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Work Plan 7, Honduras Study 2
Department of Fisheries and Allied Aquacultures
Alabama Agriculture Experiment Station
Auburn University, Alabama, USA
Delia Martinez and Eneida Ramírez
Laboratorio de Calidad de Agua
La Lujosa, Choluteca, Honduras
(Printed as Submitted)
Chemical budgets of aquacultural facilities are useful for water quality management and quan-tification of effluent impact on receiving waters. Characterization of farm effluents is prerequisite to estimating the carrying capacities of local estuarine systems for shrimp culture.
The percentage of world shrimp production in Central America is only about 7% (Rosenberry, 1994), but the industry is important to local economies. In Honduras, over $70 million was generated by shrimp culture in 1994 placing the industry third in national export earnings. Sustainability of the industry is economically critical, and could depend on estuarine water quality. A 2-yr baseline of water quality has been established for the major shrimp producing estuaries (Teichert-Coddington, 1995). The objective of this study was to characterize farm effluents and formulate a budget for nitrogen and phosphorus.
Materials and Methods
Characterization of Water
Six farms peripherally located to the Gulf of Fonseca in southern Honduras were sampled during 1993 to 1994. Farms occupied estuaries dominated by rivers (riverine) or the Gulf (gulf embayments) (Teichert-Coddington, 1995). Intake and effluent water samples were collected from ponds every two weeks during a production cycle. Ponds were first drained and then refilled. Production cycles were sampled during dry and wet seasons, which have distinctive yield differences (Teichert-Coddington et al., 1994; Teichert-Coddington and Rodriguez, 1995). The number of ponds sampled and the seasons comprised by the sampling varied by farm (Table 1).
Water was analyzed for total settleable solids (American Public Health Association (APHA) et al., 1992), nitrate nitrogen by cadmium column reduction to nitrite (Parsons et al., 1992), nitrite nitrogen (Parsons et al. 1992), total ammonia nitrogen (Parsons et al. 1992), filterable reactive phosphate (Grasshoff et al., 1983), chlorophyll a (Parsons et al., 1992), total alkalinity by titration to 4.5 pH endpoint, salinity, and 2-d and 7-d BOD. Total nitrogen and total phosphorus were determined by nitrate and phosphate analysis, respectively, after simultaneous persulfate oxidation (Grasshoff et al., 1983). Organic phosphorus was calculated from the difference of total phosphorus and filterable reactive phosphate. Dissolved inorganic nitrogen (DIN) was the sum of nitrate, nitrite and total ammonia nitrogen (TAN), and organic nitrogen was the difference of total nitrogen and DIN.
Nutrients discharged during draining were determined from water samples collected at 100%, 50%, 25%, 12.5% and 0% of pond volume during a draining event.
Table 1 (24 K file). Characteristics of farms sampled for study.
Inputs and outputs of total nitrogen and total phosphorus were quantified for ponds that had complete record sets. Records of feed, fertilizer, water exchange, stocking rates, and harvest weights were obtained from the producer. Concentrations of P and N were determined by analysis of intake, effluent and drainage water in each pond. Evap-oration and rainfall were not taken into account, but their effects on the nutrient budgets were probably minimal (Briggs and Funge-Smith, 1994). The general balance equation is:
Sin + Fin + Fertin + PVin + WEin = Sout + PVout + WEout ±Ø
where S = shrimp; F = feed; Fert = fertilizer;
PV = pond volume; and WE = water exchange.
Feed from various farms was analyzed for N and P composition. Manufacturer's proximal analyses of feed were used in instances where independent analyses were not available. Nitrogen and phos-phorus composition of shrimp were determined by elemental analyses (Boyd and Teichert-Coddington, 1995). Percentage of dry matter, N and P for P. vannamei were 25.5%, 11.2% and 1.25%, respectively.
Material exchange between intake and effluent water was expressed in terms of kg/ha-100 d. Calculations were based on nutrient concentration differences between intake and effluent water, daily water exchange rate, pond area and an average pond depth of 1 m. Results were grouped by estuarine type and season. Seasonal comparisons were completed for only 3 farms from which water had been monitored during both seasons. Mean differences were declared significant by non-coincidence of 95% confidence intervals. Regression analyses were used to reveal relationships between selected variables. Data were analyzed using software by Haycock et al. (1992).
Results and Discussion
Mean intake filterable phosphate, nitrate, nitrite, total N, total P and BOD2 were significantly higher in riverine than in embayment estuaries (Figure 1). Salinity was significantly higher in embayment estuaries. There was no significant difference for COD, chlorophyll a, and total alkalinity.
Fig 1 (47 K file) Characterization of farm intake water by location on Gulf of Fonseca embayments or estuaries influenced by rivers.
Figure 2 (29 K file) Mean net material (organic indicators) exchange on farms characterized by riverine or embayment location. Negative values indicate a net discharge of material.
No significant seasonal differences were found, except for total alkalinity, which was significantly higher during the dry season than the wet season.
