Late in 2013, scientists noticed something strange in the Pacific Ocean: a circular blob of extremely hot water making its way across the northeast Pacific. 

Appropriately named “The Blob,” this water mass became the largest marine heat wave ever recorded in the North Pacific. A marine heat wave is an extended period of extremely warm ocean temperatures: these hot temperatures can stress the ocean’s ecosystems, causing severe damage to marine organisms. 

The Blob reigned from 2014-2016, and the consequences were quickly apparent. Marine biologists began to observe several mass die-offs of organisms from krill to marine mammals to birds. These immediate effects of the Blob were intense and deadly. However, the ramifications of marine heat waves can persist over even longer time scales, and these long-term effects can be difficult to recognize until years after the heat wave ends. 

In 2015, a group of scientists led by Lauren A. Rogers at the Alaska Fisheries Science Center noticed a severe decline in larval walleye pollock (also known as Alaska pollock). Walleye pollock is an important Alaskan fishery that supports the economies of local communities. The scientists knew that this decline in 2015 was likely a result of The Blob. They also knew that larval fish declines can cause long lasting damage to fisheries: if there are not enough larvae that grow into adult fish, then populations may never return to a sustainable size and the fishery can collapse altogether. In the face of this potentially catastrophic problem, Rogers's team set out to find exactly how The Blob caused this decline.

They first collected data on the temperature and salinity of the ocean water in the western Gulf of Alaska. They then measured the population sizes of three early life stages of walleye pollock: eggs, larvae, and juveniles. Early life stages are especially important to fisheries because young fish are often the most vulnerable to stress. Finally, they measured the abundance of zooplankton, the prey of both larval and juvenile pollock. Zooplankton are tiny marine animals that consume phytoplankton, or plankton that produce their own food via photosynthesis. The scientists hypothesized that The Blob not only affected the walleye pollock, but also walleye pollock prey, which indirectly affects their survival.

The results of their study were surprising: the Blob was killing off walleye pollock in multiple ways. First, it led to extremely warm ocean temperatures and low salinity (i.e. more freshwater than salty water). Walleye fish eggs spend about two weeks in the deep ocean before hatching and rising up to the ocean surface to feed. These eggs rely on consistent, normal temperatures and salinity levels to rise and hatch.

Because The Blob changed both temperature and salinity, the eggs were not able to rise up as high in the water column as they normally do. This meant that less pollock eggs survived, likely because of high predation in deeper waters from adult pollock and/or the fact that the larvae that hatched from the eggs had a much longer distance to travel to reach food-rich areas.

The Blob also reduced the abundance of zooplankton prey for larval fish. After the eggs hatch, the larvae swim to the surface and begin feeding 5-6 days after hatching. These larval fish prefer to feed on copepod eggs, the eggs of a group of small crustaceans, and nauplii, which are the early life stage of many crustaceans (and also have a single eye!). The scientists found extremely low population sizes of copepod eggs and nauplii, meaning there was not enough food for the walleye pollock larvae to survive. 

Rogers and her team also observed that juvenile pollock had poor body condition, which is a measure of overall juvenile health, in response to The Blob. As the juveniles grow, they shift their diet from copepods to krill, which are small crustaceans that look like tiny shrimp. These energy-rich krill are a good source of nutrients for juveniles and help them grow and survive the harsh Alaskan winter. 

The Blob caused a decrease in the availability of krill for the juveniles to eat. On top of this low prey abundance, increased ocean temperatures from The Blob increased the juvenile pollocks' metabolisms, meaning they needed to consume more food to obtain more energy. Because the juvenile pollock could not consume enough krill to meet their bodies' demands, their health suffered.

This pollock decline can have lasting impacts on population sizes and shows that, in addition to immediate die-offs, the effects of marine heat waves can last long after the heat wave ends. These long-term impacts especially harm larval fish, which — just like any other baby — are extremely sensitive to negative and fluctuating growth conditions, in this case in a warming ocean. 

Unfortunately, the decline of the walleye pollock fishery is not a rare occurrence: other fisheries, such as the Pacific cod fishery, also saw declines associated with The Blob. Hopefully, the findings in this study can also help explain the reasons for the declines in these important fisheries.

Marine heat waves like The Blob are caused by combinations of several factors, such as high air temperatures, changing wind patterns, and regular ocean warming events like the El Niño Southern Oscillation in the Pacific Ocean. Though The Blob is over, it continues to harm marine ecosystems to this day. When looking at historical records, scientists have noticed an increase in the length and frequency of marine heat waves over the last century, and predict these heat waves will occur more and more often with global change. While marine heat waves are shorter in length compared to the long-term temperature increase we normally think about with climate change, the effects of heat waves can persist over long periods of time, with potentially huge consequences for marine ecosystems.