ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on October 22, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(9):1735-1742; doi:10.1093/icesjms/fsm153
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Dredging for edible cockles (Cerastoderma edule) on intertidal flats: short-term consequences of fisher patch-choice decisions for target and non-target benthic fauna
1 Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands
2 NIOZ, Texel, and Animal Ecology Group, Centre for Ecological and Evolutionary Studies (CEES), University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands
Correspondence to C. Kraan: tel: +31-222-369300; fax: +31-222-319674; e-mail: kraan{at}nioz.nl
Kraan, C., Piersma, T., Dekinga, A., Koolhaas, A. and van der Meer, J., 2007. Dredging for edible cockles (Cerastoderma edule) on intertidal flats: short-term consequences of fisher patch-choice decisions for target and non-target benthic fauna. – ICES Journal of Marine Science, 64.Intertidal flats in the Dutch Wadden Sea are protected by national and international treaties. Still, mechanical dredging for edible cockles Cerastoderma edule was allowed in 74% of 1200 km2 of intertidal flats. Cumulatively, between 1992 and 2001, 19% of the intertidal area was affected by mechanical cockle-dredging at least once. On the basis of a grid of 2650 stations sampled annually, we evaluate the extent to which cockle-dredging from 1998 to 2003 was selective with respect to non-target macrozoobenthic intertidal fauna. In all 4 years that comparisons could be made, to-be-dredged areas contained greater diversity of macrobenthic animals than areas that remained undredged. Targeted cockles were 2.5 times more abundant in areas that were to be dredged shortly, but other species also occurred in higher densities in these areas. Small amphipods and some bivalves occurred less in to-be-dredged areas than elsewhere. In terms of short-term responses to dredging, four non-target species showed a significant decrease in abundance 1 year after dredging. Only Tellina tenuis showed an increase a year after dredging.
Keywords: cockle-dredging, intertidal macrozoobenthos, patch-choice, short-term effects, Wadden Sea
Received 23 May 2007; accepted 11 September 2007; advance access publication 22 October 2007.
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Introduction |
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Large-scale industrial fishing such as trawling or dredging for demersal marine fauna is increasingly seen as a threat to the world’s marine biota (Roberts, 1997; Watling and Norse, 1998; Jackson, 2001; Coleman and Williams, 2002; Dayton, 2003; Rosenberg, 2003; Blundell, 2004). There is considerable evidence for short- and long-term negative effects of trawling and dredging on populations of target and non-target species (Jones, 1992; Dayton et al., 1995; Jennings and Kaiser, 1998; Collie et al., 2000; Piersma et al., 2001; Kaiser et al., 2006). However, management of such fisheries is often based on sustainable use of the stock of the target species only, without application of a more comprehensive ecosystem approach that includes non-target species (Turner et al., 1999; Murawski, 2000).
Since the late 1980s and as a continuation of old fishing rights, in most years industrial harvesting of edible cockles (Cerastoderma edule) and blue mussels (Mytilus edulis) was allowed in 75% of the intertidal flats in the Dutch Wadden Sea, a 2400 km2 area of barrier islands, shallow waters, and intertidal flats (Abrahamse et al., 1976; van de Kam et al., 2004). These areas have been designated by the Dutch government as a State Nature Monument and Protected Nature Area. The Wadden Sea as a whole is protected under the European Commission’s Bird and Habitat Directives, has Ramsar as well as Man and Biosphere (United Nations Educational, Scientific and Cultural Organization) status, and is a Particular Sensitive Sea Area under the International Maritime Organization of the United Nations. Although the scientific community voiced concern (Piersma et al., 1993a, Piersma and Koolhaas, 1997, 2001; Kareiva and Laurance, 2002; and see Swart and Van Andel, 2007, for a historical review of the conflict), the government of the Netherlands assessed single-species stock status only (Smit et al., 1998; Kamermans and Smaal, 2002).
Here, we document "patch-choice decisions" by mechanical cockle-dredgers, at the scale of the actual fisheries, not only with respect to the target species (C. edule), but also for non-target macrobenthic species. Our assessment is based on six benthic surveys carried out between 1998 and 2003 that were done just before the cockle-dredging season, covering >170 km2 of intertidal flats in the western Dutch Wadden Sea. Further, we evaluate short-term effects (1 year later) on local density by way of paired comparisons between neighbouring dredged (impact) and undredged (control) areas.
