Size means survival for young salmon
Getting bigger faster can help save juvenile Chinook salmon from a gauntlet of hungry predators ranging from birds and marine mammals to larger fish. We continue our series on the Salish Sea Marine Survival Project with a look at what helps salmon grow and prepare for life in the open ocean.
Young salmon graduating from freshwater and entering saltwater for the first time undergo profound changes in their environment, their metabolism and their food supply. Also, while growing rapidly, juvenile salmon must learn to flee from an entirely new group of predators lurking beyond the streams.
It’s no wonder that this early stage of life in marine waters has been identified as a period of great risk for all salmon populations and why it has become a major focus of the Salish Sea Marine Survival Project.
Physiologically, the transition that salmon and steelhead go through from freshwater into salty seawater is an amazing phenomenon — and a rarity among fish species throughout the world. Moving into Puget Sound, these migratory fish begin drinking more water while their kidneys reduce urine production. Cellular pumps in their gills reverse direction by moving sodium out of their bodies rather than in.
Intact estuaries with deltas and salt marshes can aid in this remarkable transition by providing adequate food and shelter as well as a gradual rise in salinity — as opposed to the sudden jolt of moving straight into saltwater.
Some Puget Sound estuaries are better than others at helping salmon make the transition. While the Skagit River in North Puget Sound has lost a good deal of its estuarine habitat, nearly the entire Duwamish River estuary in Seattle has been converted into an industrial area — despite the river being in better shape upstream in forested areas.
Food is critically important to Chinook in the early stages of life, according to Dave Beauchamp of the U.S. Geological Survey who has been conducting extensive studies on the early growth of Puget Sound Chinook. In fact, he said, for some species of salmon, the ability to locate enough prey to grow rapidly in size and weight may be the most important factor in their survival.
What young salmon eat depends on their size and what food they can catch and consume. As they grow, fish are able to prey upon larger, more energy-rich species.
Size means survival
Among Chinook, so-called “fry migrants” move into estuaries soon after they emerge from gravel in February or March, experts say. They are smaller in size than the “parr migrants,” which may stay in the stream a few months, growing larger before they head out into the estuary. Some Chinook have even evolved to stay in freshwater an extra year or more to increase their size where stream conditions are suitable.
In general, studies suggest that high streamflows or crowding among juvenile Chinook results in more of the little fish heading out into the estuary as fry. If the estuary becomes overcrowded for the available food, the fry are likely to travel even farther out into Puget Sound, where the risks become greater still.
The size of a Chinook salmon helps determine its suitable prey, but studies by Beauchamp and others have revealed both seasonal and regional differences in what they eat. One interesting difference, Beauchamp said, is that large terrestrial insects dominate the diets of Chinook in the estuaries of North Puget Sound, while the Chinook in South Puget Sound seem to eat mainly small crustaceans. In Central Puget Sound, their diets are a mixture of the two.
Adult insects are a higher quality prey, based on their energy content, Beauchamp said. The greater availability of insects in North Puget Sound could be the result of higher river flows carrying insects down from areas with more intact streamside vegetation.
“Once these fish move out of the estuaries …, they have to make some tradeoffs,” Beauchamp said. “Prime areas for food may also be prime areas for predation.”
The Chinook may travel around Puget Sound, pulled into one area by a supply of preferred foods and pushed back out by heavy predation from larger fish, birds or marine mammals. Their senses are highly attuned to chemicals released when fish of their own kind get eaten.
“Fish can smell death,” Beauchamp noted.
Away from the estuaries, the diets of Chinook tend to be dominated in late June and July by crab larvae, which also feed coho salmon and even forage fish, such as Pacific herring.
The abundance of crab larvae, as well as other prey species, can vary greatly from year to year and have a powerful influence on the growth of young salmon and ultimately on their survival, according to Beauchamp. Body mass of Chinook can increase two to four times in a single month during this period, with larger size being a strong factor in their long-term survival — a characteristic known as size-selective mortality.
“It’s a very dynamic situation,” he said. “You have good years and bad years, and it all determines how well the fish will perform.”
Crab larvae and other species that feed juvenile Chinook are influenced in turn by their own prey, which includes smaller plankton. Researchers in Puget Sound are actively gathering information about which plankton are available at different times, what affects their growth and how they influence the overall food web.
Sunlight, temperature, minerals, salinity and water circulation are all believed to affect the growth of phytoplankton, which convert sunlight into energy at the very base of the food web. One important research question is how human sources of nitrogen — such as that from sewage-treatment systems and fertilizers — may be changing the type of plankton growing in Puget Sound.
Another area of study near the bottom of the food web involves changes in plankton communities caused by climate change, which raises water temperatures and reduces streamflows at critical times of the year. A shift in growth periods among prey species could deprive salmon of high-energy foods when they are needed the most.
Michael Schmidt, who coordinates the Marine Survival Project in Puget Sound, said it has been a struggle to find answers for this bottom-up, food-web piece of the puzzle.