Mean discharge of total N, total P, BOD2, chlorophyll a, COD, total alkalinity and salinity were greater than mean intake, resulting in a net discharge of these variables from the ponds (Figure 2). On the other hand, mean discharge of nitrate, nitrite, total ammonia and filterable phosphate were less than mean intake, resulting in a net consumption of these variables (Figure 3). In summary, inorganic forms of phosphorus and nitrogen were processed into organic forms during passage through the pond.
There were some differences in material exchange depending on estuarine type. Net discharge of total nitrogen, total phosphorus, COD and chlorophyll a was significantly greater in embayment than riverine estuaries (Figure 2). Net consumption of total ammonia, nitrate and nitrite was significantly greater in riverine than embayment estuaries (Figure 3). No significant differences were detected for the remaining variables. Some of the variability among farms for material exchange of total ammonia and filterable phosphate stemmed from the use of inorganic fertilizers on some farms, and not on others. Where fertilizers were used, filterable phosphorus and forms of inorganic nitrogen were greater in discharge than in intake water (Table 2). There were no significant differences for material exchange attributable to season (Figure 4).
Total nitrogen and total phosphorus discharge from the Choluteca River
was estimated for 1994 (Teichert-Coddington, 1995). In comparison with discharge
from 11,500 ha of shrimp ponds, the river discharged 1.8 and 4.8 times more
nitrogen and phosphorus, respectively (Table 3).
Table 2 (19 K file) Influence of inorganic
fertilization on effluent concentrations of dissolved inorganic nitrogen
(DIN) and filterable orthophosphate from two farms located in close proximity
on the same riverine estuary. Number of ponds sampled in Farm A and B were
6 and 4, respectively.
Figure 3 (27 K file) Mean net exchange of inorganic materials on farms characterized by riverine or embayment location. Positive values indicate a net consumption of material.
There was a significant inverse relationship between pond volume at drainage and concentrations of total P, total N, dissolved inorganic nitrogen, filterable phosphate and settleable solids (P < 0.05). Concentrations of these variables increased as pond volume decreased. However, correlation coefficients were low for these relationships, because differences during draining occurred mostly during the last 12% of water discharge (Figure 5). Suspended solids were particularly high at the end of drainage (mean =
12 ml/l) compared with the start of drainage (mean = 0.16). Despite an increase in many nutrients and settleable solids with a decrease in pond volume, mean BOD2 remained constant from start to finish during pond drainage. There was high variation of nutrient concentration in pond drainage among farms. This variation was probably related to water control during draining; i.e., water input towards the end of drainage to provide oxygenated water to shrimp.
Most nitrogen entered ponds with the water (58%), while feed (40%) and fertilizer (2%) accounted for the remainder of nitrogen input (Figure 6). Input by shrimp was minimal, averaging less than 0.5%.
The majority of nitrogen was discharged from the ponds with daily water exchange (72%) and pond drainage (10%) (Figure 6). Harvested shrimp accounted for 16% of nitrogen removal. Nitrogen input averaged 1% higher than the sum of nitrogen in water discharge and shrimp removal. Unobserved nitrogen was apparently fixed by soils or lost through denitrification. There was high variation among ponds, even on the same farm, for unrecorded nitrogen. It is probable that inaccurately calculated water exchange rates accounted for some of this variation.
Mean farm conversion ratios of feed nitrogen to shrimp flesh ranged from 1.4 to 4.1. From 29 to 66% of nitrogen added in the feed was not utilized by the shrimp. Nitrogen conversion ratios were directly correlated with feed conversion ratios. Nitrogen discharge from ponds, i.e., negative net material exchange, consequently increased linearly with increasing feed conversion ratios (Figure 7). The nitrogen conversion ratio was also correlated with percentage of nitrogen in the feed (Figure 8); i.e., nitrogen conversion was less efficient with increasing protein content of feed. Higher protein levels in shrimp feeds did not result in better feed conversion efficiency (Figure 9). No significant differences for feed or nitrogen conversion ratios were detected between types of estuaries.
There were large seasonal differences for nutrient budgets. Production was significantly higher during the wet than dry season (Figure 10). The total quantity of feed added to ponds was not different between seasons. Therefore, the conversion of feed and protein to shrimp flesh was significantly more efficient during the wet season (Figure 10).
Most phosphorus entered the pond with feed (54%), while water (44%) and fertilizer (2%) input accounted for the remainder (Figure 11). Shrimp accounted for less than 0.5% of total phosphorus input.
Table 3 (13 k file) Discharges of nitrogen and phosphorus from the Choluteca River and the shrimp farming industry (11,500 ha) in southern Honduras.
Figure 4 (46 K file) Mean net material exchange on all farms by dry or wet seasons. Negative values indicate a net discharge of material.
Figure 5 (76 K file) Mean effluent concentration of nutrients at 0, 12, 25, 50, or 100 % of pond volume during draining.
Figure 6 (56 K file) Mean nitrogen budget of shrimp ponds.
Figure 7 (43 K file) Net material exchange of nitrogen and phosphorus in relation to the feed conversion ratio. A negative exchange value indicates net discharge of material.
