Açudes Project. Aquatic resource assessment in reservoirs of the semi-arid Brazilian Northeast (IRD/UFRPE/CNPq). Fieldwork completed during 1995-1999 [NE]

The "Açudes" Project (1995-2000) is a binatiional collaborative research and development project between IRD (France) and the Fisheries Department of UFRPE (Recife, Brazil), co-granted by IRD and CNPq, the Brazilian Research Council.

According to the World Bank, 80 countries comprising 40% of the World population have difficulties to meet their water demands. The semi-arid Brazilian Northeast (11% of Brazil area, 16 million inhabitants) with > 70,000 reservoirs ("açudes"), i.e. the World's second number of reservoirs after India, is characterized by low (< 600 mm year-1), highly unpredictable and spatially heterogeneous rainfalls. Among the 9 Northeast States, Pernambuco State (85,574 km2, 3 million inhabitants) comprises 7,128 reservoirs mostly small rural community-owned (97% ≤ 0.5 106 m3, with only 123 public reservoirs ≥ 106 m3) built on temporary streams. Most are multi-use reservoirs where water supply, irrigated agriculture, and fisheries sustained by supplemental stocking induce water quality incompatibility favored by the lack of rational management. During the 1998-1999 El Niño drought, rainfall deficit reached up to 80% in Pernambuco. Reservoirs are shallow (< 10 m), mainly eutrophic to hypereutrophic, often dominated by blooms of inedible algae, mainly cyanobacteria. The Açudes Project focuses on research plus development issues. As part of this project, the aim of our work is to increase the scientific knowledge base on the biology and ecology of those reservoirs, mainly re. fish communities and aquatic food webs, to improve their sustainable use for artisanal fisheries and aquaculture, and/or conciliate appropriate water quality for water supply. Research combines comparative approaches (reservoir survey using experimental fishing and acoustic) and experimental approaches (in laboratory, outdoor mesocosms, and aquaculture ponds). In addition to test the generality of the aquatic food web theory to shallow tropical ecosystems, and contribute to academic training at local universities, outputs of this project have practical implications for the civil society, and managers at federal (IBAMA, DNOCS, SUDENE) plus state agencies (SRH-PE, SPRRA-PE). Passing the scientific knowledge gained from research onto rural technicians, university extension programs contribute to improve conditions for communities of local fishermen, small farmers, and their families.

NE1 - Relationships between food webs, water quality, and fisheries yield in Brazilian Northeast reservoirs

Fish trophic guilds regulate phytoplankton in Brazilian Northeast reservoirs via multichannel omnivory. X. Lazzaro, M. Bouvy, R.A. Ribeiro-Filho, V.S. De Oliviera, L.T. De Sales, A.R.M. De Vasconcelos & M.R. Da Mata. Freshwater Biology, Special Issue (in press).

Strong community trophic cascades between biomasses of fish, zooplankton, and phytoplankton require key species such as efficient piscivores, size-selective zooplanktivores, and/or large herbivorous macrozooplankton (mainly Daphnia spp.). At the heart of the lake food web theory, maximum trophic cascade strengths have mostly been experimentally demonstrated in temperate oligo- to mesotrophic lakes. Appropriate comparative and experimental studies are lacking in tropical conditions where the descriptive approach still prevails. We tested the relative importance of top-down and bottom-up structuring forces on interactions between functional groups of fish, zooplankton, and phytoplankton in shallow tropical reservoirs of the Semi-Arid Brazilian Northeast. In those reservoirs, fish are organized into three trophic guilds of facultative piscivores, microcarnivores (visual zooplanktivores including juveniles of fish species, insectivores), and omnivores (filter-feeding planktivores, detritivores). These fish guilds coexist with small herbivorous zooplankton (mainly rotifers and small cladoceran species) and persistent cyanobacteria blooms, but rare macrophytes owing to large water level fluctuations. Using a comparative approach based on multivariate analyses (PCA. Clusters, Co-inertia), we assessed interactions between the relative biomasses of three fish trophic guilds, plankton components, physico-chemical conditions, morphometry, and pluviometry data from 13 reservoirs located within six major watersheds of Pernambuco State. We analyzed 27 samplings from these reservoirs (capacity 0.7-504 106 m3, mean depth 1.3-7.9 m, chlorophyll 3-129 µg L-1, conductivity 540-3,315 µS cm-1) over the 1995-1999 period.

