The status of an exploited population is ideally determined by monitoring changes in
abundance and distributional range and pattern over time. Area of occupancy is a measure
of the current distribution. Unfortunately, for many populations, scientific abundance and
distribution information is not readily available. To evaluate the reliability of
commercial fishing data for deriving occupancy indicators that could serve as proxies for
stock abundance, we investigated four questions: 1) Occupancy changes with stock biomass,
but is this change strong enough to make occupancy a sensitive indicator of population
biomass? 2) Fishing boats follow fish, but when does such activity alter the positive
macroecological relationship between occupancy and abundance? 3) When does the activity of
pursuing fish adversely affect occupancy estimates derived from catch and effort data? 4)
How does uncertainty in fishing effort data affect occupancy estimates? Spatial
simulations mimicking the dynamics of four deep-water fish species showed that
biomass-occupancy relationships can be weak. Fishers following fish can modify the spatial
distribution of target species, even reversing the sign of the biomass-occupancy
relationship in certain cases, and can affect the reliability of occupancy indicators,
which can also be impaired by error in effort data. Using commercial catch and effort data
and abundance indices for deep-sea fish populations to the west of the British Isles it
was found that only for roundnose grenadier might occupancy provide insights into biomass
changes. In conclusion, care should be taken when using occupancy for evaluating range
changes in cases where fishing might have modified spatial distributions, uncertain
commercial data are used or when the abundance-occupancy relationship is too flat.