Summary of progress and plans for penguin-related analyses, with reference to comments made on previous submissions

Modelling of penguin dynamics, including of their interaction with pelagic fish abundance, by MARAM scientists is taking place on three fronts: I) Models of the possible impact of fishing in the vicinity of penguin colonies II) Models of the impact of the total pelagic catch on penguin dynamics III) Models of Robben Island penguin dynamics using tag-recapture data. This paper summarises the current situation, offering an understanding of recent debates on certain aspects of earlier analyses, and indicates plans for future analyses on all three fronts. It should be noted that progress on I) is linked to the initiatives of the ICTT (Island Closure Task Team) of the PWG. Furthermore note that it is planned to submit results from the analyses planned for review at the annual international assessment review workshop scheduled to be held over 29 November to 3 December later this year, so that results therefrom can inform the PWG deliberations later in December from which recommendations for management of the Pelagic Fishery for 2011 will be developed

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

Update of penguin model development

Since the international workshop in December, work has concentrated on the problem of obtaining an appropriate statistical distribution for the variability of penguin adult mortality about the value suggested by the assumed relationship with pelagic abundance. The initial proposal of the beta distribution has been discarded as it has been shown that this does not work well. This progress is described in the Appendix. The second important advancement is the incorporation of tag data in the process of estimating survival. Some initial exploratory analysis has been made on the raw data, described in document FISHERIES/2011/SWG-PEL/02

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

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

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