Re-analysis of the island closure experiment results to implement the suggestions of the December 2020 International Panel

The suggestions of the December 2020 International Panel for further analysis of the results from the island closure experiment, in particular to use the same data within a common framework to facilitate comparisons, and to include month as a covariate, are implemented for all response variables except chick survival. Broadly, the results indicate that aggregated and disaggregated approaches give the same results for estimates and variances of the island closure effect (see Figure 2). This is especially a consequence of implementing the disaggregated approach with a nesting structure as advocated by the Panel, which is shown to be statistically justified. Inclusion of the month co-variate does impact results to some extent, the more so for foraging data for the west coast islands (see Figure 3). The sensitivities investigated generally make little difference to results; these checks include extensions to incorporate data prior to 2008. As to be expected, REML based estimates of CIs are somewhat wider than those based on MLE

A summary of results for the island closure experiment

A simple summary is offered of the results of the island closure experiment reported in Ross-Gillespie and Butterworth (2021), which implemented the suggestions of the December 2020 International Panel. A single result in selected for each island/response variable combination, and reasons are provided for preferring the aggregated data approach for this, with increases made to the CI estimates so as to correspond to the unbiased REML method. Integration across the results is problematic for various reasons, but a coarse summary indicates little evidence for any impact (in either direction) of fishing in the neighbourhood of island colonies on penguin population growth rates. Given that the islands in the experiment have been closed for 50% of the period since 2008, some coarse predictions for the extent of improvement in annual population growth rates (on average over time) were these islands to be closed to fishing every year in the future are: Dassen -0.5% (i.e. no improvement), Robben +0.25%, Bird 0% and St Croix +0.5%

Specification and conditioning of the hake OMP2018 Reference Set models

The specification and conditioning results of the nine 2018 Reference Set Operating Models are provided. These nine models consist of a cross of three different assumptions regarding the stock-recruitment relationship (one Ricker and two Beverton-Holt) and three assumptions regarding the central year in which the catch shifts from primarily M. capensis to primarily M. paradoxus. Estimates of current depletion for M. paradoxus range from 0.26 to 0.39 for the (generalised) Ricker, and from 0.15 to 0.41 for Beverton Holt stock-recruitment models. Current depletion for M. capensis ranges from 0.68 to 0.74 for Ricker, and from 0.08 to 0.76 for Beverton-Holt models. The Beverton-Holt based OMs generally reflect worse fits than the Ricker-based OMs in terms of the negative log-likelihood, and many of the Beverton-Holt models show little effect of changes in spawning biomass on expected recruitment. M. paradoxus is consistently estimated to be above BMSY . M. capensis is above BMSY except for the runs Beverton-Holt that produce a very flat biomass trajectory where biomass has little impact on recruitment

Details of updates made to the Reference Case specifications from the time of the 2017 MSC audit.

Details are provided of the changes that have been made to the South African hake Reference Case assessment model since the time of the 2017 MSC audit

List of possible robustness tests for the 2018 OMP review

This document provides an overview of past hake OMP robustness tests suggestions from various sources, and goes on to propose an order of priority for addressing them. The order is suggested more as a basis to initiate discussions than one with firm preferences. Table 1: Robustness tests from IWS 2017 and those flagged as most important in the OMP2010 review Table 2: Sensitivity tests arising from DWG discussions Table 3: Summary and attempt at prioritisation of the OMP2010 robustness tes

Investigating the suitability of the negative log-likelihood term for the catch-at-length data in the hake assessment model

Various technical improvements are proposed to the way catch-at-length (CAL) data are treated in fitting the hake assessment model in preparation for finalising the Operating Models for the hake OMP revision. This work has been conducted in collaboration with OLRAC and their code-checking exercise, with near identical results achieved. Results suggest the M. paradoxus resource to be robustly estimated to be at least 10% above BMSY at present for the Reference Case; similar estimates for M. capensis are also above BMSY, though more variable in sensitivity tests

Some initial projections under the OMP2014 rules averaged over all the Operating Models in the Reference Set

All the nine Operating Models in the Reference Set are projected forward under the rules of OMP2014, as well as variants of OMP2014 where the TAC cap and one of the tuning parameters are adjusted. A bias correction related to the stock-recruitment residual distribution has now been included in the projection code. The results suggest that there is scope for some increase in exploitation of the hake resource, pending results from the robustness tests. While long-term projections look largely positive, a key question is what changes should be made to the rules of OMP2014 in light of short-term projections of the TAC for the next few year

Does parasite infection definitely increase for sardine aged 2 and above on the south coast

The trend with length in parasite prevalence in sardine on the South Coast is modelled in a manner that isolates behaviour at larger lengths to enable a determination of whether the trend continues to increase from age one to ages of two and above. The results indicate that a continued increase is robustly confirmed at the 5% level of significance, and hence that there must be some movement of sardine of ages greater than one in at least one direction between the West and the South Coasts under the hypothesis that infection by the parasite can occur only on the West Coast