Abstract
The impact on the macrobenthic community of different types of rake, manual rake (MR), hydraulic rake (HR), and conveyor rake (CR), was studied on subtidal flats in the Sacca di Goro. Two sets of experiments, on sandy and muddy bottoms, were undertaken in summer 2004: the experimental protocol required that six parallel waylines for both MR and HR were raked by a shellfish farmer; areas between parallel waylines were considered as controls. Due to operative constraints, the impact of CR was assessed on an 800‐m distant, deeper sand flat. Sampling design followed a two‐way ANOVA layout with treatments and time as factors. Changes in the benthic community were apparent in the raked plots immediately after treatments, but recolonization by small infaunal species was relatively rapid. While the effects of MR and HR were comparable, CR had a greater deleterious effect on the macrofaunal community. In order to discourage the use of CR, clam fishermen should be allowed to equip their boats with HR gears identical to that used in this study.
Introduction
In Mediterranean coastal systems, and particularly in Italy, shellfish fishery has expanded significantly in the last decades, thus inducing concern for its potential environmental impact. Harvesting of soft‐sediment target species (e.g. clams, Chamelea gallina, Tapes decussatus and Ruditapes philippinarum) leads to the physical disturbance of the substratum. The Manila clam, R. philippinarum, is actively reared in northern Adriatic lagoons from Marano to Ravenna, and very large populations have developed, with an average yield of 20,000 tonnes yr−1 (Solidoro et al. Citation2000). In the Po River deltaic lagoons and channels (Northern Adriatic Sea, Italy), clam harvesting is carried out in very shallow waters (<2 m), in licensed grounds several times during the year, and thus environmental disturbance occurs from three to four times a year on the same ground. Clams are gathered using a manual sediment rake, with the clams being retained in a 2‐cm‐mesh net attached to it. This practice actually represents the only authorized harvesting practice in the whole area, being hydraulic dredging forbidden since the mid‐1980s. Conversely, other types of harvesting practice are allowed elsewhere, such as the conveyor rake and the mechanical harvesting. The former practice is carried out with a bottom‐cleaner barge. A belt conveyor, carrying a rake at one end, is lowered to the bottom while the barge moves slowly forward, thus collecting all material present in the upper 8–12 cm of sediment; the belt conveyor then dumps the whole catch into a mechanized sieve for sorting. The mechanical harvesting is carried out using small boats equipped with 25 HP engines positioned outboards amidship; the fishing grounds are shallow water areas, where the propeller can reach the bottom, resuspending the sediment and the clams, which are then collected inside a following net. Recently, a new rake design (the hydraulic rake, HR, figure ) was requested to be introduced into Northern Adriatic fishery. The hydraulic rake consists of a cage of steel bars in which the clams are collected. The mouth of the gear (45‐cm wide) is provided with a blade to cut sediment and a manifold of jets (figure ) from which water is expelled under pressure to fluidize the sediment. A pipe supplies seawater to the jets from a pump aboard the boat. All these gears (hand rake, hydraulic rake, tractor dredge, mechanical harvesting) are designed to dig clams out of the sediment, resuspending the sediment and its biota, thus disturbing the benthic habitat, both in terms of its physical structure and its biological communities.
Figure 1 The hydraulic rake. In the foreground a fisherman shows how to rake clams; in the back the motor pump that, when connected trough the rubber hose to the rake, is used to expel water through the rake jets.
Figure 2 The hydraulic rake: particular of jets from which water is expelled under pressure.
