Fisheries Research 54 (2002) 349±353
Macrofauna spatial differences within clam dredge-tracks and their implications for short-term ®shing effect studies L. ChõÂcharoa,*, J. Regalaa, M. Gasparb, F. Alvesa, A. ChõÂcharoa a
b
Universidade do Algarve, UCTRA/CCMAR, Campus de Gambelas, P-8000 810 Faro, Portugal Instituto PortugueÃs de InvestigacËaÄo das Pescas e do Mar, CRIPSul, Av. 5 de Outubro, s/n, 8700 OlhaÄo, Portugal Received 16 August 2000; received in revised form 2 November 2000; accepted 24 November 2000
Abstract In situ observations of clam dredging showed that the effects of the dredge on the benthic macrofauna may not be constant during a tow. A sand buffer forms in front of the gear approximately 10 m after the beginning of a tow, and this pushes the sediment partially aside. In this study, we analyse differences in abundance, the number of taxa present, diversity, and evenness within sections of dredge-tracks in a disturbed, ®shed area and a non-®shed area along the southern coast of Portugal. These areas were sampled by divers before and after dredge-®shing activity. At each site, three dredge-tracks were produced. These tracks were divided in three longitudinal sections (start, middle and end) and two transverse sections (track and edge). Six quadrats were used to sample macrofauna in each section of every track and edge. Our results show differences exist in macrofaunal distribution and abundance across sections of a dredge-track. These differences should be considered in any assessment of the short-term ecological impact of dredges on benthic macrofauna. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Fishing impact; Dredge; Macrofauna; Clam ®sheries
1. Introduction The short-term environmental effects of dredging on the sea-bottom have received increased attention in recent years, and several studies have characterised changes in the ecosystem due to dredge-®shing (e.g., Hall et al., 1990; Michael et al., 1990; Eleftheriou and Robertson, 1992; Kaiser and Spencer, 1996; Lambert and Goudreau, 1996). However, only a few authors, such as Lambert and Goudreau (1996) and Meyer et al. (1981), have considered the importance of analysing different sections of the dredge-track in ®shing impact studies. *
Corresponding author. Tel.: 351-289-800-900; fax: 351-289-818-353. E-mail address:
[email protected] (L. ChõÂcharo).
The most common approach in ®shing effects studies has been to perform random sampling along dredge-tracks (e.g., Nedcof and Caddy, 1971; Caddy, 1973; Michael et al., 1990). However, random sampling within a dredge-track can lead to biased data collection because, if the impact is not even throughout a track, results may be biased depending upon where samples are taken. Consequently, there is an increased heterogeneity of variance in results, as noted by Hall et al. (1990), Eleftheriou and Robertson (1992) and Pranovi et al. (2000), so the use of nonparametric, and frequently less powerful, statistical tests is necessary. Information on the effects of clam dredge-®sheries on benthos is scarce (Gaspar et al., 1998). In fact, few impacts of ®shing are well documented and biological impacts are particularly dif®cult to investigate because
0165-7836/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 2 7 2 - 7
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of the complexity of benthic communities and our limited knowledge of their natural variability (Gislason, 1994). Thus, there is the need to more adequately assess biological impacts (Currie and Parry, 1999). With the increase in Portuguese exploitation of clams (Spisula solida, Donax trunculus, Venus striatula, Pharus legumen and Ensis siliqua), there is also an increased need to understand the environmental effects of these ®sheries for appropriate ®sheries management. The aims of this study were to analyse differences in macrofauna within sections of a bivalve dredge-track in the southern coast of Portugal, to compare these differences between ®shed and a non-®shed areas, and to suggest the adequate sampling strategy to assess short-term dredging impact.
Fig. 1. Photograph of the dredge showing the sand buffer formed in front of the gear during towing.
2. Methods
front of the dredge mouth. This sand buffer pushes the sediment sideways, and limits the amount of material (both sediment and organisms) that enters the gear mouth.
2.1. Study site
2.3. Sampling strategy and laboratory procedure
The Algarve coast (south Portugal) extends from Cabo SaÄo Vicente in the west (88590 W), to the border with Spain in the east (78240 W). Two study areas were selected off the western Algarve coast. The area off Vilamoura is a S. solida ground and is heavily ®shed, whereas the presently non-®shed (control) area off Lagos was an E. siliqua ®shing ground, until it was exhausted in 1996. Both areas have similar physical characteristics in terms of coastline, bottom substratum (mostly sand) and direction of prevailing winds and currents (Dias, 1987).
