Year

2023

Degree Name

Bachelor of Marine Science (Honours)

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Marian Wong

Abstract

Estuaries are well renowned as some of the most productive and ecologically diverse ecosystems on the planet. As a result, fish across all life history (larval to adult) often inhabit estuaries due to their suitable biotic and abiotic conditions. However, not only do they house high abundances of fish species but their ability to provide ecosystem services for humans has quickly made them one of the most urbanised ecosystems on the globe. To accommodate these higher population densities boating infrastructure such as overwater structures (OWS) have proliferated throughout estuaries globally. The presence of these structures can lead to seagrass loss by reducing sunlight and changing the local hydrodynamics of waterways. As seagrass is an important nursery habitat for a wide range of fish species and used across multiple life history stages, the increase in OWS may have significant impacts on fish populations that require seagrass for completion of their life history. The aim of my thesis is to assess the effects of OWS across larval and juvenile/adult assemblages to understand how these structures may be impacting the ecology and overall population dynamics of estuarine fishes. A multi sampling approach was used where larval fishes were sampled using a novel and custom designed light trap, and juvenile and adult assemblages were surveyed using remote underwater videos at sites around OWS and in natural seagrass meadows away from OWS. The effects of OWS were assessed at 2 spatial scales: small-scale impacts (less than 5m) were analysed using generalised linear models (GLMS) and broader ‘seascape’ scale impacts over distances of 100m were analysed using generalised additive models (GAMS). Assessments over small-scales indicate OWS do not affect larval species richness, the abundance of the most commonly occurring species (Ambassis jacksoniensis) and the total abundance of larvae. In contrast, however increasing OWS area in the broader seascape reduced the total abundance IV of larvae and larval abundance of Ambassis jacksoniensis, although no effects of OWS at the seascape scale was observed for larval species richness. Similarly, there was no small-scale effect of OWS on the total abundance and species richness of juvenile and adult fishes. However, there were species-specific responses over small-scales with some species (e.g., the common toadfish and sea mullet) showing increases and decreases in abundance relative to OWS. The area of OWS in the broader seascape had no clear effect on the diversity and total abundance of juvenile and adult fishes. Similarly species-specific responses were not as common at this scale compared to the small-scale assessment, with only the abundance of yellowfin bream (Acanthopagrus australis) increasing with greater OWS area. Results from this study demonstrate that OWS can impact certain fish species depending on their life history stage and respective habitat requirements. Furthermore, my research highlights the importance of evaluating the effects of OWS at larger seascape scales, marking what is to my knowledge the first study of its kind to assess the human impacts on larval fish at this spatial scale. Overall, this thesis has improved our understanding of OWS and the associated effects on estuarine fish ecology across multiple life histories.

FoR codes (2020)

310301 Behavioural ecology

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.