The short‐term effects of fishing gears for harvesting molluscs on non‐target species and ecosystem structure have received increased attention in recent years. Impacts will depend on many factors such as marine sediment characteristics, type of gear, fishing operations, depth, tidal strength and currents (Hall et al. Citation1990; Eleftheriou & Robertson Citation1992; Kaiser & Spencer Citation1996; Kaiser et al. Citation1998; Spencer et al. Citation1998; Gaspar et al. Citation2003; Falcão et al. Citation2003; Mistri et al. Citation2004). Until now, however, few studies have been conducted on the impact of rakes (Kaiser et al. Citation2001; Pranovi et al. Citation2001, Citation2003, Citation2004; Orel et al. Citation2002; Fiordelmondo et al. Citation2003). Raking activity digs a furrow along a towpath provoking a sediment movement that occurs mainly through slumping of the sides of the furrow. Subsequently, the furrow is filled in by fine particles transported by tidal currents acting as a sediment trap. Clam harvesting by hand‐raking could add to the general disturbance regime in the environment through damage to the benthic community. Kaiser et al. (Citation2001), examining the effects of cockle (Cerastoderma edule) fishery on non‐target macrobenthic species, evidenced community changes two weeks after the initial disturbance. Furthermore, Fiordelmondo et al. (Citation2003) showed that clam hand‐raking has an impact on the benthic microbial loop. Both practices are allowed in many Adriatic fishing grounds and are used illegally in others; the direct ecological consequences of clam harvesting through hydraulic or conveyor‐raking are largely unknown, but the resuspension of fine particles has increased turbidity in the lagoon of Venice (Sfriso & Marcomini Citation1996; Sfriso et al. Citation2005). The objectives of this study are to determine the impact of the use of the manual rake, the hydraulic rake and the conveyor rake to harvest clams in the Northern Adriatic on the benthic community, and to assess short‐term community recovery time following raking.
Materials and methods
The impact of different types of rake, manual (MR), hydraulic (HR), and conveyor (CR), was studied on subtidal flats in the Sacca di Goro, in the southernmost area of the Po River delta (north‐western Adriatic Sea). In this environment, two distinct seabed types are present: sandy bottoms in the southern area, in a well‐vivified area facing the Sacca sea‐mouths, and muddy bottoms in the less‐vivified, central–eastern part of the basin. Analyses of the infauna sampled in these two habitats indicated that the macrobenthic communities were different (Mistri et al. Citation2001a): hence, we examined the effects of raking separately for each sector. Two sets of experiments were undertaken starting from mid‐June and late July 2004.
Experiment 1: Effects on muddy bottoms
On a subtidal mud flat (depth at MLWS: about 0.3 m), the effects of the manual (MR) and the hydraulic (HR) rake were assessed on two adjacent 10×50 m plots, not disturbed by clam harvesting practice. The experimental protocol required that six, parallel waylines for both MR and HR were raked by a shellfish farmer into each plot. Both gears had a 45‐cm‐wide mouth, and were provided with a 100‐mm‐long blade (HR) or 100‐mm‐long teeth (MR). Each wayline was 10 m long, and waylines were about 5 m apart from each other. The position of waylines was indicated by driving numbered wooden poles into the bottom, one at the start and one at the end of each wayline, with a rope tightened between them. Subsequent sampling was scheduled on a ×3 geometric scale, i.e. at 0, 3, 9 and 27 days from the beginning of the experiment. At each collecting time the macrofaunal community was sampled by taking, haphazardly from the furrow of each wayline, one grab of sediment with a Van Veen grab (area: 0.027 m2). The sampled position in each furrow was marked by placing a wooden pole into the grab hole, to avoid the risk of re‐sampling the same area. The areas comprised between parallel waylines, and thus not raked, were considered as controls and, at each sampling date, four haphazardly chosen control areas were sampled following the same procedure used for sampling furrows.
Experiment 2: Effects on sandy bottoms
On a subtidal sand flat (depth at MLWS: about 0.2 m), the effects of the manual (MR), and the hydraulic (HR) rake were assessed. For MR and HR the experimental protocol was the same as that used in the mud flat. Due to its draught and operative range, the impact of the conveyor rake (CR) was assessed on another subtidal sand flat (depth at MLWS: about 1.8 m), some 800 m from the previous one. For CR, trivially, waylines were different: 1.20 m wide and 70 m long. Because of economical constraints, the number of waylines for the experiment on sand were halved; hence we had three waylines each for MR and HR at sandy site 1 (with two control areas between furrows), and three waylines for CR at sandy site 2 (again with two control areas between furrows).
