Research Document - 2009/108

Modelling Northwest Atlantic harp seal populations: modifying an Excel model to R

By M.O. Hammill, B. Ferland-Raymond, L.-P. Rivest and G.B. Stenson

Abstract

The model currently used to describe the population dynamics of the Northwest Atlantic harp seal (Pagophilus groenlandicus) is a two parameter model that uses information on age specific reproductive rates, ice-related mortality of young seals, removals and estimate of pup production. Although the basic model has taken a number of forms, in recent years it has been run using Excel as a basis. However, the current framework is very slow reducing the opportunities for testing different scenarios and management approaches. In order to improve performance, the model was transferred to R which allowed a reduction in the simulation processing time. During the transfer process, the model was also modified slightly, including a change to the resampling process of pregnancy rates to include correlation among age classes within a year. The R model also builds a new removal matrix for each Monte Carlo simulation using the modeled population age structure rather than the fixed age structure used previously. The Excel and the R models produced similar population trends, but the R model consistently estimated higher populations with a slightly smaller variance. This resulted from the elimination of negative age classes and by the optimisation process that produced lower mortality rates with a smaller variance. The lower variance resulted in higher L₂₀ projections in the R model predictions.

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