By matching neighbouring impact and control areas on the basis of abiotic characteristics, the absence of randomly appointed treatment and control areas, because of the non-random distribution of dredging effort, was controlled for. In this way, spatial and temporal variation between control and impact area were (as far as possible) minimized. Finally, we examine whether changes in the abundance of various benthic species attributable to dredging is proportional to the selectivity exerted on them by the cockle-dredgers.
Dutch cockle-dredging
From 1993, following serious winter starvation of common eiders (Somateria mollissima) and European oystercatchers (Haematopus ostralegus) (Smit et al., 1998), 26% of the intertidal area in the Dutch Wadden Sea was closed to mechanical cockle-dredgers, with an additional 5% closed between 1998 and 2001. Reflecting the abundance or scarcity of cockles, the number of ships actively employed varied between a maximum of 22 and a minimum of 3 (in 2003).
Suction-dredging takes place when the mudflats are covered with a minimum of 80 cm of water (Figure 1a). The dredges (75 or 125 cm wide) have a water jet in the front to loosen the top layer of the sediment, which is then scraped off by a blade at a depth of 
5 cm. Within the dredge, a strong water flow ensures that objects with a diameter <15 mm are pushed through a screen. The remaining larger objects are sucked on board the ship for further cleaning and handling. Cockle-dredgers aim at cockles >19 mm, the minimum size for consumption. After dredging, the intertidal flats appear "scarred" (Figure 1b), and can remain so for a whole winter (De Vlas, 2000; Ens et al., 2004).
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To record position and fishing activity, mainly as an internal control, a "black-box" system (a GPS logger) was operated on board every vessel by the Association of Mechanical Cockle Dredgers based in Kapelle (Kamermans and Smaal, 2002). Cumulative seasonal data from the black boxes are screened and edited internally by the Association, which then releases maps with coloured blocks of 0.1' of latitude by 0.5' of longitude. Blocks are only coloured if >2% of the surface is touched by the dredge. Therefore, in many cases, sampling points to be dredged would not have been touched. This means that our assessment of dredging effects is conservative; i.e. a lack of statistical effects does not necessarily imply lack of ecological effects.
The Association presents an annual value for the total surface area of the tracks, a value corrected for overlap. On average, 4.3% of 1200 km2 of the intertidal flats in the Dutch Wadden Sea (Wolff, 1983) was reported to be affected annually by dredging from 1998 to 2000 (Donkers, 1998, 2000; Huijssen 2001). When converted to actual fishing tracks, an average of 1.3% of the surface of intertidal flats would have been dredged each year (Kamermans et al., 2003). Cockle beds were often dredged in consecutive seasons; 54% of all dredged stations examined here (n = 710) were dredged once between 1998 and 2003, but 25% were dredged twice during the period, 14% three times, and 7% were dredged every year.
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Material and methods |
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Study area
The study area covers a large part of the western Dutch Wadden Sea (Figure 2). With a tidal amplitude of 1.5 m at neaps, to 2 m at spring tides, it emcompasses an area of 890 km2 of intertidal flats. The area is partitioned by three main tidal channels separating the mainland and the barrier islands of Texel, Vlieland, and Terschelling. The sediment consists of sands and muddy sands, with median grain sizes ranging from 140 to 200 µm (Piersma et al., 2001; Zwarts et al., 2003). Between 1989 and 1991, all natural beds of mussels were removed mechanically from the study area (Piersma et al., 1993b; Smit et al., 1998; Ens, 2003).
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Some 170 km2 of the study area is covered with a grid of fixed sampling stations 250 m apart (Figure 2). Except for a nearshore area just south of Vlieland that was closed in 1999 and reopened in 2002, mechanical dredging was allowed everywhere. In 1998, 2000, 2001, and 2002, respectively, 14.9%, 11.5%, 15.4%, and 8.0% of our sampling stations were dredged (Figure 2). In 1999, virtually all fishing was in the eastern part of the Dutch Wadden Sea. The cumulative area affected by dredging from 1992 to 2001 was 19% of the whole Dutch Wadden Sea (Kamermans et al., 2003); in our study area, this was 24.6% from 1998 to 2002.
Sampling routines
Sampling stations were visited either on foot or by boat once a year between mid-July and early September. Although fishing was permitted from mid-August on, dredging never started before early September and continued to December (or even as late as January/February in the 2002/03 season). From 1998 to 2003 we sampled, respectively, 2326, 2539, 2731, 2749, 2762, and 2680 stations. These surveys, carried out independently of the fishery, aimed at documenting benthic food abundance to gain an understanding of shorebird distributions (Van Gils et al., 2005, 2006a).