Chlorophyll concentration in the pelagic central region of reservoirs increased with the percent biomass of omnivorous fish, and decreased with the percent biomass of facultative piscivores. Nevertheless, chlorophyll concentration was unrelated to overall fish biomass and biomass of small-sized herbivorous macrozooplankton. The ratio of fish biomass to chlorophyll and the biomass ratio of fish to zooplankton were inversely related to trophic state. The fish trophic guild assemblage, namely the biomass ratio of facultative piscivores to omnivores, not fish biomass per se, influenced not only chlorophyll concentration but also filamentous and colonial cyanobacteria density. We formulated two, not mutually exclusive, hypotheses regarding this apparent top-down control between fish guilds and phytoplankton. First, at low facultative piscivore biomass, we cannot ruled out that fish-phytoplankton regulation could arise from a littoral trophic cascade between zooplanktivorous juvenile fishes and zooplankton because we probably underestimated microcarnivores in our sampling. Second, regardless of facultative piscivore biomass, fish-phytoplankton regulation may originate from multichannel omnivory due to predominance of opportunism, omnivory and detritivory in the feeding behaviors of the various constituting consumer populations.

As a practical implication, fisheries management and stockings could be used as management tools to alter the community biomass ratio between facultative piscivores and omnivores. Depending on the ecosystem priority use, an increase in this ratio will reduce phytoplankton and cyanobacteria blooms to improve water quality in water supply reservoirs, whereas a decrease in this ratio may enhance eutrophication and fish yield to some extent.

NE2 - Quantifying piscivory and zooplanktivory in Brazilian Northeast reservoirs

Comparing selective zooplankton consumption between larval common carp, curimatã, tambaqui, and Nile tilapia. Da Mata M.R. & X. Lazzaro (in prep.)

The feeding ecology and zooplankton consumption by larvae of omnivorous fish species stocked into Brazilian Northeast reservoirs are poorly known. We quantified feeding selectivities and rates of larval common carp (Cyprinus carpio, 1.1-2.4 cm SL, 0.02-0.64 g), curimatã (Prochilodus cearensis, 3.8-6.3 cm SL, 1.47-4.54 g), tambaqui (Colossoma macropomum, 2.8-3.7 cm SL, 0.66-1.54 g) and Nile tilapia (Oreochromis niloticus, 2.4-3.1 cm SL, 0.55-1.13 g) on reservoir zooplankton in laboratory feeding trials. Consumption was assessed from reduction in densities between initial and final samples during one-hour replicated trials in 9-L aquaria, using replicated groups of 12-30 fishes. Feeding selectivity was computed using Chesson's alpha index', and feeding rate using Dodson's feeding rate constant. Larval common carp, tambaqui and Nile tilapia are visual zooplanktivores, whereas larval curimatã are filter feeders. Despite this difference in feeding behaviors, feeding selectivities were not significantly different between species. Rotífers and cladocerans were selected by carp, rotifers and copepodites by curimatã, and cladocerans by tambaqui and tilapia. For tambaqui and tilapia, feeding selectivity increases linearly with zooplankton body size. Feeding rate patterns were different between fish species, and for nauplii, rotifers, copepodites, copepods and cladocerans. Curimatã feeding rate differed from those of carp, tambaqui, and tilapia. Feeding rates were highest for carp consuming rotifers and cladocerans, and for tambaqui and tilapia consuming cladocerans but lowest for curimatã. Feeding rate increases linearly with zooplankton body size for tambaqui and tilapia. This experimental methodology is more appropriate to quantify zooplankton consumption by fish larvae than stomach content analysis, permits replication, and does not require fish sacrifice. Predation pressure of young fishes on lake zooplankton can be assessed, as well as lake carrying capacity for fish recruitment, plus natural feeding preferences and natural food requirements of young fish species reared in aquaculture.

Quantifying piscivory by opportunist pescada do Piauí (Plagioscion squamosissimus) and prey vulnerability in three Brazilian Northeast reservoirs. X. Lazzaro, R.A. Ribeiro Filho & L.A. Willadino (poster; ms. in prep.)

Native from Amazon, Parnaíba and São Francisco rivers, Pescada do Piauí (Plagioscion squamosissimus) is a pelagic carnivore and facultative piscivore feeding on prawns, fish and insects (Odonata). Pescada is one of the major predatory species introduced by DNOCS since the 1940s into the Brazilian Northeast reservoirs to enhance artisanal fishery yields based on supplemental stocking of mostly exotic Cichlids, Cyprinids, and Prochilodontids. We performed stomach content analyses on individuals captured by gillnets (15-120-mm mesh sizes, adjacent knots) from three reservoirs in Pernambuco State (Poço da Cruz, 8.7-36.6 cm SL, n=27-53; Saco I, 11.5-43.0 cm SL, n=63; Tapacurá, 17.0-28.5 cm SL, n=16). Empty stomachs were frequent (<30%). Diets were variable and dominated by fish (11-63% occurrence) and prawns (2-38%), occasionally insects, copepods (<5%), mollusks and plants (<2%). To assess vulnerability of forage prey species to pescada we determined relationships (mostly linear) between pescada standard length, and horizontal and vertical gape openings, in relation to prey body height-lenght and length-weight relationships. Pescada mouth gape varies from 2 to 6 cm (12-43 cm SL). With body height ≤ 3 cm and length ≤ 6 cm SL, much littoral fusiform soft-body fish species are vulnerable prey, in predominantly unsheltered areas. These are planktivorous and insectivorous Characiforms, i.e. piaus (Leporinus spp.), piabas (Astyanax spp., Moenkhausia spp.) and sardinha (Triportheus sp.), plus juveniles of most species.

