The exploration of changes in spatial and temporal change in fishing and tagging effort provides important context for the interpretation and evaluation of M-R estimates derived from the Chapman estimator. In particular, the evaluation encourages a wider discussion of the compatibility of the data sets that have been collected in assessment areas in relation to the assumptions of the Chapman M-R estimator.
Initial simulations that were conducted suggested that effort changes like spatial lag, spatial contraction, or constant effort impact the estimation differently, as do the distribution of the fish population itself. Thus, determining the driving factor or the cause of bias in the estimator is challenging because different aspects of the fishery come into play.
Statistical methods including Correspondence Analysis, spatial Dissimilarity Index, and Kernel Density Estimation were implemented on four assessment areas to enable the comparison of fishing and tagging effort between fishing seasons. These analyses clearly identified that changes in spatial and temporal effort had occurred during the time periods for which tagging data has been used in CCAMLR stock assessments. However, application of these methods in a robust manner as a potential correction of a M-R biomass estimate remains a difficult task.
Recommendations
- WG-SAM to acknowledge the potential impacts of spatial variability in fishing and tagging effort on stock assessment estimates derived from tagging data, in particular SSB0, spawning stock status and estimates of YCS
- Analyses to quantify variability in temporal and spatial fishing and tagging effort continue to be explored
- Implications of violations of assumptions implicit to Chapman estimators are considered further
- Metrics and methodologies to correct for the influence of spatial heterogeneity in fishing and tagging effort to be evaluated and presented to WG-FSA
- Broader consideration of the role that the management of commercial fishing activities play in structuring the spatial and temporal concentration of fishing and tagging effort over time