Recently we heard that widespread coral bleaching has occurred on the Great Barrier Reef off Queensland for the fourth time in seven years. Scientists say the frequency of these bleaching events mean there’s less and less chance for corals to recover.
Meanwhile, rising ocean temperatures caused by climate change is severely impacting tuna and other marine life in Pacific Islands waters.
The reliance of Pacific communities on tuna fisheries for food and employment means the rise in ocean temperature also has a devastating impact on many Pacific people and their families.
The rich tuna resources of the Western and Central Pacific Ocean (WCPO) also supply 60% of the world’s tuna. About half of the catch is extracted from the waters of the 22 Pacific Island countries and territories (PICTs) that span much of this large oceanic region. Four species of tuna dominate the catch; skipjack, yellowfin, bigeye tuna and albacore.
Environmental and fisheries agencies have projected that climate change could reduce the annual tuna catch from the combined waters of the 10 Pacific SIDS where most purse seine fishing occurs by approximately 10% (140,000 tonnes) by 2050, causing annual losses of 2-15% in total government revenue for many of these Pacific SIDS, and affecting the contributions of tuna fisheries to GDP.
“The first obvious impact of climate change that is already well observed is ocean warming. Unlike marine mammals, fish are ectotherm (i.e., cold blood) animals and thus very sensitive to the ambient water temperature and its changes,” says Dr Simon Nicol, SPC Principal Fisheries Scientist.
“Each species has evolved toward an optimal range of temperature that can be different for larvae and adults. Fish like tuna living in the open ocean can move to find their preferred temperature ranges. For species that are associated to a coastal habitat, it may be more difficult if there is no connectivity with other similar habitats offering more appropriate water temperature. Therefore, under the influence of ocean warming, the species composition and distribution is changing. Some species move, some disappear and some other appear.”
Dr Nicol says that the changing dynamic of the Pacific’s oceanography is likely to see some latitudinal shifts in species ranges but also longitudinal shifts as habitats become more preferable on the eastern side of the Pacific and opposed to the western side for example.
Ocean warming increases the stratification of surface water and thus limits the exchange and the input of nutrients coming from the deeper layer, thus reducing productivity by the phytoplankton at the basis of the food chain.
Dr Nicol says that the combined effects of fishing, natural variability, long-term trends of climate change and delayed effects in long-living tuna species make the analyses very complex. However, we know that tuna abundance and distribution are impacted by natural interannual climate variability like the well-known El Nino / La Nina Oscillation (ENSO). Skipjack, the most tropical tuna species, follows the eastward extension of the Pacific warm pool during an El Nino event. Climate change is expected to cause further expansion of the warm pool and the central distributions of tropical tuna species are projected to move easterly in response.
Pacific Community (SPC) Senior Coastal Fisheries scientist, Andrew Halford says the flow-on effects of mass coral bleaching events, such as those being witnessed in Australia “are being more frequently documented with evidence that fish and invertebrate communities, which rely on coral reef ecosystems for their survival, are being fundamentally changed.
“As well as being a key source of food and shelter for communities, coral reefs also buffer the erosion effects of waves and swell on island shorelines,” says Halford. “Degraded coral reefs not only support less fish and invertebrates, but they also become less effective at moderating shoreline erosion around island communities. This situation is escalating because rising ocean temperatures are also causing sea levels to rise which is working in tandem with eroding reef systems to cause more frequent inundation of coastal land and higher levels of erosion. Drowning of land has direct effects on agricultural output, another mainstay of food security in the PICTs.”
He says that water temperature is a key component of modulating ocean acidity, with increasing temperature increasing the acidity of sea water. Organisms that produce a calcium-based skeleton such as corals, clams, oysters and lobsters are vulnerable to a more acidic environment due to a reduced uptake of calcium carbonate, resulting in thinner skeletons that are more prone to damage. Water chemistry is critical to so much of the life cycle of marine organisms with documented effects on, for example, fertilisation rates, larval development, and the ability of fish to avoid predation. Changes to the chemistry of sea water due to climate change is already having a significant effect on marine productivity and the global carbon cycle upon which life depends.
“On the grandest scale, increasing water temperatures are affecting the global movement of the cold oceanic deep water which underpin the current systems of the world,” says Halford. “These currents drive the current climate patterns and if significant changes occur to these systems, they will not likely be reversed, even if global warming processes are substantially reduced.
“Substantial loss of coral reef habitats and associated ecosystems (seagrass meadows for example have suffered badly from heat waves and also cyclone related turbidity) will reduce the habitat needed for many species of fish and invertebrates to recruit to, shelter in and feed within and on. This will lead to substantial changes in the abundance and distribution of many species that are part of food fish assemblages across the Pacific,” he states.
“A potentially more significant issue are the effects of climate change on the early and relatively less well studied parts of the life cycle of fish and invertebrates,” says Halford. “Changes to the reproductive capacity of fishes and invertebrates, the access to food when they are part of the plankton and their ability to grow and recruit successfully to their adult habitats are all possible under changing climate conditions. Very small differences at the beginning of the lifecycle can have very profound changes on adult populations and their ability to persist.
“Perhaps the largest effect will be on social and socio-economic systems within the Pacific which are so intrinsically bound to the sea. Large changes in the availability of marine protein could have major flow-on effects which could de-stabilise work-life systems that have been in place for millennia.”
