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.
But what does this mean for fisheries?
Together, these three species make up the most important income for fisheries in the Baltic Sea. When assigning quotas for the stocks, not only fisheries, but also ecological factors need to be taken into account. This is known as the ecosystem based approach to fisheries, or simply ecosystem approach. For example, if cod catches are high it could result in a blooming of sprat and herring numbers, since there will be fewer cod to feed on them. But more sprat and herring would mean that there are now more fish to eat cod eggs & larvae. So basically it means that one type of fishery is dependent on the other.
In October 2014 the European Commission tabled a proposal for a multi-annual plan for the fisheries of cod, sprat and herring, often referred to as the Baltic multi species plan. The idea of the plan is to achieve joint long-term management of these species taking wider ecosystem effects into account, such as the effects that the fisheries on cod have on other species. The plan will fill an important gap in fisheries management as the pelagic species currently lack a management plan and the current management plan for cod is outdated.
Oceana has some concerns regarding the Commission proposal. Read more about the Multi species plan here.