Tag: oceanography

Scratching the Surface

By Dr. Leigh Torres, Assistant Professor, Oregon State University, Geospatial Ecology of Marine Megafauna Lab

I have been reminded of a lesson I learned long ago: Never turn your back on the sea – it’s always changing.

The blue whales weren’t 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 – ever changing. I knew this. And I know it better now.

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 ⁰C. 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 ⁰C, barely overlapping with the 2014 field season conditions.

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.
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.

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 ⁰C (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 ⁰C 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.)

The hot water we found this year across the survey region can likely be attributed, at least in part, to the El Niño conditions that are occurring across the Pacific Ocean currently. El Niño 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 – it only means that the SST satellite images are virtually useless for helping us to find whales this year.

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:

An overhead look of a blue whale foraging event as the animal approaches the surface. Note how the distended ventral (throat) grooves of the buccal cavity (mouth) are visible. This is a big gulp of prey (krill) and water. The video was captured using a DJI Phantom 3 drone in the South Taranaki Bight of New Zealand in on February 2, 2016 under a research permit from the New Zealand Department of Conservation (DOC) permit # 45780-MAR issued to Oregon State University.

Figure 2. An echo-sounder image of dense krill patches at 50-80 m depth captured through hydroacoustics in the South Taranaki Bight region of New Zealand.
Figure 2. An echo-sounder image of dense krill patches at 50-80 m depth captured through hydroacoustics in the South Taranaki Bight region of New Zealand.

Below are SST anomaly plots of January 2014 and January 2016 (Fig. 3). These anomaly plots show how different the SST was compared to the long-term average SST across the New Zealand region. As you can see, in 2014 (left panel) SST conditions in our study area were ~1 ⁰C below average, while in 2016 (right panel) SST conditions were ~1 ⁰C above average. So, what are normal conditions? What can we expect next year when we come back to survey again for blue whales across this region? These are challenging questions and illustrate why marine ecology studies like this one must be conducted over many years. One year is just a snap shot in the lifetime of the oceans.

Figure 3. Comparison of sea surface temperature (SST) anomaly plots of the New Zealand region between January 2014 (left) and January 2016 (right). The white box in both plots denotes the general location of our blue whale study region. (Apologies for the different formats of these plots - the underlying data is directly comparable.)
Figure 3. Comparison of sea surface temperature (SST) anomaly plots of the New Zealand region between January 2014 (left) and January 2016 (right). The white box in both plots denotes the general location of our blue whale study region. (Apologies for the different formats of these plots – the underlying data is directly comparable.)

Like all marine megafauna, blue whales move far and fast to adjust their distribution patterns according to ocean conditions. So, I can’t tell you what the ocean will be like in January 2017 or where the whales will be, but as we continue to study this marine ecosystem and its inhabitants our understanding of ocean patterns and whale ecology will improve. With every year of new data we will be able to better predict ocean and blue whale distribution patterns, providing managers with the tools they need to protect our marine environment. For now, we are just beginning to scratch the (sea) surface.

 

 

 

Blues Clues

Although blue whales are big, the South Taranaki Bight is bigger. So finding them is not straight forward. In fact, with little prior research in this area, the main focus of our project is to gain a better understanding of blue whale distribution patterns in the region. So, while bouncing around on the sea, we are collecting habitat data that we relate to whale occurrence data to learn what makes preferred whale habitat.

We conduct CTD casts. CTD stands for Conductivity, Temperature and Depth. This is an instrument we lower down to the bottom of the ocean on a line and along the w ay it records temperature and salinity (conductivity) data at all depths. This data describes the water structure at that location, such as the depth of the thermocline. The ocean is often layered with warm, low-salt water on top, and cooler and salty water at the bottom. This thermocline can act as a boundary above which prey aggregate.

Todd and Andrew deploy the CTD off the R/V Ikatere.
Todd and Andrew deploy the CTD off the R/V Ikatere. (Photo by Callum Lilley)
CTD cast
Example data retrieved from a CTD cast showing how temperature (green line) decreases and salinity (red line) increases as it descends through the water column (depth on y-axis).

We also have a transducer on board that we use to record the presence of biological material in the ocean, like krill (blue whale prey). This transducer emits pings of sound through the water column and the echoes bounce back, either off the seafloor, krill or fish. This glorified echosounder records where blue whale prey is, and is not.

Example display image from our echosounder (EK60) showing patches of prey (likely krill) in the upper surface layer.
Example display image from our echosounder (EK60) showing patches of prey (likely krill) in the upper surface layer.

Additionally, the research vessel is always recording surface temperature (SST). I monitor this SST readout somewhat obsessively while at-sea as well as study the latest SST satellite images. Using these two bits of data as my “blues clues”, we search for blue whales.

After a bumpy ride across the Cook Strait we had a good spell of weather last week. We covered a lot of ground, deploying our 5 hydrophones across the Bight and keeping our eyes peeled for blows. Our first day out we found three whales. Fantastic sightings. But, as we continued to survey through warm, low productivity water we found no signs of blue whales. The third day out was a beauty – the type of day I wish for: low swell and low winds – perfect for whale finding. We covered 220 nautical miles this day (deploying 2 hydrophones) and we searched and searched. But no whales. I could see from the SST satellite image that the whole Bight was really warm: about 20 ⁰C. I could also see a strip of cold water down south, toward Farewell Spit. I said “Let’s go there”.

Sea surface temperature (SST) satellite image of the South Taranaki Bight region in New Zealand that shows mostly warm water with a plume of colder water down south.
Sea surface temperature (SST) satellite image of the South Taranaki Bight region in New Zealand that shows mostly warm water with a plume of colder water down south.

After twelve and a half hours of survey effort through clear, blue, warm water, we finally saw the water temperature drop (to about 18 ⁰C) and the water color turn green. We started to see gannets, petrels, shearwaters, and common dolphins feeding. Then I heard the magic words come from Todd’s mouth: “Blow!” So began our sunset sighting. From 7:30 to 10 pm we worked with four blue whales capturing photographs and biopsy samples, and echosounder prey data.

Diving blue whale in the South Taranaki Bight, NZ (photo by Leigh Torres)
Diving blue whale in the South Taranaki Bight, NZ (photo by Leigh Torres)

This is an example of a species-habitat relationship that marine ecologists like me seek to document. We observe and record patterns like this so that we can better understand and predict the distribution of blue whales. Such information is critical for environmental managers to have in order to effectively regulate where and when human activities that may impact blue whales can occur. Over the next two weeks we will continue to document blue whale habitat in the South Taranaki Bight region of New Zealand.