Setline survey expansion and complementary data sources

This report reviews the International Pacifi c Halibut Commission staff’s rationale for proposing a series of expansions of the annual setline survey, outlines the current approaches used to estimate indices of relative abundance of halibut, and presents cost projections for each year of the proposed expansion. We also present a review of other sources of survey data that may be useful in improving these indices by providing information in regions and years for which setline survey coverage is incomplete. Rationale for survey expansion The International Pacifi c Halibut Commission (IPHC) setline survey index of abundance and associated biological data provide the most reliable and informative source of data for the annual stock assessment, and the apportionment estimates which feed directly into the current harvest policy. The current IPHC standard setline survey grid with 10 nmi station spacing covers depths from 20-275 fathoms, with coverage in 2013 ranging from 40° N in northern California to the Bering Sea Edge in Area 4CDE. The observation of signifi cant commercial harvest in deep waters, particularly in Area 4A, and in shallow waters in some areas, showed that halibut inhabit depths outside the current survey range, including during the time of the year the survey is undertaken. This led to the use of the 0-400 fathom depth range for estimating the bottom area of each regulatory area used in assessment and apportionment calculations (Hare et al. 2011). There are also gaps in the survey coverage within the 20-275 fathom range, and these gaps can be substantial, particularly in Areas 2B and 4. For these reasons, the IPHC has proposed a number of expansions to current and future survey efforts (Table 1). Until an expansion is undertaken, the degree of bias caused by unsurveyed habitat cannot be directly estimated. However, past expansions provide examples of what we could expect:  An expansion of the Area 2B survey to include the area west of Vancouver Island in 1999 showed that a WPUE index excluding this area had an estimated positive bias of 12%.  The eastward expansion of the survey in Area 3A in 1996 led to the calculation of an adjustment factor of 0.81, implying the mean WPUE from the survey of western Area 3A overestimated the mean for the entire area by 23%.  The 2011 expansion of the survey in Area 2A to fi ll in gaps in coverage off the WA/ OR coast has led us to estimate that previously the survey was underestimating mean WPUE by around 4% on average.  The expansions of the Area 2A survey to Puget Sound (2011) and northern California (2013) led to estimates of a combined positive bias of 4% in the mean WPUE of the survey excluding these stations.

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