Subtle fisheries gear model differences substantially influence catch rates of an invasive fish

When sampling wild animals, the collection methods used can impact the number, sex, size, and behaviour of individuals captured and it is important to understand these impacts when making inferences about populations. While biases between different types of sampling gear are well recognized, biases that arise from using different models of the same gear type are often not considered. To test if different models of the same gear type influence the number and type of individuals collected, we first used two different models of the same gear type (silver and black minnow traps) to collect an invasive fish species, the round goby (Neogobius melanostomus). Next, we tested if any observed differences in capture rates between the different trap models could be explained by differences in: (a) colour, by painting silver traps black and deploying them in the field; and/or (b) retention, by quantifying fish exit rates from the different trap models in the laboratory. We found that silver traps captured approximately twice as many fish as black traps, and that the fish captured in silver traps were smaller on average. Next, we found when silver traps were painted black these now black traps still had catch rates comparable to regular silver traps. Lastly, we found that fish were 13 times more likely to exit from black traps compared to silver traps. Our results suggest that different models of the same gear type can differ substantially in terms of the number and type of individuals they capture. Being aware of these slight dimensional differences between models within a gear type and understanding how even small shape differences can impact the samples collected is important when assessing populations of wild animals and when comparing results and data across studies. To explore this issue further, we conducted a literature survey and identified that of the 275 studies identified that employed minnow traps, 37% provided no description of the traps employed. We advocate that researchers provide detailed information related to the sampling gear used when collecting wild animals to inform appropriate in- ferences and improve comparative analyses across studies.