Environmental capacity for cage fish farming in large hydroelectric reservoirs: a need for sustainable growth of aquaculture in Southeastearn Brazil

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1 Environmental capacity for cage fish farming in large hydroelectric reservoirs: a need for sustainable growth of aquaculture in Southeastearn Brazil Gianmarco S. David São Paulo State Agribusiness Technology Agency, Barra Bonita, Brazil ( 2000gian@uol.com.br) Edmir D. Carvalho, Igor Paiva Ramos Fish Biology and Ecology Laboratory, São Paulo State University, Botucatu, Brazil Reinaldo J. Silva Wild Fauna Parasitology Labotatory, São Paulo State University, Botucatu, Brazil Alexandre N. Silveira Dept. of Biology and Animal Science, São Paulo State University, Ilha Solteira, Brazil Fanny Yasumaru, Caio C. Ribeiro, Daniel Lemos LAM Aquaculture Laboratory, Oceanographic Institute, University of São Paulo, São Paulo, Brazil (dellemos@usp.br) Summary box Large freshwater reservoirs are abundant in Southeastern Brazil, but a still small fish production is currently carried out in these artificial ecosystems. In the last decade, governmental efforts have been made to stimulate the sitting of cage farms, with some limited success. Water body use for aquaculture is currently limited to 1% of total area in order to prevent eutrophication and assure water quality for multiple users. Determination of carrying capacity baseline is feasible, as the importance field farming practices to prevent maximum allowable production in any given site. Aquaculture expansion into open waters is expected to increase in the worldwide next decades and raises the complex issues of access to and management of common pool resources (FAO, 2010). Brazil has one of the largest availability of freshwater in the world, distributed in several regions. Most of the Brazilian hydroelectric power plants are located in its Southeastearn portion, at the upper Paraná River basin, that drain the most industrialized and populated portion of South America and of the Southern Hemisphere. In the past fifty years most of the available large rivers in this basin have been transformed into cascades of hydroelectric reservoirs, which caused losses of aquatic biodiversity and interruption of migratory fish routes, resulting on low fishery production. Several attempts of stock enhancement were carried out, though these artificial ecosystems have persisted very likely as biological deserts. On the other hand, impoundments have enhanced water quality of the resulting lakes, but no solution for food production in these large ecosystems have emerged until cage aquaculture has been shown to be feasible. In spite of great biodiversity in local freshwaters, no competitive, native option have yet emerged, and cage aquaculture have been growing mainly based on tilapia farming. The total area of reservoirs in this country region is over 500,000 ha, with a potential for fish production of more than 2 million metric tons per year by using only 0.5 % of this. Realizing the sustainable farming potential Cage aquaculture in hydroelectric reservoirs has great potential for expansion in Brazil, but there are concerns on negative environmental impacts, particularly risks of eutrophication due to nutrient loads derived from fish faeces and feed losses. Continuous impact by agriculture runoff

