Ecological Archives A024-003-A4
Matthew R. Baker, Daniel E. Schindler, Timothy E. Essington, Ray Hilborn. 2014. Accounting for escape mortality in fisheries: implications for stock productivity and optimal management. Ecological Applications 24:55–70. http://dx.doi.org/10.1890/12-1871.1
Appendix D. Comparison of observed effort and registered effort.
Fig. D1. Comparison of observed effort (left panels, based on aerial surveys and deliveries to commercial processors) vs. registered effort (right panels, based on daily records of the number of boats registered to districts).
Observed effort was applied as our index of relative fishing pressure (f). In years where observed effort was unavailable (Naknek-Kvichak 1990–2000, Togiak 1985–1991), registered effort was used, discounted for the ratio observed:registered boats across seasons where data was available for both indices (range: 0.87–1.12 across five districts). Differences in annual mean estimates of daily observed and registered effort were minimal (Fig. D1). Where neither metric was available (1959–1965) mean values across known years were applied.
Annual mean estimates of daily records of observed boats and registered boats provided similar indices across years (Fig. D1). Daily estimates within a given season were also similar for observed and registered effort. In the beginning and end of each season, few fish are in districts and boats registered to a district may not actively participate in the fishery. To reduce the tendency for the beginning and end of the season to distort our analyses, estimates of both observed boats and observed are limited to dates corresponding to the main pulse of the run for each district, (Nushagak, Naknek-Kvichak and Egegik districts, June 15 – July 15; Ugashik and Togiak districts, July 1 – July 30). During the years of this analysis (1958–2004), the number of driftnet permits ranged from 872 to 3203 (mean= 1840 ± 276). Limited entry went into effect in 1977 (mean driftnet permits post-1977 = 1724 ± 186). The annual mean percentage of available driftnet permits fished was 93% ± 10, for years where data were available (1975–2010).
Note that in determining observed effort, from 1966–1975 and 1996–2009 observed effort was estimated on the basis of deliveries, whereas from 1976-1995 observed effort was estimated on the basis of aerial surveys. In comparing observed effort with registered effort (boats registered) in years where data from both metrics were available, we noted that estimated deliveries were higher in all systems than registered effort (on average 109%). In contrast aerial surveys were lower than registered effort in all systems (on average 82%). This reflects the fact that aerial surveys detect the number of boats actively operating in the district, which is inevitably fewer than the total boast registered to the district, whereas deliveries overestimates the number of boast in the district as some boats may make multiple deliveries within the timeframe of estimated effort (24 hours). Annual metrics of observed effort were adjusted to reflect the different means of estimating effort across years.
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