5.4.6.2 What is new since the TAR?
New Knowledge: Effects of temperature on fish growth.
One experimental study showed positive effects for rainbow trout (Oncorhyncus mykiss) on appetite, growth, protein synthesis and oxygen consumption with a 2°C temperature increase in winter, but negative effects with the same increase in summer. Thus, temperature increases may cause seasonal increases in growth, but also risks to fish populations at the upper end of their thermal tolerance zone. Increasing temperature interacts with other global changes, including declining pH and increasing nitrogen and ammonia, to increase metabolic costs. The consequences of these interactions are speculative and complex (Morgan et al., 2001).
New Knowledge: Current and future direct effects.
Direct effects of increasing temperature on marine and freshwater ecosystems are already evident, with rapid poleward shifts in regions, such as the north-east Atlantic, where temperature change has been rapid (see Chapter 1). Further changes in distribution and production are expected due to continuing warming and freshening of the Arctic (ACIA, 2005; Drinkwater, 2005). Local extinctions are occurring at the edges of current ranges, particularly in freshwater and diadromous species, e.g., salmon (Friedland et al., 2003) and sturgeon (Reynolds et al., 2005).
New Knowledge: Current and future effects via the food chain.
Changes in primary production and transfer through the food chain due to climate will have a key impact on fisheries. Such changes may be either positive or negative and the aggregate impact at global level is unknown. Evidence from the Pacific and the Atlantic suggests that nutrient supply to the upper productive layer of the ocean is declining due to reductions in the Meridional Overturning Circulation and upwelling (McPhaden and Zhang, 2002; Curry and Mauritzen, 2005) and changes in the deposition of wind-borne nutrients. This has resulted in reductions in primary production (Gregg et al., 2003), but with considerable regional variability (Lehodey et al., 2003). Further, the decline in pelagic fish catches in Lake Tanganyika since the late 1970s has been ascribed to climate-induced increases in vertical stability of the water column, resulting in reduced availability of nutrients (O’Reilly et al., 2004).
Coupled simulations, using six different models to determine the ocean biological response to climate warming between the beginning of the industrial revolution and 2050 (Sarmiento et al., 2004), showed global increases in primary production of 0.7 to 8.1%, but with large regional differences, which are described in Chapter 4. Palaeological evidence and simulation modelling show North Atlantic plankton biomass declining by 50% over a long time-scale during periods of reduced Meridional Overturning Circulation (Schmittner, 2005). Such studies are speculative, but an essential step in gaining better understanding. The observations and model evidence cited above provide grounds for concern that aquatic production, including fisheries production, will suffer regional and possibly global decline and that this has already begun.
New Knowledge: Current and future effects of spread of pathogens.
Climate change has been implicated in mass mortalities of many aquatic species, including plants, fish, corals and mammals, but lack of standard epidemiological data and information on pathogens generally makes it difficult to attribute causes (Harvell et al., 1999) (see Box 5.4). An exception is the northward spread of two protozoan parasites (Perkinsus marinus and Haplosporidium nelsoni) from the Gulf of Mexico to Delaware Bay and further north, where they have caused mass mortalities of Eastern oysters (Crassostrea virginica). Winter temperatures consistently lower than 3°C limit the development of the multinucleated sphere X (MSX) disease caused by P. marinus (Hofmann et al., 2001). The poleward spread of this and other pathogens is expected to continue as winter temperatures warm.
Box 5.4. Impact of coral mortality on reef fisheries
Coral reefs and their fisheries are subject to many stresses in addition to climate change (see Chapter 4). So far, events such as the 1998 mass coral bleaching in the Indian Ocean have not provided evidence of negative short-term bio-economic impacts for coastal reef fisheries (Spalding and Jarvis, 2002; Grandcourt and Cesar, 2003). In the longer term, there may be serious consequences for fisheries production that result from loss of coral communities and reduced structural complexity, which result in reduced fish species richness, local extinctions and loss of species within key functional groups of reef fish (Sano, 2004; Graham et al., 2006).
New Knowledge: Economic impacts.
A recent modelling study predicts that, for the fisheries sector, climate change will have the greatest impact on the economies of central and northern Asian countries, the western Sahel and coastal tropical regions of South America (Allison et al., 2005), as well as some small and medium-sized island states (Aaheim and Sygna, 2000).
Indirect economic impacts of climate change will depend on the extent to which the local economies are able to adapt to new conditions in terms of labour and capital mobility. Change in natural fisheries production is often compounded by decreased harvesting capacity and reduced physical access to markets (Allison et al., 2005).