Research Document 1998/60

SPANdex - SPANS geographic information system process manual for creation of biomass indices and distributions using potential mapping

By D.W. Kulka

Abstract

Indices of abundance based on fishing success have been used for calibrating Sequential Population Analysis (SPA) models to project future yields and provide a basis for advice on fishery management. Incorporating such abundance indices in SPA assumes that catch rate (catch per standardized tow in the case of research vessel data or catch per unit effort for commercial data) is directly proportional to fish stock abundance. To satisfy this assumption, the stock area must be stratified into sub-areas within which density could be considered constant. Potential mapping, a spatial analysis technique in SPANS (Tydac Inc.) is a versatile tool for analysing both commercial and research survey catch rate data. It can be used to derive point and variance estimates of biomass of a population. Commercial catch rates alone without a spatial dimension, even if standardized using a multiplicative model are simply a measure of local fish density, not biomass. Much like conventional methods for analysing survey data such as STRAP, and unlike previous methods for deriving commercial catch rate indices, the SPANdex method elaborated here accounts not only for variation in stock density but also for changes in spatial distribution of the stock. However, it differs from the conventional approach to analysis of survey data in that strata are derived (post-stratified) from fish density rather than depth. The technique can be used at the same time to study the distribution of the population. It provides the methods, not possible with more conventional statistical analyses to spatially analyze changes in distribution of marine species to the finest scale of the data and spatially relate these changes to environmental and other variables that might affect the population. This paper is a procedure manual providing step by step details of the method used to derive estimates of biomass and distributional analyses using potential mapping in SPANS.

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