“I had hoped it would be easier to understand the fundamental drivers back to something like temperature or nutrients,” he said. “It seems to be a much harder thing to figure out. We are still building toward the understanding that we really should be concerned about prey availability.”
Switchover to fish
Although crab larvae are often abundant, competition to eat them may grow intense. Large numbers of herring at times may result in less food and slower growth for young Chinook.
As Chinook grow in size, they begin to eat other fish, which can be as large as half their own size. The switch to energy-rich fish — if the Chinook can find them — triggers a period of rapid growth. Young herring, which spawn early in the year in most areas of Puget Sound, are usually a little too big to be eaten by juvenile Chinook.
That’s not the case in portions of Georgia Strait, where herring eggs hatch later and the tiny fish may be available to the hungry Chinook. In northern Puget Sound, a late-spawning group of herring, known as the Cherry Point stock, probably fed a lot of Puget Sound Chinook at one time, but today these herring are just a fraction of their historic population.
Small herring eventually become a primary prey fish in the Chinook diet. Other important forage fish include sand lance, whose populations may have been reduced by man-made bulkheads along the sandy shores of Puget Sound.
In nearshore areas, juvenile Chinook also have been known to eat juvenile pink and chum salmon, while in offshore areas larger Chinook may cannibalize smaller Chinook on occasion.
In the end, the amount and quality of food consumed in the first few months of life can mean the difference between life and death, since larger Chinook are likely to have better overall fitness and an increased ability to avoid predators.
Timing of migration
Estuaries may be critically important to wild Chinook, especially small ones, but perhaps they’re of less use to hatchery fish, which are reared longer and grow to larger size before their release. As a result, they tend to move more quickly to open water.
Although coho salmon in Puget Sound have been studied less than Chinook, coho are likely to exhibit similar size-selective mortality, experts say. Coho stay longer in their home stream — a full summer and winter — but they are not as large as Chinook of the same age. Recent studies have shown that some coho, like Chinook, leave the streams early and some may even stay in the estuaries their first year, but how that affects their survival is not well known.
Steelhead live in a stream for two years or more, growing to a larger size. They tend not to stay long in an estuary before moving to open waters, where predation seems to be a primary threat. If their early growth gives them an advantage over larger predators, it may not work with much larger predators, such as seals which could be specifically seeking larger prey.
Habitat is key
Early efforts to recover Chinook populations in Puget Sound focused on stream restoration, which allowed for more fish to live in the stream longer and grow to larger size. Still, in years with high egg survival, more Chinook left the stream as tiny fry.
“There was a school of thought that those little fish leaving the stream were excess production; they didn’t matter,” said Lance Campbell, a researcher with Washington Department of Fish and Wildlife. “Over the last decade, that has changed.”
Now, using advanced research tools, scientists are coming to understand that the little fish in the estuary don’t always die in extraordinary numbers. Many actually live to become spawning adults if given half a chance.
In one major study, Campbell and associates collected ear bones, called otoliths, from wild Chinook that had spawned and died in five Puget Sound streams. The otoliths contain growth rings made of minerals deposited during a salmon’s lifetime.
To reconstruct a salmon’s life history, researchers vaporize the growth-ring deposits with a laser beam and analyze the chemicals that are released. Because of differences between freshwater and saltwater, one can determine the size of the fish when it leaves the stream and enters seawater.
The proportion of early-leaving fry to later-leaving parr varied tremendously from year to year, related to streamflow and the number of young salmon in the stream, according to previous studies. In some years, up to 90 percent of the fish left their stream as fry. In other years, the ratio was as low as 20 percent fry.
Despite their small size, Chinook fry from the Nooksack and Skagit rivers in North Puget Sound not only survive, but they contribute significantly to the number of returning adult spawners. In the Skagit, for example, where about 80 percent of the outgoing migrants left the stream as tiny fry, those fish made up 36 percent of the returning adults in 2015.
In contrast to the northern rivers, fairly large numbers of fry leaving the Green/Duwamish and Puyallup rivers in South Puget Sound never made it back home. For example, in the Green River, between 47 and 73 percent of the out-migrants were fry, but less than 1 percent of those fish made it back to spawn.
Although the actual cause of mortality is unknown, Campbell points out that conditions in the estuary may make the difference. Estuarine habitat in the Nooksack and Skagit are more intact than that in the three South Sound streams, which are associated with urban areas. Factors believed to increase the risk of mortality include habitat loss and degradation caused by diking and filling, chemical contamination, disease organisms and greater exposure to predators.
Previous studies have shown that less than 5 percent of the estuarine habitat remains intact near the Puyallup and Green/Duwamish rivers, while the Skagit and Nooksack still have more than 50 percent of their natural estuary and delta habitat.
One might assume that if young fry can’t find estuarine habitat when they hit seawater, they might keep on going, Campbell noted. But it is also possible that they are unable to make the physiological transition without the help of an estuary. Perhaps a combination of harmful factors in some streams are just too much for the little fish to overcome.
Next week: Part 3: Top-down issues affecting salmon survival
Previous story: Read Part 1 of our series "Opening the black box: What's killing Puget Sound's salmon and steelhead?"