Sampling stations were found with hand-held GPS (Garmin 45 and 12, using WGS84 as map datum), and at each station one (on foot) or two (by boat) sediment cores, covering 0.018 m2 in total, to a depth of 20–25 cm were taken. The cores were sieved over a 1 mm mesh and numbers of all species were recorded; for bivalves, the age/size classes were also noted. All crustaceans and molluscs were collected and stored at –20°C for later analysis in the laboratory (see Piersma et al., 1993b; Van Gils et al., 2006a, b, for details). On average, 56% of the samples were taken by boat and 44% on foot. Both methods, based on comparisons between benthos densities at 165 adjoining positions (excluding Hydrobia ulvea) that were either sampled by boat or on foot between 1998 and 2004, yield similar results (unpublished data).
An invasion of the exotic polychaete Marenzelleria viridus began about 1998, but quantitatively we only trust and subsequently use observations from 2000 on. Rare species such as the tube-living polychaete Lagis koreni were excluded from analysis. Their scarcity made comparisons between control and impact areas impossible. The mudsnail H. ulvae was also excluded, because numbers were not sampled quantitatively by boat. Note that cockles were separated into two size classes, 
19 and >19 mm, and subsequently treated as separate species in the analysis.
Statistical analysis
We tested for differences in the number of species per station (target-sized cockles excluded) between the to-be-dredged area and the undredged area for the whole western Dutch Wadden Sea, by Student’s t-tests on log-transformed data. This procedure assumes that cores taken 250 m apart are independent samples. Data from four dredging seasons (1998, 2000, 2001, and 2002) were included.
We also determined whether cockle-dredging led to changes in the abundances of macrozoobenthic species, a posteriori matching dredged (impact) areas with undredged neighbouring ones (control areas). We made sure that paired areas were similar in terms of tidal height, water coverage during high tide, and sediment composition, to ensure that temporal as well as spatial effects would be accounted for statistically (Figure 2). Historically, industrial cockle-dredging started in the 1950s, with strong growth in the 1970s (Dijkema, 1997). Therefore, undredged areas only refer to areas not dredged during this study. However, all areas are likely to have been affected by dredging in the previous 20 years (Zwarts et al., 2003). The relative occurrence of a particular species before dredging (Table 1) is computed as the ratio of the density in to-be-dredged to undredged areas (Figure 2), Nd0/Nc0, where N denotes average density (averaged over all areas) of the focal species, and d and c designate dredged or control areas. Zero indicates before dredging and a numeral 1 to the densities 1 year after dredging.
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To detect changes in density attributable to cockle-dredging, we examined densities in paired areas after dredging, i.e. using the benthic data up to and including 2003. The changes in density attributable to dredging in autumn 1998 were then calculated with data from 1999, using stations sampled in both years only. This fisheries effect was calculated as ln (Nd1/Nc1). In the absence of any effect of cockle-dredging, the relative occurrence and the fishery effect are expected to be the same, i.e. ln Nd0/Nc0 = ln Nd1/Nc1. The null hypotheses of no fishery effect was tested with 1000 bootstrap samples (Manly, 1997) of paired control (Nc1) and impact (Nd1) average densities after dredging and before dredging (Nd0 and Nc0). This generated 1000 combinations of dredging-related alterations and the accompanying selectivity index (Nd0/Nc0) for each species. Average densities were based on paired area combinations; 25 in total over all years (Figure 2). Observed values were then compared with bootstrapped values (Manly, 1997). Statistical analyses were performed with either SYSTAT or R (R Development Core Team, 2006).
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Sampling points that were to be dredged had on average 25% more macrozoobenthic species (target-sized cockles excluded) than eventually undredged stations (Figure 3) (Student’s t-test, t = 4.65, d.f. = 8, p < 0.01). The Shannon–Wiener index, based on log(x + 1)-transformed abundances, gave similar results (p < 0.01), with mean values for to-be-dredged areas between 0.48 and 0.53, and for undredged areas varying between 0.37 and 0.43. This means that from 1998 to 2002, cockle-dredgers concentrated their fishing in intertidal areas with relatively high macrozoobenthic species diversity.