Data treatment
At each sampling date, the contents of the grab were gently washed on a 0.5 mm sieve. Material retained on the sieve was fixed in 8% buffered formalin, and stained with rose bengal to facilitate sorting and identification. The abundance of individuals of each taxon, identified at the species level when possible, was measured for each sample. The following univariate measures were determined for treatments (MR, HR, CR, and control (C)): the total number of species (S), the total number of individuals (N), α‐diversity (as Shannon–Wiener's H′), and evenness (as Pielou's J′). Sampling design followed a two‐way ANOVA layout with treatments and time (days from the beginning of the experiment) as fixed factors. Prior to analysis, data were checked for homoscedasticity (Levene's test), and logarithmic transformation was used whenever necessary (Underwood Citation1997). The power of each performed ANOVA was assessed a posteriori through the calculation of the non‐centrality parameter φ (Sokal & Rohlf Citation1995a), and Pearson–Hartley charts (Sokal & Rohlf Citation1995b), under α = 0.05. To establish which taxa contributed most to either the similarity or dissimilarity between groupings of data, the similarity percentage analysis (SIMPER) routine was carried out: the contribution of each species to the Bray–Curtis measure was calculated after square‐root transformation, and the species ranked in order of their contribution to separating each group (Clarke & Warwick Citation1994).
Results
Experiment 1: Effects on muddy bottoms
In table , ANOVA results for the effects of raking on community parameters are shown, together with statistical power. On muddy bottoms, significant differences were found only for the factor treatment, while the factor time and the interaction term were always not statistically significant. A significant reduction between control and raked areas was observed for the total number of species (Tukey HSD test; S: C vs MR, P = 0.002; C vs HR, P = 0.04). The benthic community at the HR treatment showed a significantly higher evenness (J′: C vs HR, P = 0.01), while a difference in α‐diversity resulted between the two raked plots (H′: MR vs HR, P = 0.04), with values significantly higher at HR. Similarity within groups (SIMPER analysis) was very high (C: 79%; MR: 65.8%; HR: 63.8%), while, between groups, highest dissimilarities were observed between C and HR (34.1%), and MR and HR (34.6%). The list of species contributing to dissimilarity (cut off: 70%) is shown in table .
Table I. Two‐way ANOVA for treatments (MR, HR, C) and time relative to macrofaunal community descriptors at the muddy site (T×T: interaction term).
Table II. Abundance (ind m−2) for control (C) and treatments (MR, HR) at time 0, and similarity percentage (SIMPER) analysis at the muddy site (cut‐off: 70%). MR, manual rake; HR, hydraulic rake; C, control.
Experiment 2: Effects on sandy bottoms
A one‐way ANOVA on values of S, N, and H′ at the control sites at time 0 showed that there were slight but significant differences (all P<0.05) in community structure at the two, 800 m‐distant apart, sandy control sites, so data were analysed separately.
In table , ANOVA results for the effects of MR and HR on community parameters are shown, together with statistical power. Both factors, treatment and time, were significant (except for S and N, where the factor time was not significant), as well as the interaction term, thus suggesting a different response of benthic communities living in sandy areas to sediment disturbance. Hence, the community at the HR treatment showed parameter values which were closer to the control (S: C vs MR, P = 0.049; MR vs HR, P = 0.003; N: C vs MR, P = 0.001; MR vs HR, P = 0.001; J′: MR vs HR, P = 0.001; H′: C vs MR, P = 0.009; C vs HR, P = 0.02; MR vs HR, P = 0.001). Similarity within groups was high (C: 58.9%; MR: 71.2%; HR: 78.38%). Highest dissimilarity was observed between C and MR (43%); table shows species contribution to such dissimilarities.
Table III. Two‐way ANOVA for treatments (MR, HR, C) and time relative to macrofaunal community descriptors at the shallow sandy site (T×T: interaction term).
