When Morton comments about Miller's study in regards to 50% of the Fraser Sockeye dying once they enter the river it should be noted that assumes an unbiased estimate of abundance at the Mission hydroacoustic site and/or catch estimates upstream of Mission and estimate of abundance on the spawning grounds. It is not uncommon to have the Mission estimate exceeding the spawning ground estimates and sometimes the other way around. There can be biases in either direction from the ocean right up to the spawning grounds. Years of extreme environmental conditions, abnormal migratory behaviour or high mortality can impact this. When you add management adjustments (pMAs) to this which help ensure escapement targets are met those additional fish may or may not be real fish. Estimating mortality is not such a clear-cut, easy thing to figure out. So, when folks hear Morton claiming certain percentages of Fraser Sockeye enroute loss just keep the above in mind. It doesn't mean everything is rosy, but hopefully it will give you an appreciation of all the variables involved in estimating mortality in these studies.
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Topic: Salmon tally ‘disturbing’ (Read 2593 times)