Research Document - 2015/006

Evaluation of the sustainability of a flexible system of total allowable annual catches of narwhals (Monodon monoceros)

By P. R. Richard and R. Young

Abstract

In response to Resource Management’s requests for science advice on a proposed system of flexible catch limits of narwhals, various model scenarios were run to determine the sustainability of allowable harvest limit carry-overs or credits between years over a five-year period. The modelling was similar to the robustness trial method used for Potential Biological Removal (PBR) calculations. Population numbers are projected over a period of 100 years with harvest rates based on PBR updated from estimates of the population every ten years. Results of model runs indicate that modelling scenarios with harvest debits or credits from year to year were not different from the base model with no credit or debit between years. All runs projected above Maximum Net Productivity Level (MNPL); the population size at which the combined size and growth rate of the population produces the largest number of animals per year, i.e., largest productivity. These results indicate that a flexible harvest limit system can be sustainable, as long as the five year total remains less than or equal to five times the PBR, as was the case in our model runs. To add realism to these long-term population projections, the model scenarios were also run with process error σprocess = 0.05. Results of the various scenarios were close to or better than the base model case, with no credits or debits from year to year. Model runs with a recovery factor fixed at 1 had a reduced number of end populations above MNPL but the debit or credit scenarios gave similar results to the base model without credit or debit between years. These results assume that actual hunting losses (i.e., struck and lost) are not different from those used to calculate Total Allowable Landed Catches in previous science advice and that there are no implementation errors in this flexible catch limit system.

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