Shift of the ecosystem
Complex interactions between overfishing, eutrophication, and changing ocean circulation and freshwater inputs and climate change could facilitate ecosystem shifts. This has already been examined for the circumpolar arctic in the Arctic Climate Impact Assessment Scientific Report, and globally under the IPCC Fourth Assessment Report and the reports of the Millenium Ecosystem Assessment. These complex ecosytem shifts, in part driven by climate change, but also driven by other human impacts, will be a key area of importance in the future.
Effects on the Mediterranean Sea
A recent example of these complex interactions may be seen in the Mediterranean Sea. The combination of higher water temperatures, overfishing and nutrient influxes may result in the presence of agal blooms and jellyfish. In the Mediterranean, algal blooms are boosted by nitrate and phosphate influxes from farming and human wastes. Jellyfish also benefit from the reduction in the number of natural predators like loggerhead turtles and the bluefin tuna, which have been drastically reduced by over-fishing. Once jellyfish are predominant, it can be difficult for juvenile fish populations to re-establish that predator-prey relationship.
Reduced river flows during hotter summers might also lead to increased numbers of jellyfish near the shore, as freshwater currents no longer keep the jellyfish offshore. The predominance of jellyfish and algal blooms in coastal waters and adjacent to beaches also reduces the attractiveness of tourism for those beaches.
Effects of overfishing and climate change
Though these changes are not predominantly climate driven, there has been a very direct link between overfishing and ecosystem collapse, and the introduction and success of an invasive jellyfish species in the Black Sea, illustrating the complexity and vulnerability of ecosystems under the threats of multiple uses and environmental change. Similarly, overfishing of cod in the east coast of Canada off the Grand Banks has resulted in the introduction of a new ecosystem and fishery that is based on shellfish and molluscs.
Part of the dilemma for fish stocks and other ecosystem impacts are difficulties in separating climatic impacts from impacts of overfishing and increasing destruction of coastal and estuarine habitat. This parallels the interaction between human use and development, and climatic impacts on the coastline. In ecosystems under stress, there is also greater likelihood of the introduction and survival of invasive marine species in coastal and marine waters, which can affect the overall ecosystem, its health and population and important fish species.
Local, artisanal, sport and commercial fisheries, and other forms of marine harvesting; tourism and recreation; will also be affected by climate change. These impacts can include higher sea surface temperatures, stratification of the seas and alteration of the food web, which in turn can affect nutrients, spawning and growth of important fish species. In addition, higher sea temperature can facilitate the introduction, and the emigration and immigration of valuable fish species.
Climate change is already seen to be having an affect on coastal and estuarine fisheries. For example, the UK State of the Seas of 2005 report discusses some of the changes occurring in the food web, which in turn affects fish populations. Due to shifting water temperatures, there have been shifts in fish populations in the North Sea, where fish stocks in the Norwegian and EC waters are moving to Icelandic waters. Parallel shifts have also been seen in the Barents and Bering Sea.
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