Research Document - 2013/009
Bayesian State Space Biomass Dynamic Modelling and Assessment of 4VWX Silver Hake 1993-2012
By Cook, A.M.
Abstract
The population dynamics of Northwest Atlantic Fisheries Organization division 4VWX silver hake (Merluccius bilinearis) was examined using a logistic biomass dynamic model with parameters estimated through a Bayesian state space framework. Due to changes in both the population dynamics and fishery distribution the time series was truncated to 1992-2012. The modelled biomass was projected one and two years ahead under a range of landings scenarios including (0, 9.89 12,15,18, 45.1 t = FMSY). To provide further context on the status of the population, several additional indicators, including age structure, recruitment levels, fish condition and habitat use were provided. Furthermore, simple correlation analyses were performed to relate recruitment, fish condition and bottom temperatures.
Overall, 4VWX silver hake are currently at the highest biomasses estimated throughout the 1993:2012 time block. Biomass has increased over the past ten years and has been high and relatively stable for the last four years. Fishing mortality has decreased over the past five years and is currently at the lowest level in the last twenty years. Biomasses are projected to be lower in 2013 than in 2012 under all total allowable catch scenarios examined. In 2013 and 2014, the probability that biomass will fall below 80% of BMSY was 0.1 and 0.15, respectively. There is evidence of a strong 2009 year class which has shown up in both the 2010 and 2011 summer research vessel surveys, and has likely made a significant contribution to the fishery landings in each of 2011 and 2012. There were some interesting correlations between environmental variables, habitat preferences and biological characteristics, which may prove useful in describing some population characteristics and should be examined more closely in future silver hake assessments. Furthermore, the model used here was an aggregated biomass model which does not fully capture the dynamics of a species, such as silver hake, which has sporadic recruitment events. It is suggested that age or stage disaggregated models continue to be explored for future assessments.
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