The sandeel's sense of sand

Sandeels behavior monitored

Sandeels regularly burrow as a natural part of their behavior in sandy areas, probably to hide and rest, for example at night and in winter to save energy. The fish has an elongated body which is adapted to digging and living in gravel and sand, a so-called fossorial lifestyle.

But according to the researchers' new discoveries, sandeels will not burrow into any type of sand. In the study, the researchers conducted an experiment in which they monitored the burrowing behavior of coastal bivalves when they are given the opportunity to choose different sediments that can be characterized based on different grain sizes. 

Here, the researchers found that the small sandeel (Ammodytes tobianus) has clear preferences for sandy bottoms with grain sizes of 0.5-2.0 mm rather than both finer and coarser sediment types. 

To the researchers' surprise, it is very close to the same type of sand that the lesser sandeel, also kvown as Raitt's sandeel, (Ammodytes marinus) which is a closely related species to the small sandeel, prefers. The lesser sandeels preferences for sand were also mapped over 20 years ago by researchers from DTU Aqua. Some of the same researchers are also involved in the new study on the small sandeel. 

The new knowledge has important implications for the understanding of the small sandeel's choice of habitat and geographical distribution. Unlike the lesser sandeel, the small sandeel's role in ecosystems is less well known because it has not been the subject of as intensive scientific research as the lesser sandeel. 

"Although knowledge about the small sandeel is limited, there is no reason to believe that the small sandeel's contribution to the functioning of the ecosystems is different from that of the lesser sandeel. The small sandeel plays an especially important role in coastal areas and inland Danish waters where it dominates," says Ole Henriksen.

Marine sediment extraction can potentially put pressure on sandeels

The new knowledge about the behavior and selection of sandy bottoms with specific characteristics of the small sandeel makes it particularly vulnerable to pressure from human activities that operate in and affect habitats consisting of gravel and sand.

The research is also part of a new EU project that focuses on the importance of marine gravel and sandbanks, which are often overlooked as a vital ecosystem.

It is worth noting that activities such as marine sediment extraction, which we humans carry out in coastal areas in the form of e.g. sand extraction, can be a pressure factor for many sandeels, if you remove precisely the type of sand that sandeels like. 

And this is not unthinkable, because coarser sand holds better and is not affected by waves to the same extent as fine sand will be when sand feeding is used as a means of coastal protection.

"Yes, if the sand that is removed locally happens in the sandeel's preferred habitat, it can be problematic depending on the extent, because then it can potentially lead to displacements and changes in their habitats, which in turn can have an impact on the ecosystems where the small sandeel serves as a food source for other marine animals and supports a larger industrial fishery. I have previously heard local fishermen are worried about marine sediment extraction and sand nourishment," says Ole Henriksen and elaborates: 

"With the Danish government's latest plan for climate adaptation, where one billion is set aside for sand nourishment, I think it is appropriate to consider what ecosystem effect it may have locally when you have such activities. They are busy ecosystems that house a myriad of life that lives partially or fully buried in the seabed and acts as a feeding chamber, nurseries and key habitat for a wide variety of fish.” 

Ole Henriksen points out that their new study also sheds light on how light affects the sandeel's choice of sand – a further insight into how the interaction between environmental factors can have great complexity and affect the behavior and distribution of fish in the wild.

DTU Aqua has done the study Effects of sediment type and light availability on the burying behavior of small sandeel (Ammodytes tobianus) together with other researchers from the University of Melbourne, Marine Scotland Science and the Danish Health Data Agency. The research is part of the former project PELA and current EU project MARHAB.