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ICES Journal of Marine Science: Journal du Conseil 2003 60(2):393-401; doi:10.1016/S1054-3139(03)00014-6
© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
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Evaluation of shell damage to the clam Chamelea gallina captured by hydraulic dredging in the Northern Adriatic Sea

V Moschino, M Deppieri and M.G Marin*

Department of Biology, University of Padova Via U. Bassi, 51/B, 35131 Padova, Italy

*Correspondence to M. G. Marin; fax: +39 049827 6199. e-mail: mgmar{at}civ.bio.unipd.it.

The impact of experimental hydraulic dredging was assessed on Chamelea gallina populations in two sites along the north-western Adriatic coast (Lido and Jesolo) by detecting and quantifying shell damage caused by fishing operations on both captured and discarded clams. Various levels of stress were applied, the highest being that used by commercial fishing vessels, which employ high water pressure and mechanised sorting and the lowest manual sampling of clams by scuba divers. Water pressure and sorting significantly increased shell damage, the highest levels always being observed in commercially dredged clams. At Lido, damage was mostly due to the action of the mechanised sorter; at Jesolo, the effect of high water pressure was more clearcut. Moreover, clams collected at Jesolo had both higher mean damage level and higher numbers of damaged individuals compared to the Lido samples. These differences seem to be mostly related to differing bottom features in the two sites. A positive relationship was observed between damage level and clam size: small-sized samples (length <17 mm) were less damaged than medium-sized ones (25 mm > length >17 mm) and commercial size clams (>25 mm) showed the highest damage level. The severe and harmful physical impact of hydraulic dredging was apparent in captured and then discarded animals, a small fraction of which appears able to recover, as shown by the presence of clams with repaired shells.

Keywords: Chamelea gallina, clam, hydraulic dredging, shell damage, Adriatic Sea

Received 30 July 2002; accepted 27 January 2003.


   Introduction
Top
 Introduction
Materials and methods
 Results
 Discussion
 References
 
Many studies have been carried out to assess the impact of fishing gear for harvesting molluscs on non-target species and ecosystem structure (Hall et al., 1991; Currie and Parry, 1996; Collie et al., 1997; Kaiser et al., 1998; Hall-Spencer et al., 1999; Pranovi et al., 2001). Research on commercially targeted species has been concerned mainly with gear efficiency or stock assessment (Dare et al., 1993; Dare and Palmer, 1994; Fifas and Berthou, 1999) and only few studies have estimated the effects of fishing on population sustainability (Brust et al., 2001; Repetto, 2001; Ortiz et al., 2002). The need to establish adequate criteria for management of the resource has focused particular attention on the effects of various fishing techniques commercially used to harvest bivalve molluscs. The impact of fishing gear must be considered especially harmful on discarded animals, which are dredged, sieved and then returned back into the sea. These organisms are disturbed and may suffer physical damage, both of which may reduce their survival potential. Simulated fishing disturbance causes a delay in the reburrowing response of the cockle Cerastoderma edule (Coffen-Smout and Rees, 1999) and a reduction in the ability to perform an escape response in the whelk Buccinum undatum (Ramsay and Kaiser, 1998). Jenkins and Brand (2001) also demonstrated a decrease in the ability to swim and escape predators in captured undersized great scallop, Pecten maximus. Dare (1974) found that up to 13% of mussels (Mytilus edulis), which passed through a rotary sorting machine, showed shell damage and that many of them apparently suffered some internal damage which impaired their long-term survival out of water. Gaspar et al. (1994) detected a series of shell margin breaks in the razor clam Ensis siliqua collected from a heavily dredged area, and suggested that these disturbances to shell growth were the results of repeated dredge damage.

