Factors affecting the habitat usage of estuarine juvenile fish in northern New Zealand

Abstract

New Zealand’s estuaries are coming under increasing pressure from anthropogenic change and degradation. Few, if any remain in a pristine state. Such environments play an important role as nursery areas for juvenile fish. However the processes underlying the patterns of juvenile fish habitat use are poorly understood. This thesis examined abiotic and biotic factors, influencing the distribution and abundance of juvenile fishes over multiple spatial and temporal scales within 9 northern New Zealand estuaries and one offshore island, and the impacts of changing environmental conditions on the foraging success and health of juvenile fishes. Small fish assemblages, abiotic variables (e.g. sediment grain size, suspended sediments, current velocity) and biotic variables (e.g. benthic invertebrates and predatory fishes) were quantified along a 20km environmental gradient in the Manukau Harbour over two seasons. Fish assemblages showed strong spatial and temporal variability along the gradient, with densities declining from the sheltered, muddy upper Pahurehure Inlet to the clearer, deeper, faster-flowing sandy areas of the lower harbour. BIOENV and CCA analysis revealed that depth, current velocity and turbidity were the most strongly correlated with the fish assemblages. Other contributors retained in the CCA forward selection included mysid abundance, quantity of biogenic habitat and percentage of fine sediments (<64μm). Dietary composition shifted seasonally in response to changes in food source availability, with a significant increase in consumption of mysids in spring (contributing 66% of total dietary biomass). During spring, the density of new recruits were positively correlated with mysid abundance. Diversity of juvenile fish and dietary breadth were highest at mid harbour sites, which also contained the most biogenic structure. The impact of turbidity on juvenile fish was assessed using snapper (50-90mm FL) as a model species. In experimental aquaria, increasing suspended sediment (TSS) levels resulted in decreased foraging success. Longer-term (one month) exposure to increased TSS resulted in higher weight losses, higher mortality rates and significant increases in gill deformation (epithelial hyperplasia, and fusion of the lamellae) which impaired respiratory function. Other sublethal effects included increased coughing and gulping at the surface, higher respiration rates and decreased activity, which are consistent with the effects of anoxia. A field survey of seven northern estuaries revealed that juvenile snapper had significantly lower condition indices in the more impacted estuaries characterized by increased sedimentation and catchment urbanisation. Higher levels of gill deformation and parasite loads were also recorded, particularly for Manukau and Mahurangi Harbours. Dietary analysis revealed a change in foraging tactics with increasing turbidity from active (probably visual) selection of pelagic prey (zooplankton; 0.5-0.71mm), to larger, slower moving benthic prey (≥ 0.71 mm). Increasing TSS can thus potentially restrain juvenile snapper condition and growth by reducing the overall food supply available and perhaps relative nutritional values by reducing their ability to select optimal prey sizes, Results suggest that physiological stress in fishes in response to increased TSS can decrease immunological competence and growth. Fish and macro invertebrates in seagrass, mangroves, sand and mudflats were sampled from seven northern estuaries over late summer to assess the relative value of specific habitats for juvenile fish. Different habitats supported different species assemblages of fish and invertebrates which varied with latitude, geographical setting (east/west) and between and within estuaries (tidal position). A small number of species such as yellow-eyed mullet and exquisite goby had more ubiquitous distributions. Benthic faunal diversity, abundance biomass and productivity tended to be highest in seagrass habitat (particularly subtidal seagrass), followed by sand, mangroves and mud habitats. Results support the paradigm that seagrass meadows are a valuable nursery habitat in northern New Zealand, particularly for species such as snapper and trevally. Ontogenetic dietary shifts were evident for the majority of fish species, with meiofaunal crustaceans (0.5-1mm) predominating. Newly recruited fish exhibited an obligatory planktivorous stage, with a gradual transition to larger crustaceans such as mysids, gammaridean amphipods and caridean shrimps and crabs. Habitat-related differences in diet were also evident, reflecting benthic prey availability and diversity. Overall, these findings indicate that ongoing large-scale environmental changes within estuaries are affecting the functioning of fish nurseries both directly (by reducing the fitness of individual fish) and indirectly (by reducing the area of biogenic habitats such as subtidal seagrass beds). This highlights the need for resource management to include linkages to catchment level effects on estuarine habitats and the juvenile and small fish assemblages that are associated with them.