The principal objective of VIT is to study the population dynamics of a population subject to fishing effort. This study is done by an analysis of pseudocohorts on size frequency data, given knowledge of certain parameters. More specifically, VIT endeavors to study the exploitation rate, the interaction between fishing gears and the reaction to changes in the exploitation pattern, through virtual population and yield-per-recruit analysis. The theory behind these analysis and the equations used are presented in section 5.

Models used in this package are those traditionally employed in population dynamics, with some modifications that are presented in the text. Descriptions of these models can be found, among other places, in Beverton and Holt (1957), Ricker (1975), Laurec and Le Guen (1981), Pitcher and Hart (1982) and Sparre et al. (1989).

The program conducts a virtual population analysis (VPA), and assuming a steady state, reconstructs the population. There are two methods in the analysis of pseudocohorts, depending on the equation used to relate catch and fishing mortality to population numbers: the classic catch equation (Gulland 1965) or the cohort analysis Pope's equation (Pope 1972) (and section 5.4 further in this volume). Program VIT allows the use of both methods (VPA and cohort analysis) upon both catch-at-age and catch-at-length data.

Once the population has been reconstructed, it is possible to carry out yield-per-recruit analysis and studies of the transient effects of changes in the fishing pattern or the level of fishing effort. These later analyses follow the evolution of a population that is not in equilibrium. It is also possible to do sensitivity analysis on model parameters. Because the hypothesis of a steady state is central to the mechanics of the program it is not possible to work with time series of data.

Therefore, to carry out the various analyses it is necessary to have age or size frequency distributions (up to a maximum of 200 classes), for one or more fishing gears (up to a maximum of 4). It is assumed that such distributions are representative of the real age or size distributions of the catch for the time period used in the analysis (generally one year). Although the size interval of age or size classes is obviously fixed, it is possible to have an open final class, different from the others, which includes all individuals above a certain value (size or age) and that is denominated the plus (+) class.

In addition to the above mentioned data it is necessary to enter a number of parameters: growth, length-weight relationship, natural and fishing mortalities, percentage mature by size or age, and proportions caught by each fishing gear.

The program is structured in two parts, the first one relates to data input (frequency distribution of size or age, and initial parameters) and the choice of population assessment method to be used: VPA or cohort analysis (section 3.3). The method chosen and the population data input in this first part, except the parameters, cannot be modified during a run . Within this first part it is necessary to specify the output filename which contains the complete results of the analysis. This name is unique for each run of the program (section 4.4).

It is at this point, as long as the parameters are not modified and before the program goes into its second part, that the population rebuild by VPA or cohort analysis is stored in the computer memory for future use.

The second part starts with the appearance of the Main menu on the screen. The different options of this menu allow to save the general information (data and filenames) entered in part one (section 3.4.1), to view a synthesis of the results of the population assessment (section 3.4.2), to obtain the full results of the VPA or cohort analysis (within the output file) (section 3.4.4.1), to modify the parameters (sections 3.4.3 and 3.4.4.2), and execute any of the analyses through the different menus (section 3.4). Results of the analysis yield per recruit (section 3.4.5), transition (section 3.4.6) and sensitivity (section 3.4.7) will appear in the output file. Transient analysis require an age-structured population, that can be obtained by transforming size frequencies into age frequencies through the size to age menu (section 3.4.8). During the program run, the options that appear in the Main menu can be repeated as many times as required, because all options revert to the main menu. Particularly, every time a parameter is modified the program carries out a new VPA (from now we will refer to the analysis method, VPA or cohort analysis simply as VPA, considering its more general sense) before it returns to the Main menu.

In general the results of the analysis are not shown on the screen, except for the summary information on the evaluation results (section 3.4.2) or information on how to make modifications to the input. To see the results in full it is necessary to exit the program and check the output file (section 4.4).

The output file does not contain any graphs, instead it has numeric tables with comments. However, this file was designed so that it can be easily imported into a spreadsheet program for graphing.

Size frequency data have to be entered in a file according to a format specified in section 4.1. Initial parameters describing growth, length-weight relationship, natural and fishing mortality and catch proportions by gear can be entered from the keyboard or by creating a formatted parameter file as it is described in section 4.2. Certain options of the transition analyses require new information which has to be provided in another file (section 4.3). All extra input required for the rest of the analyses has to be entered with the keyboard.

In summary, to operate the program it is sufficient (i) to have a size or age frequency data file, (ii) to know or to choose a set of initial parameters, and (iii) to decide on the population assessment method to be used. The following table summarizes the requirements for the use of this program.

Computer: IBM PC and compatibles

Math co-processor recommended

Operating system: MS-DOS version 3 or later

Data: Up to 200 size or age groups with constant group intervals

admits a plus (+)class

up to 4 fishing gears

not appropriate for historic data series

Parameters: Von Bertalanffy growth (L¥ , K, t₀)

Length-weight relationship (a, b)

Natural mortality (M)

Terminal Fishing mortality (F_term)

Proportions by fishing gear

Maturity proportions by group

VPA options: Catch equation

Cohort analysis (Pope)

Units: Any, but the same for parameters and data