What swims between the algae of the rocky intertidal zone?

What swims between the algae of the rocky intertidal zone?

You can find a very lively ecosystem in the rocky intertidal zones along Iceland‘s coastlines. One highly abundant species using this habitat is the brown algae Ascophyllum nodosum, also commonly known as knotted wrack in English and Klóþang in Icelandic. This species is harvested for multiple purposes such as food or fertilizers. But what can we find swimming between the algae and are there differences between harvested and unharvested areas? This is a question Jón Tomas Magnússon and colleagues have addressed.

Juvenile cod and saithe caught in the multi mesh nets.

But first things first, what do we mean by rocky intertidal zones? Generally, the intertidal zone lies between the lowest point the water edge reaches during low tide, and the highest one during high tide. Species in this habitat have to adapt to rapidly changing environments throughout the tidal cycles where they can go from fully under water to mostly dry. But this habitat can also serve as important area for juvenile fish who use it as nursery grounds, seeking shelter in rockpools, between rocks, or underneath algae.

Fieldwork in the rocky intertidal zone in Breiðafjörður.

In their recently pubished study “Icelandic intertidal fish communities and effects of knotted wrack (Ascophyllum nodosum) harvesting”, Jón and colleagues placed multi-mesh nets in the rocky intertidal area of Breiðafjörður for full tidal cycles to document what fish are caught moving into or out of the intertidal. They first set up this system in June 2019 before the harvest and then repeated it once per season until June 2020. The collected information on what species are present.

Among the fish species that they caught in the nets were the commercially highly important cod (Gadus morhua) and saithe (Pollachius virens) who use the rocky intertidal as nursery and feeding grounds. While the results currently show no significant impact of the harvesting activities on the observed fish, the authors point out that this was a very small-scale study. Knowing how our activities, in this case the harvesting of knotted wrack, can impact commercially important species especially in their most vulnerable juvenile stages is crucial. Jón Tomas hopes that future studies will follow that will continue to monitor the potential impacts on the fish in the rocky intertidal zone across a bigger region as well as over multiple years.

If you are interested to read more, especially when you are curious about the impacts of seaweed harvesting and the ecology of intertidal fish, find out more here.

Measuring two sculpins that were caught in the net.