BREMERTON — Pulsating gelatinous sea creatures, which float through the water like alien life-forms, appear to be on the increase in portions of Puget Sound.

Jellyfish are not welcomed by most biologists, who have learned that these translucent creatures tend to show up when the ecosystem is troubled. They’ll take a bite out of the lower portion of the food web, gobbling up plankton that could otherwise feed herring and other forage fish — which, in turn, are important prey for salmon, birds and marine mammals.

Casey Rice of NOAA’s Northwest Fisheries Science Center said there is a lot to learn about jellyfish, but one thing seems clear. When the number of jellyfish is high, the number of forage fish, such as herring, is low.

The cause and effect are not well understood. Some say humans have inadvertently invited the jellyfish into local waters by releasing excess nutrients from sewage into the water. The nutrients produce tons of algae, which feed larger plankton, which are eaten by jellyfish in competition with forage fish.

In some cases, overfishing has reduced forage fish, which leaves the food supply wide open for jellies. Ironically, jellyfish are caught in fishing nets, creating a major nuisance for fishermen.

Since they have few predators, jellyfish tend to stick around too long, maintaining their troublesome ways. They seem to survive better than fish in low-oxygen conditions. And when food supplies run low, they may out-survive fish by drifting about and using very little energy.

It is often said that jellyfish are an energetic “dead end” in the food web, since they eat many things but almost nothing eats them.

Jellyfish are among the most ancient multicellular organisms on Earth. Their multistage life cycle begins with a fertilized free-swimming larva that settles on a hard surface. The larva grows into a polyp with a short stalk and a mouth ringed by tentacles for constant feeding. Eventually, the polyp breaks away, floats around and develops into the pulsating medusa, the stage typically identified as a jellyfish.

Some experts believe that rock and concrete bulkheads, installed to prevent beach erosion, encourage jellyfish by providing good habitat on which polyps can grow and develop.

In South Puget Sound and Southern Hood Canal, where poor water conditions are marked by low oxygen and increased acidity, the greatest numbers of jellyfish have been found, according to a 2011 study headed by Correigh Greene, Rice’s associate at NOAA.

The study compared the biomass of fish with that of jellyfish in five areas throughout Puget Sound. Jellyfish were greatest, at 96 percent of total weight, in both Admiralty Inlet and Hood Canal. Jellies were 93 percent of total weight in South Sound and 91 percent in Central Sound. In contrast, fish were more prevalent in Rosario Strait, where jellyfish made up only 39 percent of the weight and near Whidbey Island, where they totaled 34 percent.

The differences between North and South Puget Sound — including the relative abundance of different types of plankton — suggest that two distinct food webs may be in operation, one favoring forage fish, one favoring jellyfish.

Questions remain about how humans might be encouraging jellyfish and whether short- and long-term climate variations may shift things in favor or against jellyfish versus forage fish.

“The important thing,” said Rice, “is that people are trying to understand this stuff. It is exciting to compare notes and talk about how things are being pushed around at the bottom of the food chain. Our observations so far are consistent with going down the jelly road.”