Physical and ecological impacts of climate change relevant to marine and inland capture fisheries and aquaculture

This chapter reviews the physical and ecological impacts of climate change relevant to marine and inland capture fisheries and aquaculture. It is noted that the oceans are warming but that this warming is not geographically homogeneous. The combined effect of temperature and salinity changes due to climate warming are expected to reduce the density of the surface ocean, increase vertical stratification and change surface mixing. There is evidence that inland waters are also warming, with differential impacts on river run off. Increased vertical stratification and water column stability in oceans and lakes is likely to reduce nutrient availability to the euphotic zone and thus primary and secondary production in a warmed world. However, in high latitudes the residence time of particles in the euphotic zone will increase, extending the growing season and thus increasing primary production. While there is some evidence of increased coastal upwelling intensity in recent decades, global circulation models do not show clear pattern of upwelling response to global warming at the global scale. However, current climate models are not yet sufficiently developed to resolve coastal upwelling and so the impacts of climate change on upwelling processes require further work. There is also evidence that upwelling seasonality may be affected by climate change. Sea level has been rising globally at an increasing rate, risking particularly the Atlantic and Gulf of Mexico coasts Climate change implications for fisheries and aquaculture – Overview of current scientific knowledge 8 of the Americas, the Mediterranean, the Baltic, small-island regions, Asian megadeltas and other low-lying coastal urban areas. Ocean acidification has decreased seawater pH by 0.1 units in the last 200 years and models predict a further reduction of 0.3-0.5 pH units over the next 100 years. The impacts of ocean acidification will be particularly severe for shell-borne organisms, tropical coral reefs and cold water corals. Climate change effects marine and inland ecosystems are in addition to changes in land-use, including changes in sediment loads, water flows and physical-chemical consequences (hypoxia, stratification, salinity changes). The consequences of these processes are complex and will impact community composition, production and seasonality processes in plankton and fish populations. This will put additional pressure on inland fish and land-based, water intensive, food production systems, particularly in developing countries. Many effects of climate change on ecosystem and fish production processes have been observed. While a slight reduction in global ocean primary production has been observed in recent decades, a small increase in global primary production is expected over this century, but with very large regional differences. Changes in the dominant phytoplankton group appear possible. In general terms, high-latitude/altitude lakes will experience reduced ice cover, warmer water temperatures, a longer growing season and, as a consequence, increased algal abundance and productivity. In contrast, some deep tropical lakes will experience reduced algal abundance and declines in productivity, likely due to reduced resupply of nutrients. The intensification of hydrological cycles is expected to influence substantially limnological processes, with increased runoff, discharge rates, flooding area and dry season water level boosting productivity at all levels (plankton to fish). Climate change is expected to drive most terrestrial and marine species ranges toward the poles, expanding the range of warmer-water species and contracting that of colderwater species. The most rapid changes in fish communities will occur with pelagic species, and include vertical movements to counteract surface warming. Timing of many animal migrations has followed decadal trends in ocean temperature, being later in cool decades and up to 1–2 months earlier in warm years. Populations at the poleward extents of their ranges will increase in abundance with warmer temperatures, whereas populations in more equatorward parts of their range will decline in abundance as temperatures warm. More than half of all terrestrial, freshwater or marine species studied have exhibited measurable changes in their phenologies over the past 20 to 140 years, and these were systematically and predominantly in the direction expected from regional changes in the climate. Differential responses between plankton components (some responding to temperature change and others to light intensity) suggest that marine and freshwater trophodynamics may be altered by ocean warming through predator-prey mismatch. There is little evidence in support of an increase in outbreaks of disease linked to global warming, although spread of pathogens to higher latitudes has been observed. The paper summarises the consequences of climate change along temporal scales. At “rapid” time scales (a few years) there is high confidence that increasing temperatures will have negative impacts on the physiology of fish, causing significant limitations for aquaculture, changes in species distributions, and likely changes in abundance as recruitment processes are impacted. Changes in the timing of life history events are expected, particularly affecting short lived species, such as plankton, squid, and small pelagic fishes. At intermediate time scales (a few years to a decade), temperature-mediated physiological stresses and phenology changes will impact the recruitment success and therefore the abundances of many marine and aquatic populations, particularly at the extremes of species’ ranges, and for shorter-lived species. At long time scales (multi-decadal), predicted impacts depend upon changes in net primary production in the oceans and its transfer to higher trophic levels, for which information is lacking. Considerable uncertainties and research gaps remain, in particular the effects of synergistic interactions among stressors (e.g. fishing, pollution), the occurrences and roles of critical thresholds, and the abilities of marine and aquatic organisms to adapt and evolve to the changes. Regarding freshwater systems, there are specific concerns over changes in Physical and ecological impacts of climate change relevant to marine and inland capture fisheries and aquaculture 9 timing, intensity and duration of floods, to which many fish species are adapted in terms of migration, spawning, and transport of spawning products, as a result of climate change. The chapter concludes with specific anticipated responses of regional marine ecosystems (Arctic, North Atlantic, North Pacific, coastal upwelling, tropical and subtropical regions, coral reefs, freshwater systems and aquaculture systems) to climate change.