Researchers at the Estonian Marine Institute of the University of Tartu described diel variations in fish assemblage composition in a shallow near-shore area in the Baltic Sea by using beach seining. The article about the study was recently published in the journal Boreal Environment Research.
Imre Taal, junior researcher at the Estonian Marine Institute, said that to his knowledge this was the first study that described diel variations in the fish assemblage composition of a non-tidal brackish surf-zone environment using an annual data set. Studies like this are needed to improve knowledge about shallow near-shore fish community dynamics.
Lauri Saks, researcher at the Estonian Marine Institute, added that this study was useful for getting an overview of the subject because shallow coastal zones in general can be often described as the spawning and/or nursery area for most of the fish that inhabit the Baltic Sea. Especially for those species that make up the bulk of catches for coastal fishermen. “Also, while the majority of fish species that inhabit the surf-zone do not have a specific commercial value per se—e.g., there is no commercial fishery for the sand goby in the Baltic—these species may have a crucial role in the food chain of the sea. The sand goby may be an important food source for the young pikeperch, which is a highly valued commercial fish species,” he noted.
Thus, knowledge about the forces shaping the fish communities that support the abovementioned coastal fisheries is needed for the sustainable management of fisheries.
Importance of Time
The beach seining was conducted monthly during the ice free season, thus, almost for an entire year. To properly describe the diel changes in the surf-zone fish assemblage, monthly sampling was conducted four times a day: at dawn, noon, dusk, midnight.
The results of the study demonstrate that the composition of fish assemblage observed at a certain time of day cannot be directly extrapolated to another time of day or regarded as a representation of the whole astronomical day. For example, the small sandeel was present in samples taken at noon, while the European smelt was mostly abundant in the midnight samples.
According to Taal, these results reflect that crucial data, such as the significance of the habitat for certain species, could be overlooked if sampling is limited to a specific diel time period. “Our results indicate that the time of day when fish sampling is undertaken in the Baltic Sea is an important factor, and should be considered in planning future research on shallow near-shore fish assemblages.” He added that this methodology is relatively simple and inexpensive, and thus the method could also serve monitoring purposes.
Many Topics to investigate
Taal said that the Baltic Sea is a splendid study system for investigating the factors that form fish communities on a small scale. “The diversity of the coastline and the different ecological conditions throughout the Baltic Sea provide us with a study system that allows controlling different factors—natural or anthropogenic,” he said. For example, one of the most visible changes in the Baltic coastal fish communities in the past decade has been the invasion of round goby.
As the initial dataset was collected before the invasion, the research group has the unique opportunity to study the effect of such invasions on fish assemblage composition in a coastal shallow near-shore area by comparing the current, “invaded” system with “pristine” conditions.
This article was funded by the European Regional Development Fund through Estonian Research Council.