Using the bootstrap to investigate the effects of varying tow lengths and catch sampling schemes in fish survey

In this paper we explore the application of bootstrap methods to analyse of errors in trawl survey indices of abundance at age, with focus on the relative participation of the three sampling levels: haul design, sampling of lengths and sampling of ages, and their implications for survey accuracy. The method consists of resampling these three sources of variability, independently and together, following the sampling scheme and comparing the results. Our results show that although the haul design component is the main source of variability of abundance at age, the importance of catch sampling (sampling of lengths and ages) may be substantial, especially at low abundance levels. Furthermore, we have used our method as a survey simulator; as an example we have performed a simulation with an alternative sampling scheme, i.e. reducing the tow duration and spending the saved time performing more hauls. The simulation shows that the alternative scheme improves the accuracy of abundance at age. This survey simulator may be used as a tool to evaluate other alternative sampling schemes. We have used the cod (Gadus morhua) data from the Flemish Cap survey as a case study but the method may be adapted to different survey procedures.