Scientists searching for evidence of climate change in Alaska’s Cook Inlet say they’ve found the fingerprints of global warming in the falling numbers of prized Chinook salmon, but admit they chose to ignore the heavy bootprint of an invasive species in local waters.
Their peer-reviewed study published in Global Change Biology earlier this month looked at 15 salmon populations inhabiting Inlet streams from 1980 to 2015 and highlighted productivity for “the two coolest (Chulitna and Little Susitna rivers) and two warmest (Deshka River and Alexander Creek) study sites.”
It noted the Inlet harvest of the largest of the Pacific salmon has fallen from a peak of 134,489 in 1993 to 10,838 in 2012. It has remained generally low since, and the fish have been shrinking in size, which some scientists think could be linked to lack of food in the ocean.
Some scientists have suggested the big fish simply can’t compete with their smaller competitors in a North Pacific Ocean now stuffed with salmon. Others have blamed predation from sharks and marine mammals.
And then there is the over-arching issue of global warming.
The latest study concluded that for “all populations and years, productivity decreased when maximum weekly temperatures exceeded approximately 18°C (64.5 degrees F) during spawning and incubation and approximately 15°C (59 degrees) during rearing. The nonlinear relationships were strongly influenced by data from the coldest and warmest streams.”
The warmest streams were the Deshka and Alexander Creek in the lowlands of the Susitna Valley, and the researchers noted that while the study’s “productivity model fit differed substantially among populations” in the many Susitna Valley streams home to kings, it veered off the scale with Alexander Creek showing “the greatest model error, suggesting that our models were missing important drivers of productivity for this population.”
That would be a scientific observation of uber proportions.
An often weedy, always winding and slow-moving waterway, Alexander Creek was almost four decades ago contaminated with northern pike apparently introduced by people. By the 1990s, according to an Alaska Department of Fish and Game study, the pike – a voracious predator – were overrunning the drainage.
By 2015, those biologists were reporting that “northern pike are widespread throughout the system. A large portion of the drainage is shallow and densely vegetated, making it ideal northern pike habitat.”
They now refer to the pike as an “infestation.”
Biologist Dave Rutz, now the director of the state Division of Sport Fisheries and an authority on northern pike, has observed that where pike move into weedy, slow-moving tributaries to the Susitna River such as the Alexander, their presence “can lead to over 90 percent predation on juvenile salmonids.
Alexander Creek was by 2016 the poster child for the ways in which pike can depress salmon populations.
Prior to the pike infestation, it sometimes produced nearly 10,000 Chinook per year. A 2011 survey found only 83 kings had returned to spawn, a 72-fold decline.
The problem has been widely publicized. It has been discussed repeatedly by the Alaska Board of Fisheries, and it was written about often by Anchorage-area news media as what was once a vibrant, Alexander Creek king salmon fishery faded and died.
Exactly how far the pike problem has spread beyond Alexander Creek is an unknown, but Alexander Creek is a disaster.
By 2013, U.S. Geological Survey scientists studying the situation were reporting that pike had been so efficient in killing young salmon – their preferred prey – that they were having trouble finding enough young kings to eat and had been forced to start feeding on Arctic lampreys and slimy sculpins as well in order to survive.
“…Pike consume other native fish species when juvenile salmonids are less abundant,” they reported in the peer-reviewed journal Ecology of Freshwater Fish. “Implications of this trophic adaptability are that invasive pike can continue to increase while driving multiple species to low abundance.”
“Alexander Creek once supported a multimillion-dollar sportfish industry with 10, full-time lodges, along with numerous guides, air and boat charter services, and boat rentals – all which are no longer in operation since the king salmon fisheries plummeted,” Rutz wrote.
The authors of the Global Change study – “Watershed‐scale climate influences productivity of Chinook salmon populations across southcentral Alaska” – chose to ignore this well-known environmental problem and focus on temperature, precipitation, ice and a few other climate-related issues.
While conceding that pike were “likely a primary driver of the decline in Alexander Creek,” they said they chose to ignore “multi‐species interactions due to a lack of time‐series data.”
As a result, Alexander Creek joined the Deshka as one of the two watersheds suffering the most because of global warming. But the study overall found climate factors generally hard to sort out.
