Coastal communities mainly depend on fisheries for their food security, livelihood and economic development. Fish is the major nutritional food for the urban and coastal communities. Majority of population depends on the fish for micronutrients, such as zinc, iron and omega-3 fatty acids. This also helps the people to avert the malnutrition due to lack of micronutrient. In global fish catch, tropical marine fisheries make a significant contribution of approximately 50% to a worth of US$96 billion. In tropical region, fisheries provide employment opportunities for coastal communities through harvesting and postharvest plants and operations.
Tropical fisheries are threatened by several factors such as overfishing, habitat degradation, pollution, sedimentation, invasive species besides various physical and biogeochemical changes related to climate change, global warming, sea level rise, deoxygenation, acidification and altered concentration of nutrients. Carbon dioxide driven changes are expected and may increase in the forecoming decades and that can affect the physiology, behaviour and interactions of coastal and oceanic tropical marine fish species, leading to its spatial distribution and abundance.
Impacts of climate change on tropical fisheries
Climate change is considered as one of the important factor that threatens the tropical marine fisheries and which in turn has effects on the communities that depend on these resources, within and outside the tropics. The physical and biogeochemical changes associated with climate change and global warming has immediate response on the tropical marine fish resources. The changes in the physical and biogeochemical properties of the ocean can trigger the rising concentrations of greenhouse gases, particularly, carbon dioxide emissions. Actually tropical oceans are the ones that first showed the signals of climate change. The ocean is warming by absorbing the 90% of the greenhouse gas which is accumulated through rising anthropogenic activities.
From the beginning of the twentieth century, the sea surface temperature in the tropical ocean has risen by nearly about 0.75 °C, with accelerated warming of the ocean. Throughout the twenty first century tropical ocean warming is expected to amplify and there will be an increase of about 0.8 ± 0.3 °C by 2100. Warming of the tropical ocean increases the stratification of the upper water column, thereby reducing the exchange of oxygen between the oxygen rich and oxygen depleted water layers respectively.
Due to the warming, rising of sea level and loss of land ice has occurred which is also a threat to mangroves, sea grasses and coral reefs which is the primary fish habitat and spawning grounds for tropical fishes. It is predicted that the sea level rising in all tropical oceans will continue in the twenty first century also. In addition to sea level rising and other important extreme events such as tropical cyclone winds and rainfall also causes threats to coastal habitats and fisheries.
Impacts on Net Primary Production
Net Primary Production (NPP) of Phytoplankton in the oceans and coastal areas supports the food web of the marine fishes. The production of phytoplankton is dependent on the appropriate temperature, light and dissolved nutrients supplied by the Oceans. The increased ocean warming has increased the stratification of layers in the surface zone of water and this in turn may inhibit the supply of nutrients to the photic zone. These changes in light, nutrients, grazing and ocean stratification changes are projected to decrease tropical NPP by 7–16% by 2100 relative to 2006 to 2015 and in turn will result in decrease in fisheries catch.
Impacts on the marine ecosystems
Marine ecosystems are affected by the stresses induced by physical and biogeochemical changes and these effects have direct or indirect effects on the abundance and reproduction of fish and invertebrate species. Coral reefs, mangroves and sea grasses habitats for fish lifespans in the marine ecosystems and such ecosystems are vulnerable due to anthropogenic stressors.
Half the world’s coastal wetlands, including sea grass meadows, salt marshes and mangroves, are lost through climatic and non-climatic drivers. Like corals, specific temperatures are required for all fishes and invertebrates for their growth reproduction and survival. For tropical marine species, are particularly sensitive to thermal tolerances than temperate species. Particularly marine fishes and invertebrates shift to deep waters or isotherms areas where there is a prevailing environmental condition which can favour the growth and survival. This shift of the marine fishes can decrease the environmental richness of the areas where there is a thermal stresses.
Due to the global warming and its interactions the oxygen content in the water will be decreased causing ocean deoxygenation and thereby it affects the aerobic capacity of fishes and invertebrates. This can have effects on the metabolic functions that can impair growth and reproduction in fishes and invertebrates. Due to ocean deoxygenation, body size of some marine fishes and invertebrates, also reduces especially in the tropics region fishes.
Changes in Marine Fish Stocks
The catch of the tropical fisheries have expanded vast from 1950 and annual catch has increased from 7.1 million tonnes (US$7.3 billion) to 50 million tonnes in 2016 (US$89.7 billion. But climate change has threatened the fish stocks in marine ecosystems. The tropical marine ecosystem, species and catches have changed from 1950 due to the changes in temperature and phytoplankton production etc. Furthermore, the catch from tropical ecosystems has shown dominant of warm water tolerant species since the 1970s.
Effects on communities and economies
The influence and effects of climate change on tropical fisheries have a major role on the profitability and employment opportunities of fish processing industries in extra tropical regions as the extra ex tropical regions depends on the tropical fisheries. Raw materials for the world’s tuna processing industries are often supplied by distant water fleets or imports from tropical regions. The effects of climate change and its related changes can have negative impacts on the abundance and distribution of tuna. This has threatened the developing countries which depend on the tropical fisheries resources for food security, employment and economic development and also the investments made by the other countries in distant water fishing in tropical fisheries by the developing countries. Tropical marine resources support important aspects of sustainable development, especially the UN Sustainable Development Goals on food security (SDG2) and economic development to eliminate poverty (SDG1).
Adaptation and mitigation measures
Practical and effective adaptations are required in tropical regions to sustain their fisheries. The stressors related to climate and others are categorized in to three broad categories: ecosystem based solutions, built environment solutions and policy based solutions. Ecosystem based adaptations are the management, conservation and restoration of fish habitats and fish stocks to provide better ecosystem services in spite of climate change, including the mitigation of greenhouse gas emissions. Built environment adaptations are designing coastal infrastructure to cope with sea level rise that can restore the migration of mangrove and sea grass habitats. Policy based adaptations include practices to sustain catches within social governance and economic systems. Therefore understanding the risks of climate change is necessary and immediate attention is required to tropical fisheries for sustainable development.