Characterizing diets of predatory fish species in Brazilian Northeast reservoirs and Lago Paranoá. R.A. Ribeiro & X. Lazzaro (Ribeiro's dissertation; ms. in prep.)

NE3 - Growth, mortality, and recruitment of omnivorous curimatã in Pão de Açúcar reservoir. X. Lazzaro

Assessing growth, mortality, and recruitment parameters of curimatã (Prochilodus cearensis) in Pão de Açúcar reservoir, from FiSAT software (FAO-ICLARM). X. Lazzaro (report; ms. in prep.)

NE4 - Interaction effects of omnivorous fish species on plankton, water quality, and extensive polyculture production. Z.M.P. Nunes & X. Lazzaro

Quantifying interactions between omnivorous tambaqui, curimatã pacu, Nile tilapia, and common carp on plankton and water quality of tropical ponds. Z.M.P. Nunes & X. Lazzaro (Nunes' PhD dissertation; ms. in prep.)

The knowledge of trophic interactions between omnivorous fishes on specific growth rates and water quality can be used to optimize polyculture productivity. We performed a factorial design experiment crossing the presence or absence of common carp (Cyprinus carpio) plus curimatã pacu (Prochilodus marggravii) with the presence of Nile tilapia (Oreochromis niloticus), tambaqui (Colossoma macropomum) or their absence. Our objectives were: (a) to evaluate the addition effects of carp and curimatã consortium on tilapia and tambaqui growths; (b) to compare the addition effects of tilapia or tambaqui on curimatã and carp growths; (c) to estimate the carrying capacity of experimental ponds; and (d) to determine the effects of tilapia and tambaqui, alone or combined with the consortium of curimatã and carp, on plankton communities and water quality. The experiment comprising 5 triplicated treatments was performed in fifteen 120-m2 ponds and lasted for 90 days. Neither supplementary food was supplied to fishes, nor water level was compensated for evaporation; only nutrients were added. Tambaqui growth and yield were reduced in presence of carp and curimatã (repeated-measures ANOVA, p = 0.014 and marginal p = 0.057, respectively). Tilapia and carp yields and growths were not affected by the presence of other fishes. Tilapia or tambaqui presence had no significant effect on curimatã growth. However, curimatã yield was higher in absence of tilapia (P = 0.040), revealing a trophic niche interference between tilapia and curimatã. Water in polyculture ponds was more turbid than in monoculture ponds (P < 0.05). Chlorophyll-a was marginally higher in polyculture ponds than in monoculture ponds (P = 0.07). Principal component analysis (PCA) revealed that fish biomass, mainly that of common carp, was responsible for 28% of overall data variance. Turbidity and chlorophyll-a increased with fish biomass, whereas dissolved oxygen contents decreased. Selective predation of tilapia on cladocerans, allowed rotifers and copepods to dominate within the zooplankton. Progressive pond enrichment during the culture was unveiled by an increase in orthophosphate concentration and in the Cyanophycea:Chlorophycea density ratio. At experiment completion, Cyanophycea participation in phytoplankton density was greater, as well as rotifers participation in total zooplankton density. Equation expressed in terms of initial biomass indicated that a maximum 582 kg.ha-1 fish yield over 90 days could be reached with an initial biomass of 75 kg.ha-1, i.e., a 2,320 kg.ha-1.yr-1 overall yield, extrapolated on a yearly basis.

Biomass interactions between omnivorous tambaqui and Nile tilapia: Effects on fish yield and plankton in tropical ponds. Z.M.P. Nunes & X. Lazzaro (Nunes' PhD dissertation; ms. in prep.)

