Incorporating Integrated Ecosystem Assessment Products into Stock Assessments for the Gulf of Mexico: A Case Study for Gag Grouper, Natural Mortality, and Recruitment

Significant progress has been made within the Gulf of Mexico in terms of identifying and incorporating ecosystem products into single-species stock assessments.  For gag grouper (Mycteroperca microlepis), ecosystem considerations tested within the 2012 Stock Synthesis assessment model included time-varying natural mortality and recruitment anomalies due to environmental factors.  Various methods either linking natural mortality to red tide indices (1980-2010) or parameterizing a red tide fleet were tested.  An index of recruitment anomalies (2003-2013), estimated from the CMS, a biophysical model which simulated the transport and survival of spawned eggs and larvae, was linked as an environmental effect on recruitment deviations.  Consideration of red tide mortality improved model fit and was incorporated into the base model.  While model results suggest that red tide causes mortality on adults, the mode of mortality remains unknown.  Natural mortality rates for adults estimated from an individual-based OSMOSE model and an Ecopath with Ecosim model were consistent with the red tide mortality hypothesis.  Recruitment anomalies explained about one-third of the variation in the estimated stock-recruitment deviates and informed recent years of the assessment where cohort strength is poorly estimated.  Incorporation of ecosystem considerations has the potential to reduce uncertainty within estimated model parameters and derived quantities.