CIMEC’s Work Elucidates Reference Points from Stock Assessment Results
CIMEC investigator Marc Mangel is lead author on “A perspective on steepness, reference points, and stock assessment,” published in The Canadian Journal of Fisheries and Aquatic Sciences. This publication is a milestone in work funded by PIFSC and FED/SWFISC and has profound implications for the interpretation of stock assessments.
Background: One of the major objectives of stock assessments is the determination of Reference Points (RPs) that are frequently used to help guide policy. Common RPs include: unfished biomass B0; the biomass leading Maximum Sustainable Yield (MSY) BMSY ; the rate of fishing mortality that gives MSY, FMSY; and the ratio of Spawning Biomass Per Recruit when the population is fished at BMSY to the Spawning Biomass Per Recruit of an unfished population SPRMSY.
It has become common in the last 20 years to discuss the resilience of a population to fishing using a parameter named steepness, which is defined as the fraction of recruitment from an unﬁshed population when the spawning stock biomass declines to 20% of its unﬁshed level. Fixing both steepness and adult natural mortality rate is common in stock assessments. Mangel and his collaborators have shown that in such cases, with either of the commonly used stock recruitment relationships, many important RPs are then fixed before any analysis is done.
Significance: This work has implications for how stock assessments are conducted and interpreted, particularly as assessments are being attempted on stocks with progressively more limited data.
One option is not to fix steepness and the natural mortality but to estimate them from the data. Here, thorough simulation analyses can be used to determine what kinds of data would be necessary so that steepness and natural mortality could be estimated as part of a stock assessment. Utilized by Brodziak and Mangel, this approach is facilitated by developing prior distributions for steepness using reproductive ecology.
Alternatively, one may seek a SRR for which RPs are not fixed when steepness and life history parameters are fixed. Work in this direction is in progress by Dick and MacCall and its feasibility has already been demonstrated in a stock assessment of cowcod, done in conjunction with CSTAR graduate student researcher Maria DeYoreo.
An alternative interpretation of these findings is that management policy with biomass targets or rebuilding plans on a fixed time table with specified probability is often overstepping what can realistically be expected from a defensible assessment of an individual stock.
Point of contact: Marc Mangel (firstname.lastname@example.org)
 (CJFAS, Volume 70,pgs 930-940 (2013). Co-authors are Jon Brodziak, EJ Dick, Alec MacCall, and Steve Ralston (Fisheries Ecology Division, Southwest Fisheries Science Center, FED/SWFSC), Robyn Forrest (Fisheries and Oceans Canada, Nanamio, BC), and CSTAR undergraduate research fellow Roxanna Pourzand.
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