An assessment of beaked redfish (S. mentella and S. fasciatus) in NAFO division 3M (with an approach to the likely impact of recent 3M cod growth on redfish natural mortality)

Abstract

The 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 used with the same framework of previous assessments and with the tuning of the 1989-2010 EU survey. Recent survey results suggest that the beaked redfish stock has not been able to hold its growth and sustain an above average level, suffering instead a severe decline on the second half of the 2000’s. For several reasons, the most likely hypothesis to justify this unexpected downward trend on stock size is an increase in mortality other than fishing mortality. From the sensitive analysis, carried out for a set of natural mortality options, a natural mortality of 0.4 (fixed for ages 4-6 through 2006-2010, and ages 7+ on 2009 and 2011) was adopted. This is the lowest possible level of natural mortality giving assessment results in line with the recent survey declines and at the same time with key diagnostics very close to the best ones, obtained with a higher natural mortality of 0.55. A 2011-2007 retrospective XSA was also carried out. When compared to previous assessments, these retrospective present more consistent trends namely as regards female spawning biomass and average fishing mortality, coupled with a non systematic bias signal. Very high fishing mortalities until 1996 forced a rapid decline of abundance, biomass and female spawning biomass. With lower fishing mortalities since then, the stock decline was halted. The weak 1991-1997 year classes kept the stock size at a low level till 2003, basically sustained by the survival and growth of the existing cohorts. Recruitment at age 4 increased from 2002 till 2006, when the 2002 year class was at an historical high, and from 2006 to 2008 fell as fast as it went up, still continuing to decrease on most recent years and being on 2010 just bellow average. Above average year classes coupled with fishing mortalities in the vicinity of F0.1 or even lower allowed a rapid growth of biomass and abundance since 2003 and sustained the stock at a high level on 2007-2008. However the stock decreased on the last couple of years despite low catch and, being still above average level, there are no signs that the present decline rate is slowing down. Female spawning stock component experienced a similar decline. Short and medium term stochastic projections were obtained for female spawning stock biomass (SSB) under Fstatusquo (average 2008-2010 fishing mortality), together with SSB and yield medium term probability profiles. Keeping fishing mortality at its present low level well below F0.1 will sustain on the short term the female spawning stock biomass above the SSB interval from where all the abundant year classes from the past decade where generated. But on the long term it will be natural mortality to determine the future of beaked redfish as a fishery resource. The average 2012-2013 Fstatusquo catch for beaked redfish will be 2 of 3,200 tons. According to 2008-2010 observed catch data from the Portuguese, Spanish and Russian National Sampling Programmes on board, a beaked redfish annual catch of 3,200 tons would correspond to an overall 3M redfish catch (including the shallower golden redfish catches) near 7,000 tons (6,840 tons)