Baleen whale migration: data analysis (part 2)

by Enrico Villa

Looking for Part 1 of this article? Click here!

Author's note: since we operate commercial whale watching trips supported by land-based lookouts, there are several sources of bias potentially affecting our sighting data. For this reason, the analysis below should be considered a work in progress - a simple, preliminary attempt to find patterns in the data we collected. Furthermore, the hypothesis we formulate here cannot be put to test based on our data alone, so that the main goal here is to stimulate discussion and, hopefully, contribute to further studies on larger, more appropriate datasets.

Remember from Part 1: if there is a lot of phytoplankton in March (for which we use chlorophyll concentration as a proxy), you will find a lot of krill in April. With a high concentration of phytoplankton in April, you will see a high abundance of krill in May, etc. For this reason, to achieve a better graphical representation of the correlation between chlorophyll concentration and baleen whale sightings, the number of sightings for any month is superimposed with the chlorophyll value referring to the month before.

ANALYSIS OF BLUE WHALE AND FIN WHALE DATA

Mouse over the interactive charts below to see the number of baleen whale sightings for each spring month and the chlorophyll value corresponding to the previous month. The vertical axis represents number of sightings. To read cholorophyll values (expressed in mg/m³), mouse over the green line in correspondence to any month.

2010

RESULT: a positive correlation is observed between chlorophyll concentration (month before whales are seen) and sightings, for both Blue whales and Fin whales.
OBSERVATION: chlorophyll concentration in May (superimposed with June sightings) drops below 0.2 mg/m^-3. As a consequence, there is too little food left and we have almost no baleen whale sightings in June.
OBSERVATION: since Blue whales only feed on krill and Fin whales also feed on fish, Blue whales may be more sensitive to drops in primary production.

2011

RESULT: a positive correlation is observed between chlorophyll concentration and Blue whale sightings, but NOT for Fin whale sightings.
HYPOTHESIS: Cholorohyll was still reasonably high in April (so that there was krill in May) but, possibly, competition with Fin whales drove the Blue whales away from our study area, as they have higher requirements for food availability.
OBSERVATION: Since Fin whales also feed on fish, the foraging habitat may be more suitable for them than for Blue whales.

2012

RESULT: like in 2010, a positive correlation is observed between chlorophyll concentration and sightings, for both Blue whales and Fin whales.
OBSERVATION: unlike in 2010, in 2012 we had more baleen whale sightings in April than in May, because chlorophyll values were higher in March than in April, while it was the other way around in 2010.

2013

RESULT: No positive correlation found between chlorophyll concentration and baleen whale sightings, neither for Blue whales nor for Fin whales. The trend for cholorophyll is the same as in 2012 (higher in March than in April), but we did not have more whales in April than in May.
OBSERVATION: unlike 2012, in 2013 chlorophyll concentration keeps being high in April, even if it was higher in March.
HYPOTHESIS: While chlorophyll concentration dropped in April, phytoplankton turnover was so high that it could still sustain large aggregations of krill, which means plenty of food for baleen whales. Possibly, when the drop in cholorophyll concentration does not bring it below - let say - 0.2 mg/m^-3, we should not expect a decrease of baleen whale sightings on the following month.
HYPOTHESIS: There could also be a packing effect. Since krill is still abundant in May, whales that arrived in the study area in April may be incentivated to stay longer. At the same time, more whales add up as they reach the Azores from their breeding grounds and we end up having more sightings ('old' whales + 'new' whales).

We could call this phenomenon whale packing and the hypothesis could be tested by comparing the time of residence of baleen whales in years when Chlorophyll drops dramatically in April with the time of residence in years where chlorophyll remains reasonably high. For such comparison, resightings of individuals within the same season or satellite telemetry data from tagged animals could be used; a statistically significant difference would support the whale packing hypothesis, while the lack of it would disprove it.

2014

RESULT: same situation as in 2013, but with higher values for chlorophyll concentration, so that the number of whale sightings was higher than in 2013.
OBSERVATION: unlike in 2013, the chlorophyll value for May was still relatively high (well above 2.0 mg/m^-3), so that we had baleen whales also in June.

2015

RESULT: same scenario as in 2014.

2016

RESULT: same scenario as in 2011, except sightings of both Blue whales and Fin whales are exceptionally extended to the end of July, due to the concentration of chlorophyll being still high in May (and still above 2.0 mg/m^-3 in June).

2017

RESULT: same scenario as in 2014, but with even higher chlorophyll in May and even June.

2018

RESULT: same scenario as in 2010.

2019

RESULT: same scenario as in 2012.

FINAL REMARKS

How do baleen whales know in advance how much krill there is going be off the Azores on any particular season?
Probably, they don't! However, whales do remember the spots that, historically, turned out to be good foraging grounds and they keep visiting them year after year.

Baleen whales are aware there are yearly fluctuations in food availability in any particular spot and they can definitely live with that.

In years where krill is relatively scarce in the Azores, they will not linger for long. However, they are not all migrating at the same exact time, so that we never, ever had a spring without baleen whales!

A Whale migration is an unbelievably long journey, especially if you consider the round trip. Baleen whales migrate because having two distinct habitats (one for breeding, one for feeding) is an energetically favoured strategy, despite the high cost of moving between them.

Baleen whales leave the Azores some time in late spring when food here becomes scarce. They know there will be more to eat in the summer at higher latitudes.
In other words, they do not go all the way to Norway, Iceland, Greenland, etc. to keep fit and they would be happy to stay longer in the Azores if krill kept being plentiful.

In a world where climate change is such a serious issue, it is important to keep monitoring the migration routes of animals such as baleen whales, as well as their timing.

The chlorophyll data used in this article were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC.
J. G. Acker and G. Leptoukh, “Online Analysis Enhances Use of NASA Earth Science Data”, Eos, Trans. AGU, Vol. 88, No. 2 (9 January 2007), pages 14 and 17.