Exploration of a MICE approach to evaluate the impact of fish abundance on penguin survival

A MICE approach is used to investigate the possible relationship between penguin annual survival proportions and sardine biomass. The approach is applied for the West and South coasts separately, as well as for the two coasts combined. Logistic relationships are fit for the coasts combined and West coast models, both of which indicate an increase in survival proportion with higher biomass. The survival proportions estimated are compatible with those obtained from independent analyses of other (primarily tag-re-sightings) data. However, the MICE results do also indicate an additional marked reduction in survival proportions over the first decade of this century, which cannot be attributed to changes in sardine abundance. Had it not been for this additional reduction, it is estimated that current penguin abundance would be more than what it was forty years ago, which serves to emphasise the importance of a better understanding of what could have caused such a reduction. A possibility is negative effects, over an extended period, of the Treasure oil spill in 2000. However, the number of these cumulative additional mortalities is substantial, and discussion is needed as to whether this Treasure hypothesis on its own provides a plausible explanation, or other reasons for this additional mortality also need to be sought. (One possibility is a reduction is reproduction over this period, which could be a consequence of oiling, but that would need to have been substantial – by about 80% over almost a decade.) After allowing for these effects, the estimated annual survival proportions during the beginning and end of the period analysed (mid-1980s to the present) are only some 3% less than is needed for the penguin population to stabilise. One possible reason that has been put forward to explain this non-sustainability is predation by seals that have expanded beyond their “natural” pristine abundance, given that some earlier space-limitation constraints no longer apply. The numbers of additional penguin deaths leading to population decreases over these initial and final time intervals are shown to be compatible with such a hypothesis, in that the requisite extent of such seal predation would need to be of the order of an average of only 0.01 penguins per seal per year.

A recently added Appendix C (responding to some further suggestions received) finds that adding anchovy to sardine as a component of the effective biomass impacting penguin survival generally does not seem to provide models with notably better explanatory power. The management implications of the Holling Type III (logistic) functional response form adopted for the Base Case MICE are important. This is consistent with the penguin abundance data from the mid 1980s to mid-1990s, but the situation is not as clear cut for the most recent decade. Further consideration of this should, however, first await discussion (and possible inclusion in this MICE approach) of other recent effects including (but not necessarily limited to) the impact of shipping noise on penguins in the Eastern Cape and reports of perhaps large numbers of penguin mortalities in illegal gill nets set close to some of the penguin breeding colonies. Further inputs on such other factors possibly making a relatively large contribution to the penguin decline do also need to be considered in moving forward with this MICE approach, and are to be welcomed, so that alternative models and assumptions can also be progressed.