Table IV. Abundance (ind m−2) for control (C) and treatments (MR, HR) at time 0, and similarity percentage (SIMPER) analysis at the shallow sandy site (cut‐off: 70%). C, control; MR, manual rake; HR, hydraulic rake.
table shows ANOVA results for the CR treatment site. Except for S and N (but the statistical power was so low that the risk of Type II error was real), the factors treatment and time were significant as well as the interaction term. The community parameter values were always lower in the treated areas with respect to the control, suggesting a higher and time‐prolonged impact of the conveyor rake on the benthic community (e.g. H′: C0 vs CR0, P = 0.0003; C3 vs CR3, P = 0.001; CR0 vs CR27, P = 0.0002; CR3 vs CR27, P = 0.0002). SIMPER analysis showed high similarity within groups (C: 63.9%; CR: 66.1%), while an average dissimilarity of 37.2% was observed between them (table ).
Table V. Two‐way ANOVA for treatments (MR, HR, C) and time relative to macrofaunal community descriptors at the deeper sandy site (T×T: interaction term).
Table VI. Abundance (ind m−2) for control (C) and treatment (CR) at time 0, and similarity percentage (SIMPER) analysis at the deeper sandy site (cut‐off: 70%). C, control; CR, conveyor rake.
Discussion
Field‐based ecological studies on processes such as disturbance/recovery can be used to make predictions of the potential environmental effects of fisheries. Together with intensity and frequency, the gear used in a particular fishery also needs to be considered in developing hypotheses about potential fishing impacts. This study considered three different gears: manual, hydraulic, and conveyor rakes, used in Italian shellfish fishery. Our experiment demonstrated that macrobenthic community structure in raked areas differed from unraked areas, but the impact of different types of rake was different.
Benthic communities that utilize a particular habitat have adapted to their environment through natural selection, and the impact of fishing gears on the habitat structure and biological community can be scaled against the magnitude and frequency of bottom disturbance due to natural causes (De Alteris et al. Citation1999). For instance, Currie & Parry (Citation1996), studying the impacts of scallop dredging on soft sediment communities, found that reductions in faunal density caused by dredging were small when compared with annual changes in population density. Kaiser et al. (Citation1998) found that immediately after fishing the composition of the community in stable sediments was significantly altered, while in mobile sediments the effects of fishing were not detectable. By contrast, Pranovi & Giovanardi (Citation1994) found that hydraulic dredging produced considerable long‐term negative effects on the bottom environment of Venetian lagoon, hypothesizing that the slow recovery of the infaunal community was related to the low energy conditions of the lagoon environment.
The timing of our experiment was chosen to coincide with a period of calm weather to allow accurate sample collection and raking. Although the effects of a single passage of a rake may be relatively limited, chronic raking disturbance may produce long‐term changes in benthic communities (Jennings & Kaiser Citation1998). Human‐induced disturbance (like clam raking) may be characterized as either short‐time (“pulse”) or continuous (“press”) (Glasby & Underwood Citation1996). Manipulative experiments testing for effects of fishing gears may mimic either types of disturbances: some studies monitor effects of a single disturbance event (e.g. Hall & Harding Citation1997; Kaiser et al. Citation1998; Gaspar et al. Citation2003; this study), others quantify cumulative effects of repeated disturbances (e.g. Eleftheriou & Robertson Citation1992; Kaiser & Spencer Citation1996; Tuck et al. Citation1998). Information based on “press”‐type experiments is particularly useful for long‐term management of soft‐sediment ecosystems. Conclusions from “pulse” disturbances are also useful, being less affected by confounding temporal changes, since abundances of animals may not be naturally variable at these temporal scales (Glasby & Underwood Citation1996). In the Sacca di Goro, clam fishermen work over much larger areas and repeatedly rake the same area several times a year, hence resulting in a far higher level of bottom disturbance than that used in our experiment. It should be stressed that the benthic communities resident at the experimental sites were composed almost entirely of small, r‐strategist species, mostly polychaetes. It has been shown that, in different habitats, fishing disturbance removes larger and more K‐strategist species (Bergman & Hup Citation1992; Eleftheriou & Robertson Citation1992), and select for species with a facultative response to disturbance, with communities becoming dominated by rapid colonist species. In our study sites the communities were already dominated by small and short‐lived species, due to either natural environmental conditions or fishing pressure, and are therefore less likely to show effects than other community types. As an example, the lack of significant effects on intertidal macrofauna after trawling for fishing was attributed to the predominance of subsurface feeding polychaetes and to naturally high levels of disturbance in their study habitat (Brylinsky et al. Citation1994).
