It's a strange event indeed, I can't say I strongly agree that this is heat related since our first indication that something was wrong came on July 9th during our standard bi-monthly shellfish collection for PST testing at Nahku Bay (59.478461, -135.337864), and temperatures had been low if anything for the month.
The mussels we collected that day during a -0.3m tide were pretty strong smelling, not rotten but definitely off. The first wave of dead mussels washed ashore on July 14th, possibly earlier but this was the first report we received. I took the pictures included in my LEO observation on July 16th, and the temperatures were only just then beginning to climb into the upper 70s and lower 80s. We monitor sea surface and air temperature pretty regularly and nothing this year has indicated unusually warm water or air, granted we only collect these data discreetly at a single point in time weekly. The oddball parameter in this equation that sticks out to me is salinity.
Skagway experiences steady southerly winds in the summer and we have two rivers fed by high elevation glaciers pumping freshwater into the northern Taiya Inlet all summer - salinity is typically between 5 and 10 ppt this time of year. For the last three weeks salinity in Nahku Bay has been below 1 ppt and lots of what I assume are freshwater rotifers have been in our phytoplankton samples. I've been hearing reports that this has happened before, maybe ten or more years ago, but no environmental data are associated with this previous event and I haven't gotten a solid date.
On July 16, the tide level was at 0.4m and most of the mussels I examined above the water line appeared to be dead, byssal threads disintegrated and most of the shells opened up with meat inside decaying as shown in the pictures. We haven't had a low enough tide to check on the lower strata of mussels to see what mortality looks like a little lower in the intertidal zone, but our collection on 7/09/21 was from this level and the mussels seemed off. I'm at a total loss, and wishing we had deployed a continuous temperature logger to monitor SST (sea surface temperature) in the weeks leading up to this event. This could be the catalyst to get that going. Thank you for your attention on this matter, I hope this is at least isolated to Skagway and not happening in a major way elsewhere in SE AK.
Follow up Comment by Reuben Cash on July 23, 2021: I have some additional photos taken during this morning's -0.9m low tide. My hopes that lower strata mussels had avoided whatever was killing the population further up in the intertidal zone have been dashed - it's the same story at least in Nahku Bay right down to the lower limit of mussel colonization. We collect from an old shipwreck that was previously covered in Mytilus, on the remains itself and all around on the rocky substrate in a 50-meter buffer at least. The shipwreck is now half bare with those shells remaining mostly dead. We tried to collect a standard 100 gram mussel meat sample for our PSP testing and it took two of us 45 minutes to locate enough live specimens to fill the whirlpak, and I had to range onto the rocks bordering Nahku to the west. We count wildlife weekly during our sampling, and this morning there were more mew gulls than we've ever counted on the beach foraging in the wrack - hopefully there is not anything in the dead mussel meat that will cause issues in the foraging birds. I'm sticking with my original estimation for mortality at between 70 and 90%, I'll head over to other shellfish beds over the weekend to verify that it's widespread elsewhere too.
Editors Comment: please note this is the same location as original post, as Reben explains:* This is the same location, at Nahku Bay (this is the Tlingit name, locals call it Long Bay). However, this is also being observed within a ten mile radius at other beaches (Yakutania Point, Smuggler's Cove, Skagway Harbor), all the places I can access shellfish beds by road. We have a lead on a boat to go further down the inlet and check in places where the salinity might be closer to typical marine levels.
Follow up Comment by Reuben Cash on 10-28-21: We checked around the inlet to see how widespread the event was, it seems to be limited to the upper Taiya, attenuating in sync with rising salinity levels as you travel away from the Taiya and Skagway River estuaries.
I'm copying Mandy Lindeberg at the NOAA Auke Bay Lab here, since she knows much more about mussels. I did talk with Margo Reveil, one of our local oyster farmers in Kachemak Bay who knows a lot about mussels as well. She said the mussels do not do well at salinities that low (as you have in the response to the observation) and she has heard that mussels can release their byssal threads when those conditions occur, to move away. That might be consistent with what Reuben observed too.
Mandy Lindeberg at the NOAA Auke Bay Lab writes in reply:
Wow, interesting. That area has a huge biomass of mussels due to a lack of predators (because of low salinities). The "lethal" low salinity is unusual and hard to validate. For intermittent exposure to low salinities mussels are usually very tolerant so this could be a case of a prolonged event. Agreed, wishing for data loggers!
Blue Mussels (mytilus edulis) have an ideal environment for the growth and reproduction dependent on specific levels of salinity and temperature.
The publication Recent Changes in the North Sea: Temperature and Salinity describes how sea temperatures, air temperatures, and ocean salinity interact in specific ways. The PDF version can be found in the "documents" tab to the right. As Mr. Cash reported, there are two rivers fed by glacial runoff that add freshwater to the northern Taiya Inlet. Higher outside temperatures increase melting and precipitation, increasing the input of freshwater and would decrease salinity. With warmer water temperatures, the molecules expand and make more room to hold salt molecules, increasing salinity. Colder water temperatures make water molecules compact, decreasing salinity.
According to a published fact sheet for blue mussels (also found in the document's tab) states: "Highly tolerant of a wide range of environmental conditions, the blue mussel is euryhaline and occurs in marine as well as in brackish waters (Baltic) down to 4%, although it does not thrive in salinities of less than 15% and its growth rate is reduced below 18%. Blue mussels are also eurythermal, even standing freezing conditions for several months. The species is well acclimated for a 5-20 °C temperature range (41°F - 68°F), with an upper sustained thermal tolerance limit of about 29 °C or 84.2°F for adults."
Mr. Cash states the salinity was below 1ppm and temperatures may have been too high, making these two hypotheses for this event very plausible.
Attached below is weather data for Skagway at the time of this event. Chyna Perez-Williams
Blue mussel
Observations From Members 