PERSPECTIVES ON ECOLOGICAL CARRYING CAPACITY OF LAKES AND RESERVOIRS USED FOR CAGE AQUACULTURE IN LATIN AMERICA

As a consultant for the US Soybean Export Council for 11 years, I have visited many natural lakes and multi-use water reservoirs in Latin America that are currently used for cage aquaculture of tilapia and rainbow trout. As a generality, more river dam reservoirs are used for tilapia farming and more natural lakes are used for trout farming.

Some of the key observations from more than a decade working in this area:

• Every water body responds differently to nutrient loading and carrying capacity cannot be simply reduced to an equation.

• Water turnover rate (hydraulic retention time) is a key factor driving the response of a water body to nutrient loading. Lakes and reservoirs can be classified as having short (< 1 yr), intermediate (1-10 yrs) and long (>10 yrs) retention time. The risk of failure to recover from eutrophication increases with retention time.

• Not every water body used for cage aquaculture is operating at carrying capacity. However, in water bodies with multiple cage aquaculture projects, the tendency has been to test the limits of available capacity.

• Carrying capacity varies according to prevailing conditions. High carrying capacity is favored by high turnover rate, water level, and wind. Minimum carrying capacity occurs under drought conditions when river flow and reservoir level are low, and winds are calm. Fish production schedules to meet peak market demand do not necessarily match the dynamic and seasonally fluctuating carrying capacity limit.

• Naturally occurring events such as lake turnover or upwelling of hypolimnetic water can lead to oxygen depletion and fish kills. The effects can be localized or system wide and are difficult to manage, often leading to disease outbreaks.

• Water transparency and the depth of the photic zone regulates oxygen production by photosynthesis. The risk of dissolved oxygen depletion increases when SDV is < 2 m. All other things being equal, carrying capacity is maximized when photosynthetic oxygen production is maximized under intermediate, mesotrophic conditions.

• There are numerous social issues around cage aquaculture in Latin America, including fisheries-aquaculture conflicts, thievery and piracy, poor coordination and communication among stakeholders, and weak governance systems.