Harbor porpoises become increasing players in the Puget Sound food web
With a population growth of about 10 percent per year in inland waters, harbor porpoises are having an undetermined but growing effect on food dynamics in Puget Sound.
An explosive growth in the number of harbor porpoises in Puget Sound could be creating a ripple effect through the food web, with potential consequences for salmon, seals and even orcas.
Harbor porpoises are notoriously difficult to study, and diet studies so far suggest that young salmon are not normally on their menu. But experts acknowledge limitations in the diet studies, and some believe that these marine mammals might well be consuming young salmon.
In any event, harbor porpoises do appear to be eating large numbers of so-called forage fish, which are key prey for salmon that ultimately feed the endangered Southern Resident killer whales.
Unlike harbor seals, which are known to eat young salmon and steelhead, nobody has implicated harbor porpoises in the troublesome declines in Chinook salmon nor in the ongoing losses of Southern Resident killer whales. Still, the rapid growth in harbor porpoise numbers — up to 37 percent per year in the main body of Puget Sound — means that these animals can no longer be ignored while trying to determine who is eating whom and why some species are doing so much better than others, experts say.
“The number of marine mammals is certainly an important question,” said Chris Harvey, an ecologist at NOAA’s Northwest Fisheries Science Center. “There are many of them; they are large; and they are warm-blooded, which means they must eat many, many times their body weight to keep their furnaces stoked.”
In fact, some studies have shown that high-energy harbor porpoises typically eat between 5 and 10 percent of their body weight each day, or between 18 and 36 times their weight each year. For each harbor porpoise, that is likely to be between 6 to 15 pounds of seafood every day.
"...they must eat many, many times their body weight to keep their furnaces stoked."
—Chris Harvey, ecologist, NOAA’s Northwest Fisheries Science Center
More than 11,000 harbor porpoises now occupy Washington state’s inland waterways, according to a recent aerial survey of marine mammals conducted for the Navy. That includes about 2,700 harbor porpoises in the main portion of Puget Sound (south of Admiralty Inlet), an area where harbor porpoises had practically vanished by the 1990s.
Harbor seals, which eat many of the same fish as harbor porpoises, now number somewhere between 12,000 and 15,000 in the inland waterways, with about 3,000 of those found in the main body of Puget Sound, including Hood Canal, according to Steve Jeffries, marine mammal biologist for the Washington Department of Fish and Wildlife. Harbor seal numbers generally leveled off about 20 years ago and now appear to be declining.
Seals have been a hot topic for the Southern Resident Killer Whale Task Force, appointed by Gov. Jay Inslee to find ways to save the orcas from extinction. Seals are known to eat Chinook salmon, the primary food for resident killer whales. That has made the seals in Puget Sound a potential target, as task force members ponder whether actions to reduce the number of seals could increase the food supply for orcas.
So far, the role of harbor porpoises has gotten little attention among scientists, and the issue has never come up in formal discussions with task force members.
More study is needed to learn how an increasing number of marine mammals is affecting other creatures in Puget Sound, said Robert DeLong of NOAA’s Alaska Fisheries Science Center in Seattle.
“We shouldn’t ignore them,” DeLong said of the harbor porpoises in Puget Sound. “That is the one thing that has changed in the predator-prey dynamics south of the Tacoma Narrows Bridge over the past two decades. They are an important part of the food-web story.”
Harbor porpoises return to Puget Sound
During the 1940s, harbor porpoises were common in Washington’s inland waterways — including Puget Sound, the San Juan Islands and the Strait of Juan de Fuca — as well as shallow bays along the coast.
“We have seen them thus within 30 feet of shore at Steilacoom (South Sound) and far out in Puget Sound,” according to Victor Scheffer and John Slipp, writing in their 1948 monograph “Whales and Dolphins of Washington State.” “In southern Puget Sound, the harbor porpoise occurs at all seasons — rarely singly, usually in groups of 2 to 5, occasionally [in groups of] 10 to 12..."
Smaller than most porpoises, the harbor porpoise, Phocoena phocoena, can grow to about 5 feet long and up to about 150 pounds for males. Unlike many marine mammals, adult females are generally a little larger than males.
To observers, their backs, flippers and dorsal fins appear as a dark gray or brown, while their lighter-colored sides blend toward an underbelly of white.
