Evidence of habitat fragmentation affecting fish movement between the Patos and Mirim coastal lagoons in southern Brazil

Evidence of habitat fragmentation affecting fish movement between the Patos and Mirim coastal lagoons in southern Brazil

Marcelo D. M. Burns^I; Alexandre M. Garcia^II; João P. Vieira^I; Marlise A. Bemvenuti^I; David M. L. Motta Marques^II; Vinicius Condini^I

^IFundação Universidade Federal de Rio Grande, Depto. de Oceanografia, Laboratório de Ictiologia, CP 474, 96201-900 Rio Grande, RS,Brazil. e-mail: [e-mail] (MDB), [e-mail] (AMG), [e-mail] (JPV), [e-mail](MAB), [e-mail] (VC)

^IIUniversidade Federal do Rio Grande do Sul, Instituto de Pesquisas Hidráulicas, CP 15029, 91501-970 Porto Alegre, RS, [e-mail] (DMLMM) – send reprint requests to MDMB

ABSTRACT

The communication between the Patos and Mirim lagoon systems occurs via a natural channel called São Gonçalo. In 1977, a dam was built in this channel to prevent entrance of saline waters from Patos Lagoon estuary into the Mirim Lagoon. Our results showed an abrupt discontinuity in salinity and fish species distribution along the study sites. Sites below the dam showed salinity values higher than zero, whereas sites above had zero salinity values across all sampling periods. Marine and estuarine fishes (e.g., mullets Mugil platanus, M. curema, silversides Odontesthes argentinensis, Atherinella brasiliensis, sardine Brevoortia pectinata, and white croaker Micropogonias furnieri) were not captured above the dam. If these juvenile fishes could enter the Mirim lagoon in greater numbers, they probably would enhance catches in the artisanal fishery. We hypothesized that the São Gonçalo dam acts as a barrier hindering the entrance of salinity water and fishes inside the Mirim Lagoon.

Key words: dam, fish migration, artisanal fishery, estuary.

RESUMO

A comunicação entre o sistema lagunar Patos-Mirim ocorre através de um canal natural denominado São Gonçalo. Em 1977, foi construída uma eclusa com o objetivo de evitar a entrada deágua salgada, proveniente do estuário da Lagoa dos Patos, para o interior da Lagoa Mirim. Os resultados mostraram uma descontinuidade abrupta na distribuição da salinidade e dos peixes ao longo daárea de estudo. Estações de coleta abaixo da barragem tiveram valores de salinidade acima de zero, enquanto os valores foram iguais a zero nas estações acima da barragem durante o período amostrado. Peixes marinhos e estuarinos (p.ex., tainhas Mugil platanus, M. curema, peixes-rei Odontesthes argentinensis, Atherinella brasiliensis, sardinha Brevoortia pectinata) não foram capturados acima da barragem. Caso entrassem em abundância na Mirim tais espécies poderiam contribuir positivamente para os desembarques da pesca artesanal nessa região. Dessa forma,é sugerida a hipótese de que a barragem do São Gonçalo atua como uma barreira impedindo a entrada deáguas salobras e peixes marinhos e estuarinos para o interior da Lagoa Mirim.

Introduction

The Patos-Mirim lagoon complex has a diverse flora and fauna and has long supported fisheries, agriculture, industry, and municipalities. These lagoons are located in the coastal plain of Rio Grande do Sul state (southern Brazil), although part of the Mirim lagoon is located in Uruguay (Fig. 1a). Starting in the 60's, the Brazilian and Uruguayan governments implemented policies to promote economic development around the Mirim lagoon. Dams were established to allow agricultural and industrial activities. The most important of these was a dam built in the São Gonçalo Channel in 1977 to prevent the entrance of salinity waters inside the Mirim Lagoon (Fig. 1b) where freshwater is extracted for rice irrigation. Prior to the dam, intrusion of salinity waters and occurrence of marine and estuarine fishes (e.g., whitemouth croaker Micropogonias furnieri, mullet Mugil platanus, flounder Paralichthys orbignyanus), were common inside the Mirim Lagoon (Cunha, 1953; FAO, 1972; Machado, 1976).The effects of this salinity barrier on fish assemblages within Patos and Mirim lagoons have not been investigated.

