If marine ecosystems are to be managed efficiently, the impact from all the different human pressures, such as fertiliser run-off or commercial fishing, needs to be taken into account. Indicators (or metrics) are easily understood descriptions of different aspects of the environment, which are used to monitor ecosystems.
A number of indicators are required to identify all the different causes of changes that take place in ecosystems. These changes can be brought about by top-down impacts, such as fishing, which targets species in the upper levels of the food chain but have repercussive effects on the lower levels of the food chain. They can also be bottom-up impacts caused by environmental changes, which directly affect species in the lower levels of the food chain but drive changes higher up the food web. For example, agricultural pollution leads to increased nutrient levels which can encourage toxic algal blooms.
Partially funded by the EU IMAGE1 project, this study examined the impacts of fishing and environmental pressures on 14 temperate exploited groundfish (bottom-dwelling fish, such as sole and halibut) communities in the Mediterranean Sea and North-East Atlantic Ocean.
The researchers developed a model to help them understand these impacts, which categorised the marine food chain into three levels. The two upper levels are targeted by fishing: piscivores (that primarily eat other fish) and fish prey of the piscivores. The lower level was resource species, such as small creatures that eat plankton, which are eaten by the prey species and which are particularly affected by bottom-up environmental pressures, such as pollution or climate change.
Trawl survey data, from 1995-2006, were used to develop two classes of metrics: abundance of species and size structure. These metrics represent changes to the groundfish communities at food chain level (i.e. piscivores, prey and resource species) and changes to individual populations. Trends identified in the different metrics were combined to determine the most likely causes of the observed trends.
Results of the study found evidence of changes at the population level in all groundfish communities in all the study areas. There was evidence of both changes being driven by environmental pressures (bottom-up impacts) and changes driven by changes in fisheries activities. In most cases there were changes caused by bottom-up processes and some evidence of reduced fishing impacts.
A range of possible pressures responsible for the changes were identified. For example, for a few prey species, increased mortality through fishing reduced both abundance and size of the fish species. For piscivores, changes were probably due to a combination of pressures, including an increase in stock size and reduced mortality.
At the group level, for prey and piscivorous species, multiple pressures were probably responsible for the predicted changes. For example, the most likely reasons for piscivore changes in many areas were an increase in the availability of prey (caused either by enhanced availability of resource species for the prey due to an increase in available nutrients, or by less exploitation of the prey species), and a decrease in exploitation of the piscivore species.