World Fisheries Congress 2012: Special ICES Session on Management Strategy Design and
Implementation
Session A: Science to underpin sustainable fisheries
Sub-Topic a) How were uncertainties quantified and communicated?
Title: Quantifying resource risk for highly variable species in MSE: should pristine biomass provide the yardstick?
Authors: CL de Moor and DS Butterworth
Abstract:
A core and difficult component of Management Strategy Evaluation (MSE) is the appropriate quantification of resource risk
for the performance statistics amongst which trade-offs are considered. Later in the process, a selection needs to be made
of the level to which this should be tuned, preferably in a manner which ensures consistency over time. Typically the
associated Management Procedures (MPs) are tuned to lower percentiles of the distributions of biomass levels expressed
as fractions of pristine abundance (B0), a yardstick which is also often used in specifying target and limit reference points.
Especially where fisheries on lower trophic level species are concerned, reference points higher than typical in a single
species context are often advanced from the ecosystem perspective of addressing the food needs of natural predators of
the species which is under harvest. Thus recently the Marine Stewardship Council (MSC) has followed CCAMLR in requiring
a target reference point at 75% of pristine abundance for such forage species, rather than the 30-50% range typical for
MSY abundance as usually selected in a single species context. However, for species with highly variable abundance, this
approach can prove problematic to apply operationally. Estimates of B0 can be highly sensitive to which stock-recruitment
relationship is chosen from amongst a number all providing almost equally satisfactory fits to the data. Furthermore, as
more data become available, perceptions of the extent to which recruitment varies naturally can change appreciably, with
implications for lower percentiles of abundance distributions when expressed as fractions of B0. These problems are
illustrated for the South African sardine and anchovy resources, where some time ago they necessitated moving away from
the use of B0 for specifying acceptable resource risk levels in the process of selecting MPs. This raises questions concerning
the wisdom of the MSC’s recent clarification of its expectations on the additional precaution that should be applied by the
Fisheries Assessment Methodology to such species. Instead for these South African fisheries, acceptable risk levels (with
concomitant target reference points) are considered on the basis of the extent to which the distributions of abundance
levels are moved lower under TACs specified through application of an MP, compared to the corresponding distributions
for these abundances in the absence of fishing.