James, G. (2006). Assessment of fish populations in the Avon-Heathcote Estuary. National Institute of Water and Atmospheric Research.
Commissioned by the Christchurch City Council (CCC), this report is the second of six planned annual surveys of the Avon-Heathcote Ihutai estuary designed to monitor changes in fish species diversity and abundance before and after the 2010 construction of an ocean outfall to remove wastewater discharge from Te Ihutai. Trawling for adult fish in the main channels was predominantly used to integrate long term data and describe recreational fishing experiences, while beach seining for juvenile fish at high-tide margins was used to reflect recent events in the estuary.
Overall, the abundance of most fish species was similar to 2005 data, suggesting that the sampling intensity was sufficient to reliably assess changes over time. Juvenile yellowbelly flounders were much less common in 2006 compared to 2005, however, annual variation in short lived species is not unusual. What was notable from this study was that fish diversity seems to have declined since the 1960s. The relative abundance of sand flounder also had decreased since the 1960s, whereas adult yellowbelly flounder were larger and more abundant than in 1965.
The report concluded that these changes could be due to several factors, including commercial fishing pressure and increasing sea temperatures alongside wastewater discharge into the estuary. They recommended future surveys continue to be conducted in November to prevent sea lettuce clogging sampling nets.