Sustainable use of marine bio-resources
Environmental change is affecting marine ecosystems while overfishing is degrading their biodiversity. The overexploitation of certain fish stocks is a major socio-economic issue, however, most fishery policies do not take into account environmental change and its affect on fish populations. This situation is unprecedented in the marine biosphere and it is important to know how ecosystems will respond so managers can formulate adaptive strategies and manage vulnerable stocks. The CPR dataset has already described alterations in plankton distributions, abundance and phenology in association with climate-induced changes in sea surface temperatures, which all affect fish recruitment. A recent CPR study showed how the breeding success of seabirds in the North Sea was linked across 4 trophic levels (phytoplankton, zooplankton, sandeels, seabirds) which are ultimately controlled from the bottom-up by environmental forcing.
Northerly geographical range extensions or changes in the geographical distribution of fish populations have been recently documented for European Continental shelf seas and along the European Continental shelf edge. These geographical movements have been related to regional climate warming and are predominantly associated with the northerly geographical movement of fish species with more southern biogeographical affinities. These include the movement of sardines and anchovies northward in the North Sea and red mullet and bass extending their ranges northward to western Norway. Records were also observed over the last decade for a number of Mediterranean and north-west African species on the south coast of Portugal.
Regional climate warming in the North Sea has also affected cod recruitment via changes at the base of the food web (cod, like many other fish species, are highly dependent on the availability of planktonic food during their pelagic larval stages). Key changes in the planktonic assemblage recorded by the CPR survey caused by the warming of the North Sea over the last few decades have resulted in a poor food environment for cod larvae and hence eventual recruitment success. This research is an example of how both the dual pressures of over-fishing and regional climate warming have conspired together to negatively impact a commercially important fishery. Recent work on pelagic phenology has shown that plankton communities, including fish larvae, are very sensitive to regional climate warming with the response to warming varying between trophic levels and functional groups. These changes, again seen in the North Sea, have the potential to be of detriment to commercial fish stocks via trophic mismatch. Work will continue to monitor and describe, in the long-term, pelagic variability and diversity in the core areas of the CPR survey over the Northwest European shelf and in the eastern and western Atlantic with a focus on applications to fisheries research, in particular ecosystem based management, and interpretation of climate change effects on fisheries.
Since we are only just beginning to understand the complexity of bottom-up controls on ecosystem structure, our appreciation of their full ramifications will continue to improve with continued monitoring of the plankton as the global climate changes. How these bottom-up controls of ecosystem productivity interact with top-down effects, such as fishing, will be an important focus for CPR research as it becomes a necessary component of ecosystem management models. For example, long-term monthly changes in the frequency of jellyfish nematocysts (stinging cells) in CPR samples show an increase in frequency of gelatinous zooplankton in the North Sea. In many other marine regions worldwide a proliferation of jellyfish is seen as an indicator of ecosystem degeneration. Since some jellyfish feed on fish eggs, fish larvae and zooplankton, they can exert both top-down and bottom-up control of fish recruitment. Genetic analysis will be used to speciate individual nematocysts in CPR samples to help understand the changes in the North Sea gelatinous zooplankton community and whether these changes need to be included in fisheries management models.
As molecular methods develop we are gaining ever more insight into population structure and local adaptations. For example, analyses of single nucleotide polymorphisms in cod have revealed nuclear loci in cod differentiation between North Sea and Baltic stocks that may reflect adaptations to different environments, and indicate that these stocks must be managed separately; such knowledge is essential for restocking programmes, for example. A future area of study we wish to pursue is to determine whether apparent changes in plankton distributions reflect geographical movement or changes in the abundance of locally adapted genotypes.
