Effects of Shrimp Trawling Upon Macrobenthic Fauna in Patos Lagoon Estuary
INTRODUCTION
Over the last 25 years there has been an increasing interest in the effects of the fishing activity, not only for target species, but also for non-target ones. Traditionally, research on the impact of bottom trawling has focused on target species (
, 1999). The study of its consequences upon the benthic fauna started with an analysis of organisms captured by fishing gear (e.g. , 1955). However, for the development of sustainable fishing activities, the effects of fishing upon the ecosystem must be recognized as a whole ( , 1997).
That requires, among other things, studies on the impact of the physical disturbance by fishing upon the structure of the benthic communities, including species that are not captured by fishing gear. As emphasized by (1997), the ideal would be that such studies involved experimental approaches. In that sense, several field experiments have been conducted in the northern hemisphere in order to investigate the effects of different kinds of fisheries using dredges and trawling nets
( and , 1992; , 1995; and
1996; , 1998; , 2000;
, 2000; , 2000).
A great part of the work developed by those authors shows that trawling can modify substrate and habitat complexity, thus bringing injury to the benthic species. However, in a literature review, it was found that the great majority of studies were conducted considering only the effects of deep and mid-depth oceanic commercial fishing, not dealing with areas of great ecological importance such as estuaries.
Estuarine areas, such as southern Patos Lagoon, are of great ecological importance as nursery, feeding and refuge areas for many species of fish and invertebrates with ecological and economic relevance , 1989). In spite of the prohibition of bottom fishing gear in Patos Lagoon, this activity is commonly observed in the estuarine area during periods of
shrimp harvesting (D , 1991, , 1991).
According to (1991), trawling can destroy the shrimp habitat and damage small shrimps.
Fishing of the shrimp (
, 1967) in the estuarine region of Patos Lagoon takes place during the summer, particularly in the months of January
and April ( , 1991; , 1991), when
capture and trawling are highly intense, which may cause an acute impact upon the ecosystem. Also during the summer, an intensification takes place in the reproduction and recruiting processes of most benthic macroinvertebrates in the region 1997b), and eventual disturbances during this period could cause serious reflexes upon the structure of the benthic associations.
In spite of the potential ecological implications of the shrimp trawling activity upon bottom organisms, no study has yet been developed in order to identify and characterize eventual disturbances of such activity upon soft-bottom benthic associations in estuarine inlets in Patos Lagoon.
The present study aims to evaluate the effects of two different shrimp artisanal trawling nets (coca and berimbau nets) upon the soft-bottom macrobenthic community, through a field experiment conducted in the estuarine region of Patos Lagoon.
Lagoon is a huge, 10,360 km lagoon, with a length of 279 km, and maximum width of 57 km. It presents a southbound reduction, where the last cell, accounting for about 10% of its body, shows estuarine features (Figure 1). Local tidal amplitudes are less then 1m. Larger fluctuations in water level and salinity are associated with less predictable changes in meteorological conditions, such as rainfall intensity and wind direction (KLEIN, 1997). Winter and early spring are usually the rainy season.
The two main estuarine environments are constituted by the open, deep central body and shallow, protected coastal inlets (ASMUS, 1997). Such protected, shallow (less than 1.5m- deep) environments present higher benthic macrofauna abundance and diversity than the central portion of the estuarine region (BEMVENUTI, 1997a).
The present study was developed in the largest estuarine inlet in Patos Lagoon, called Arraial Bay where small-scale trawling is quite common for the shrimp harvest. Arraial is a protected shallow bay, with a mean depth of 0.5 m ( , 1996). Its cating a HALL-SPENCER
GRAHAM
COLLIE
COLLIE
BERGMAN HUP TRUSH CURRIE
PARRY, TUCK LINDEGARTH
SANCHEZ SMITH
(DAY
´INCAO VALENTINI,
PÉ REZ
FARFANTE
D´INCAO VALENTINI
(BEMVENUTI,
BONILHA
et al.
et al.
et al.
et al.
et al. et al.
et al. et al.
et al.
et al.
Farfantepenaeus paulensis
et al.
