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Large Numbers of Deceased Coho in Brothers Creek - The Threat from Urban Runoff.

The beginning of the annual salmon surveys looked quite promising with the first returns of coho and chum salmon in Brothers Creek during the third week of October. Unfortunately, this turned out to be a story of tragedy for the coho salmon. Many healthy coho that were among the first to arrive perished soon after entering Brothers Creek. Our contact within the Capilano Hatchery suspects the pre-spawn mortality be related to the presence of 6PPD-quinone in the lower reaches of the stream.

6PPD is a chemical used to protect motor-vehicle tires from cracking and the potential danger of tire failure. The fact that the chemical is linked to coho mortality is now not speculative. Many studies dating back to the 1990’s particularly in Washington State, support this conclusion. Those studies found that 6PPD-quinone is one of the main contributors to Urban Runoff Mortality Syndrome (URMS). As tires wear on pavement, they leave behind the toxic 6PPD chemical which, when exposed to sunlight, becomes highly toxic. This accumulates on local roads, and when the first heavy rainfall arrives, as experienced in early October, the chemical is washed into the storm sewer system and then into local creeks.

URMS can be fatal to coho but strangely, does not seem to affect pink or chum salmon. The current hypothesis is that, at the time of the first heavy rain storm in West Vancouver, only pink salmon had returned into the creeks. However, with additional rainfall coho and chum entered the creeks following their normal pattern and encountered concentrations of 6PPD before creek flows rose high enough to dissipate the contamination. Sadly, many coho quickly perished before having a chance to spawn. The females that were examined still contained full egg sacks.

It should be noted that this is an unprecedented event that has not, to our knowledge, been previously observed in our creeks. A combination of a very dry summer and sudden precipitation in the fall that coincided with the first coho to return is most unfortunate sequence of events. It is truly a tragedy to lose so many otherwise healthy pre-spawned coho.

However, recent stream surveys of Brothers and Hadden Creeks indicate the danger may be over. Good numbers of healthy coho have more recently arrived and the sudden-mortality effect seems to have thankfully passed. This event also highlights the value of our stream survey program which quickly identified the issue and may have the resolution to assist with mitigative measures in the future.

Submitted by:

Dr. Steve Macdonald, Director and John Barker, Past President

November 7, 2023

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