Performance evaluation of fixed-station sampling design for a fishery-independent survey with multiple objectives

Fixed-station sampling design was widely used in fishery-independent surveys because of its characteristics of convenient sampling station setting, but the non-probabilistic (fixed) nature made it more uncertainty of drawing inferences on population. The performance of fixed-station sampling design for multispecies survey has not been evaluated, and we are uncertain if the design could detect the temporal trends of different populations in multispecies fishery-independent survey. In this study, spatial distribution of abundance indices for three species with different spatial distribution patterns including small yellow croaker (Larimichthys polyactis), whitespotted conger (Conger myriaster) and Fang's blenny (Enedrias fangi) were simulated using ordinary kriging interpolation as the "true" population distribution. The performance of fixed-station sampling design was compared with simple random sampling design by resampling the simulated "true" populations in this simulation study. The results showed that the fixed-station sampling design had the power to detect the seasonal trends of species abundance. The effectiveness of fixed-station sampling design were different in different species distribution patterns. When the species had even distribution, fixed-station sampling design could get high quality abundance data; when the distribution was uneven with heterogeneity or patchiness, fixed-station sampling design tended to underestimate or overestimate the abundance. Evidently, the estimates of abundance index based on the fixed- station sampling design must be calibrated cautiously while applying them for fisheries stock assessment and management. This study suggested that fixed-station sampling design could catch the temporal dynamics of population abundance, but the abundance estimates from the fixed-station sampling design could not be treated as the absolute estimates of populations.