Traditional data flow chart of fisheries data.
One or more data boxes may be missing.Explanation
The traditional flow of information and data when an annual assessment is conducted using high-quality data sets can be presented in a flow diagram.
%SPLITSLIDE Measurements to be undertaken and data to be collected relates to how this flow diagram is constructed. As seen from the diagram, the following basic data is typically collected each year: \begin{itemize} \item Catch statistics \item Length distributions \item Age determination \item Weights \item Surveys \item Other measurements \end{itemize}
Details
The approaches to fish stock assessments vary considerably depending on data availability and the purpose behind the assessment. In the straight-forward case of annual assessments with annually collected age readings of high quality, the process is as follows:\\
Each year, data is collected by estimating the amount landed, along with biological sampling from the fishery and surveys. \\
Traditionally, the biological samples from the fisheries are used to compute the catches in numbers at age (along with various other important indicators such as mean weight at age).\\
The survey (or catch per unit effort from the fleet) data provides information on the relative stock status, in the form of indices of abundance. Similarly, underwater visual surveys provide indices of abundance which can in principle be used to monitor trends in abundance.\\
Acoustic measurements and sighting surveys (counts) of whales are of a different nature from the most common (indirect) measurements of marine animals when used as absolute abundance measures. Such direct measurements, which in principle provide absolute counts rather then indices, will not be covered in any detail, but they can also be used as indices, i.e. in the same manner as any other survey data.\\
The same applies to tag-return data. Such data can be extremely important, but have mainly been used to obtain information on migration between regions.\\
Many of the above monitoring methods are commonly scaled up to give an absolute measure of abundance. Thus, in principle one can scale results from an underwater visual survey to obtain an estimate of total abundance. It will be seen, however, that this is a very dangerous procedure and will almost never be recommended.\\
Most commonly the assessment process consists of annual data collection followed by data summaries for each annual data set. The data summaries are subsequently used with mathematical models of the population dynamics to estimate the state of the stock and provide advice on its utilization.\\
In many formal statistical assessment models the procedure commonly deviates a bit from the above in that the data tend to be used in less of a summary form. Thus, length distributions are typically only used in simple summary form, not raised to the catches. Each such data set is simply used as a different piece of information in a so-called likelihood function.