Mid-Atlantic Ridge: Turtles & Whales

Somewhere in the middle of the Atlantic ocean, but also at the edge of the Mid-Atlantic Ridge, real sea mountains rise above the sea floor, creating an ideal pit stop for marine animals, such as whales and turtles. From the previous articles you may remember how the Azores archipelago originated and how its origin contributed to the increased biodiversity now observable; there is however yet another topic we need to discuss.

Not long ago, loggerhead sea turtles (Caretta caretta) were studied to understand their migratory patterns and their growth and developmental success within the Azores archipelago. Genetic studies revealed that individuals found among the area in question originate by 90% from the southeastern United States.

Credit: Visitazores

Do you remember when Marlin and Dory from the animation movie “Finding Nemo” joined a group of turtles that were drifting within a strong current to rescue Nemo? Here the picture is quite similar because between late June and early November loggerhead sea turtles with a ~ 5 cm long carapace, called hatchlings, enter the unknown ocean from the nesting beaches of North Carolina and Florida to be transported by the Gulf Stream. Years later, these turtles, now grown up to ~ 25 cm in carapace length, appear around the Azores’ seamounts.

Before going further it is necessary to highlight that the transition from hatchlings to oceanic juveniles is thought to be associated with high mortality (up to 50%) due to nest destruction, hatchling emergence success, hatchling and neonatal survival and dispersal patterns. In addition, it is one of the most difficult life cycle phases to study. In fact, hatchlings from nests produced in the eastern North America appear in the Azores archipelago – 5000 km away – after approximately 3 years. Their abundance corresponds to the yearly environmental changes as well as changes in the production of nests. All this obtained knowledge is the result of years and years of meticulous observation and spotting of the individuals along predetermined transects of the area around the Azores archipelago.

Cross-correlation between the derived annual relative abundance of loggerhead sea turtles in the Azores calculated from the POPA visual sightings database (2001 – 2015), and annual nest counts from Florida core index beaches for 0 to 12 year lags. (see Vandeperre et al. 2019)

Once the loggerhead sea turtle juveniles reach the Archipelago, they visit different seamount peaks in dependence of food availability. Years pass before they are ready to return back to their origin as mature individuals, this time aged between 36 and 42 years. Long life, late maturity and low fecundity are the reasons why Caretta caretta are considered an endangered and threatened species. A silver lining whatsoever is the fact that stochastic effects appear to be quite irrelevant in the recruitment success, which is a plus considering that nowadays, humans have drastically contributed to the increase of these effects.

During winter, the waters around the Azores are visited by many species of whales, especially by blue, fin and sei whales. Beside feeding throughout winter in the Northern Hemisphere, blue and fin whales breed. Males exhibit reproductive displays represented by acoustic calls similar to songs with frequencies ranging around 20 Hz (humans can hear frequencies from 20 Hz to 20 kHz). The more intense and beautiful the song, the more chances males have to attract a female.

Additionally, blue whales seem to produce singular calls all year-round, which is thought to be associated with social interactions and feeding behaviour. Sei whales, however, do not produce distinctive songs, but are more similar to calls occurring in doublets and triplets, which represent contact calls between conspecifics.

A 5 year study of acoustic monitoring has shown an increased presence of blue and fin whale calls around the Azores archipelago in autumn, with progressive intensification during the winter season, and decrease in the springtime. Finally, a complete absence of blue and fin whale songs was recorded during the summer season when these cetaceans migrate towards high-latitude feeding areas. The summer season is also characterised by the lowest concentrations of chlorophyll a (present within algae), which in consequence represents the annual minimum in zooplankton concentrations.

The lack of phytoplankton (level 1 organisms) determines a lack of zooplankton (level 2 organisms) and therefore carnivores & top predators (level 3 & 4 organisms) have less food source. Credit: National Geographic

Sei whales, on the other hand, showed different call patterns with peaks in autumn and spring. Conversely to blue and fin whales, sei whales migrate north through the Azores towards the Labrador Sea during spring and back south towards the tropical breeding grounds during autumn. So the Azores archipelago in this case is only a brief stop for sei whales.

Observe the call peaks that vary depending on the species & season. Credit: Romagosa et al., 2020

Another interesting fact to point out is that fin whales feeding over deep waters display more vocalisations during night to increase the possibility to forage more vertically migrating prey near the seamounts.

The importance of studying vocalisation patterns is huge – it serves to understand the presence of different species of whales within a certain area, track their migration patterns to some extent but also understand the types of calls emitted for different reasons (communication between conspecifics, breeding, foraging, etc.). Although such studies have already been applied in different places around the world, this particular study is very recent, indicating the lack of such information, especially for the Atlantic Ocean.

Finally, the Azores archipelago not only hosts an incredibly high biodiversity of organisms, but it is also a stop for migratory species that benefit from this area so that they can propagate through time. Knowing how these species move and especially when, is certainly a peculiarity that must not be excluded when establishing laws regarding marine conservation.

  • Vandeperre, F., Parra, H., Pham, C.K., Machete, M., Santos, M., Bjorndal, K.A. and Bolten, A.B., 2019. Relative abundance of oceanic juvenile loggerhead sea turtles in relation to nest production at source rookeries: implications for recruitment dynamics. Scientific reports, 9(1), pp.1-12.
  • Romagosa, M., Baumgartner, M., Cascão, I., Lammers, M.O., Marques, T.A., Santos, R.S. and Silva, M.A., 2020. Baleen whale acoustic presence and behaviour at a Mid-Atlantic migratory habitat, the Azores Archipelago. Scientific reports, 10(1), pp.1-11.

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