Bag Limit Builder: concepts & considerations

The Bag Limit Builder is designed to evaluate single-species bag limits or aggregate multi-species bag limits. The phrases ‘catch per fishing event’ and ‘bag limit per fishing event’ are used as generic terms, where ‘event’ could be day, week, hour, or some other measure of fishing time or effort. It is essential for the catch dataset to reflect the intended fishing time or effort of the proposed bag limit regulation. The most direct example, and perhaps most common, is daily bag limits. To evaluate options for daily bag limits, the catch data must be presented as fishing records in the units of catch in numbers per day, thus, enabling estimation of the effect of a daily bag limit against current (or historical) daily catches. This way, catch data provides a frame of reference for existing fishing practices, against which bag limits (i.e., proposed changes to existing practices) can be evaluated.

After specifying a bag limit in the control panel, the immediate effect of the bag limit is calculated as (short-term or immediate) percentage catch reduction. This calculation is made using a bootstrap analysis, which makes use of the degree of imprecision in the catch data to calculate an expected range (between a minimum and maximum) of percentage catch reduction. The bootstrap analysis makes the following assumptions in calculating the effects of a proposed bag limit. First, fishing events (fishers) exceeding the bag limit will instead typically achieve the bag limit. Second, fisher behavior will not change because of the bag limit, nor will the quantity of fishing effort that occurs (i.e., bag limit will neither reduce nor enhance entry to the fishery nor shifts in effort other fisheries). Finally, the effect of the bag limit is immediate, meaning it does not account for longer-term effects on the size of the fish stock; accordingly longer-term effects may differ as the fish population responds to regulatory changes applied to fishers.

The Bag Limit Builder can be used to analyze single-species bag limits (single level of grouping variable selected) or aggregate bag limits (multiple levels of grouping variable selected). Importantly, Bag Limit Builder uses trip identifier, column one in the loaded data set, as a unique identifier of each fishing trip. Summation by unique identifier occurs in the Bag Limit Builder prior to analysis and presentation of results to users. Thus, non-unique (repeat) identifiers are presented as the summation of catches reported in column two. This summation feature enables the use of a grouping variable to examine single-species bag limits or aggregate bag limits. When multiple group levels are selected, these should correspond to shared trip identifiers, thus, summation by trip will result in aggregate total catch per trip for the selected set of species (or group levels). Analysis of total catch reduction follows this aggregation step.