Modelling the impact of the South African small pelagic fishery on African penguin dynamics

221 Pages·2013·1.42 MB·English

Modelling the impact of the South African small pelagic fishery
on African penguin dynamics
William Michael Lewin Robinson
Thesis presented for the degree of
Doctor of Philosophy
in the Department of Mathematics and Applied Mathematics
University of Cape Town
November 2013
Supervisors: Douglas S. Butterworth and E´va E. Plag´anyi

Declaration
I hereby declare that this thesis has not been submitted to this or any other university for a
degree, either in the same or different form.
I know the meaning of plagiarism and declare that all of the work in the thesis, save for that
which is properly acknowledged, is my own. In that context, it is to be noted that the general
linear models presented in Chapter 3 are an extension of an approach first proposed by Branda˜o
and Butterworth (2007).
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Contents
Declaration iii
Abstract ix
Acknowledgements xi
Abbreviations xiii
1 Introduction 1
1.1 The ecosystem approach to fisheries with a focus on impacts on seabirds 2
1.1.1 Concern over global fishing intensity 2
1.1.2 Ecosystem considerations in fisheries 4
1.1.3 Implementations of EAF/EBFM 5
1.1.4 The importance of forage fish 7
1.1.5 Current trends in fisheries management 8
1.1.6 Ecosystem modelling 10
1.1.7 Seabirds as ecological indicators 11
1.2 Review of approaches to modelling seabird–fishery interactions 13
1.2.1 CCAMLR Ecosystem Monitoring Programme 14
1.2.2 Other seabird–fishery interaction studies 15
1.3 African penguins 16
1.3.1 Population trends 16
1.3.2 Prey abundance and distribution 18
1.3.3 Predation 18
1.3.4 Oil pollution 20
1.3.5 Other influences on penguin dynamics 21
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1.4 The South African fishery for small pelagics 22
1.4.1 Recent prey distribution shift 24
1.5 The management procedure approach 25
1.5.1 Description of the MP approach 26
1.5.2 Management of the South African small pelagic fishery 29
1.6 Past approaches to assessing African penguin–small pelagic fishery interactions 32
1.7 Thesis objectives and outline 33
2 Data 35
2.1 Moult and breeder counts 36
2.1.1 Double Gaussian method 38
2.1.2 Results 40
2.1.3 Discussion 41
2.2 Re-sightings of banded birds 43
2.3 Major oil spills 46
2.3.1 Apollo Sea 46
2.3.2 Treasure 47
2.4 Pelagic biomass 48
2.5 Data series for GLM analyses 50
2.5.1 Fledging success 50
2.5.2 Breeders per adult moulter 51
2.5.3 Active and potential nests 52
2.5.4 Survival 52
2.5.5 Foraging trip parameters 53
2.5.6 Sardine and anchovy catches 53
2.5.7 Sardine and anchovy biomass 54
3 GLMs relating penguin demographics and foraging behaviour to forage fish
abundances 75
3.1 Fishery closures experiment 76
3.2 Part A—GLM analyses conducted in 2010 77
3.2.1 Response and explanatory variables 78
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3.2.2 Results 82
3.2.3 Analysis 82
3.2.4 Discussion 84
3.3 Part B—GLMs including updated data 85
3.3.1 Response and explanatory variables 85
3.3.2 Results 87
3.3.3 Analysis 88
3.3.4 Discussion 89
4 Penguin–fish interaction model 125
4.1 Basic dynamics 126
4.1.1 Annual adult mortality 128
4.1.2 Age at first breeding 129
4.2 Population model 129
4.2.1 Adult mortality 131
4.2.2 Biomass–mortality relationship 132
4.2.3 Reproductive success 133
4.2.4 Immigration 135
4.3 Model fitting 135
4.3.1 Fitting to moult counts 135
4.3.2 Fitting to tag data 136
4.3.3 Objective function minimized 138
4.3.4 Parameter estimation 138
4.4 Results and discussion 140
5 Penguin population projections 159
5.1 Linkage with the small pelagic OMP 159
5.2 Future sardine biomass spatial distribution 161
5.3 Projected penguin numbers 162
5.4 Results 164
5.5 Sensitivity tests 165
5.6 Discussion 165
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5.6.1 Base case results of linkage with the pelagic OMP 165
5.6.2 Sensitivity tests 167
6 Conclusions 175
6.1 Thesis summary 175
6.1.1 Moult count aggregation 175
6.1.2 GLMs for penguin demographics and foraging behaviour 175
6.1.3 Penguin–fishery interaction model 176
6.1.4 Penguin population projections 179
6.2 Future research 181
6.2.1 Moult count aggregation 181
6.2.2 GLMs for penguin demographics and foraging behaviour 181
6.2.3 Penguin–fishery interaction model 182
