|dc.description.abstract||Estuaries are important nursery habitats for many fish species, both in New Zealand and worldwide, characterised by high levels of primary production, shallow warmer waters, and affording relative protection from predators. Many estuaries are under constant threat from anthropogenic stressors due to the vicinity of industries that can cause contamination. It is necessary to first understand how larval fish use estuaries in order to understand the potential effects of anthropogenic impacts on their life history stages. Few studies in New Zealand have examined the use of estuaries by pre-settlement larvae. The majority of research undertaken in New Zealand on larval fish has been completed in coastal waters, examining the effects of seasonality, distribution patters, and growth of larvae. The objectives of this research were to increase understanding as to how larval fish use Tauranga Harbour. The study used a stationary channel net at Bridge Marina to intensively sample over one summer, and fortnightly over the ensuing 16 months, in an effort to discover which species of larval fish are found within the harbour; if they were more abundant on the flood or ebb tides; if they were more abundant during the day or night; if any patterns of seasonality were present; and if the abundance was affected by environmental variables such as water temperature or current speed. A choice chamber experiment was conducted to assess if larval kingfish (a species not found in the estuarine sampling programme), were able to differentiate between two different water types - estuarine and oceanic, in an effort to assess if larvae may use olfactory cues when choosing a habitat.
The larvae of at least 19 species from 18 families were caught during the intensive summer sampling, and at least 13 species from 11 families during the fortnightly sampling, with no new species found during the fortnightly sampling. Three taxa (anchovy Engraulis australis, Tripterygiidae and Gobiidae) were the most abundant over the entire sampling period, making up the majority of all catches. Larvae were found in higher abundances at night compared to the day, and no significant difference was found between abundances on the flood and ebb tides. Seasonality had an effect on larval fish abundance, with higher abundances caught during spring and summer. The kingfish larvae in the choice chamber experiment appeared to display a preference for the oceanic water over the estuarine water.
The findings of this study indicate that larval fish of a range of species use Tauranga Harbour potentially as a nursery. Not surprisingly perhaps, estuarine species were found in the highest abundances. Larval abundance in the estuarine water column is affected primarily by the diel cycle, with larvae displaying behaviours of diel vertical migration and possibly selective tidal stream transport. Abundances of larvae peaked during spring and summer when the water temperature begins to warm after winter, coinciding with the spawning seasons of many fish species. Larval kingfish displayed an attraction towards oceanic water, or perhaps an avoidance of the estuarine water. This suggests that olfactory stimuli are important when choosing potential habitats, signifying that larval fish of estuarine species may actively seek out refuge within an estuary using olfactory cues. This study provides a good benchmark for future studies on larval fish within Tauranga Harbour and other estuarine systems.||