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Quick DipsAssessing Fish StockHuman activity on our lakes can damage aquatic life by increasing suspended sediments or altering lake levels according to a study by NIWA (the National Institute of Water and Atmospheric Research). Suspended sediment blocks light, disrupting the process of photosynthesis and reducing the number and distribution of aquatic plants. Long periods of low lake levels can harm shallow plant communities, which might take months or years to recover. Suspended sediment and fluctuating lake levels can mean less food for lake animals that live among the plants. The NIWA study isparticularly looking into the effects of catchment and shoreline developments such as hydroelectric generation, forestry, agriculture and marinas. It is also assessing how aquatic biota including trout and native fish are affected, either directly orthrough changes to feeding behaviour and distribution. Programme leader Dr Mark James says the research will help scientists, lake managers and developers to predict and mitigate the potential harmful effects of sediments and lake level changes on shore and lake life. This research is particularly important with the deregulation of the electricity industry and the sale of hydro stations to different operators, and will complement work commissioned by Genesis Power, Mighty River Power and Meridian Power (all formerly ECNZ), Trustpower, Waiau Fisheries and Wildlife Trust, and Clutha Fisheries Trust on lakes Coleridge, Manapouri, Taupo, Te Anau, Rotoaira and Waikaremoana. James says a big concern for many of New Zealand's large lakes is the possible decline in recreational fish stocks. Researchers have found to their surprise that changes in water clarity have little effect on the behaviour of some fish species, and they can feed in complete darkness. Many overseas studies have suggested vision alone is used to detect prey. A few very large native koaro were caught in the least clear lake in the study, Pukaki, and are among the largest fish of this species caught anywhere in New Zealand. Recent work has also found that most of the trout caught in Lake Hawea were rainbows, but in Lake Wanaka most were brown trout. This may be related to differences in food supplies or in spawning grounds, and hence the number of juvenile fish entering these lakes. |
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