Temperature changes were “weakly negative” for some streams, “weakly positive” for others, and often negligible. Overall, the study underlined how hard it is to find significant climate impacts on salmon in the Cook Inlet region though the overall influence of climate on Alaska salmon is well documented.
When the North Pacific Ocean went through a cold phase in the 1970s, Alaska salmon harvests and returns were dismal. They are now at record levels but with some big differences among regions and species.
Sockeye salmon harvests are booming in Bristol Bay, but declining around the Gulf of Alaska. Chinook catches are depressed almost everywhere. Pink salmon are so plentiful in Prince William Sound – thanks to some help from hatcheries – that they’re straying into Cook Inlet in search of less crowded places to spawn.
It is a big complicated picture, but some things have been known for a long time.
Fall floods hurt production as the latest study found. That problem has long been known. Generally, increased summer rainfall appeared to improve survival conditions for young Chinook, which might be explained by water levels staying close to or under vegetated streams banks which shelter the young fish from predators.
Productivity in the Inlet’s coldest stream, the Chulitna River, improved with higher maximum temperatures for spawning and increased average temperatures for rearing,, the study said. But productivity in the two warmest streams, the Deshka River and Alexander Creek, crept down slightly
Like Alexander Creek, the Deshka is now also home to some pike, but they are not the problem there that they are in Alexander Creek. Whether warmer waters hurt Alexander Creek fish or simply boosted predation is an unknown.
Pike are at their best as predators in water of 60 to 65 degrees. Juvenile Chinook are most comfortable in 55-degree water.
Whatever the case, there are clearly myriad factors affecting the big drop in Inlet Chinook numbers.
And few of them are easily controlled by humans.
The entire world is struggling with the issue of how to reduce the emissions of the greenhouse gas carbon dioxide without destroying a global economy still largely fueled by hydrocarbons which produce carbon dioxide as a by-product.
And dealing with prolific pike – which have left no debate about what they can do to salmon populations in weedy slow-moving streams – has not proven much easier.
In 2008, the Alaska Board of Fisheries banned fishing for kings on the Alexander because of the precipitous rate at which returns of the big fish were falling. The population continued to shrink.
The state’s sustainable escapement goal for Alexander kings – the number of fish biologists believe need to escape the nets of commercial fishermen and spawn to maintain a health run – was set at 2,100-6,000 fish in the early 1990s.
Chinook spawners disappeared from the tributaries upstream of Alexander Lake in 1992 and have not returned. By about 1998, they were gone from the upper mainstem of the creek.
Spawning is now mostly isolated to Sucker Creek. Rutz said that by the mid-2000s, the natural, ecological picture of the situation was clear.
Pike were thriving in good pike habitat, and salmon – both kings and cohos – along with grayling and rainbow trout were holding out in the prime habitat for those fish, primarily fast-moving water over cobbled or rocky stream beds.
Most of that habitat is in Sucker Creek or the upper Alexander.
In 2011 the state began a pike suppression effort to aid salmon and trout in the creek. The Alaska Legislature approved $635,000 from the Alaska Sustainable Salmon Fund to fund a two-year, pike-control program. It approved another $563,000 for another two-year project in 2014-2016.
Since then, the Matanuska-Susitna Borough and the National Fish Habitat Partnership have financially joined in the effort to suppress pike numbers in a watershed that is now facing a second problem with invasive elodea, a noxious, aquatic weed.
“Unfortunately, since the discovery of elodea in 2014, it has already established throughout 90 percent of Alexander Lake and fully covers the Sucker Lake complex,” the Partnership’s website says. “Further, it is expanding downstream in Alexander and Sucker creeks. This habitat change may now be creating better habitat for predatory northern pike, further exacerbating the existing impacts of pike predation on juvenile salmon and other fish.”
Some progress has been made. The five-year average return of spawners is 737. That’s up 26 percent from the 10-year-average, but still only about a third of the minimum goal.
Still, the 1,297 Chinook that returned to the river last year was the most since 2005, and the U.S. Fish and Wildlife Service has agreed to fund the 2021 pike control effort.
Still, a long-term solution appears almost as elusive as a quick fix to global warming.