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October 21, 2014

A Baltic Sea chain reaction



Big fish eats small fish.

Well…of course there are exceptions to this but on a general scale this is what happens below the waves. Scientists usually express these food interactions with complicated looking figures called food webs. A food web shows what a species eats and what it’s being eaten by. Here in the Baltic Sea, an important example is the cod (Gadus morhua), which plays a central role in the Baltic Sea ecosystem and the associated food webs. Cod feeds on many organisms but the most important in terms of fisheries are sprat (Sprattus sprattus) and herring (Clupea harengus), which in turn eat cod eggs and larvae. Consequently, those species are strongly connected to each other since they eat each other at different stages of their life. 

High numbers of cod means high predation on sprat & herring, which could mean a reduction in their numbers. On the other hand, a low number of cod can result in higher numbers of sprat & herring. But those fish also eat small animals called zooplankton which in turn eat even smaller animals such as algae. Consequently, high numbers of sprat & herring results in low numbers of zooplankton, which in turn results in high numbers of algae, which can result in algae blooms, polluting vast areas of the Baltic Sea making it toxic to people and animals. This is just a simple example showing how a whole ecosystem is connected, where each individual species has a key role, and a small change can lead to a chain reaction, sometimes referred to as trophic cascades.