Material and Methods

Patos (10,227 km²) and Mirim (3,750 km²) lagoons have a drainage basin of 263,876 km² (Seeliger et al., 1996). The lagoons are connected by a 75-km long, 200 to 500-m wide, 6-m deep natural channel (called São Gonçalo) that periodically changes flow direction (FAO, 1972) (Fig. 1a). Fishes were sampled at six stations (Fig. 1b) encompassing Patos Lagoon estuary (A1, A2), São Gonçalo Channel (B1, B2) and Mirim lagoon (C1, C2). From March 2004 to February 2005, five beach seine hauls were made monthly in each station. All captured fishes were preserved in 10% formalin and later identified, counted and measured to the nearest mm. Each month, water temperature, water transparency (Secchi disc), depth, salinity and dissolved oxygen were measured at each station.

Frequency of occurrence (FO%) was calculated as the ratio of the number of occurrences of a species divided by the total number of samples (x 100). Percent numerical abundance (PN%) was calculated as the total abundance (n) of a species, divided by the total abundance (N) of all species captured (x 100). Species showing FO% e" the average FO% in each site collection were considered frequent fishes, whereas those with FO%< the average FO% were considered rare fishes. A similar method was applied to the PN%, resulting in High Abundance (PN% e" average) and Low Abundance (PN%< average) categories. Finally, combining FO% and PN%, fishes were classified in terms of relative importance in four groups: 1. High abundance and Frequent (e"PN%, e"FO%), 2. High Abundance and Rare (e"PN%,<FO%), 3. Low abundance and Frequent (<PN%, e"FO%) and 4. Low abundance and Rare (<PN%,<FO%) (Garcia et al., 2004). Variations in abiotic factors (salinity, water temperature, water transparency, depth and dissolved oxygen) among sites were tested using one-way ANOVA (Zar, 1996).

Results and Discussion

There was an abrupt discontinuity in the fish species distribution along the study sites, especially between A1, A2, B1 and B2, C1, C2 (Table 1). Marine and estuarine fishes, such as mullets (Mugil platanus, M. curema), silversides (Odontesthes argentinensis, Atherinella brasiliensis), sardine (Brevoortia pectinata), and white croaker (Micropogonias furnieri), which regularly use or reside year-round at the estuarine area (A1, A2) (Chao et al. 1985) were not captured inside the Mirim Lagoon (C1, C2). Although these species occurred from the estuary (A1) to the São Gonçalo Channel below the dam (B1), they were absent from stations located above the dam. Mullets, silversides, sardine and white croaker have high commercial value and comprise important items of the artisanal fishery in the lower reaches of Patos Lagoon (Reis et al., 1994). The most parsimonious explanation for this marked difference in spatial distributions seems to be the occurrence of a dam between B1 and B2.

The dam acts as a barrier hindering the entrance of salinity water inside the Mirim Lagoon (Fig. 2). Among the physciochemical factors examined here, salinity had the greatest differences between sites above and below the dam (F= 93.1, p< 0.0001). Sites A1, A2, and B1 were the only locations showing values higher than zero, whereas B2, C1, C2 had zero salinity values across all sampling periods. In fact, salinity was the only abiotic factor showing a discontinuous spatial distribution that coincides with the one described for the fish fauna (Fig. 2).

To our knowledge, the present results provide the first concrete evidence of habitat fragmentation in the upper reaches of the Patos Lagoon estuary. This fragmentation did not allow that estuarine zone extends towards the Mirim Lagoon. If valuable commercial fishes were able to enter the Mirim lagoon in high numbers as juvenile and/or sub-adults, they could augment artisanal fishery catches in this area. In fact, before the dam construction, historical records show that marine and estuarine species were important components of fishery landings in Mirim lagoon (Machado, 1976).

Studies elsewhere (Holmquist et al., 1998; Agostinho et al., 2002; Carolsfield et al., 2004) have shown that dams can alter habitat and fish assemblage structure and disrupt links among adjacent aquatic systems such as marine, estuarine and freshwater habitats. Future studies, with a higher sampling effort along the São Gonçalo Channel, should evaluate the degree to which loss of connectivity between the two lagoon systems affects not only the fisheries, but also ecological processes in this region.

Acknowledgements

We thank numerous colleagues who assisted in the field, Luiz R. Malabarba for help with species identification, Kirk O. Winemiller for reviewing a first draft and the CNPq for providing a post-doc fellowship for AMG.

Literature Cited

Received April 2005

Accepted November 2005

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Publication Dates

• Publication in this collection
  13 Dec 2007
• Date of issue
  Mar 2006

History

• Received
  Apr 2005
• Accepted
  Nov 2005