By Cathrine Boerseth
Edited by Katyanne M. Shoemaker
People don't like having their homes destroyed and neither do animals; bears don’t like it, birds don't like it, fish certainly don't like it and neither do the tiniest planktonic animals that people often forget even exists. Some of these tiny animals are meroplanktonic, which means they only float around in the early stages of their lives, to grow up as adults they need somewhere to settle down, a nice home with a good foundation; for many organisms that means a hard surface like rocks or a coral reef.
Sadly, in the waters of northern Paraná state, many of these nice hard (and already rare) surfaces were destroyed by destructive fishing methods like trawling. The meroplanktonic larvae were still floating around in the water, but there were few places for them to settle down. In the biological world one thing always affects another and so did the lack of appropriate habitat in our case; fish eat the organisms living on and around rocky reefs and so the lack of hard bottom substrates meant a lack of food for the fish, and so the populations declined.
But what if we made new homes for these animals and what if those homes were so sturdy and strong that trawlers wouldn't be able to break them? Well, that’s exactly what researchers did between 1997 and 2013 when they deployed a number of artificial reefs along the Paraná coast. But what exactly is an artificial reef? An artificial reef can be made out of rocks, concrete blocks or even sunken ships. They are man-made structures, preferably with different holes and crevasses, placed under water to provide shelter for marine organisms. Bacteria and algae are usually the first organisms to arrive, meroplanktonic larvae settle and grow up to be anything from anemones to crabs; all of these animals attract fish looking for food and they in turn attract larger fish and other predators. After a while, the ecosystem on the artificial reef grows to become a place with both food and shelter for all kinds of marine organisms.
However, even after the artificial reefs were in place, many questions were still unanswered: would meroplanktonic organisms come to settle? Would they attract fish? Would those fishes reproduce? Would the ecosystem of the artificial reefs be anything like a natural rocky reef? The answer to the two first questions was discovered to be a big YES, but what about the other questions? That's what I wanted to find out! Exiting stuff, now what?
To answer those questions, we decided to look at fish larvae and fish eggs. To capture them we used a net attached to an underwater scooter (so cool, I know), and a light-trap. With the scooter and light-trap we were able to capture larvae very close to the artificial reef; the net captured eggs and the smallest fish larvae while the trap attracted larger larvae. We also sampled at a distance from the artificial reef (would the abundance of larvae and eggs be different there?) and at a natural rocky reef habitat nearby (the beautiful archipelago of Currais). We collected as many samples as the weather and waves allowed between the July of 2014 and April of 2016.
The samples were collected using a light-trap (left) and a net attached to an underwater scooter (right).
So what did the data show?
The number of fish larvae and fish eggs was in fact higher on the artificial reef compared to samples taken at a distance from the reef. Furthermore, the fact that the samples contained eggs and very small newly hatched larval fish means that fish are either reproducing on the reefs or close by. Additionally, many of the fish larvae collected on the artificial reefs belonged to species that are known to live on rocky reef habitats; most of the other species found were pelagic, which means they live in the open water. What does it all mean? Well, it means that the artificial reef is beginning to act like a natural reef (great!), but it still has a way to go. Fish are still more abundant on the natural reef and many of the fishes on the artificial reef are more like visitors, like the pelagic species. They are all welcome of course! The artificial reef provides food and shelter; many of the visitors attracted by delicious food become food themselves, but that's ok, it's all part of the food network.
It may sound like artificial reefs are the solution to all of our problems and you may want to stand up with your hands in the air shouting: let's put artificial reefs in all the seas in all the world! Then everything will be great again, right? That would be amazing, but unfortunately, as with most things in life, it's just not that simple. There are many factors to consider because deploying an artificial reef is in itself a human intervention in nature and could cause more harm than good, careful research in each individual case is essential!
What can we learn from all this? Nature finds a way. Humans are destructive; in order to get our way and build our houses, we destroy houses of so many other animals. Fortunately, given time, many ecosystems are resilient enough to come back to life. Artificial reefs may not be the answer to all our problems, but on the coast of Paraná it appears that a tiny piece of a suffering ecosystem may actually be getting back on its feet.
About Cathrine:
Biologist and currently preparing to defend my masters’ dissertation in the field of biological oceanography at the University São Paulo. As a true Norwegian I fell in love with the ocean scuba diving in the freezing waters of the north. I have been living in Brazil for four years now and I can't wait to discover where life will take me in the future. What I know with certainty is that I want to work and live close to nature, that being in the beautiful tropics of Brazil or in the wonderful Arctic of Norway (or somewhere in between).
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