Dead krill at Praia Formosa

A few days ago a friend told me that he had witnessed thousands of shrimps dying at Praia Formosa.

He told me that the sea was calm that day (at 21-07-2015), only very small waves, almost like a lake. Then he noticed there was something unusual, many dead reddish shrimps were floating on the water surface (some of them were still alive), so many that people swimming could feel shrimps touching their skin.

I decided to go there to check it myself. And I did found thousands of specimens dead in the beach sand:
Euphausia-gibboides-die-off

Species identification

I collected biological material to try an identification. I found out that they were not shrimps but krill. The species is Euphausia gibboides, a krill species that occur at Madeira latitudes. Here is a photo of a specimen I have collected:
Euphausia gibboides-specimen
The specimen was 21 mm long.

Some of the features used to identify it were its round eye, a pointed rostrum, a mid-dorsal spine and a lappet (small process extending dorsally from the end of the 1st segment of the 1st antenna) forward and outward-directed [1].

Euphausia-gibboides-distribution

See Euphausia gibboides distribution map [2] in the Atlantic:

This species eats microplankton and is itself an important prey for fish, birds and whales [3].

It do not live in the coast between rocks like some shrimp species do. It is not benthic (living in the bottom of the sea), they are creatures that live in the water column.

thermocline

This species lives in and below the thermocline (thermocline is the temperature transition layer between the mixed layer at the surface and the deep water layer). Lives in cool subsurface layers, and adults migrate down to 300-400 m by day [4].

So, I would not expect to find this species near the coast.

What could have happen to make the krill die in mass at the beach?

What made the krill die in mass?

Was this a natural die-off, a natural phenomenon, maybe as consequence of sea changes? Is it related to fishing activities or to a pollution event?

Was the krill captured offshore by fishing boats and released near Praia Formosa where they would not find the cool layers they need to survive?

In Madeira there is no continental shelf, the bottom of the ocean gets very deep not far away from the shore. Maybe somehow they end up near the beach by night, brought by the waves, and when the day came they were not able to go back to higher depths where the temperature is colder. If so, they were stuck there at Praia Formosa at a depth of only a few meters where the quite waters of that day, warmed by the sun, would prove fatal to them.

Did the krill die as a consequence of some pollution event? Was the water contaminated that day, like some people had claimed? It would only make sense if it was a specific kind of pollution that would affect only this species. I did not hear of dead fish or the death of any other kind of shrimp (I think none of the benthic shrimp species were found dead that day). Personally I think it was not a case of pollution.

Maybe it was only some natural phenomenon related to sea streams and water temperature changes.

Tagged on: , ,

2 thoughts on “Dead krill at Praia Formosa

  1. atownsend@ucsd.edu'Annie Townsend

    https://scripps.ucsd.edu/zooplanktonguide/species/euphausia-gibboides

    Hi,
    I found your observation of stranding of a euphausiid species on the beaches of Madeira interesting. From the photo you posted above, it appears that the eyes are bilobed and not rouned; also I cannot see the spine on the 3rd abd. segment. Is your specimen eyes’ damaged? Please see the above website and compare to your specimens if you still have them. I want to be able to confirm your identification.

    Thank you,
    Annie W. Townsend
    Retired. UCSD/SIO
    USA

    1. MadeiraNaturalist Post author

      Hello Annie

      Thank you for your comment.
      The eyes are not bilobed, they have shrunk after they dried but they were quite round when the specimen was still wet laying on the beach. Yes, the spine on the 3rd segment is not evident in that photo. The spine is visible in the following image:
      Euphausia-gibboides-spine

      I’ve checked the photos on your link and they do match my specimens too.

      In another time I have also identified a specimen of Meganyctiphanes norvegica that was on the sand beach, but it was only one specimen, nothing like the thousands of specimens of Euphausia gibboides that I had found before.

Leave a Reply

Your email address will not be published. Required fields are marked *