D
Moderate dynamics 1994). It is considered as a nursery,
´INCAO
(MAZO,
METHODS Study Area
Patos 2
sediment is mainly composed by fine sand, indi
L. G. Angonesi† and C. E. Bemvenuti†
† Departamento de Oceanografia.
Laboratório de Ecologia de Invertebrados Bentônicos Fundação Universidade Federal do Rio Grande.
CEP 96201-900, Brasil luangonesi@hotmail.com
ANGONESI, L. G and BEMVENUTI, C. E., 2006. Effects of shrimp trawling upon macrobenthic fauna in Patos Lagoon Estuary. , SI 39 (Proceedings of the 8th International Coastal Symposium), 1368 - 1372. Itajaí, SC, Brazil, ISSN 0749-0208.
The present study aims at assessing the effects of two different shrimp artisanal trawling nets (Coca and Berimbau nets) upon the soft-bottom macrobenthic community, through a field experiment conducted in the estuarine region of Patos Lagoon. The experiment was performed with two treatments (Coca and Berimbau net trawling) and a control, where no trawling was performed. Six replicates were conducted in each treatment and the control area.
Immediately after trawling biological samples were taken from the first 5 cm of sediment. Statistical analysis evidenced harmful effects of both trawling nets on the estuarine gastropod , but did not detect harmful effects of trawling nets upon the benthic community structure. However, a clear trend toward a decrease in descriptive parameters of the benthic community submitted to both treatments was observed relative to the control, and the MDS plot showed a higher similarity between samples collected in the control as contrasted to the treatment areas. These results can be explained by the high recruitment of dominant peracarid species, which is reflected by an expressive variance, or by the natural resistance to disturbances verified in estuarine organisms.
Journal of Coastal Research
Heleobia australis
Artisanal fishing, environmental disturbance, soft-bottom.
ADDITIONAL INDEX WORDS:
ABSTRACT
feeding and refuge area for several species of fish and invertebrates with economical and ecological importance in the
region (G , 1996; G and 1997;
and 1998). The experiment was
conducted in a non-vegetated muddy bottom next to the Pombas Island in Arraial Bay (Figure 1).
The experiment was performed with two treatments: 1) Coca net trawling; 2) Berimbau net trawling; and a control, where no trawling was performed. Trawling was conducted by foot, without help from any vessel. Coca net was towed by 2 people and Berimbau by only one person. Six replicates were performed in each treatment and the control area. Each replicate covered areas with 4 x 8 meters, which were randomly distributed near Pombas Island (Figure 1). A total of five trawling runs in each replicate was defined by a previous pilot study. Immediately after trawling biological samples were taken from the first 5 cm of sediment in both treatments and control areas, with a 10-cm-wide PVC tube extractor (0.008m ).
The collected material was sieved at the field with a 0.3-mm nylon mesh to retain the macrofauna, and the captured organisms were fixed in 10% formalin in environment water.
In the laboratory the biological material was stained with the vital stain Bengal rose, sorted, identified to the least possible taxon, and preserved in 70% ethanol. Species richness and total density (expressed in number of individuals/0.008m ) were determined for each species in each biological sample.
carried out on log-transformed data, log (x+1), and Bray- Curtis index. Statistical analysis was performed using the software Primer 5.0 computer package and Statistica 98 edition.
In the first 5 centimeters of sediment, a total of 11 benthic macroinvertebrate species were found. Out of such total, 9 species were registered in the control area, 10 in the Coca net treatment and 11 in the Berimbau net treatment (Table 1). The specific composition was dominated by the infaunal species (Tanaidacean) and the epifaunal
species (Pelecipoda) and
(Isopoda).
The highest mean densities were found in the control (490 ind/0.008m ), decreasing to 406 ind/0.008m in the Berimbau trawling net and 344 ind/0.008m in the treatment with the Coca trawling net (Table 1) A decrease can be observed in the mean density of the treatments in relation to the contro (Figure 2a), but no significant statistical difference was found. The Analysis of
Diversity indexes (H´Shannon) changed little (Table 1), as similar values were found among both Coca (H=2.23) and Berimbau (H=2.19) treatments and the control (H=2.31).
However, there was a tendency to a decrease in this variable's values on both treatments relative to the control, as illustrated Virtually identical for all treatments (0.8), being slightly higher in the control (0.84) (Figure 2c).
