Use of Simulation Analyses to Investigate Yellowfin Tuna (Thunnus Albacares) Growth Models in the Atlantic Ocean Incorporating Gear Selectivity

The growth rate of a fish is a fundamental component used in stock assessments to help determine the population size and the fishery pressure affecting the species. There has been recent debate within the stock assessment community regarding which type of growth model best represents the true growth rate of yellowfin tuna, Thunnus albacares in the Atlantic Ocean; specifically, whether assessments should use a traditional von Bertalanffy growth curve or a so-called “two-stanza” growth curve, which has one growth rate for smaller individual tuna and another for larger sizes. Using a simulated population based on known biological parameters from the stock in the Atlantic Ocean, and the Stock Synthesis 3 program available through the National Oceanic and Atmospheric Administration (NOAA), a simulated yellowfin tuna population is compared to each model in order to determine the merits of each growth rate assumption. In addition, gear selectivity during fishing operations often affects the length composition data from fisheries dependent sources. Gear types such as trawls and longlines tend to select for adult tunas since they are deployed deeper in the water column and therefore have been shown to have a logistic shaped selectivity curve. Gear types such as gillnets are more apt to target schools of tunas of the same size and therefore create a dome-shaped selectivity curve. The simulated population was further used to determine the effects of different growth rates on gear selectivity within each of the growth models.