“In brisk, uneven succession they breach, loom up, and subside close together, their forward motion visible at a distance by the passage of the triangular dorsal fins — up, over and down, as if mounted on revolving wheels thrust briefly above the surface and withdrawn,” wrote Scheffer and Slipp in their 1948 monograph.
For unknown reasons, the large number of harbor porpoises declined in Puget Sound over the next 25 years, although they were still seen in scattered groups in the San Juan Islands and Strait of Juan de Fuca. One leading theory for the decline is that the porpoises were caught and killed in gillnets as the amount of fishing increased following World War II. Other possible explanations include noise and disturbance from humans, habitat loss and pollution.
From 1970 to 2000, harbor porpoises were nearly absent in the main body of Puget Sound, researchers say. Then, starting in 2000, they began returning in greater and greater numbers, a rapid growth that cannot be explained solely by natural reproduction, which is about 4 percent per year.
From 1995 to 2014, the overall population in Washington’s inland waters grew by about 10 percent per year, according to a 2016 study led by biologists at Cascadia Research Collective in Olympia. Their analysis looked at 19 years of winter bird surveys conducted from airplanes by the Washington Department of Fish and Wildlife, surveys that also recorded the sightings of marine mammals.
The rapid growth of harbor porpoises in the main body of Puget Sound was about 37 percent per year from 2000 to 2014, according to the analysis. During that time, growth in the Strait of Juan de Fuca was about 9 percent. Meanwhile, the population seemed to be leveling off around the San Juan Islands following a period of rapid growth.
A female harbor porpoise has the potential of giving birth to a single calf each year, although actual birth rates vary. Because harbor porpoise numbers have been exceeding their natural growth rate, some of the increase must be from animals coming into Washington’s inland waters from somewhere else — most likely from Canada to the north and coastal waters to the west, according to Tom Jefferson, a research consultant who led the latest population study.
“I see this comeback as sort of an optimistic sign for those who work on conservation wildlife issues,” Jefferson said.
He noted that a similar increase in harbor porpoises has been observed in San Francisco Bay to the south, where the animals made a comeback after a 60-year absence.
But Jefferson also works on issues related to the highly endangered vaquita porpoises, whose population has dropped to less than 30 animals in the Gulf of California and Sea of Cortez in Mexico — the only place this species exists.
“It’s depressing to see this population decline and decline,” Jefferson said of the vaquitas. “They may not be around in another year or two. That’s why seeing populations of harbor porpoise showing signs of recovery is a good thing. You need to have some positive [news] now and then.”
Harbor porpoises must be eating something
Limited studies conducted so far in the Salish Sea suggest that harbor porpoises eat a wide variety of small schooling fish, such as Pacific herring, walleye Pollock, Pacific hake, eulachon and Pacific sanddab. They also are known to consume Pacific sandlance, Pacific sardine, northern anchovy and shiner perch, as well as squid.
So far, salmon and steelhead have not been identified as a source of food for harbor porpoises, based on diet studies that examined the stomach contents of dead porpoises. But these studies have their limitations, said Brad Hanson, a marine biologist with NOAA’s Northwest Fisheries Science Center who has been involved in the work.
Most of the carcasses were of animals that died and washed up on a beach, so they may have been sick and not eating normally, if at all. A few others were porpoises that drowned in fishing nets when young salmon may not have been around.
Jeffries of WDFW said harbor porpoises, like harbor seals, appear to be generalists in their diet choices, going after “shiny little fish” that catch their attention. For that reason and given the limitation of existing studies, he does not rule out the possibility that harbor porpoises would eat salmon if they encountered them in the right size and abundance.
There is also the case of two harbor porpoises found dead near the mouth of the Columbia River in the spring and summer of 2012, according to Debbie Duffield, a biology professor at Portland State University. The stomach of one porpoise contained 93 tiny metal tags that identified the digested prey as young Chinook salmon tagged at an upstream hatchery. The other animal had eaten a tagged winter steelhead.
Harbor porpoises may eat salmon if given the chance, Duffield said, but those were the only tags found during 10 years of research, which involved the examination of about 18 dead porpoises per year, on average.
DeLong of NOAA said one must look carefully at both where and when samples are taken for diet studies, because the abundance of small fish available to harbor porpoises varies by location and time of year.
The same goes for harbor seals, DeLong said, expressing criticism about recent studies that use limited information about diets to project the number of Chinook salmon eaten by harbor seals. It is a critical issue with potentially serious fallout for orcas that depend so heavily on Chinook. Samples taken within a limited time and place should not be used to generate estimates of the total predation over an entire year and throughout all of Puget Sound, he added.