Since the 1970s, exploitation of the Chamelea gallina beds in the Northern Adriatic has increased considerably, due to improvements in fishing technology such as the introduction of hydraulic dredging and mechanised sorting. The hydraulic dredge consists of a cage of steel bars in which the clams are collected. The mouth of the gear (3 m wide) is provided with a blade to cut sediment and a manifold of jets from which water is expelled under pressure to fluidise sand. A large pipe supplies seawater to the jets from a pump aboard the vessel (Mattei and Pellizzato, 1997). When the boat reaches a suitable fishing ground, the stern anchor is cast, the dredge is lowered to the bottom and water under pressure is injected through the pipe. The vessel tows the hydraulic dredge below the bow by warping on the stern anchor, and moving backwards (Fig. 1). The towing technique makes the gear motion very regular on flat sandy bottom, and this probably contributes to the high efficiency of this fishing system (Froglia, 1989). When fishing is completed, the cage is hauled up and the whole catch is dumped into a collecting box and then conveyed to a mechanised sieve for sorting. Commercial size clams, larger than 25 mm, are sorted and retained, whereas undersized clams and non-target species are rejected and thrown back into the sea and are considered by-catch. Since the early 1990s, a general weakness of the C. gallina populations was observed, as revealed by the evaluation of their physiological status (Viarengo et al., 1998) and increasing occurrence of irregular mortality events (Ministry for Agricultural Policy, 1998); these conditions, the intense fishing effort on the resource and the specific features of the technique used for harvesting clams resulted in a dramatic reduction of fishable clam biomass (Del Piero and Fornaroli, 1998; Del Piero et al., 1998). In this study, we evaluated the mechanical stress due to dredging and sorting by identifying and quantifying shell damage on both captured and discarded C. gallina.


Figure 1
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  		Fig. 1 Fishing system using hydraulic dredge (from Froglia, 1989, modified).

 

   Materials and methods
Top
 Introduction
 Materials and methods
Results
 Discussion
 References
 
To evaluate damage levels in clam shells caused by hydraulic dredging, four experimental surveys on natural C. gallina grounds were carried out in February, May, July and October 2000 in two fishing areas (separated by ~8 nmiles), characterised by differing sea bottom conditions, along the west coast of the Northern Adriatic (Fig. 2). Sediment at Jesolo is mainly fine sand whereas it is coarser at Lido with medium-sized sand prevailing and with the presence of a large number of empty broken shells (Brambati et al., 1983).


Figure 2
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  		Fig. 2 Map of the study area in the Northern Adriatic, showing the two sampling sites: Jesolo and Lido.

 
Clam samples were collected by commercial fishing vessels from a water depth of about 5 m, using three methods to highlight the effects of water pressure and the mechanised sieve, the two main components which may cause damage to the organisms:
dredging at low water pressure (inlet pressure ~1 bar, the lowest allowing dredging), without sorting (LP treatment);
dredging at high water pressure (inlet pressure ~2.5 bar), without sorting (HP treatment);
dredging at high water pressure (inlet pressure ~2.5 bar) and mechanical sieving for sorting, as in commercial fishing (HPS treatment).

Measurements were also performed on clams collected from by-catch during the fishing of HPS samples (i.e. commercial fishing system). Undersized animals were split into two categories according to their length: the medium-sized sample (MS, 25 mm > length >17 mm) and the small-sized sample (SS, length <17 mm). This subdivision was made to examine the possible relationship between damage level and clam size.

About 400 clams per treatment were checked: shell damage was identified and then shell length measured. To define a damage index, six categories were scored (from 1 to 4), as listed in Table 1. All damage categories imply exposure of soft tissue, which increases according to the score (Fig. 3). The percentage of clams showing major damage was used to quantify mortality caused by dredging, since organisms having crushed umbo or shell must be considered as unable to recover.


Figure 3
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  		Fig. 3 Various types of damage detected in C. gallina: (a) intact shell, (b) shell with previous damage repaired, (c) shell with both valves scratched, (d) shell with edge chipped, (e) crushed umbo and (f) crushed shell.

 


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  		Table 1 Damage categories and scores.

 
Statistical comparisons were performed using the non-parametric Kruskal–Wallis ANOVA for the damage levels, the one-way ANOVA for the mean shell length and G-test (Sokal and Rohlf, 1981) for the mortality frequencies.


