Island closure feasibility study power analysis results for Dassen and Robben islands

The power analysis for penguin responses to fishing around Dassen and Robben islands has been completed. Response variables considered are chick condition, active nest proportion, fledging success, chick growth, foraging path length, and foraging trip duration. Although complete results are given here only for the first two of these variables, overall there are clear indications regarding the residual variances associated with these variables. Those variances are now estimable for Dassen and Robben islands with sufficient reliability to allow a decision on whether an experimental closure programme could yield definitive conclusions regarding the impact of fishing close to island colonies on penguin demographics within a realistic time span. Thus the purpose of the feasibility study has been achieved and the study can be concluded, with the island closure experiment commenced for these two islands. Definitive results from the experiment can be expected for both islands with about 2–4 years’ additional data

Results of an alternative pelagic biomass series for the relationship with penguin adult survival

A key component of the Robben Island penguin population model (Robinson and Butterworth 2011a,b) is the relationship between annual penguin adult survival and the abundance of small pelagic fish. This is of primary importance as it enables one to produce penguin population projections based on different future small pelagic abundance levels. The sardine and anchovy OMPs may then be designed to take into account the risk to penguins of reaching a low pelagic abundance level

Robustness of the penguin population model estimator

The robustness of an estimator may be investigated by assuming that the “true” operating model is in fact different (for example, a constant may have a different value or a key relationship may have a different functional form to that assumed by the estimator). Data generated from this “reality” are then fitted using the estimator. The distributions of the results are then compared to the corresponding “true” values of the operating model

Island closure feasibility study power analysis

This paper indicates how it is planned to extend the general linear model (GLM) analyses by Robinson (2013) to estimate the power of the Island Closure Experiment. Statistical power reflects the probability that an experiment will detect an effect if it exists

Comments on the benefit to penguins of fishing restrictions around Robben Island predicted by Weller et al. Robben Island penguin model simulations

The following comments relate to results in a recent publication (Weller et al., 2014) that are pertinent to the discussions of the Island Closure Task Team, specifically the impact of fishing around Robben Island on the penguin population. Three scenarios were explored in that paper, each starting with 3500 adult penguins

GLMs relating penguin demographics to pelagic catches close to islands and to pelagic abundance

Earlier GLM analyses of the impact of pelagic fishing in the vicinities of Robben and Dassen Islands on the dynamics of penguins breeding on those islands are extended to cover a wider set of data series, and to consider relationships involving sardine and anchovy separately as well as together. This in turn allows the estimation of the change in penguin population growth rate to be expected from suspending pelagic fishing in the vicinities of these islands. Interpretation of results is confounded by poor precision which is a consequence of the shortness of the time series. Likely the most that could be said with some confidence is that the results of the analyses do not support the hypothesis that suspending fishing around Robben and Dassen Islands would enhance penguin reproductive success there

Full description of the Robben Island penguin-fish interaction model and base case Bayesian results

Work on the penguin–fish interaction model has progressed to the point where a reasonable base case for Robben Island has been achieved

Penguin population models for Robben Island

As an initial illustrative exercise, a population model for Robben Island penguins, which includes dependency of reproductive success and survival rate on pelagic fish abundance, is fitted to moult count information for the colony. The results indicate a strong dependence of adult penguin survival rate on sardine abundance west of Cape Agulhas. However, the logistic transformation used to ensure respect of biological constraints on these demographic parameters leads to some problems in extending the approach to Bayesian estimation. Further work will explore use of the beta distribution and the incorporation of further data for the penguin population in the fitting process

Updated results from a Spatial Age-Structured Model of African Penguin populations for use in linking to the pelagic OMP testing process

Updated results of an earlier penguin model are presented. The model focuses on four colonies 1) Robben Island; 2) Dassen Island; 3) Boulders and 4) Dyer island, although projections will narrow the focus further to the first two of these. Some progress has been made in terms of fitting the model adequately to historic data. However, there are still problems with the fits such that the current model is not yet at the stage where it can be linked to the pelagic OMP (Operational Management Procedure) testing process to take account of the relationship between the breeding success and also natural mortality of African penguins and the abundance of both anchovy Engraulis encrasicolus and/or sardine Sardinops sagax. Despite extensive investigations, in many cases no overly strong functional relationships between penguin breeding success and pelagic abundance have been found. As a result, functional relationships are being investigated between the natural mortality of African penguins and pelagic abundance, with results suggesting that a relationship exists at Robben Island, but not at Dassen Island. Dassen and Robben Islands are split and modelled separately for these and other reasons

Penguin island closure feasibility study analysis results update: random effects models applied to both Western and Eastern Cape islands

This Addendum updates results from previous papers using random effects instead of fixed effects models for the year factors. Results are now given for Bird and St Croix islands in addition to those for Dassen and Robben islands. The results from random effects models are qualitatively unchanged from those given earlier from the fixed effects models, except that the periods required to obtain statistically significant results are extended somewhat as a result of the removal of the negative bias in the residual variance estimates for the earlier fixed effects models