|dc.description.abstract||Abstract: The primary data sources for this study were (1) 17 years of commercial fish catch data from over 30 selected estuarine fish species or species groups; and (2) estuarine habitat maps for Queensland, Australia. Queensland appears to be the best study region as it had the largest extent of soft sediment estuarine habitats, comprising seagrass, salt marsh and mangroves, and offers long-term commercial fish catch data compared to other states in Australia. As a first step, the relationship between catch (both as catch-per-unit-effort (CPUE) and total catch) for commercially caught species in their dominant fisheries (trawl, line, net or pot fisheries) and estuarine geomorphic spatial metrics were extracted from digital habitat maps in geographic information systems (GIS). For preliminary analyses, I examined 13 geographical regions in Queensland based on their importance to commercial fisheries. Spatial metric characteristics such as Euclidean distance, patch density and landscape connectivity for 273 estuaries along the Queensland coast were then calculated within 90 fish catch grids, which provided inshore fish catch data from 21 species groups.
The collective spatial characteristics of estuarine habitats such as size and structural connectivity showed significant correlation with fish catch, with r2 values > 0.7 for 17 commercial species groups. More detailed multiple regression analysis and non-metric multidimensional scaling (nMDS) plots showed significant links between geomorphic coastal features such as structural connectivity and nearshore fisheries production. The relationship was best explained by connectivity indices for mangroves, salt marsh and channels, further suggesting the fundamental importance of connected tidal wetlands to the fish catch.
Climate-driven dependencies of fish catch were explored by using CPUE, rainfall, coastal air temperature and the Southern Oscillation Index (SOI) and catch time-series for specific combinations of climate, seasons and regions. Surplus production models where applied to the commercial fish catch data using the program CLIMPROD. In addition to habitat configuration, climate, particularly rainfall, played a major role in the species composition of the fish catch. Up to 30 % of Queensland’s total fish catch and up to 80 % of the barramundi catch variation for specific regions were explained by rainfall, often with a time-lag response to rainfall events. Temperature had similar influences on fish and prawn catches, with high temperatures having a negative effect on the prawn catch in the Gulf of Carpentaria.
The importance of estuary-scale habitat connectivity to fish populations was assessed by studying fish movement in relation to the extent of flooding for several commercially and recreationally important species in and out of a small mangrove creek using a stationary passive integrated transponder (PIT) system, augmented by underwater digital video recorders (DVRs) for visual sensing. A high-resolution digital elevation model derived from airborne light detection and ranging (LIDAR) and aerial imagery was used to estimate inundation pattern of intertidal habitats to compare with movement of PIT tagged fish. Investigations of fish movement at a detailed scale confirmed the broad-scale findings that habitat connectivity and close spatial proximity of resources are key factors for estuary-dependent fish species in tide-dominated systems to access different habitats. Access to habitat resources such as mangroves is more limited in time than previously thought and is dependent on fish species and size, thus suggesting importance of other adjoining habitats. Fish species and size also affected time of residency and diel pattern of habitat use.
The analyses presented in this thesis also allow an evaluation of the economic consequences of climate parameters on estuarine fisheries. Species-specificity in climate-catch relationships suggested a strong need to develop forecast models and manage estuaries for future climate change by adjusting the quota for the more sensitive species. In addition, my findings on the role of estuarine habitat connectivity could guide the construction of a network of protected marine areas of various structural configurations that can optimise ecosystem services. The demonstrated value of habitat connectivity further warns against future reduction of connectivity by habitat destruction and climate change. The new techniques developed for assessing the spatial ecology of estuarine fish and their habitat utilisation and site fidelity behaviour will assist evaluation of the role of estuarine habitat inter-dependencies in supporting coastal fish populations.
The fish catch data set used for fisheries management is valid for broad-scale investigations. However, it can be insufficient for managing certain stocks, e.g., fish targeted by recreational fishers. Data sets can be significantly improved by in-corporating locally collected recreational fish catch data, which can provide detailed information for particular river systems and allow better assessments of potential overfishing.
The results of this study suggest that sound ecosystem-based fisheries management requires a diversity of information, an improved catch record system, a species-specific approach to managing the impact of climate change, and ecologically meaningful protection areas with high habitat connectivity.||