Field season marked by low flow and high pre-spawn mortality

Pink salmon that entered Hogan Creek on the high tide died by the thousands after they were stranded without enough water flows to return to the bay. Photo courtesy of Brad von Whichman
Pink salmon that entered Hogan Creek on the high tide died by the thousands after they were stranded without enough water flows to return to the bay. Photo courtesy of Brad von Whichman

It was a very strange year for the Prince William Sound Science Center field crews collecting data on pink salmon for the pedigree component of the Alaska Hatchery Research Project (AHRP) in Prince William Sound this last summer.

The science center has been involved in doing field work for AHRP, a large multi-organization study on straying and reproductive success of hatchery-origin pink and chum salmon, in wild streams since the pilot study in 2012. This summer a reduced field crew focused on three of the five streams in western Prince William Sound sampled for the “pedigree component,” the part of the study that is designed to determine the reproductive success of hatchery-origin pink salmon compared to their wild counterparts in streams.

For the pedigree component, crews have been walking up and down five different streams in Prince William Sound collecting detailed data from as many dead fish as possible since 2015 (the field study was carried out in 2013 and 2014, but with much lower field effort). Every day during the spawning period in August and September they count the number of alive versus dead salmon and take samples from dead fish. Researchers record their length and height,  their sex, the sampling location in the stream (from the intertidal zone up into freshwater reaches of the streams), at what life stage they died (pre- or post-spawn), whether they were preyed upon, how long they have been dead by noting the color of their gills, and any unusual characteristics of the fish. They also remove an otolith (ear bone), which is used to determine if the fish was of hatchery or wild origin, and heart tissue for DNA extraction.

In a normal year, field crews work long hours under drenching rain and high river flows in August and September.

This year, virtually no rain led to extremely low flows and field crews observed unprecedented pre-spawning die offs and unusually late migration into the streams. Pete Rand, the lead researcher for the AHRP field component in Prince William Sound reported that, normally, the fish are entering the streams by the second week in August and sample numbers rise quickly. This year, the crews took virtually no samples in the month of August.

The hatchery wild crew. Photo courtesy of Prince William Sound Science Center

According to Rand, the fish finally started, what was for many, an ill-fated journey into the streams after some rain in early September. The rain stopped and the rivers dried up again, but many of the fish had already entered the streams on the high tide. Soon thousands of fish were restricted to tide pools without enough water to return to the bays. They all suffocated.

 “During the first 10 days of September, our dead fish count in one of our streams rose from virtually none to nearly 30,000 dead pink salmon, all dying prior to spawning,” Rand recounts. “Our field crews estimated 10,000 died over a single night. We have never documented anything like that in the past.”

Although Rand worries about the effects on future production, this didn’t cause any problems for the pedigree study. Because this is near the end of the project, researchers just wanted to know if the fish, who would by now be grandchildren of the original fish sampled in 2015, survived to return to the stream as adults. With this information researchers can make a family tree from the fish returning to the streams every two years and determine the relative reproductive fitness of hatchery-hatchery, hatchery-wild, and wild-wild spawning pairs. The field crews were able to collect around 30,000 samples in September, which should be enough to accomplish the objectives of the study. However, how climate change might affect streams and salmon in our region may be an even bigger question.

 “This may be our ‘new normal’, with natural spawning restricted to a very narrow time window, and fish spawning under very crowded conditions,” Rand concludes. Time will tell how these recent climate conditions play out for pink salmon in Prince William Sound.


Teal Barmore covers news for the Prince William Sound Science Center’s research and monitoring programs. More of her work can be seen at pwssc.org/news.