Monday 15 September 2014

Migration Update #1

One week and one-thousand kilometers later, three of our tagged whales are heading west...

Now that I am back in Christchurch, my updates to Megaptera14 are going to be about once a week. Monday nights (NZ time) work best.  But I do promise to keep updating you all so long as these tags are transmitting!

For this week's update, I've prepared some graphics. Top of the order is an update on where our whales have headed (this will be the standard lead-off position in the weeks ahead as well - Figure 1 is almost always a map when you're dealing with a ball-playing geoscientist!). There are several interesting points to make about the whale movement data presented in Figure 1 (below).

First, the migrating whales are moving west/northwest along very similar paths.  This 'route fidelity' is fascinating to me as none of these whales are ever in the same place at the same time. They might very well pass over the same chunk of ocean floor (~5000m below them!), but they're doing so at different times (n.b. - the tracks symbolized by triangles are from 2007! The larger colored circles are the 2014 data.). If there is an outbound migration corridor around Rarotonga, it is most certainly to the west-northwest. This gives us strong evidence to suggest potential 'no-long-line-fishing' zones during certain months of the year - a triangle connecting Rarotonga, Niue, and Palmerston would be a great start!

Figure 1 - Humpback Whale movements near Rarotonga (mid September, 2014)

 Second, our 2014 data is much higher temporal resolution than our 2007 data. In other words, we are receiving more transmissions per day from each tag in 2014 than we did from each tag in 2007. This is GREAT PROGRESS as the temporal resolution of the data dramatically impacts the level to which we can interpret each whale's movement behaviors! More data points means we have a greater chance of capturing turns, slow-downs, speed-ups, etc. I am very excited by the temporal quality of our data this year.  Here's hoping the tags keep transmitting for several months!!

Third, I might be seeing things, but it sure looks like 112726.14 got a call from Niue yesterday and decided to stop in for a visit. 'She' should arrive in Niue late on Sept. 15 local time if she keeps following the same trajectory. Now you might think I'm really starting to see things if you believe me when I say that it looks like one of the 2007 whales did a very similar thing just before its tag dropped (medium grey triangles approaching Niue from the east-southeast). Maybe polynesians really did 'ride whales' when they were island hopping during the past millennium?

Fourth, the Distance Traveled versus Day of Year plot shows that this year's migrators are a fast bunch.  They've been swimming ~6-7 km/hr for the past week straight without stopping - day or night. Cool.

For those of you who love data (like me), I've made a couple extra plots for your viewing pleasure and metacognitive pain. Figure 2 presents a couple pie charts. The one on the left shows the relative proportions of location data received by the 6 different Argos system satellites (MA, MB, NK, NN, NP, SR). The pie chart on the right shows the relative distribution of the location data's quality (3 is the best quality, zero is the worst quality - A and B could be good or could be bad, the Argos system couldn't tell). Clearly we're getting mostly 'B' quality data. This is not ideal, but it's not necessarily bad either...

Figure 2 - early to middle September (2014) Rarotonga humpback whale satellite transmission data - by Argos satellite (left) and data quality (right)

Figure 3 shows all of the whale locations the Argos system has received as a function of hour of the day. The vertical axis is plotting the 'error radius' of the whale location as determined by the Argos system (all those 'B' quality locations have largely 1-5 km errors - which is not so bad considering the spatial scale of the migrations). The parabolic curves are plotting when each of the Argos system satellites are able to receive radio transmissions (i.e. the vertical axis for the parabolic curves is basically how high in the sky the satellite is during its passage over the region). I can't see any clear pattern relating satellite position, or satellite count, to error radius. And it looks like our chosen transmission periods don't have many holes/gaps; we deliberately pre-programmed the transmitters to save battery and shut-down when the 'NP' satellite was passing as it was the sole satellite during its passages over Oceania this time of the year.

Figure 3 - Relationship between satellite transmission data quality (as Argos error radius), time of day, and receiving satellite position.

Perhaps the BEST thing about Figure 3 is that it shows quite clearly that we're capturing both sides of dawn and dusk nicely. Crepuscular (homework = look it up!) behaviors are common in the biosphere and I quite deliberately made sure that we set up the tags to collect data across these important times of the day. I'd estimate that half our data was collected during periods of darkness and half our data was collected during periods of daylight. The possibilities indeed are endless.

And I'm curious as to whether or not last week's class 2 solar flare wrought any havoc on the navigational systems used by these whales. Another topic for another day, perhaps?

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