Fish assemblages and life history patterns in estuaries along the Otago coastline, New Zealand

Understanding the ways in which fish use estuaries is crucial to develop effective interrealm management plans that take into account all components of these highly connected ecosystems. Fish assemblages of New Zealand estuaries are poorly studied, and knowledge of the life history of fish that commonly occur in estuaries remains limited. This thesis examined the distribution, occurrence and movement of fish in estuaries along the Otago coastline. The role of estuary-ocean connectivity on estuarine fish assemblage structures was investigated across seasons. Life histories of fish were explored by combining community ecology and otolith microanalysis approaches. Comparison of ichthyofaunal composition between 12 estuaries with varying degrees of connection to the marine environment showed marked variations in fish assemblage patterns. Fish species with marine affinity commonly occurred in the shallow littoral habitats of permanently open estuaries. Intermittently closed and open lakes or lagoons (ICOLLs) were found to be dominated by species with potential marine larval rearing but are also known to form landlocked populations. Forsterygion nigripenne, an endemic estuarine species that is generally assumed to have marine larval phase, occurred in almost all permanently open systems and in one ICOLL. Greater fish abundance was recorded in ICOLLs than in permanently open estuaries. Seasonal comparisons showed higher fish abundance in summer than winter. Salinity and temperature were best predictors of fish abundance in both estuary types. The distribution and occurrence of fish also varied between different sections of a permanently open estuary. A year-round sampling of fish in the Waikouaiti Estuary showed distinct seasonal and spatial differences in fish assemblages. Juveniles and adults of fish species with marine larval phase occurred more frequently in the upper reach of the estuary. F. nigripenne occurred in large numbers at different life stages in the middle reach of the Waikouaiti Estuary. Conversely, primarily marine species that use estuaries for short periods dominated the lower reach. Fish abundance was higher in the middle reach where the water was more turbid. Warmer months of the year saw higher abundances and species richness. Among all physicochemical variables, salinity and turbidity were best predictors of fish abundance in the Waikouaiti Estuary. The presence of F. nigripenne in large numbers at different life stages throughout the year in the middle section of the Waikouaiti Estuary raised the question as to whether this species actually has marine larval phase. Thus, otolith microchemistry was used to investigate how F. nigripenne uses estuaries. A laboratory experiment was conducted to understand the extent to which the chemistry of the ambient water affects the otolith elemental profile of F. nigripenne. Results of the experiment indicated a positive and direct relationship between the chemical composition of F. nigripenne otoliths and the ambient water. Otolith microchemistry of wild-caught F. nigripenne collected from two permanently open estuaries showed relatively consistent elemental profile spanning across the otolith without prominent peaks. The results indicate that F. nigripenne is primarily an estuarine-resident species that completes its entire life cycle within estuaries. The inter- and intra-estuarine fish assemblage structure patterns presented in this thesis may reflect the impacts of stressors (both global and local) and effects of management actions that has been implemented. Future deviations from findings of the present study, in terms of distribution and occurrence of fish in estuaries, may indicate changes in the freshwater-estuarine-marine continuum because of stressors and/or management actions taken by decision makers