   Results
Top
 Introduction
 Materials and methods
 Results
Discussion
 References
 
At Lido, no significant differences were observed between LP and HP clams, whereas in HPS samples the damage level was significantly higher, when compared with the HP value, in February (p<0.001), May (p<0.001) and July (p<0.001) (Fig. 4). At Jesolo, the situation was different: in three out of four samplings, LP samples were significantly less damaged than HP ones, which were significantly different from HPS samples only in May (p<0.05). Manually collected clams showed the lowest damage levels with average values frequently close to 1 (i.e. no damage) (Fig. 4).


Figure 4
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  		Fig. 4 Damage levels in C. gallina collected at Lido and Jesolo using various fishing methods (M, manual; LP, low water pressure; HP, high water pressure; HPS, high water pressure and mechanised sorting). Mean±s.e., n~400. Kruskal–Wallis ANOVA: *p<0.05; **p<0.01; ***p<0.001.

 
Seasonal comparisons exhibited no differences among LP samples at Lido, whereas HPS samples showed more variability, with the greatest damage levels occurring in May. At Jesolo, the lowest damage value in HPS clams was observed in October, being significantly different in comparison with the February and May HPS values (p<0.001).

The highest number of damaged animals was found in May in HPS samples from both sites, peaking at 35% at Lido; in the other months, higher percentages were observed at Jesolo (Table 2). In by-catch samples from the HPS treatment, the percentage of damaged clams ranged from 9.4 to 26.8% and the highest values were always recorded at Lido (Table 3). The percentage of clams showing shell repair after previous damage ranged from 1 to 8.6% (Table 4).


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  		Table 2 Percentages of clams showing shell damages in the various treatments from two sampling sites; mean±s.e. are also reported.

 


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  		Table 3 Percentages of discarded clams (from by-catch of HPS samples) showing shell damages.

 


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  		Table 4 Percentages of clams with previous damage repaired.

 
The lowest mortality values were observed in HP samples at Lido and in LP ones at Jesolo: significant differences were found when comparing these samples with HPS ones, which exhibited the highest mortality in both sites (Fig. 5). Mortality was significantly higher at Jesolo than at Lido during all seasons (p<0.001), reaching 19.6% in February (Fig. 5).


Figure 5
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  		Fig. 5 Mortality values, calculated on the basis of the percentage of crushed umbos or shells, in C. gallina collected at Lido and Jesolo using various fishing methods (LP, low water pressure; HP, high water pressure; HPS, high water pressure and mechanised sorting). Bottom graph: comparisons between HPS samples from two sampling sites. n~400, G-test: *p<0.05; **p<0.01; ***p<0.001.

 
Generally, significantly lower average shell lengths were observed in manually collected clams, except at Lido in October and in July, and at Jesolo in July. In dredged samples, mean shell length at Lido was variable, with the lowest values recorded in LP samples in February and October, and in HP ones in May and July. At Jesolo, LP clams were almost always significantly smaller than those collected using HP or HPS treatments (Fig. 6).


Figure 6
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  		Fig. 6 Shell length in C. gallina collected at Lido and Jesolo using various fishing methods (M, manual; LP, low water pressure; HP, high water pressure; HPS, high water pressure and mechanical sorter). Mean±s.e., n~400. ANOVA: *p<0.05; **p<0.01; ***p<0.001.

 
Small-sized clams showed lower damage levels with respect to medium-sized clams and HPS commercial size samples. Significant differences were found when comparing Jesolo samples, whereas at Lido only in October did HPS dredged clams show significantly greater damage than smaller categories (p<0.001) (Fig. 7).


Figure 7
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  		Fig. 7 Damage levels of commercial size (HPS), medium-sized (MS) and small-sized (SS) clams collected using high water pressure and mechanical sorter at Lido and Jesolo. Mean±s.e., n~400 Kruskal–Wallis ANOVA: *p<0.05; **p<0.01; ***p<0.001.

 

   Discussion
Top
 Introduction
 Materials and methods
 Results
 Discussion
References
 
High water pressure and mechanised sorting both increase the physical impact on clams, since the highest levels of damage were always detected in commercially dredged clams. These results confirmed the observations of Dare (1974) on dredged and sorted mussels. The significant difference observed when comparing damage levels in manually collected clams (M) and LP samples can be considered mostly due to stress sources other than water pressure and mechanised sorting, such as the contact with the cutting blade and dumping into the collecting box.