Polyculture fish farming uses an appropriate combination of species and densities. This practice permits to efficiently exploit available resources by maximizing fish-fish and fish-environment synergisms, while minimizing antagonistic effects. To assess polyculture densities of tropical species we performed a factorial design experiment where three biomass levels, 0, 75 and 225 kg.ha-1, of Nile tilapia (Oreochromis niloticus) and tambaqui (Colossoma macropomum) were cross-classified to evaluate biomass and interaction effects on specific growth rates and water quality, as well as to establish pond carrying capacities. Our 95-day experiment combined eight replicated treatments, using sixteen 120-m2 earthen ponds, 0.80 m in depth, arranged in. Ponds were organically and chemically fertilized prior to the experiment during which fishes were not supplied with supplementary food. Tilapia growth increased with tilapia biomass (P < 0.05). At low initial tilapia biomass (TI75, 75 kg.ha-1), the presence and increase in tambaqui biomass (TI75+TA75, TI75+TA225; P < 0.001) favored tilapia growth. At high initial biomass (TI225+TA75), tilapia had low growth rate probably because ponds reached carrying capacity. Except for treatment TI225+TA75, tambaqui growth was reduced by its own increase in biomass from 0.16 down to 0.04 g.g-1.d-1 (P < 0.05). Tilapia presence (TI75) reduced even more tambaqui growth (P = 0.003). However we detected an interaction (P = 0.008), as tambaqui growth reduction due to tilapia presence was attenuated by tambaqui biomass increase. Unexpectedly, in treatment TI225+TA75 tambaqui growth at low initial biomass was enhanced by high tilapia biomass, as tambaqui probably controlled tilapia recruitment via predation on larvae and juveniles. As interpolation between total yield and initial tilapia and tambaqui biomasses, 75 kg.ha-1 of tambaqui plus 150 kg.ha-1 resulted in a 600 kg.ha-1 total fish yield within 3 months. High transparency plus low turbidity and chlorophyll that promoted the growth of filamentous algae at pond surface and bottom and increased pH characterized water in monoculture tambaqui ponds. Cyanobacteria dominated phytoplankton, whereas nauplii and calanoid copepods dominated zooplankton. Phytoplankton growth inhibited the development of filamentous algae in tilapia monoculture ponds that were characterized by the highest turbidity and chlorophyll values. As consequences, quickly utilized nutrients, particularly PO4-P, reached lowest concentrations (p = 0.046), whereas rotifers and copepods dominated zooplankton. The first four PCA components demonstrated that fish biomass explained 70% of water quality data variability.

Applicability of polyculture based on synergistic effects between omnivorous fish species in small Brazilian Northeast reservoirs. Z.M.P. Nunes & X. Lazzaro (Nunes' PhD dissertation; ms. in prep.)

Small (0.1-5 ha) reservoirs scattered within semi-arid Brazilian Northeast may be well fitted for semi-intensive fish production in polycultures. We thus performed a 90-day experiment, replicated in 0.5-ha ponds, to analyze the effects of a polyculture using four omnivorous fish species at two biomass levels on water quality and fish growth performance. Low and moderate biomasses (75 and 207 kg.ha-1) of Nile tilapia (Oreochromis niloticus, 63%), tambaqui (Colossoma macropomum, 31%), curimatã pacu (Prochilodus marggravii, 2%), and common carp (Cyprinus carpio, 2%) were combined in presence of piscivorous tucunaré (Cichla ocellaris, <2%) to control excessive tilapia recruitment. To mimic reservoir conditions, water lost by evaporation and infiltration was not replaced. In addition, to avoid hypereutrophication detrimental to the multi-usage of reservoirs, manure and fertilizers were only moderately used. High water temperature promoted rapid fish growths, taking advantage of fast nutrient uptake by phytoplankton. Physical, chemical and biological variables did not differ significantly between the two treatments (P > 0.05). Tambaqui and common carp displayed heterogeneous growths revealing food limitation even at low biomass level. The 75 kg.ha-1 biomass level promoted the best fish growth and production. Stocked at 1.8-1.9 kg.ha-1, tucunaré did not efficiently controlled tilapia fingerling recruitment.

NE5 - Characteristics of fish communities in Brazilian Northeast reservoirs

Fish communities in semi-arid Brazilian Northeast reservoirs: biological and ecological bases for fisheries and water quality management. X. Lazzaro (ed.), V.S. De Oliveira, Z.M.P. Nunes, L.T. De Sales (book in prep.)

Table of contents: Climatic and hydrological characteristics of semi-arid Brazilian Northeast region - General characteristics of studied reservoirs in Pernambuco State - Biology and ecology of fish communities in reservoirs - Allometric relationships of fish species - Fish trophic guilds and predator-prey relationships - Gillnets selectivity - Estimates of relative fish species abundance, biomass, and spatial heterogeneity from experimental catch plus acoustic survey - Pelagic trophic interactions between fish and plankton - Productivity assessments of fish and fisheries - Fisheries management recommendations for multi-use reservoirs - Polyculture model based on omnivorous fish species combination.