In rare situations, field surveys can estimate absolute population abundance. More typically, indices of abundance are used. Indices ideally are proportional to abundance, either of the entire population or of the fraction sampled, such as a range of length or ages. Applications range from using the index alone as an indicator of stock status to using indices in formal assessment models of various levels of complexity.
Abundance indices are developed from catch per unit of standardized effort U. The underlying relation is a variant of eqn :
where Ct is catch in numbers, Et is standardized fishing-effort rate (e.g., tows, hauls, or sets per unit time), Nt is average population abundance, and q, the catchability coefficient, is often assumed constant.
The assumption of constant q is reasonable in a designed study, where methods remain constant and are
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