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Using the paired comparisons between undredged and to-be-dredged parts of the otherwise similar intertidal areas (Figure 2), targeted C. edule (>19 mm length) were 2.5 times more abundant in the area to be dredged later that season (Table 1). Mytilus edulis was 12.8 times more abundant, Nereis diversicolor 1.4 times, Ensis americanus 1.3 times, and Macoma balthica 1.2 times (Table 1). Species that occurred less in to-be-dredged areas than elsewhere were mainly small amphipods, polychaetes such as Scoloplos armiger, and the bivalves Abra tenuis and Tellina tenuis (Table 1).
Mechanical dredging for cockles had a significant negative short-term effect on the abundance of five species and a positive effect for one species, which corresponds to 27% of the intertidal benthic species examined (Table 1, Figure 4). However, the relationship between fishery effect and relative occurrence could be characterized as fuzzy (Figure 4). The abundance of most species seemingly neither increased nor decreased after 1 year proportional to their relative occurrence in dredged areas. Not surprisingly, consumption-size cockles showed a density reduction by cockle-dredging on top of natural mortality. The same effect was present for M. edulis, Heteromastis filiformis, Crangon crangon, and E. americanus, even if the last three were not in particularly high densities in dredged areas. Small cockles (19 mm), Carcinus maenas, and the polychaete M. viridus revealed no additional negative or positive fishery effect. Only T. tenuis demonstrated a significant increase in abundance after dredging.
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Discussion |
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Most research dealing with fisheries effects necessarily describes ecological effects at spatial and temporal scales smaller than that of the actual fisheries (Collie et al., 2000; Kaiser et al., 2000; but see Thrush et al., 1998). Contrasting scales potentially obscure changes in benthic communities directly related to these fisheries. Our research is based on a long-term benthic research programme that happened to correspond to the actual extent of cockle-dredging in the western Dutch Wadden Sea (Figure 2; Piersma et al., 2001; Van Gils et al., 2006b).
The experimental design that compared paired undredged and later dredged areas before and after dredging (Figure 2) to the best of our knowledge has not been used before on such a large scale and should allow assessment of short-term fishery effects. The paired areas were matched by location, height in the intertidal zone, and sediment characteristics, with the aim to minimize possible effects attributable to the non-random choice of fisheries locations. However, the possibility that areas selected by fisheries changed in a different way than unfished areas for other reasons than the fisheries itself cannot be excluded; changes may simply be related to the fact that the areas were biologically richer. Yet, under this constraint, we believe that our design is the best possible approach. The present analysis disentangled fisheries and natural mortality, and only the density change attributable to fisheries mortality is considered in relation to relative occurrence. This means that, in the absence of an effect of cockle-dredging, the relative occurrence and the fishery effect are the same (ln Nd0/Nc0 = ln Nd1/Nc1). To express the fisheries effect as the ratio of dredged-to-undredged densities before and after dredging (ln Nd0/Nc0–ln Nd1/Nc1), though appealing, would produce spurious correlations when combined with relative occurrence (Nd0/Nc0). This is because density changes consist of natural mortality plus recruitment in the undredged area and additional dredging mortality in the dredged area. Both these independent mortalities would be incorporated in the error of the relative occurrence and the fisheries effect.
Contrary to the general impression at the time (and see Beukema et al., 1998, who refer to distinct cockle beds), mechanical dredging in the late 1990s focused on intertidal areas with the greatest abundance of large cockles (>19 mm) and the highest species diversity. Species targeted by shorebirds for food (M. edulis, N. diversicolor, A. marina, and M. balthica) were in greater densities in to-be-dredged than in undredged areas.
Although some species we observed were only present in modest densities in areas targeted by cockle-dredgers, short-term effects of dredging were noted. Because our estimates of short-term dredging disturbance are conservative, because of the rough assignments of dredged and non-dredged sampling locations (see Dutch cockle-dredging practice), and because we did not perform a random search for correlations but rather evaluated species one by one, we believe that application of a correction for multiple testing would lead to an unreasonable increase of type-II error (Nakagawa, 2004; Garamszegi, 2006). On longer time-scales, the dredged areas were characterized by reduced settlement of cockles (and M. balthica), which may relate to loss of fine silts and an increase in median grain size (Piersma et al., 2001; Zwarts et al., 2003). Changes in sediment composition influence the feeding performance of deposit-feeding bivalves (Drent et al., 2004), including small cockles (Kang et al., 1999; Rossi et al., 2004). Such change in the sediment may also explain the increase in abundance of Tellina tenuis a year after being dredged. T. tenuis prefers very coarse sediments, often near the low water mark (Dekker and Beukema, 1999; own data). A previous meta-analysis on the impact of mechanical fisheries on non-target species also noted that species from coarser sediments were less likely to be affected (Collie et al., 2000).