Besides the direct effects of clam collection devices on non‐target species, and hence on the benthic community, indirect effects caused by sediment resuspension and turbidity might also cause benthic changes. In the Sacca di Goro there is a substantial sedimentary input from the Po River deltaic branches (IDROSER Citation1994), but sediment nourishments are needed to maintain sedimentary characteristics, especially in areas where clams are reared. It is generally assumed that clam harvesting causes sediment erosion, even if Castaldelli et al. (Citation2003) recently suggested that manual harvesting by means of MR has a negligible impact on sediment physicochemical properties. In contrast, in the Venice Lagoon mechanical harvesting causes greater physical disturbance (Pranovi et al. Citation2001, Citation2003, Citation2004; Sfriso et al. Citation2005). It must be stressed, however, that mechanical clam harvesting is carried out by means of a gear (the locally called “rusca”; Pranovi et al. Citation2004) whose digging action is obtained by means of a 25 HP outboard engine located on the side of the boat. Such a highly disruptive technique for clam fishing has significantly contributed to the increase of the sediment suspension and removal in the shallow bottoms of the Venetian Lagoon (Sfriso et al. Citation2005), but disturbance due to the rusca fishing is not comparable with the milder one due to MR and HR. Moreover, just the burrowing activity of clams causes a major disturbance to the surface sediments and enhances resuspension by tidal currents (Sgro et al. Citation2005): the overall impact of continuous bioturbation by clams was found to be a significant process in destabilizing sediments and increasing resuspension/turbidity, more than the intermittent disturbance by manual raking, regardless of the intensity of fishing activity (Sgro et al. Citation2005).
What is clear from those previous studies and the present one is that the impact of fishing practices on (and the recovery rate of) non‐target fauna is highly variable according to sediment type, local environmental conditions and the type and frequency of fishing process employed. This might complicate any attempt to make general predictions and hence to make sensible management decisions that relate to the sustainability of such practices. In the present study, given the relatively shallow penetration depth of the rakes (between 5 and 10 cm), the type of sediment (sandy and muddy) on which clam fishing is practised in the Sacca di Goro, and the relatively high factors of disturbance found all year round (macroalgal blooms, anoxic and hypoxic water, high summer temperatures; Mistri et al. Citation2001a, Citationb; Citation2002; Mistri Citation2003, Citation2004), relatively minor changes in the benthic community might have been anticipated with very rapid recovery. From our results, at least manual (MR) and hydraulic (HR) raking is unlikely to have persistent effects on infaunal communities of the Sacca. Recolonization by small infaunal species was relatively rapid, while the effects of MR and HR were comparable. Conversely, we found that the conveyor rake (CR) had a greater deleterious effect on the macrofaunal community than MR and HR. In conclusion, the mild disturbance due to MR and HR caused a little (and comparable) response to the biota, and this result can be useful for decision‐makers facing the problem of combining the protection of the environment with fishermen's considerations. Thus, in order to discourage the use of CR, clam fishermen of the small local Adriatic fleets should be allowed to equip their boats with HR of the same size to that used in this study. There are advantages for clam fishermen when fishing with HR, since the gear is less fatiguing than MR; since the effects on non‐target species and, thus, on the benthic community, are not different to MR, we suggest that the gear could be locally (and legally) adopted.
Acknowledgements
This study was funded by the Council Department of Ferrara Province. Thanks are due to E. Turolla and G. Caramori (IDEA Ferrara) for valuable help in field sampling, and to two anonymous reviewers for constructive criticism.
Related Research Data
References
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