2 and urban sewage is today a critical issue in many southern Brazilian rivers, and any other new source of nutrient loads may break out some precautionary advices. While poor technologies and inadequate sitting of fish cages can degrade aquatic ecosystems, aquaculture could enhance fishery production in reservoirs if properly planned and located. The preservation of strategic, public natural resources such as freshwaters require government planning to set limits to any activity with potential for eutrophication. Governmental agents and private undertakers agree about the risks of auto pollution by cage aquaculture, and regulations limited the occupation of hydroelectric reservoirs to 1% of the area of each site, but no objective evaluations proved this limit to be suitable. The environmental sustainability of cage aquaculture depends on the harmonization between farming practices and the hydrological particularities of the proposed site. Natural resources must be used without causing drastic, deleterious changes in the structure and function of the ecosystems in which it is inserted. The main risks are associated with eutrophication processes. In freshwater, tropical reservoirs, phosphorus emission is the key factor for eutrophication. To quantify the limit amount of fish that can be produced in each site, it is necessary to know how much phosphorus is loaded per ton of fish produced, and determine the carrying capacity of the site, i.e., the amount of phosphorus that can be loaded in a given period without surpassing the threshold of eutrophication. The former can be calculated considering FCR and feed phosphorus content, while carrying capacity is much more complicated to assess. Carrying capacity is determined by intrinsic limnological characteristics of each site, and its estimative demands limnological field surveys, including detailed bathymetry, hydrodynamics, water flushing rate, initial water nutrient content (total phosphorus and nitrogen), sedimentation, water, conductivity, profiles of dissolved oxygen and temperature, turbidity and chlorophyll a. Assessing baselines of environmental carrying capacity are: The main factors considered in the determination of the carrying capacity of a given site - Mean depth: the deeper allows more intensive production; - Flushing rate: calculated as the theoretical time needed to fully exchange the water volume of a site. Low flushing rates allows more intensive production; - Initial water nutrient content: the cleaner is the water, with less nutrients, higher is the amount of nutrients that can be loaded by aquaculture; - Sedimentation rate: more intense sedimentation keeps water clean, allowing more intensive production. These assessments are made using mass balance models to estimate the amount of nutrients that can be loaded without triggering eutrophication. If the nutrient load related to the production of a tonne of fish is known, than the maximum allowable production of a given site can be calculated. For instance, the simulated carrying capacity for tilapia cage-culture in two sites (20 16 S; W and S; W) in the same reservoir (Ilha Solteira) and with similar areas (31.6 and 30.8 km 2, respectively), but different water retention times (33.0 versus 21.6 days, respectively) and mean depths (6.0 versus 10.4 m) resulted very different (3,982 versus 7,768 metric tons per year), even though the more productive site displayed higher initial phosphorus (12.7 versus 16.1 mg/m 3 ). Model calculations assumed a standard emission of 13.5 kg of phosphorus per ton of produced tilapia based on average 1.5 FCR and 1.5 % total phosphorus feed content though feeding management may drastically affect total allowable fish production. Simulating a 20 % FCR increment, from 1.5 to 1.8, phosphorus emission would increase 25 % (18.0 kg P/metric ton of fish produced) resulting in a 25 % reduction in the compatible

3 production according to site carrying capacity. Accordingly, the increment of 6.7 % in phosphorus feed content (from 1.5 to 1.6 % total feed P) would result in 9 % decrease in total allowable production in the farming site. Some reservoir sites register significant nutrient increase related to cage farming, although by now apparently not enough to reach the eutrophication threshold due to limited scale of farming operations. Seasonal variations in hydrological features were also relevant, with reduction in the carrying capacity related to massive nutrient afflux from agricultural and sewage runoff, especially in summer, when water temperature is more favorable for tilapia farming. Water temperature during winter months is frequently below 20 o C, when tilapia cultivation is less profitable due to reduced growth and potential increase of diseases and parasites. Is this condition, efficiency of nutrient use by fish may be lower, with potential higher nutrient emissions to the environment. Careful climate zoning for tilapia cultivation could assist proper planning of cage sitting according environment specificities. Environmental planning for aquaculture Carrying capacity studies have been carried out by our research group in the last four years in 19 different sites in the upper Paraná river basin, concentrated on Ilha Solteira and Chavantes reservoirs. Fish production at the studied sites seems up to now compatible with local carrying capacity for assimilation and recycling of nutrients derived from farming. Regarding water quality, conditions may be considered excellent, with no signals of surpassing eutrophication thresholds probably due to limited scale of farming operations. Specific models are needed for the management of aquaculture in the ecosystems considered here. Measures for the effective planning for farming public waters require further discussion and guidance at governmental levels, in order to reach a truly sustainable aquaculture, in all its socioeconomic and environmental interfaces.

4 Feeding management: important component for nutrient releases in farming operations Effect of FCR and feed composition on phosphorus loads by cage culture P load kg/ton fish produced ,0 1,2 1,4 1,6 1,8 FCR 2,0 2,2 2,4 2,6 2,8 3,0 10 TP feed kg/ton - Field farming practices strongly affects P loads per ton of fish produced, evidencing the importance of quality feeds and selection of low stress sites

5 site 1 (rio P a n ta n o ) site 2 (rio P o n te P e n sa ) total allowable fish production (tons) site 2 (rio Ponte Pensa) site 1 (rio Pantano) P load per ton of fish produced - Total production in each site is strongly affected by specific P load per ton of fish produced, and can be almost tem fold lower when low quality feed and high FCR are associated. Distribution of hydroelectric power plants in Brazil

6 Hidrodynamics Carrying Capacity calculations requires detailed ecological studies Preserved vegetation and deep waters are favorable for cage farming

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