Although the mechanical stress applied was similar, some differences were noted when comparing data from the two sites: damage to clam shells at Lido seemed to be mostly due to the action of the mechanical sorter rather than to water pressure, as shown by the significant differences between HP and HPS samples whereas the effect of high water pressure was more evident at Jesolo, HP samples being more severely damaged than LP ones.

Lower mortality and smaller mean shell length values were often observed in HP samples from Lido, whereas an increase in both mortality rates and mean shell lengths were found when physical impact increased at Jesolo. This may be due to the large amount of empty broken shells in sediments at Lido which caused compaction of the catch inside the dredge, a situation which became particularly severe when high water pressure was used. Tight packing of material inside the dredge may have reduced shaking of clams and consequently reduced shell damage, and thus limited size selectivity through the mesh.

Relatively large numbers of damaged individuals were found in both captured and discarded clams, highlighting the severe impact of this type of fishing gear. More than 30% of commercially fished clams showed shell damage in samples collected at Lido in May and in those from Jesolo in February, May and July (Table 2). Clams collected at Jesolo had not only a higher mean damage level, but also higher numbers of damaged individuals with respect to the Lido samples. The influence of ground features in determining levels of physical impact on dredged organisms is further confirmed.

Lastly, useful information is provided by the by-catch samples, considering that all undersized clams captured by fishing gear, sieved and then rejected into the sea, may contribute to restocking of natural populations. Discarded animals thrown back into the sea may die as a direct consequence of physical damage incurred during fishing or indirectly due to predators or disease. Damage levels increase proportionally with increasing clam size: small-sized samples (length <17 mm) were less damaged than medium-sized samples (25 mm > length >17 mm) and HPS samples (>25 mm) showed the highest damage levels. At both sampling sites, a considerable fraction (up to 27%) of discarded clams displayed shell damage: these damaged undersized clams probably have reduced survival capability. It has been demonstrated that captured and then discarded animals generally undergo high levels of predation and mortality (McLoughlin et al., 1991; Kaiser and Spencer, 1995). In this context, some additional considerations may be made by the observation and quantification of clams with shells repaired after previous damage. Due to the characteristic aspect of repair, the probability that the observed damage was produced by hydraulic dredging is very high. Thus, the frequency of repaired shells may represent the frequency of clams which have encountered a hydraulic dredge at least once in their life, before being harvested, and survived despite the damage undergone. Although underwater observations in the study areas did not reveal high predation pressure, most predators being crabs, the low percentage of repaired shells indicates that only a small fraction of damaged discarded clams may be able to recover.

The overfishing which C. gallina populations in the Northern Adriatic have undergone since the introduction of hydraulic dredging has caused a dramatic reduction in fishable clam biomass (Del Piero and Fornaroli, 1998; Del Piero et al., 1998), mainly due to the efficiency of the gear used and the high number of vessels employed. It is estimated that the efficiency of the hydraulic dredge is close to 100% for commercial size clams (Froglia, 1989), and that the same area in the clam ground may be dredged more then ten times per month (Pellizzato, personal communication). Shell length measurement reveals that most large individuals are removed by fishing activities, as observed in other exploited populations (Costa et al., 1987; Ramón and Richardson, 1992). Although the maximum length recorded for this species was 50 mm (Froglia, 1989), no specimens were collected with a shell length exceeding 26–27 mm, very close to the minimum legal commercial size of 25 mm. Considering that C. gallina is one of the most important resources of the local fisheries, the capability of clam populations to sustain fishing effort needs to be thoroughly evaluated.


   Acknowledgements
 
This study was carried out with funding from the Commission of the European Communities, Agriculture and Fisheries (FAIR) specific RTD programme, CT98-4465, "Evaluating and improvement of shellfish dredge design and fishing effort in relation to technical conservation measures and environmental impact" (ECODREDGE).


   References
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 Materials and methods
 Results
 Discussion
 References
 

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