The influence of seascape spatial features on the fish and macroinvertebrates in seagrass beds Jane E. Jelbart Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy at the University of Western Sydney, Hawkesbury August 2004 © J.E. Jelbart 2004 Acknowledgements I would like to thank my supervisor Dr Pauline Ross for her support and guidance. Her diligence and experimental design and research skills were a great contribution to the thesis. She always encouraged me to aim high with this project and ensured I had the support to do so. I am also very grateful to Rod Connolly from Griffith University for his ideas and guidance on experimental design and for discussions about the meaning of results. His expertise on fish ecology, seagrass habitat and estuarine research was invaluable. I am indebted to the team of people that assisted me with the fieldwork component of the project including Robin Janus, Caroline Herlihy, Karen Stephenson, Marcus Scnell, Chris Baker, Sonia Claus, Andrew Hayes, Brendan Haine, Paul Thomas, Vimla Rao, Nicholas Gay, Karen O'Neill and Bethany Alexander. Financial support made this project possible from the University of Western Sydney (UWS) in the form of a Hawkesbury Postgraduate Award (3 year stipend); The NSW Royal Zoological Society (Ethel Mary Read student research grant); Gosford City Local Council (student ecological research grant); the Centre for Landscape Ecology Management UWS (9 week stipend); and the Research Office UWS, which financed my attendance to two international conferences. Special mention must be made of the support, camaraderie and great assistance provided by Sonia Claus without whom this project would have been so much harder. Finally special thanks go to my parents (Elizabeth and Ray), Robin Janus and Nicholas Gay. Statement of Authentication The work presented in this thesis is original except as acknowledged in the text. I hereby declare that I have not submitted this material, either in whole or in part, for a degree at this or any other institution. Jane E. Jelbart Table of Contents Abstract 1 1 General Introduction 3 1.1 Landscape / seascape ecology 3 1.2 Seagrass habitat in estuaries 4 1.3 Fragmentation of seagrass habitat 5 1.4 Importance of seagrass for estuarine fauna 6 1.4.1 Importance of seagrass for fisheries 7 1.5 Structural features of seagrass habitat that promote the abundance and diversity of estuarine fauna 8 1.5.1 Small scale structural features 8 1.5.2 Influence of size and shape of seagrass bed 9 1.5.3 Are there edge effects in beds of seagrass for fish? 11 1.5.4 Habitat heterogeneity of a seagrass bed 12 1.5.5 Position of seagrass bed within the estuary 14 1.5.6 Proximity to other habitats, e.g. mangroves 16 1.6 Marine and Estuarine Protected Areas 17 1.7 This study 18 2 General Methods 19 2.1 Study locations 19 2.2 Identification and mapping of the estuarine habitats 19 2.2.1 Description of Zostera capricorni 23 2.2.2 The shoot density and blade length of Z. capricorni 23 2.3 Fish survey techniques 26 2.3.1 Identification of fish and macroinvertebrates 27 2.4 Pilot study: Influence of tidal states 27 2.4.1 Methods 28 2.4.2 Results and Discussion 30 3 A test of the species area relationship and landscape ecology theories 39 3.1 Introduction 39 3.1.1 The species-area relationship 39 3.1.2 Location of the seagrass bed within the seascape 41 3.1.3 Study Aims 41 3.2 Methods 42 3.2.1 Study area and description 42 3.2.2 Seagrass bed categories 42 3.2.3 Estimation of habitat characters 43 3.2.4 Fish Survey 46 3.2.5 Data analysis 47 3.3 Results 52 3.3.1 Habitat heterogeneity 52 3.3.2 Size of the seagrass bed 56 3.3.3 The perimeter to area ratio of a seagrass bed 56 3.3.4 Testing for the passive sampling hypothesis 56 3.3.5 The distance of the seagrass bed from the estuary mouth 57 3.4 Discussion 64 3.4.1 Comparison of day & night sampling 64 3.4.2 Heterogeneity of seagrass and the assemblage of fish 64 3.4.3 Small beds have greater fish densities and species richness 65 3.4.4 Passive sampling hypothesis 67 3.4.5 Location of the bed influences the fish assemblage 68 3.4.6 Implications for the species-area relationship and marine conservation 69 4 Do the edges of seagrass beds influence small fish and macroinvertebrates? 