EFFECTS OF MODEL DESIGN AND ENVIRONMENTAL VARIABLES ON JUVENILE U.S. SOUTH ATLANTIC KING MACKEREL (<i>Scomberomorus cavalla</i>) ABUNDANCE
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As growing human populations put an increasing demand on finite ocean resources, fisheries management tools rely ever more on high quality inputs and a comprehensive understanding of model factors. Stock assessment modeling for South Atlantic (SA) King Mackerel, <i>Scomberomorus cavalla</i>, uses inputs such as abundance indices, growth parameters, and fisheries landings. However, one underlying assumption in this modeling system is there are measurable connections among life stages. A juvenile abundance index developed from the SEAMAP-SA Coastal Trawl Survey (CTS) is presumed to represent ecological recruitment. Very weak correlations to other life stage proxies suggested a deficiency with the juvenile abundance index accuracy and indicated data exploration into the index formulation was needed. Examination of CTS juvenile length frequencies support that the smallest juveniles appear in the summer and spring juveniles are from overwintering of the previous year class. Juvenile abundance indices developed using year class (year in which a fish is spawned) rather than year of sampling (as done for previous assessments) showed substantial differences, in particular lowering AIC values indicating an improvement in model accuracy. Evidence of seasonal and regional variation with CTS juveniles prompted the development of separate age 0 and age 1 indices. Correlations of these indices to fisheries-derived year class strength suggested the age 0 index to be the best indicator of initial juvenile king mackerel abundance while the age 1 index reflects abundance after high early life stage mortality. Data exploration also was conducted for potential environmental variables impacting age 0 and age 1 abundance. Relationships were found between age 0 abundance and freshwater input, the Gulf Stream, hurricane activity, and predator abundance and between age 1 abundance and freshwater input and hurricane activity. This research provides not only valuable baseline knowledge for SA king mackerel juveniles, but also findings pertinent for their management.