ARCIA et al. ARCIA
Kalliapseudes schubartii
Erodona mactroides Munna peterseni VIEIRA,
KAPUSTA BEMVENUTI,
Field Experiment
Data Analysis
RESULTS
2
2
2 2
2
For each biological sample, species richness (expressed in number of species), density, abundance of individuals, the Shannon-Wiener (H') (log ) diversity index and the Pielou evenness index (J) were determined. The species densities and
total densities were compared through a 1-way Analysis of Variance. Normality and heterogeneity of variances were tested by using Kolmogorov-Smirnov and Levene´s test, respectively.
The multivariate statistical analysis (MDS-plot and ANOSIM) was
e
.
variance only detected significant harmful effects from both trawling nets on the density of the estuarine gastropod
both adults and juveniles (p<0.05).
by Figure 2b. On the other hand, the evenness indexes were Heleobia australis,
Figure 1. The Patos Lagoon estuarine region. The study site in detail. The symbol ( ) represents the experiment location.*
Journal of Coastal Research Special Issue 39, 2006
Despite a tendency of depreciation to some benthic community descriptors represented in Figure 2, the MDS plot did not show well-defined clusters between samples performed in each treatment and through ANOSIM, the non-clustering between treatments and Control was confirmed (Global R=0.095). However, it can be seen that 5 out of 6 control samples cluster to some extent in the center of the plot (Figure 3).
A trend toward a decrease in the descriptive parameters of the benthic community submitted to the treatments with trawling nets is observed relative to the control (Figure 2). However, through univariate and multivariate statistical analyses, no significant differences were detected among these parameters.
That only happened in the population level for the estuarine gastropod . These results can be discussed based on three main suppositions.
The first one relates to the high recruitment of peracarid
species (mainly and
) which reflected in an expressive variance influenced by a strong gregarism found in this group. The great majority of the benthic macroinvertebrates present in the region breed during the summer. This experiment, conducted in the
summer, may have been influenced by possible confounding recruitment influences. These three peracarid dominant species have a marsupial pouch, and their offspring soon after hatching stay in the same tube as their progenitor ( , personal observation) or in little superficial tubes located very near the
progenitor (as ) ( , 1987). Because of
this pattern, variance was high and no difference in treatments was found. Additional replicates would have increased the power of analysis to allow detection of significant differences with small or medium effect, as discussed by and (1997). Therefore, our failure to accept the alternative hypothesis should not be taken as an acceptance of the null hypothesis. Rather, we consider the ANOVA results to be inconclusive. Greater replication might well have allowed us to reject the null hypothesis (p<0.05).
The second supposition is in fact that estuarine organisms are naturally under frequently stressful conditions (salinity, temperature, sediment deposition, predation, among others). In typical estuarine areas, environmental variable fluctuations are so broad to the point of establishing a limit on the number of species that might colonize such ecosystems ( , 1989;
., 1989).
Even more rigorous conditions take place in the estuarine region of the Patos Lagoon, with its choked-lagoon features ( , 1994) where wind action, fluvial precipitation and its long and narrow mouth determine an intense hydrodinamics, substrate instability and do not allow for the formation of stable salinity gradients, thus resulting in a low benthic macrofauna diversity ( , 1992). Space- time monitoring of the benthic macrofauna in the estuarine region of the Patos Lagoon has revealed low species richness with a specific composition dominated by organisms of estuarine origin ( , 1997a), such as the ones found in the present study. In order to survive under such conditions, animals living within this habitat need to be resistant, or to have mechanisms to resist to sediment resuspension (by winds, mainly in shallow bays), sedimentation and others variables.
The benthic fauna in the experiment location may have a natural
DISCUSSIONS
Heleobia australis
Kalliapseudes schubartii, Munna petreseni Sinelobus stanfordi
S. stanfordii K. schubartii
et al
sensu
et al.
BEMVENUTI
BROWN
WILSON
MCLUSKY
DAY
KJERFVE
BEMVENUTI
BEMVENUTI
Figure 2. Descriptive variables (Mean Density (A), Shannon diversity index (B) and Pielou evenness index (C)) in the two treatments and control area.