During September 1999, both sampling areas were sampled by divers before and after dredge-®shing activity, within a depth range 7±10 m. At each site, three dredge-tracks were produced. At both areas, three 50 m dredge-tracks were created (approximately 1 min towing), following usual ®shing procedures. These tracks were divided in three longitudinal sections (start, before the formation of the sand buffer; middle, approximately 3 m after the formation of the sand buffer; and end, the last 3 m of the track), and two transverse sections (inside the track, and track-edge). Sediment from six quadrats (0:0625 m2 0:15 m depth) from each section of every track was collected by Scuba divers, before and after the ®shing disturbance. Samples were sieved in situ through 1-mm-mesh bags. The macrofauna collected were preserved on board in 70% ethanol. In the laboratory, organisms were identi®ed to species level, and counted.
2.2. Gear description Bivalve dredges used on the south coast of Algarve are large, heavy, iron structures, with a 25-mm-mesh net bag and a toothed lower bar to the mouth. The gear's mouth can be up to 150 cm wide and have teeth up to 50 cm long, which act as a rake when the dredge is dragged through the sediment (Gaspar et al., 1994). After a dredging tow, a clearly de®ned track can be found on the sea-bottom. The edges of this track are formed by a slope about 20 cm high. In situ and video observations of clam dredging show the presence of a sand buffer (Fig. 1), which forms approximately 10 m after the start of a tow, in
2.4. Data analysis Macrofaunal abundance was converted to abundance per square metre (A) and total taxa (T), Shannon±Wiener diversity (H) and evenness (E) indices (Legendre and Legendre, 1979) were calculated. The effect of transverse and longitudinal
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(®shed area) were these interactions also observed in macrofaunal abundance and the number of taxa present (Table 2). Application of the Tukey HSD test (Fig. 2) showed that signi®cant differences in macrofaunal abundance and number of species occurred between the dredgetrack and its lateral edges at the start of the track, at both Vilamoura and Lagos. At Lagos, this section of the track also showed signi®cant differences in the Shannon±Wiener diversity index of the track and edges. Macrofaunal community structure along the middle length of the dredge-track did not differ signi®cantly between the track and the edges at Vilamoura, but signi®cant differences in the diversity index of these sections were found at Lagos. For both areas, no signi®cant differences in any of the parameters analysed were observed between the track and the edges of the end of the dredge-track. Signi®cant differences between longitudinal sections of the dredge-track were found at the Vilamoura study area. In this ®shed area, signi®cant differences in diversity and evenness were found between the start and the middle, and between the middle and the end of the dredge-track. At Lagos, only evenness was signi®cantly different, and then only between the middle and end track sections.
dredge-track sections on H, E, A and T, in Lagos and Vilamoura was examined by eight conventional twoway ANOVA (F-test). As more than two comparisons were made in each, all analyses were followed by a means comparisons Tukey test to compare means between dredge-track sections and the results displayed on ®gures. Statistical analyses were conducted using the STATISTICA 5.0 program. 3. Results In both the ®shed and non-®shed study areas, differences in macrofaunal composition were observed following dredging. The most signi®cant reduction in diversity, abundance and the number of taxa present occurred at the Lagos study area (Table 1). Two-way ANOVA analysis showed signi®cant differences in diversity, evenness, abundance and the number of taxa present between track sections at the Lagos study area. Off Vilamoura, only macrofaunal evenness was signi®cantly different across longitudinal sections of the track. The results indicate interactions in macrofaunal diversity between longitudinal and transverse sections of the dredge-track at both the studied areas, though only off Vilamoura
Table 1 Mean and standard deviation of the Shannon±Wiener diversity index, evenness, abundance and number of taxa, before and after dredge disturbance, at both Lagos and Vilamoura sampling sites Lagos
Shannon±Wiener diversity index (H) Evenness (E) Abundance (A) Number of taxa (N)
Vilamoura
Before
After
Before
After
1:21 0:14 0:87 0:05 694 323 15 5
0:76 0:20 0:90 0:12 195 135 83
0:88 0:15 0:82 0:08 160 54 52
0:64 0:22 0:86 0:23 154 78 62
Table 2 Summary of p values of eight two-way ANOVA, ®xed effect for Shannon±Wiener diversity index (H) and evenness (E), abundance (A) and number of taxa (T), at both the Lagos and Vilamoura sampling sitesa Lagos
Vilamoura
H 1ÐTransverse sections 2ÐLongitudinal sections Interactions a *
E *
0.0088 0.1644 0.0023*
0.2667 0.0001* 0.2482
A
T *
0.0000 0.0024* 0.1421
*
0.0001 0.0458 0.0087*
H
E
A
T
0.4879 0.0449 0.0013*
0.3161 0.0001* 0.1753
0.2482 0.7954 0.0020*
0.1490 0.2744 0.0008*
1ÐTransverse sections of the dredge-track: edge and track, 2Ðlongitudinal sections of the dredge-track: start, middle and end. p < 0:05.
