Folly and fantasy in the analysis of spatial catch rate data

Spatial catch per effort data can provide useful indices of population trends provided that they are averaged so as to correct for effects of changes in the distribution of fishing activity. Simple, nonspatial ratio estimates should not be used in such analyses. The averaging for any time period must necessarily make some assumptions about what catch rates would have been in spatial strata that had not yet, or were no longer, being fished. Ignoring the unfished strata (averaging only over the areas that were fished) amounts to assuming that they behaved the same as the fished strata and can lead to severe hyperdepletion in abundance indices for fisheries that developed progressively over large regions.