Research Document - 2016/055
Science Advice on Theoretical Harvest Reduction Scenarios and Sustainable Catches of NWA harp seals?
By M.O. Hammill, G.B. Stenson, and A. Mosnier
Abstract
A population model was used to examine changes in the size of the Northwest Atlantic harp seal population between 1952 and 2014, and then extrapolated into the future to examine the impact of different harvest simulations on the modelled population. The estimates of 2014 population numbers at age, pup production, natural mortality (M), and carrying capacity (K) obtained from the fitting model were used as the starting point for the reduction scenarios. We were asked to determine the catches necessary to reduce the harp seal population to 6.8 million or 5.4 million animals assuming catches consisting of 90% Young of the Year (YOY) or 50% YOY, and occurred over different time periods (5, 10,and 15 years). Also, we were to determine the level of catches possible after each of these reduction scenarios that would be meet the management objective (i.e., a 95% probability of remaining above the Limit Reference Point which was defined as 2.4 million) for a period of 15 years. The impacts of different Canadian catch options on the projected population were tested under two scenarios. The first scenario (Model A) assumed that reproductive rates and Greenland catches were similar to that seen over the past 10 years. The second scenario, referred to as Model B, assumed that both future reproductive rates and Greenland catches behave in a density-dependent manner. The predicted changes in the population trajectory were affected very strongly by the age composition of the harvest used to reduce the population, the speed in which the reduction was achieved and whether the scenario used a population whose dynamics were assumed to be similar to what has been seen in the past 10 years or assumed to vary in a density-dependent manner.
The results of the modelling exercise indicated that more animals would need to be removed if the population reduction was to be achieved rapidly, or with a harvest comprised primarily of YOY. Under Model A, once the target level was achieved, the catch levels that would ensure a 95% probability of remaining above the Critical Reference Limit were much lower than the harvest levels allowable during the reduction phase. Under Model B, the numbers of animals needed to be removed to achieve the reduction target of 6.8 million animals, were similar to the numbers of animals needed to reduce the population to the same level, but under Model A. However, with Model B and a reduction target of 6.8 million animals), much higher harvests were allowed over the 15 years following the reduction due to the increased reproductive rates and reduced Greenland catch that were assumed. However, catch levels needed to reduce the population to 5.4 million were much higher when density dependence was assumed than under the Model A scenario and harvests had to be reduced considerably to permit the population to remain above the reference limit point. Under all scenarios, the uncertainty associated with estimates of population size increased considerably as time since the last survey also increased. These simulation results are very sensitive to model assumptions and should be considered for illustration only.
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