6.3 Concluding remarks 184
References 187
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Abstract
Under an ecosystem approach to fisheries, managers aim not only to achieve a good yield from
targeted stocks, but also to preserve the functioning of the whole ecosystem. The population
demographics of predators such as penguins that are dependent on forage fish for food can give
an indication of the health of the ecosystem in which they live.
The rapid decline in the African penguin Spheniscus demersus population in the twenty-first
century prompted the re-classification of the IUCN conservation status of the species as “en-
dangered” in 2010. Reasons suggested for the decline include scarcity of quality accessible prey,
predation, and oil spills. The South African fishery for small pelagics catches predominantly
sardine Sardinops sagax and anchovy Engraulis encrasicolus, which are the two most important
prey species of African penguins. This project aims to quantitatively assess the past and future
impact of the fishery on penguin population dynamics and abundance trends.
The working group which provides scientific advice on the management of the sardine-anchovy
fishery convened a task team to conduct a feasibility study to evaluate the power of an experiment
to detect an effect on penguin demographics and foraging behaviour of fishing restrictions around
penguin breeding colonies. The colonies considered include Dassen Island and Robben Island off
the west coast of South Africa and St Croix Island and Bird Island in Algoa Bay. The extent of
relationships between penguin demographic parameters at these colonies and extractions of forage
fish was explored with general linear models, taking account of fish abundance through biomass
estimates from surveys. Under the assumption that prey densities at neighbouring islands should
vary in approximate synchrony, the differential effect of fishing was evaluated. Results indicate
(perhaps surprisingly) that penguin reproduction is better when catches are larger. Furthermore,
the CVs of the standard deviations of the residuals of the response variables were estimated.
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These estimates are necessary to determine whether an island closure experiment will have the
power to detect an effect of fishing within a reasonable period.
A model of the Robben Island penguin population dynamics was developed. Penguin adult
annual mortality was related to the sardine 1+ biomass observed west of Cape Agulhas in
the November hydroacoustic survey for small pelagic fish. Data from moult counts and re-
sightings of tagged penguins were integrated in a rigorous and statistically defensible manner.
Results suggest that the rapid growth of the colony during the 1990s was driven primarily by
immigration. Furthermore, the sardine–penguin mortality relationship predicts that on average
penguin survival decreases only when the sardine 1+ survey biomass west of Cape Agulhas is less
than approximately one-quarter of the maximum level observed (1343 thousand tonnes in 2003).
The population model was used to calculate ten-year projections of penguin numbers. Adult
survival rates were based on future sardine abundances predicted under the sardine and anchovy
operational management procedure (formula for setting total allowable catches) currently being
tested and its associated base case operating model. Projections were compared to equivalent
scenarios without fishing. Results indicate that the effect of fishing is likely to be rather
small, especially when compared to uncertainties in the dynamics related to the variable spatial
distribution of the sardine population.
The coupling of the penguin population model to the sardine operating model provides an
objective assessment of the impact of fishing on Robben Island penguins. This is an example of
how a fishery can be managed considering not only single-species goals but also ecosystem-related
effects. Such quantitative consideration and fitting to data of a non-target predator is rarely
found in fisheries management. This work advances implementation of an ecosystem approach to
fisheries, which is important in the context of increasing pressure worldwide to account for the
foraging needs of dependent predators.
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