Effects of density dependence in growth and natural mortality on F_MSY and maximum sustainable yield

In fisheries management, it is generally assumed that density-dependent processes are confined to the pre-recruit stage of a fish. Consequently, current calculations of fishing mortality reference points include density dependence (DD) solely in recruitment dynamics. However, several studies have reported the importance of DD in growth and natural mortality (M). In this study, we tested the effect of DD on estimates of fishing mortality producing maximum sustainable yields (FMSY), comparing these estimates to those resulting from constant growth and M. We simulated fish populations with a wide range of life-history traits (LHTs), applying either constant or density-dependent growth and M and derived FMSY for each stock. Results showed that including DD in growth and M led to higher FMSY and often also higher yield estimates (MSY). The effect of DD was found to be greater in growth than in M. Additionally, FMSY estimates derived from surplus production models fitted to generated stocks with density-dependent growth and/or M were generally closer to true FMSY, compared to those from age-based long-term predictions assuming constant growth and M. However, due to the inherent uncertainties in estimates of stock dynamics and biological reference points (BRPs), caution is suggested before implementing DD in FMSY calculations. Equilibrium yield curves relating production to stock size were right-skewed, shape parameters from production models were slightly below 1 for scenarios assuming constant growth and M, but declined to 0.5 with increasing DD.