Friday, July 10, 2009

Tyee Test Fishery Uncertainty

The graph above is from a powerpoint presentation given by the head of the DFO Stock Assessment Division for North Coast to the Skeena Watershed Iniative Congress in Terrace earlier this year.

It basically shows how many times the Tyee Test Fishery over or under-estimated the sockeye run size since 1971. Remember, the in-season Tyee numbers are corroborated by post-season examination of the Babine River Fence counts, so this is how DFO knows how accurate the in-season numbers were.

Quick Tyee Primer here:

As for the graph, over the longterm it shows just about as many 'overs'(19) as 'unders'(16)....which really just reinforces how inaccurate it is as a measuring device. But, if you look from 1995 onwards the trend is definitely more towards over-estimates. Look at 2002 with it's amazing over 100% error. It appears the average overestimate is about 20% from 1995 onwards.

The propensity for over-estimates is of course disconcerting. We end up with situations like 2008 where because Tyee overestimated the sockeye run size by 36% the 2008 commercial fishery significantly exceeded exploitation rate targets. Moreover, the exploitation rates steelhead were exposed to for 2008 are still not known. But given the size of the over-estimate, it appears they are well over the old SWC ceilings.

Of course, uncertainty in Tyee is a major known weakness in stock assessment, but it is the failure of DFO to adequately use any sense of precaution in interpreting its results that creates the major problems. Unleashing the marine fishing fleets prior to adequately ensuring escapement numbers in stocks, especially depressed stocks, is not a recipe for successful, sustainable fisheries management.

Let's keep watch for any evidence of precaution in how the 2009 fishery is managed.
Graph courtesy and © copyright DFO North Coast

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