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Fig. 2. Arrows show signi®cant differences among sections of the area affected by the dredge (track and edges), as determined by Tukey HSD test, for the population parameters analysed: Shannon±Wiener diversity index (H); evenness (E); abundance (A); number of taxa (T).
Differences between longitudinal sections of the lateral edges of the dredge-track were also observed. In both study areas, these differences occurred only between the start and middle sections of the edges. Similarly, in both ®shed and non-®shed areas, the number of taxa present varied signi®cantly between these two sections. Furthermore, differences in the diversity index were found at Lagos, and macrofaunal abundance differed signi®cantly between these two sections of track edge at Vilamoura. 4. Discussion As expected, macrofaunal composition changed following dredging in both the ®shed and non-®shed areas (e.g., Bergman and Hup, 1992; Kaiser et al., 1998). Therefore, changes within track sections can be directly related to the ®shing impact. This was more evident at the Lagos site, where abundance, number of taxa, diversity, and evenness of the macrofauna were all higher before ®shing. In this non-®shed area, more differences also occurred between track sections, and ®shing was most disruptive in this more stable ecosystem.
In contrast, at the Vilamoura site, macrofaunal composition must be adapted to the long-standing ®shing impact to which it has been exposed and fewer changes were caused by dredging. This would explain why, in the ®shed area, only the evenness of the macrofaunal community was signi®cantly different between dredge-track sections, whereas in the non-®shed area, signi®cant differences between track sections were observed for abundance, number of taxa, diversity, and evenness. Therefore, greater caution is needed when sampling the macrofauna of dredge-tracks in non-®shed areas, which are commonly used as control areas during studies. If ®shing activity may explain overall differences in macrofaunal composition between tracks at the ®shed and non-®shed areas, differences among track sections may also result from the gear behaviour, as also noted by Eleftheriou and Robertson (1992) and Meyer et al. (1981). Our observations showed that approximately 10 m after the start of dredging, a sand buffer forms in front of the gear. This results from the clogging of the mesh. After formation of this buffer, sediment and organisms are ``mixed'', and then pushed downwards and sideways away from the gear. This explains the
L. ChõÂcharo et al. / Fisheries Research 54 (2002) 349±353
signi®cant differences between the start and middle sections of the track, and between the track and its edges. However, after formation of the sand buffer and the subsequent ``mixing'', the sediment and organisms pushed along the track or to the edges become more homogeneous, and fewer differences can be observed in macrofauna composition between track sections. 5. Conclusions Thus, the decision where to sample within a dredgetrack will depend on the objective of a study. Speci®c analysis of the effects of the gear on the ecosystem (e.g., the ®shing impact on a particular taxa or sediment type) can be most clearly determined if sampling is conducted at the start of the dredge-track, before the sand buffer forms. However, more general analysis of the impact of ®shing should also consider changes caused by the formation of a sand buffer, which affects the larger part of a dredge-track. Therefore, we suggest that for ®shing impact studies, the behaviour of the gear should be assessed before sampling, and sampling design must consider eventual differences between longitudinal and transverse sections of the area affected by the gear. Acknowledgements This study was supported by the FAIR ECODREDGE (PL-4465) project. Thanks are also due to Rita Constantino for her help with laboratory processing of samples and to IPIMAR for logistical support. We are also grateful to two anonymous reviewers for their criticism and comments. References Bergman, M., Hup, M., 1992. Direct effects of beam-trawling on macrofauna in a sandy sediment in the southern North Sea. ICES J. Mar. Sci. 49, 5±11.
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