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Ocean heatwaves like "The Blob" cause lasting damage to marine ecosystems

Recent research details The Blob's effects on the Alaska pollock fishery

Keira Monuki

Marine Biology

University of California, Davis

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.

a white fish with dark top and spotted sides

NOAA Fishwatch on Wikimedia Commons 

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.

Comment Peer Commentary

We ask other scientists from our Consortium to respond to articles with commentary from their expert perspective.

Ashley Marranzino

Marine Biology

University of Rhode Island

I think this research highlights two important issues scientists can run into when trying to address questions involving unprecedented, one-off or infrequent events like oil spills or marine heat waves.

As scientists, we are frequently expected to answer questions about how an entire ecosystem will respond to an event. But ecology just isn’t that simple. There are so many moving pieces at play that scientists cannot fully understand how all of those pieces work together and what could happen if one is altered. While it may seem logical that changing salinity will impact fish biology, it is not always intuitive how or why. I would never have guessed that lower salinity levels would decrease hatching rates in some fish because the change in buoyancy ultimately  forces the eggs to incubate in a suboptimal location.

I also think this research emphasizes why scientists need to conduct general surveys of areas to collect baseline data and determine what “normal” looks like in a system. It may not seem worthwhile to spend time and money on these surveys when everything is healthy, but  scientists rely on the understanding of a “normal” system in order to classify impacts from natural and environmental disasters. This team of researchers was able to connect the dots and determine how The Blob was changing fish populations only because they had a sense of what  “normal”, pre-Blob fish and plankton populations looked like in the area.

Ecosystems are complex and predicting how they’ll respond to a given event is nearly impossible. But it’s that complexity that also makes them elastic and often able to withstand disturbances. The trouble is that “The Blob” is a disturbance so large, it extends the ecosystem past its bounds, like an overstretched rubber band. Salinity levels change, crustacean eggs disappear, and krill populations drop. Individually, these changes may not matter. But collectively, they overstretch pollock populations, almost to the point of snapping. I’m curious if the study could have been bolstered by any kind of magnitude measurements. Not as a way to find a single cause, but to better understand how the different forces within marine ecosystems interact. 

Allison Fritts-Penniman

Ecology and Evolutionary Biology

This is a really great example of why we need to study all life stages of a species to understand how environmental changes impact them! Also, while the pollock were the focus of this study, their results make me want to know more about the populations of the invertebrate species that  they mentioned as prey, such as copepods, nauplii, and krill. They noticed these prey species were at lower abundance that year, and I wonder if that has had any long term effects for those species, and for other predators of those species? 

Sarah Heidmann

Fish Ecology

University of the Virgin Islands

This article illustrates the ecological story of how one event affects one species. It makes me wonder: how many unstudied species were also affected by The Blob? Not just prey species, but all kinds of producers and consumers throughout the food web. In a complex ecological setting  such as the one you described, one period of extended abnormal conditions can cause many cascading effects throughout the ecosystem. As these events start happening more and more often, affecting more and more species, they will have lasting consequences that populations may not be able to recover from.

I think this is also a good example of ecological damage that would be difficult to remediate. You can’t just recreate an entire cohort of fish; the best we can do is to focus on preventing future extreme events and other environmental stressors.