Research Document - 2013/054
Update of SURBA+ for 2J3KL cod
By N. Cadigan
Abstract
SURBA+ is an age-based and survey-only stock assessment model that provides absolute estimates of total mortality rates (i.e., Z’s) and relative estimates of stock size. However, SURBA+ requires external information about the catchability pattern of the age-based survey indices used for estimation, and SURBA+ estimates of Z’s are sensitive to assumptions about catchability.
The data used in the model were the DFO 2J3KL RV survey indices for cod ages 2-12 during 1983-2012. Z’s are modeled as a separable function of age and year effects in a standard SURBA model; however, this approach provided a poor fit to the 2J3KL cod survey index and a modification was used (i.e., SURBA+) in which Z’s are modeled as a random walk over time for each age, and Z’s are auto-correlated across ages. An update of this model is presented here.
Results indicated that 2J3KL cod increased in abundance (total and recruitment) and biomass (total and spawners) during 2004-09 and changed little since then. In 2012 the spawning stock biomass was 15 % (95 % confidence interval, 6-36 %) of the limit reference point. Average Z at ages 5-11 was low (about 0.15) during 2005-2008 but increased substantially to 0.84 in 2009 and then declined. In 2012 average Z was 0.27 (0.10, 0.73).
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