Using an overlapping dataset (1998–2002 rather than 1998–2003), but with different spatial resolution (272 km2 blocks, rather than the 25 paired area comparisons used here), statistical technique and size assignment (16 mm in Van Gils et al., 2006b, 19 mm here), Van Gils et al. (2006b) also found densities of small cockles to remain stable in dredged blocks (note that we found a non-significant density increase; Table 1). Those authors showed that the detrimental effects of cockle-dredging on top predators such as red knots (Calidris canutus) was due to loss of body condition of cockles of edible size rather than to density changes.
Decreases of C. edule and M. balthica, attributable to a single-dredging event, occurred over many rather than single years, and were a consequence of decreased spatfall in the years following dredging and increases in median grain size in the dredged area (Piersma et al., 2001). The absence of a short-term dredging effect on M. balthica, as we found in our study, is indeed consistent with this. That these species continued to decline up to 8 years after dredging (Piersma et al., 2001) suggests that single and multiple dredging events may reduce the system’s resilience and shift intertidal soft-sediments to alternative states of reduced species richness (Scheffer et al., 2001). The failure of seagrass (Zostera noltii) to re-establish, the ongoing decline of M. balthica numbers in the western Dutch Wadden Sea (own data), and the non-recovery of bivalve stocks after intensive cockle-dredging south of Vlieland and Richel (Piersma et al., 2001) might be symptomatic of this state-shift.
To allow cockle-dredging, the Dutch government through its Ministry of Agriculture, Nature Conservation and Food Safety, had to give exemption from nature conservation laws, with due recognition of the precautionary principle and Ramsar’s wise use principle. The compromise reached in parliament was that dredging for cockles should be allowed when more than 70% of the predicted energy intake of common eiders and European oystercatchers could be covered by the sum of the estimated availability of edible cockles, blue mussels, and trough shells (Spisula subtruncata) in the Wadden Sea and the nearshore waters of the North Sea. This requirement assumes that all biomass is harvestable (which it is not; Zwarts and Wanink, 1993), that birds are omniscient (which they may be; Van Gils et al., 2006a) and free of travel costs (which they are not; Goss-Custard et al., 2004; Van Gils et al., 2006a), and that 30% of the required intake can be covered by other food sources (not a robust assumption according to Smit et al., 1998; Camphuysen et al., 2002; Atkinson et al., 2003). These mollusc-eating birds are in greatest numbers in the Wadden Sea in their non-breeding season, which they have to survive in good condition to be able to complete their migration back to the breeding sites the following spring (Goss-Custard et al., 2004; Van de Kam et al., 2004).
The clear outcome of this work is to support the application of a whole ecosystem approach to fisheries management (e.g. Kaiser et al., 2000; Murawski, 2000; Scheffer et al., 2005), with application of the wise use principle as advocated by European legislation. This principle states that any economic activity should prove itself harmless before it is allowed. The following example is relevant for the Wadden Sea. The approval of cockle-dredging decreased numbers of small mussels M. edulis in the dredged intertidal areas (Figure 4). Consequently, the re-establishment of mussel beds in the western Dutch Wadden Sea was prevented (see also Herlyn and Millat, 2000; Hiddink, 2003), even though it represented a long-term conservation goal of the Dutch government (LNV, 1998).
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Acknowledgements |
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During most of the fieldwork, we were based on the Royal NIOZ-research vessel RV "Navicula", and we thank K. van de Star, T. van der Vis, H. de Vries, and J. Tuntelder for their help. We thank Vereniging Natuurmonumenten for permission to work around the island of Griend and to use their warden’s hut. A large number of volunteers and students contributed to the collection of the field data, especially M. van den Berg, K. Prinz, and J. Fermer. C. Raaijmakers made a large contribution to the work in the laboratory. We thank J. van Gils, R. Dapper, and K. Camphuysen for help with databases and mapping programs, and J. de Vlas and M. de Jonge for use of their photographs. Three anonymous referees are thanked for valuable comments on an earlier version of the manuscript.
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