Warming ocean temperatures can lead to a variety of changes in marine ecosystems, including:

• Changes in abundance of living marine resource stocks. As water temperatures warm due to climate change, they may create conditions that are more or less favorable for certain marine species. Increasing temperatures tend to be detrimental to species near the warm end of their geographic range, but may enhance productivity at the cool extent of the range. These effects can alter the success of fishing operations and influence the effectiveness of management measures.
• Changes in species locations. Long-term changes in the geographic distribution of species reflect shifts in temperature, other physical ocean conditions, and prey availability. Some species move to track their preferred environmental conditions, including temperature. As species shift, fishing vessels may have to travel further to catch their target species, or they may no longer have access to the stock.
• Increased pathogens. Warmer water temperatures can facilitate the spread of pathogens that affect seafood safety. For example, toxic strains of Vibrio bacteria increase in oysters under warmer water conditions, creating a risk of illness or even death from consuming raw shellfish. Such conditions threaten the health and well-being of seafood consumers as well as the economic viability of the fishery.

Marine heatwaves can cause abrupt changes in marine ecosystems and create shocks that can affect fishery operations and supply chains.

• Coral bleaching. In severe cases, marine heatwave-induced coral bleaching events can change reefs from a coral-dominated state to an algal-dominated state, reducing habitat for particular reef fish species. This habitat change can alter fish community composition and reduce the abundance of species traditionally targeted in the fishery. Such changes can impact fishing incomes and food security.
• Lethal effects. The extreme temperatures associated with marine heatwaves can result in direct mortality of some species, particularly sessile species living in the intertidal zone or shallow coastal waters. These species experience the greatest heat stress and are unable to move to cooler waters. Mortality events eliminate fishing opportunities for harvesters of those species and may affect nutrition and livelihoods in the community.
• Altered fishery-ecosystem interactions. By altering where and when species occur in the ecosystem due to changes in suitable thermal habitat, marine heatwaves can change how fisheries interact with other species, including threatened and endangered species. For example, as humpback whales shifted their foraging grounds off California during a heatwave, whale entanglements in the Dungeness crab fishery increased, resulting in constricted fishing seasons.

Severe weather events are increasing in frequency and severity as a result of climate change, creating impacts that include:

• Risks to safety-at-sea. Increased storminess poses serious risks to fishing vessels, compromising the safety of fishers who are caught out in the storms or who choose to fish in marginal weather. Fishers who are most dependent on their catch for their food or income are more likely to risk fishing in storms, compromising their safety and lives to avoid hunger or debilitating economic hardships.
• Reduced number of fishing days. To avoid risks to safety at sea, fishers may choose to stay in port on more days due to storms. In addition, large storms can destroy vessels or port facilities, without which fishing is not feasible. Both effects reduce the number of days fishers can be on the water, which has implications for income, livelihoods, and nutritional needs in some communities.
• Disruptions to seafood supply chains. Large storms such as hurricanes and cyclones can destroy or severely damage infrastructure needed to transport, process, distribute, or sell seafood products. These effects can be felt over large geographies, with fisheries in one region of the world potentially being affected by storm-related supply chain disruptions in another region.