An Invisible Force Driving Sunfish North

By Yoochan An, Marine Biology ‘27

On August 30th, 2023, a mola mola fin was photographed in Puget Sound. Mola molas, commonly known as sunfish, reside primarily in temperate and tropical waters. Temperate and tropical waters range from about 20ºC to 30ºC, while Puget Sound ranges from about 7ºC to 11ºC. Given the drastic difference in the mola mola’s natural habitat and the environment of Puget Sound, the reason for its appearance is puzzling to many.

One possible explanation for the mola mola’s appearance would be upwelling. Upwelling brings nutrient-rich water from the deep ocean to the surface, providing phytoplankton with necessary nutrients for photosynthesis and other biological processes. These influxes of nutrients stimulate algal blooms across the coast. Sunfish feed primarily on algae and zooplankton, and it is possible that these blooms caused sunfish to migrate to higher latitudes in search of food. The month in which the mola mola was discovered coincides with the upwelling season (between April and September), supporting this potential explanation. However, this theory does not explain why that particular sunfish risked its life traveling so far north to cold water regions.

Another potential cause is ocean surface warming and acidification. It is estimated that ocean temperatures are rising by 0.2°C every decade world-wide. At the same time, increased CO2 concentrations in the atmosphere result in higher concentrations of dissolved CO2 in the ocean. Ocean acidification puts stress on tropical environments, causing coral bleaching in tropic regions and loss of biomass and biodiversity altogether. Coral communities provide a breeding ground for many fish and an abundance of plankton and algae. The combination of rising ocean temperatures and loss of coral communities drives organisms from the equator toward the poles in search of congenial conditions and food.

Tropical latitudes are not the only regions experiencing increased surface ocean temperatures. It is estimated that water temperatures in Puget Sound have increased by 0.5°C to 1°C since 1970. Though these numbers seem small, slight changes can have big impacts. Additionally, areas such as the Hood Canal have experienced maximum surface temperatures of 20°C, which is much higher than maximum surface temperatures of ~15ºC in 1970. This temperature provides an environment suitable for sunfish to survive. And while the Hood Canal wasn’t where the sunfish was spotted, its sighting warns of a change in Puget Sound as a whole if warming trends continue. In addition, studies show that climate change is expected to bring higher levels of precipitation and thus more runoff to coastal waters, bringing nutrients from land into the waters and fueling algal blooms.

The appearance of sunfish in Washington’s cold coastal waters indicates that the ocean we know is shifting and warming. As climate change persists, it is likely that the appearance of non-native species may increase.