab<\/em><\/p>\n

I have been reminded of a lesson I learned long ago: Never turn your back on the sea \u2013 it\u2019s always changing.<\/p>\n

The blue whales weren\u2019t where they were last time. I wrongly assumed oceanographic patterns would be similar to our last time out in 2014 and that the whales would be in the same area. But the ocean is dynamic \u2013 ever changing. I knew this. And I know it better now.<\/p>\n

Below (Fig. 1) are two satellite images of sea surface temperature (SST) within the South Taranaki Bight and west coast region of New Zealand that we surveyed in Jan-Feb 2014 and again recently during Jan-Feb 2016. The plot on the left describes ocean surface conditions in 2014 and illustrates how SST primarily ranged between 15 and 18 \u2070C. By comparison, the panel on the right depicts the sea surface conditions we just encountered during the 2016 field season, and a huge difference is apparent: this year SST ranged between 18 and 23 \u2070C, barely overlapping with the 2014 field season conditions.<\/p>\n

\"Figure<\/a>
Figure 1. A comparison of satellite images of sea surface temperature (SST) in the South Taranaki Bight region of New Zealand between late January 2014 and early February 2016. The white circles on each image denote where the majority of blue whales were encountered during each field season.<\/figcaption><\/figure>\n

While whales can live in a wide range of water temperatures, their prey is much pickier. Krill, tiny zooplankton that blue whales seek and devour in large quantities, tend to aggregate in pockets of nutrient-rich, cool water in this region of New Zealand. During the 2014 field season, we encountered most blue whales in an area where SST was about 15 \u2070C (within the white circle in the left panel of Fig. 1). This year, there was no cool water anywhere and we mainly found the whales off the west coast of Kahurangi shoals in about 21 \u2070C water (within the white circle in the right panel of Fig. 1. NB: the cooler water in the Cook Strait in the southeast region of the right panel is a different water mass than preferred by blue whales and does not contain their prey.)<\/p>\n

The hot water we found this year across the survey region can likely be attributed, at least in part, to the El Ni\u00f1o conditions that are occurring across the Pacific Ocean currently. El Ni\u00f1o has brought unusually settled conditions to New Zealand this summer, which means relatively few high wind events that normally churn up the ocean and mix the cool, nutrient rich deep water with the hot surface layer water. These are ideal conditions for Kiwi sun-bathers, but the ocean remains highly stratified with a stable layer of hot water on top. However, this stratification does not necessarily mean the ocean is un-productive \u2013 it only means that the SST satellite images are virtually useless for helping us to find whales this year.<\/p>\n

Although SST data can be informative about ocean conditions, it only reflects what is happening in the thin, top slice of the ocean. Sub-surface conditions can be very different. Ocean conditions during our two survey periods in 2014 and 2016 could be more similar when compared underwater than when viewed from above. This is why sub-surface sensors and data collection is critical to marine studies. Ocean conditions in 2014 and 2016 could both potentially provide good habitat for the whales. In fact, where and when we encountered whales during both 2014 and 2016 we also detected high densities of krill through hydro-acoustics (Fig. 2). However, in 2014 we observed many surface swarms of krill that we rarely saw this recent field season, which could be due to elevated SST. But, we did capture cool drone footage this year of a brief sub-surface foraging event:<\/p>\n