Shared-catch simulation model

Paired counts that are both correlated and overdispersed are hard to model well. So, we built a model for exactly that: a negative-binomial trivariate reduction, reparameterised so its parameters represent quantities of interest that are intuitive and meaningful. See how it works by adjusting the dials in the interactive panel below.

Each longline set has a logbook (LB) count and an electronic monitoring (EM) count. Each count comprises fish seen by both methods (A = ) plus fish seen only by that method (LB-only = , EM-only = ). M is the mean count, and congruence f is the fraction of it that is shared. The box on the left shows the model parameters; the ten stacks are sets randomly generated from the model. Non-zero bias β distributes the unshared fish unevenly between the two methods, without changing M. With κ = 0 each part of the count is generated by a Poisson distribution; with κ > 0, by a negative binomial (higher variance).

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Built by Sea Through Science as part of a project to evaluate electronic monitoring of Belize-flagged longline vessels, and how this method compares with logbook data — a project run in collaboration with JPEC Consulting for The Nature Conservancy.

Tuna silhouette: Thunnus albacares by T. Michael Keesey, PhyloPic, public domain.