Annual spawning surveys of Waituna tributaries
Waituna Lagoon is a very popular trout fishing destination with ‘the lake’ receiving about 2,240 angler visits a year. Some people consider that you need to be more hardy than most to fish the lagoon, with its frequent blustery westerly. The increase in ownership of three- and four-wheeled motorbikes enables easy access to the ‘breakout’ – where the lake opens to the sea – the most popular spot for fishing. Many anglers prefer to fish the lagoon when it is tidal.
As part of Fish & Game’s management responsibilities we carry out annual spawning surveys on the Waituna tributaries. As all of the trout from the Waituna system spawn in these tributaries it allows us to closely monitor trends in the population. This allows us to adjust any regulations that influence harvest if this is required, such as reducing the bag limit. Also, any significant reduction in the population may be a sign of a decrease in the health of the lagoon ecosystem. However, because trout are relatively long-lived their numbers may not be a very timely indicator of lagoon health, i.e. their numbers my not change unless the lagoon becomes algal dominated. Numbers in the lake’s tributaries remain high with indications in excess of 3,000 adult trout.
When spawning, between May and August, brown trout swim upstream into the Lake’s tributaries. The female trout then forms a ‘redd’ in which to lay her eggs. Redds are formed by the female thrashing her tail while on her side, using the force of the water to displace the gravel. In the pocket that forms she lays her eggs, with the male then adding his contribution, before she works her tail again and the eggs are buried and protected. Trout tend to form their redds at the head of riffles.
Fine sediment that finds its way into the stream from bank erosion or farm runoff can clog up the redds and prevent oxygen reaching the eggs and suffocate them.
Unfortunately, more than 50 years of stream cleaning resulted in the unavoidable removal of gravel along with the fine sediment on some stretches of the Waituna Creek. Environment Southland recognised this significant issue and has replenished gravels in one or two stretches. Monitoring of spawning in these sections since show that trout use has increased significantly.
Fish & Game also capture more than 100 trout annually from the spawning migration to assess their overall health and condition. There has been no significant change in size or condition of trout over the last decade.