Table 1.Macrobenthic species composition and mean densities at each treatment. = Control; = Coca's net and = Berimbau's net. “J” symbols meaning juveniles.C K B
10.8 9.7 10.8
J 32.5 21.2 17.5
3.7 2.5 4.5
J 5.2 4.8 2.8
3.5 4.7 5.5
J 26.2 17.7 16.7
J 93.7 66.2 64
47.8 26.5 23.2
J 21.2 6.8 9.7
21.2 33.5 25.7
J 88 70.5 104.8
55.5 32.2 55.2
J 37.7 23 39.5
19.8 9.8 11
J 21 10.3 13.5
0 0 0.7
2 4 1
Gamaridea 0 0.3 0.2
TOTAL
Richness (species number) 9 10 11
Diversity (H'shannon) 2.31 2.23 2.19
Eveness (J Pielou) 0.84 0.8 0.8
C K B
POLYCHAETA
MOLLUSCA
CRUSTACEA
489.7 343.7 406.2 Heteromastus similis.
Heteromastus similis Laeonereis acuta Laeonereis acuta Nephtys fluviatilis Nephtys fluviatilis Erodona mactroides Heleobia australis Heleobia australis Kalliapaseudes schubartii Kalliapaseudes schubartii Munna peterseni Munna peterseni Sinelobus stanfordi Sinelobus stanfordi Dyastilis sympterigiae Kupellonura sp.
resistance to the “slight” disturbance caused by trawling nets.
A
y.
The magnitude with which species abundance and diversity can be affected by trawling depends on the duration, extension, and frequency of the physical disturbance, the kind of net used (configuration, weight), sediment characteristics, kind of substrate covering, time of year, and the intrinsic ability of the population or association for persisting or recolonizing after a disturbance has occurred ( , 1989; , 1992;
and , 1996; ., 1998). In the present study, the immediate effects generated by the different kinds of nets along with the fishing effort could not be detected in the benthic community structure by the methods used (third supposition). However, the MDS plot evidenced a higher similarity among samplings performed in the control area compared to samples subjected to treatment with trawling nets, which were more spread in the plot (though no statistical significance was found ANOSIM). According to
and (1993), this fact in itself may be indicative of a certain level of stress. These authors say that multivariate analyses, which explore variability of abundance in the same set of species and changes in species identities, show a clear increase in variability/heterogeneity among samples with increased levels of stress. They propose that an index could be constructed from the relative variability between impacted and control samples. We think that the development of techniques for early detection of disturbance-induced effects on organisms based in multivariate analysis could lead us to more conclusive results.
However, the Analysis of Variance evidenced harmful effects of both trawling nets on the estuarine gastropod , both adult and juveniles. This gastropod is an epifaunal species and presents a wide vertical distribution in muddy-sand bottoms in Patos Lagoon. This species is the most abundant in this region and the main component of crab and shrimp diet, which use the estuary as nursery and feeding area
( , 1997b).
trend may be observed toward a decrease in the descriptive parameters of the benthic community submitted to treatments with trawling nets in contrast to the control. However, no significant differences were detected through statistical analyses, except in the population level for the estuarine
gastropod is a highly important
species in Patos Lagoon, as it is the main component of crab and shrimp diet, which use the estuary as nursery and feeding area. It can be concluded that due to an expressive data variance, caused by the high recruitment of dominant peracarid species, no difference was found between treatments. Nevertheless we have to consider that animals living in estuarine unstable areas need to be resistant, or to have mechanisms to resist to natural
disturbances, and that the effort made in this experiment may have been too weak for the estuarine community structure under stud
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MUS,
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BEMVENUTI, BEMVENUTI,
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ODEBRECHT, CASTELLO,
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SEELIGER, ODEBRECHT, CASTELLO,
BERGMAN, HUP, BONILHA,
BROWN, WILSON,
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CURRIE, PARRY,
D´INCAO
DAY JR. HALL, KEMP, YÄ Ñ EZ- ARANCIBIA
GARCIA, VIEIRA, BEMVENUTI, GERALDI,
GARCIA, VIEIRA,
GRAHAM,
HALL-SPENCER, FROGLIA, ATKINSON, MOORE,
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Heleobia australis. H. australis
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