Causes and Effects of Bias in Anticipated Mark Rates in Mark-Selective Fisheries for Coho Salmon.

Abstract

One issue that has been raised regarding mark-selective fisheries is that observed mark rates for coho have consistently been lower than predicted mark rates. As a result of this, more unmarked fish are encountered per marked fish landed in mark-selective fisheries than is expected preseason. One possible consequence of unexpectedly high encounter rates is that more unmarked fish may be killed as a result of incidental mortality in mark-selective fisheries than is being projected pre-season. To evaluate this potential problem, we examine the magnitude of the bias in preseason mark-rate projections, look for possible explanations of the bias, and evaluate the post-season impacts of mark-selective fisheries on unmarked coho. Each year the STT reports anticipated and observed mark rates for coho as well as quotas or catch expectations, expected incidental mortality, observed catches and estimates of unmarked coho released and incidental mortality in mark-selective fisheries. These values are reported in Table I-9 of the 2010 Review of Ocean Salmon Fisheries (STT 2011a) and in table I-8 in the same document for prior years back to 2000. In most years, post-season estimates of mark rates in commercial mark-selective fisheries are not available, so we focus here primarily on recreational fisheries