Figure 8 (25 K file) Conversion ratio of feed nitrogen to shrimp flesh in relation to percentage of nitrogen in the feed during dry and wet seasons.
Figure 9 (19 k file).Feed conversion ratio in relation to the percentage of nitrogen in the feed.
Figure 10 (51 K file) Mean shrimp production, total feed usage, and feed conversion ratios during dry and wet seasons. Bars indicate 95% C.I.
Figure 11 (41 K file) Mean phosphorus budget
of shrimp ponds.
The majority of phosphorus was discharged from the ponds with daily water exchange (54%) and pond drainage (4%) Harvested shrimp accounted for 10% of phosphorus removal. Almost a third of input phosphorus was not observed in the sum of phos-phorus discharged with water and harvested with shrimp, and was apparently fixed by the soils.
Mean conversion ratio of feed phosphorus to shrimp flesh was 6.3. Phosphorus conversion ratios were significantly greater (less efficient conversion) during the dry season (9.4) than during the wet season (3.7). Mean phosphorus conversion was not significantly different between type of estuary.
Much of the Honduran shrimp industry investment is from domestic sources, so a goal of the industry is to be sustainable for the next generation of Hondurans. The goal can be reached only if estuarine water quality is given prudent attention. Water quality depends on the assimilative capacities of estuaries and rate of nutrient discharge from farms. Farm nutrient discharges cannot exceed the assimi-lative capacities of the estuaries for sustainable production.
Estuarine assimilative capacities vary widely based on distance from the gulf, offshore currents, tidal fluctuation and estuarine hydrography (Ward and Montague, 1995). Hydrography of riverine estu-aries in Central America is strongly influenced by seasonal rains, which cleanse estuaries during flooding. In Honduras, estuarine nutrient concen-trations are significantly higher during the dry than wet season (Teichert-Coddington, 1995). Conditions become progressively worse with distance upstream from the gulf (Teichert-Coddington, 1995). The pro-bability of low estuarine DO is higher during the dry season. Estuarine water quality is therefore more imminently critical, and of a lower assimilative capacity during the dry season.
Farm management practices can be modified to minimize nutrient discharge, particularly during the dry season when shrimp production is historically low (Teichert-Coddington et al., 1994). Inorganic fertilization can probably be eliminated in riverine estuaries, and reduced in embayment. Studies in Choluteca indicated that the use of inorganic fertili-zers was of uncertain value during the wet season, and did not result in increased shrimp yield during the dry season (Green and Teichert-Coddington, 1990; Rodriguez and Teichert-Coddington, 1995). The current study demonstrated that inorganic nitrogen and phosphorus effluents were higher in fertilized than in unfertilized ponds. If fertilization increases nutrient discharges without increasing shrimp production, then estuaries will be needlessly en-riched, and assimilative capacities will be more quickly reached at lower shrimp production levels. Dry season fertilization of ponds should therefore be stopped. A self-regulated moratorium on fertilizer use in riverine estuaries has, in fact, been employed by the Honduran National Association of Aquaculturists.
Feed accounted for 54% and 40% of nitrogen and phosphorus input, respectively, to ponds. Nitrogen appears to be a limiting factor to estuarine primary productivity, and should be controlled. This study indicated that nitrogen and phosphorus discharge increased with higher FCRs, and nitrogen discharge increased with higher feed protein levels. Yields of shrimp stocked at 5 to 10/m2 were not different in Choluteca (P > 0.05) when using a 20% or 40% protein feed (Teichert-Coddington and Rodriguez, 1995). Low protein feeds should be used with semi-intensive shrimp culture under current feeding practices. Higher protein feeds have not demonstrated increased shrimp yields, but probably contribute to nitrogen discharge.
The current study indicated that mean dry season production was significantly lower than mean wet season production, yet similar quantities of feed were used during both seasons. Feed conversion ratios and wastes were consequently higher during the dry season. These data indicate that mean feeding rates had not been adjusted during the dry season to reflect lower dry season shrimp growth. Rodriguez and Teichert-Coddington (1995) decreased feeding rates by half during a dry season trial without any impact on shrimp production. Current studies are focusing on the combination of higher feed protein and lower feeding rates to reduce total nutrient effluents.
Other management practices to reduce nutrient wastes in semi-intensive culture should be tested. Feeds with longer water stability are being employed sporadically, but controlled studies on the benefits of these feeds are unknown. Pond designs may have to be altered to gain further reductions in nutrient discharge. Mean area of the ponds in this study was 21 ha. It is hard to sample shrimp populations and distribute feeds efficiently in ponds of this large size.
Smaller ponds would be more expensive to build, but the savings in feeds, and reduction of nutrient discharge might make smaller pond designs more sustainable.
This study was made possible by collaboration of the Dirección General de Pesca y Acuicultura, Secretaría de Recursos Naturales, Government of Honduras and shrimp producers of the Honduran National Association of Aquaculturists (ANDAH). Jaime López assisted in the laboratory.
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