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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.3. Impacts of Eutrophication

1.3.3. Case Studies of Effects of Eutrophication (suite)

Fisheries on the Prairies of Central Canada and U.S. Mid-West

Thousands of water bodies of different sizes are scattered over the Canadian and U.S. prairies. They range from small trenches, marshes and ponds to large reservoirs and lakes of over 30,000 km² (i.e., Lake Winnipeg and Lake Manitoba). Most prairie lakes are naturally eutrophic with large fluctuations in their nutrient and dissolved oxygen content as well as in biological production. Primary production is represented by dense algal blooms, which are composed mostly of cyanobacteria. Secondary production is characterized by high abundance of zooplankton and benthic invertebrates. Due to the abundance of food, the lakes have a high potential for fish production. However, some of the lakes develop winter anoxia causing fish mortalities, i.e., winter kill, the gradual depletion of dissolved oxygen in water column under ice and snow cover obstructing penetration of light for photosynthesis and production of oxygen by algae in the absence of aeration through the lake surface. Other lakes undergo collapses of their algal blooms during summer, resulting in oxygen depletion and fish death, i.e., summer kill (see Hypereutrophy above).

Prairie lakes and reservoirs are exploited for commercial and sport fishing where fish production is substantially higher than in neighboring, oligotrophic lakes on the Canadian Shield (Table 1.1.). Species composition is also different, with prairie lakes favoring percids and esocids while oligotrophic lakes favor salmonids. Commercial, prairie fish production accounts for one third of Canada's freshwater fish catch. Sports fisheries are an integral part of the tourist industry, and eutrophic prairie lakes are as popular as the Shield lakes for sport fishing.

Table 1.1. Comparison of fish yields in eutrophic praire and oligotrophic Precambrian Shield lakes in Canada.

A viable fish farming industry was developed in Canada based on the natural high productivity of small eutrophic lakes with abundant benthic orgnisms to support growth of rainbow trout (Salmo gairdneri). This aquaculture is an increasingly popular industry, utilizing small (i.e., up to 30 ha) shallow, winter kill lakes for rainbow trout production. The lakes are stocked after the ice is out (i.e., April) with 4 to 8 g rainbow trout fingerlings from commercial fish hatcheries. During the 5 to 6 month ice-free period, the fish grow to marketable size (i.e., 200 to 300 g) and can be caught using gillnets. The success of this method depends on the absence of competing fish species (due to the winter kill), the abundant secondary production, and the absence of summer kill. The annual average yields are 20 to 300 kg/ha

Milk Fish Aquaculture in Laguna de Bay, Philippines

Laguna de Bay is a major freshwater body located on Luzon Island, Philippines. Aquaculture practiced in Laguna de Bay in fishpens, which are artificial enclosures constructed of bamboo stakes and fish netting, represents approximately 40% of the fish production through aquaculture in the Philippines (Figure 1.14.). A major drawback of this convenient method for culturing milkfish (i.e., Chanos chanos, locally called bangus) are frequent fish kills caused by the collapses of Microcystis blooms, which vary considerably in severity from year to year. Annual fish yields in the lake are approximately 450 kg/ha and are sustained entirely by the natural biota of the Laguna de Bay.

Laguna de Bay is an example of both positive and negative impacts of eutrophication. While the abundant algae provide an adequate source of food for the milkfish stocks, their vulnerability to sudden meteorological changes, such as heavy overcast and torrential rains, resulting in occasional massive fish kills, makes the operation risky. However, thousands of fish farmers make their living in Laguna de Bay, and the fish-pen aquaculture occupies an important place in the local economy.

Figure 1.14. Fish pens in Laguna de Bay, Philippines.

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