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Strange colonial animals called pyrosomes are thriving in the north-east Pacific Ocean as it warms up, especially during marine heat waves. But according to a computer model, their success means less food is available to animals higher up the food chain, including fish.

鈥淚t could have huge implications for energy flow throughout this ecosystem, and how many fish we can catch,鈥� says at Oregon State University.

Pyrosomes, sometimes called sea pickles, are colonies of individual animals called zooids that feed on plankton. Some species grow gigantic, but they are typically just a few inches long. Pyrosomes live in relatively warm waters and were never seen off the coasts of Oregon and Washington until recently, Gomes says.

But during a major marine heatwave dubbed 鈥渢he Blob鈥� that started in 2013, a species called Pyrosoma atlanticum 鈥� found worldwide, despite its name 鈥� bloomed in huge numbers. There have been further blooms during subsequent marine heatwaves.

Gomes and his colleagues have now fed data on the changing abundance of various organisms, taken from long-term surveys, into a computer model of the ecosystem of the northern California current, which flows from north to south down the west coast of North America. According to the model, the arrival of pyrosomes could have a big effect.

The issue is when pyrosomes are abundant, there is less food for other plankton feeders, such as krill, sardines, small jellyfish, pteropods, small invertebrate larvae and the like. But whereas other plankton feeders are often eaten by larger animals, pyrosomes are seldom eaten and instead sink to the seafloor when they die.

Most of the energy and nutrients in their bodies therefore ends up trapped on the seafloor, says Gomes. They are a dead end in ecosystem terms.

What remains unclear is why pyrosomes are not eaten. It could be that predators aren鈥檛 used to them and will start to consume them over time.

鈥淏ut the other idea is that they鈥檙e not really worth eating,鈥� says Gomes. Pyrosomes are relatively low in calories, he says, and while their bodies are gelatinous like those of jellyfish, they are also harder and may be less palatable.

Another reason the ecosystem is becoming less productive is animals鈥� metabolic rates rise in warmer water, causing them to use more energy. 鈥淵ou can think of it as, there鈥檚 more consumption for the same amount of seafood being produced,鈥� says Gomes.

The numbers of Chinook salmon and cod in the region have declined since the heatwaves began. But it is too soon to conclude the heatwaves are to blame, he says, because fish populations can vary for many reasons including natural boom and bust cycles.

Gomes also cautions the model does not yet include factors such as falling oxygen levels as waters warm. 鈥淚t鈥檚 a first stab at trying to understand how marine heatwaves are changing the ecosystems of the north-east Pacific,鈥� he says.