In a new study, the prime predators of the Baltic Sea at the top of the food web are losing weight, and the study connects the deteriorating health of gray seals and cod with changes in bottom-living crustaceans, isopods, and amphipods.
Researcher at the Department of Aquatic Resources at the Swedish Agricultural University, Lena Bergstrom, said that it is essential they understand how the food web works when managing a fishery. Controlling the way the fish and fisheries are changing is not enough. The availability and quality of food are at least as essential.
The study's authors, in collaboration between various universities, examined how the health and abundance of individual species have changed over two decades in the Bothnian Sea and the Baltic Proper. The scientists studied seal, cod, herring, sprat, isopods, amphipods, and zooplankton: species that all occur at different levels in the Baltic Sea food web. The system appears complex, and various species can be both predators and prey. A good instance is herring eating zooplankton and some bottom fauna while herring itself is eaten by cod and seal.
Primarily, the study revealed that there are links between the health of both cod and seal with the availability of bottom-living animals. Regarding the seals, the link is indirect through that the herring it eats is influenced by the availability of the bottom-living animals. In the two situations, there is a connection to eutrophication and climate change.
The lead author of the study and researcher at the Department of Ecology, Environment, and Plant Sciences (DEEP) at Stockholm University, Agnes Karlsson, said that oxygen levels in Baltic Sea have cut down since the 1990s, in significant part because of eutrophication, creating vast oxygen-free areas. This case results in less living space for the bottom-living prey animals. Such a situation like this has, among other things, led to the fact that the isopods have become fewer and smaller, making them a poorer food choice for cod.
In the research, the fat content and the low weight of herring in the Bothnian Sea have recently improved due to the supply of bottom-living crustacean, the amphipod has increased. Agnes Karlsson said that however, the upturn is relative because the amphipod in the Bothnian Sea collapsed in the early 2000s and what they new see are signs of a recovery.
After a period of hefty rain that changed the water quality of the Bothnian Sea, the bottom-living crustaceans were almost eliminated. According to Lena Bergstrom, wit climate change, they will likely see similar extreme events more frequently in the future. If activities that lead t eutrophication are not reduced, oxygen shortage in the Baltic Sea will likely continue, leading to further reductions in the numbers of bottom-living animals. This can have far-reaching effects for the economy, concerning the fish species that are essential commercially. To manage a fishery, there is a need to control the environment and the food web.
