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An assessment of beaked redfish (S. mentella and S. fasciatus) in NAFO division 3M based on revised 2005-2008 catches (is a retrospective biased assessment necessarily useless in terms of scientific advice?)

AuthorsÁvila de Melo, A.; Saborido-Rey, Fran ; González-Troncoso, Diana; Skryabin, Ilya; Alpoim, R.
Issue Date2009
PublisherNorthwest Atlantic Fisheries Organization
CitationScientific Council Research (SCR) Document 09/29 (2009)
AbstractThe 3M redfish assessment is focused on the beaked redfish, regarded as a management unit composed of two populations from two very similar species: the Flemish Cap S. mentella and S. fasciatus. The reason for this approach is the historical dominance of this group in the 3M redfish commercial catch until 2005. However a new golden redfish fishery (S. marinus) started on September 2005 on shallower depths of the Flemish Cap bank above 300m. This new reality implied a revision of catch estimates, in order to split recent redfish commercial catch from the major fleets on Div. 3M into golden (S. marinus) and beaked (S. mentella and S. fasciatus) redfish catches. An Extended Survivor Analysis (Shepherd, 1999) was performed using the previous XSA framework and the 1989-2008 EU survey abundance at age matrix as the tuning input file. The sequence of negative-positive regions on the log q’s residuals that mark the 1989-2000 interval lead the authors to consider an alternate XSA framework: the 1989-2008 EU survey series was sliced into “three consecutive surveys” (EU survey I, 1989-1993; EU survey II, 1994-2000; and EU survey III, 2001-2008) corresponding to the three regions of the log q’s matrix of residuals. This option resulted on improved catchability diagnostics but with a worst XSA performance in terms of retrospective bias, correlation with the tuning survey and convergence. Sensitivity analysis shown that the model runs better with long survey series based mean log q’s, despite the well defined residual patterns inherent to this option. Very high fishing mortalities until 1996 forced a rapid and steep decline of the 3M beaked redfish stock. With lower fishing mortalities since then the stock decline was halted. But the weak 1991-1997 year classes kept the stock size at a low level till 2002, basically sustained by the survival and growth of the existing cohorts. The cohort’s size at age 4 increased continuously from 1998 onwards, reaching an historical high with the 2002 year class. The 2000-2004 sequence of above average year classes pumped abundance and 4+ biomass to high levels at the beginning of 2008. The reproductive potential of the stock has been well above average over the past ten years and female spawning stock biomass (SSB) is now increasing at a faster rate. But in 2008 female SSB was still far away from a target of 40,000 tons, beyond which the two consecutive above average recruitments from the beginning of the series occurred.
Retrospective analysis show more severe retrospective patterns from 2008 to 2009 (however, and regardless the downward revisions of their trajectories, exploitable biomass and SSB on the final year of most recent assessments have very close estimates). Assuming that the retrospective bias observed between the two last assessments can be repeated between the present and next year assessment, cohort dependent retrospective log linear functions were adjusted to observed log XSA2009/2008 ratios, for abundance and fishing mortality. These functions provide the initial population and fishing mortality at age of a projection with two sets of age/cohort specific bias correctors that can be used to minimize in the short term likely discrepancies on SSB and yield trajectories due to retrospective patterns between consecutive age based assessments. 2 Consecutive medium term stochastic projections of female SSB and yield, starting in 2007, 2008 and 2009, run under constant 2006-2008 fishing mortality and bellow average recruitment. The purpose was to assess the impact of retrospective patterns on the consistency of predicted SSB and yield trajectories from sequential projections. The possibility of a retrospective 2010/2009 replicate of the (severe) retrospective bias observed between the 2009/2008 XSA’s, was taken into account by a final 2009 bias corrected stochastic projection. Regardless the generally poor diagnostics of the XSA’s, the retrospective biases between consecutive assessments, and the adopted low productivity regime, under the present average level of fishing mortality the SSB trajectories (50% percentile) given by the (2007-2009)-2016 projections are similar and gave the same signal: survival and growth of the above average year classes from the first half of this decade will drive SSB above the 40,000 tons target within the next couple of years. And even if the 2009 projection departs from a “retrospective corrected” initial population and fishing mortality, SSB is still expected to reach the vicinity of its target level in a foreseeable future. The translation of these SSB projections into yield (for the 5% percentile) predicts a catch within the 12,200-14,800 tons interval, while for 2011 the same fishing mortality is expected to generate a catch between 13,300 and 15,700 tons. All of these high probability predicted catches associated with predicted SSB’s, remained well above 10,000 tons on 2010-2011
Description56 páginas, 15 tablas, 13 figuras.-- Scientific Council Meeting
Publisher version (URL)http://archive.nafo.int/open/sc/2009/scr09-029.pdf
Appears in Collections:(IIM) Artículos
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