Biomass decline is a vital threat for small-scale fisheries, but lack of data affects our ability to understand both biomass development and fishers’ adaptation. This study contributes to the literature on cost-effective, survey-data-based methods in data-poor and development-oriented settings. Based on original survey data from Nigeria, we find that 58% of respondents perceive a decline in fish abundance, in particular top-predator biomass. However, we also find signs of strategic behavior by respondents. Subsequently, we use multinomial logit and probit models to analyze the consequences of biomass decline for livelihoods. Our empirical findings support our theoretical prediction that biomass decline may crowd out some fishers while net favoring others, based on heterogeneous competitiveness. Furthermore, crowding-out status overlaps with high financial vulnerability. This emphasizes that biomass decline not only affects the cost of fishing, but may also adversely affect the fisheries pro-poor functions.