Inanga spawning success

This pilot study aims to investigate inanga spawning success in the Uretara and Te Mania streams from April to June 2022.  The study was carried out by researcher Rae, Hee-Yun Jung.

Abstract (read the full research report here)

Inanga (Galaxias maculatus) is one of the five galaxiid species that make up most of the whitebait catch. Inanga are classified as “Declining” and the causes preventing inanga spawning success can vary between streams therefore, it is essential to locate spawning sites and identify threats to put in place enhancement and better management strategies. This pilot study aims to investigate inanga spawning success in the Uretara and Te Mania streams from April to June 2022 through spawning monitoring and egg searching based on historical salt wedge locations. The salt wedge location and the issues preventing spawning success were also determined and mouse predation was investigated using different pest monitoring methods.

Spawning occurred from multiple spots in the monitoring sites and the eggs were successfully found at both streams. In the Uretara stream, a few eggs were found attached to kikuyu grass whereas intensive spawning occurred in the Te Mania stream and a high density of eggs were found mostly among the tall fescue. However, spawning did not occur in all monitoring sites. The cause of unsuccessful spawning was likely contributed to by high water flow and outflows from the stormwater drains in the Uretara stream, and maybe a high salinity level in the monitoring area at the Te Mania stream. Egg mortality was likely associated with predation, steep bank slope, and short height and low density of riparian vegetation at both streams.

In the Uretara stream, the upper limit of the salt wedge was found approximately 900 m upstream from the spawning sites found in this study which was much further upstream than the estimated salt wedge location in this project. In the Te Mania stream, spawning occurred within 180 m downstream from the upper limit of the salt wedge. However, salinity was measured a few days after the spring tide therefore, the salt wedge location cannot be precisely accurate.

In the mouse predation monitoring, no clear evidence was found that mice were predators of inanga eggs. Mice visited into the tracking tunnels and disregarded the inanga egg bait, and in the trail camera experiment, mouse predation on inanga eggs was not detected.

Inanga return to the same sites for spawning year after year therefore, better management and improvements need to be put in place to support spawning success in the spawning grounds. Management should also examine the stressors preventing spawning success such as habitat loss/degradation, access for migration, predation, fishing pressure, water quality and sediment effects.

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