70 4.1 Introduction 70 4.2 Methods 72 4.2.1 Selection of the seagrass beds 72 4.2.2 Defining the edge, inner and central regions of the beds 74 4.2.3 Fish and macroinvertebrate survey 74 4.2.4 Data analysis 76 4.3 Results 77 4.4 Discussion 89 4.5 Conclusion 92 5 The influences of seagrass patch area, perimeter length and perimeter to area ratio on small fish and macroinvertebrates 93 5.1 Introduction 93 5.2 Methods 96 5.2.1 Construction of the artificial seagrass units. 96 5.2.2 Size, shape and arrangement of the artificial seagrass units. 96 5.2.3 Sampling of fauna 97 5.2.4 Data analysis 99 5.3 Results 102 5.4 Discussion 103 5.5 Conclusion 117 6 Fish assemblages in seagrass beds can be correlated with proximity of mangrove 118 6.1 Introduction 118 6.2 Methods 120 6.2.1 Study areas and descriptions 120 6.2.2 Experimental design 120 6.2.3 Fish Survey 123 6.2.4 Univariate data analysis 124 6.2.5 Multivariate data analysis 125 6.3 Results 126 6.3.1 Univariate analysis: abundance and density of fishes 126 6.3.2 Multivariate analysis: composition and abundance of fish assemblages 130 6.4 Discussion 138 6.5 Conclusion 141 7 General Discussion 142 7.1 Seagrass beds contain small fish and macroinvertebrates142 7.2 Fish assemblages in seagrass beds support landscape ecology theories 142 7.2.1 The spatial structure (i.e. size and shape) of seagrass beds 143 7.2.2 The position of a seagrass bed within the estuary 144 7.2.3 The proximity of seagrass beds to mangrove forests 145 7.2.4 The patchiness or heterogeneity of Z. capricorni beds 146 7.2.5 Edge effects in seagrass beds 147 7.2.6 Self-similarity across multiple scales 148 7.2.7 Seascape approach required 148 7.3 Patch size and the species-area relationship 149 7.4 Models to explain the greater densities of small fish in small compared to larger seagrass beds 151 7.4.1 The "settle and stay" model 151 7.4.2 The 'grain' or scale of response of small fish 152 7.4.3 The absence of large predators in small seagrass beds 152 7.4.4 Life-history strategies (r and K selection) 153 7.4.5 The source or sink model 154 7.5 Reasons for correlations of fish and macroinvertebrate in the inners of the small beds 154 7.6 No change in the abundances of macroinvertebrates between small and large seagrass beds 157 7.7 Implications for seagrass regeneration projects 157 7.8 Implications for fragmentation of seagrass beds 158 7.9 Conservation and protection of seagrass beds for estuarine fauna 159 7.10 Final Comment 161 8 References 162 List of Figures Figure 2.1. A map of Broken Bay (-33.6(S, 151.3(E) showing the Pittwater estuary and the Brisbane Water estuary and its location within Australia. 20 Photo 2.1. The southern end of the Pittwater showing the extensive recreational use of the estuary (Geoscience Australia 2001). 21 Photo 2.2. The western side of the Pittwater estuary showing a Z. capricorni seagrass bed in the foreground and A. marina mangroves. 21 Photo 2.3. A picture of a small and patchy seagrass bed in the Pittwater estuary. 22 Photo 2.4. A photo of a Z. capricorni bed and A. Marina mangroves in the Brisbane Waters estuary. 22 Figure 2.2. The number of seagrass shoots measured in accumulated quadrats in two types of seagrass beds (patchy or uniform) in the Pittwater estuary. 24 Photo 2.5. The small seine net (8 x 2m, 1mm mesh) used for the surveys in Chapters 2,3,4 and 6. 25 Figure 2.3. The length (cm) of seagrass blades measured from sampling seagrass shoots in one quadrat in the Pittwater estuary, January 2000. 25 Figure 2.4. The average number of fish species and fish individuals collected during the day and night sampling from each haul of the net in the pilot study (Pittwater 2000). 32 Figure 2.5. The average number of fish species and fish individuals collected during the three tidal states (low, mid & high) from day and night sampling in the pilot study (Pittwater 2000). 33 Figure 2.6. The plots of the average number of fish species or fish individuals per net and the water depth during sampling in the day. The results are shown for all water depths sampled (from 22 to 140 cm) 34 Figure 2.7. The plots of the average number of fish species or fish individuals per net and the water depth during sampling in the day. The results are shown for water depths from 30 to 100 cm in the pilot study (Pittwater 2000). 35 Figure 2.8. The plots of the average number of fish species or fish individuals per net and the water depth during sampling in the night.