Effects on the food web may depend on which fish harbor porpoises and harbor seals are eating. Both species eat hake at times, and hake are known to eat salmon. Both also eat herring, a fish that may compete with young salmon for food during one life stage, then become a key food for salmon during later life stages.
While populations of harbor seals and harbor porpoises may be considered generalists when it comes to diet, there is some evidence that individual animals can become specialists in predation, according to Barry Berejikian of NOAA’s Northwest Fisheries Science Center. During experiments in South Puget Sound, Berejikian was trying to determine how many steelhead get eaten by harbor seals by placing tiny acoustic transmitters on young steelhead released in the wild. Surprisingly, indirect evidence shows that at least four transmitters were ingested by a single seal, while other seals had none.
The effects of dramatically increasing or decreasing a marine mammal population can generate ripple effects through the entire food web, something called a top-down effect. At the same time, problems among the microscopic plankton that feed the tiniest fish can produce a food shortage up the line, known as a bottom-up effect.
Much of the research has been focused on harbor seals because harbor porpoises and other top predators are more difficult to study, Berejikian said. For example, fecal samples from seals can be picked up from their haul-out areas on land to figure out what they are eating.
“All along, I would have loved to do something with harbor porpoises,” Berejikian said. “It’s not that we think harbor seals are driving everything [in the ecosystem]. But we are trying to quantify their effects. Then the remainder is everything else.”
Connecting the dots in the food web
Puget Sound’s food web is so complex that it cannot be described easily in words. Line diagrams on paper that point from prey to predator and back may help people visualize the connections, but such diagrams don’t capture the location of the interactions, nor the depth of the water, nor the time of year — all of which are critical factors in determining whether a species eats or gets eaten.
That’s where a computer model of the food web comes into play, says Harvey of NOAA who is playing a key role in developing a food-web model of Puget Sound. The model will help scientists understand how changes in one population can lead to changes in many others.
”The amount of food of a particular type that a predator eats changes as a function of the amount of food that is out there,” said Harvey. “We refer to that as a functional response. As the amount of, say, herring increases, the amount that a predator is eating will increase until the predator can’t eat anymore, or until the prey decreases and the predator is not capable of catching those fish.”
If the herring population declines, then predators will need to find something else to eat or their population will decline … and so on.
“There are many species of prey out there, and every one has its own strategy on how to make it in this world.”
—Chris Harvey, ecologist, NOAA’s Northwest Fisheries Science Center
“There are many species of prey out there, and every one has its own strategy on how to make it in this world,” Harvey said. “Responses are often unpredictable, because you have so many feedbacks. But models can hopefully provide hypotheses to see how that works.”
The modeling effort, funded by the Salish Sea Marine Survival Project, is built upon the Atlantis Model, developed by Australian government scientists. Early results are expected next year.
If diet studies can determine what various species are eating, then the food web model should be able to predict changes in the real world that would occur under various management scenarios — such as if harbor porpoise populations were to suddenly increase, or if harbor seals were removed from Puget Sound, or if herring or hatchery salmon were boosted with the help of humans.
While resident killer whales depend heavily on Chinook salmon, transient killer whales — a distant cousin — eat mainly marine mammals, including both harbor seals and harbor porpoises. The number of transients visiting Puget Sound has been increasing in recent years.
The presence of transient orcas could be reducing the number of marine mammals, experts say. That reduction, in turn, could benefit populations of small fish, which could help salmon, which could provide more food for the endangered fish-eating killer whales.
When the Atlantis model is up and running, it should be able to track the growth or decline of various fish and marine mammal populations and to trace their general movements, Harvey said. One could hypothesize that harbor porpoises will eat fish of a certain size and abundance if they’re in their general area. The model can predict the resulting effects on other species.
The model also is designed to run through time. “With harbor porpoise, we could assume the populations were what they were in 1990 and grow the population to its present level,” Harvey said. “Does that change the amount of salmon or the amount of herring or the diets of harbor seals? That is what we would like to find out. What effect is one change having on the rest of the system?”
It is generally assumed that harbor seals are the main culprits when it comes to eating large numbers of threatened Chinook salmon, which are desperately needed to feed the Southern Resident killer whales, Harvey said. “But it is also possible that harbor porpoises are playing a role. They may be eating a lot of food that harbor seals used to eat. A food web model is absolutely essential to answer questions like this.”