Predation potential of the invasive green crab (Carcinus maenas) and other common predators on commercial bivalve species found on Prince Edward Island.
KEY WORDS: predation, crab, Carcinus maenas, Cancer irroratus, starfish, Asterias vulgaris, snail, Euspira heros, aquaculture, bivalve
The culture of bivalve species represents one of the most important aquaculture industries in Atlantic Canada. In the year 2003, about 80% of the total bivalve aquaculture production originated from Prince Edward Island (PEI) (Department of Fisheries and Oceans 2004). The most important species cultivated in PEI are the blue mussel (Mytilus eclulis) and the eastern oyster (Crassostrea virginica) (Department of Fisheries and Oceans 2004). The PEI shellfish industry has also shown, over the last decade, an increasing interest in diversifying cultured bivalve species (Brown et al. 1995). Two new native species, the quahog (Mercenaria mercenaria) and the soft-shell clam (Mya arenaria), are presently undergoing grow-out trials by the aquaculture industry.
Predators play an important role in structuring marine benthic communities (see reviews by Connell 1983, Commito & Ambrose 1985, Menge & Sutherland 1987, Ambrose 1991, Ebenhoh et al. 1995). Studies have identified a large spectre of endo- and epibenthic predators in soft- and hard-bottom communities including invertebrates (e.g., polychaetes, nemerteans, crabs, gastropods, echinoderms) and vertebrates (e.g., fish, birds, mammals). Because most bivalve aquaculture operations occur directly in the marine environment, cultured animals are vulnerable to predators. Predation at culture sites is a major concern of bivalve farmers (e.g., Lindsay & Savage 1978, Flimlin 1993, Rosenthal et al. 1995, Beal & Krauss 2002, Walton et al. 2002). For instance, commercial bivalve aquaculture ventures in the United States spend a fair proportion of their budget every year in antipredation material and techniques to control quahog mortality caused by predators (Menzel 1989).
Many potential predators were identified by PEI shellfish farmers and various stakeholders to be threats to their operations (Prince Edward Island Aquaculture Alliance 2000). These predators include the common starfish (Asterias vulgaris), the moon snail (Euspira heros) and various crab species including the green crab (Carcinus maenas), which was accidentally introduced to North America in the 19th century (see review from Audet et al. 2003) and the rock crab (Cancer irroratus). The green crab was noted for the first time off southeastern shores of PEI in 1997 (Gillis et al. 2000, Audet et al. 2003). It has since progressed toward the northwest of the island on both shores (Audet et al. 2003). Wherever they are found, green crabs have been shown to affect their new environment at various ecologic levels (e.g., Glude 1955, Ropes 1968, Hughes & Elner 1979, Elner 1981, Grosholz et al. 2000, Trussel & Smith 2000, McDonald et al. 2001, Walton et al. 2002, Floyd & Williams 2004).
Without adequate knowledge of the biology and the impact of these predators on cultured bivalve populations, little can be done to suggest antipredation techniques. The objective of this study is to evaluate and document the predation behavior of various species to understand and establish the priority of implementing predator control measures and further research and development projects. A first experiment was carried out by determining mortality rates of four PEI commercial bivalve species in the presence and absence of the invasive green crab and the native rock crab. A complementary experiment was also undertaken to compare the predation potential of the green crab to other predators commonly found off the coast of PEI (common starfish and moon snail). It was expected that mortality rates of the prey species would increase in the presence of predators, would vary according to the predator species and mortality rates per predator species would decrease with increasing prey size.
MATERIALS AND METHOD
Experimental Enclosures and Prey Species
All experiments were carried out at the Ellerslie Shellfish Hatchery, PEI (Canada), in the fall of 2000. Trials were held in thirty 38-cm wide x 54-cm long x 16-cm high plastic crates. These crates were divided in two with a PVC separator tightly fixed with silicon. This gave us a total of 60 water-tight experimental enclosures (38-cm wide x 27-cm long). Each side opposing the PVC separator had a small hole for water outflow (1 cm from the crate rim). Enclosures were filled with a 10-cm-deep layer of homogeneous sandy sediments. Sediments were sieved through a l-mm mesh prior to use (removal of macro-invertebrates) but were not sterilized. Crates were randomly placed in 2 large tanks (15 per tank). These tanks allowed us to collect the water outflow from all crates and insured that the invasive green crabs could not escape inside the hatchery. Experimental enclosures were individually supplied with seawater. The water was pumped from the Bideford Estuary (20-23 ppt) through a sand filter. The water was then heated to 15[degrees]C in holding bins and then supplied to the experimental enclosures by gravity. Experimental enclosures were left for 1 wk to allow the sediments to stabilize.
Animals were obtained from various PEI locations: eastern oysters from Malpeque Bay, soft-shell clams from North River and quahogs from the Ellerslie hatchery. Quahogs were of the notata variety. Three size-classes were used to determine the preferred size-classes of predators: 0-15 mm, 15-25 mm and 25-40 mm. Green crabs and rock crabs for experiment 1 were collected from Pinette River and Basin Head, respectively. Green crabs and moon snails were collected from St. Mary's Bay, whereas common starfish were from Tracadie Bay in experiment 2. In both experiments, only male crabs were used. Green crabs in prolonged intermolt phase (orange or red coloration) were used to eliminate potential carapace weakness in recently molted crabs (green coloration). Bivalves were deposed on the sediments in each enclosure 24 h before predators were introduced (1 predator per enclosures except in control treatments). This allowed the infaunal species an opportunity to burrow into the sediments. The sediment layer was thick enough to allow quahogs and soft-shell clams to cover themselves completely and extend their siphon without being exposed. Small-size mesh nets (1 cm aperture) were affixed using clips on enclosures to prevent predators from escaping. Photoperiod was maintained at 8 h of light and 16 h of darkness in all experiments.
Predation Trials Between Green Crabs and Rock Crabs
Trials were designed to have one of the crab species combined with one of the four prey species at a time (single-choice experiment). Ten individuals per size-class were used for a total of 30 prey per single experimental enclosure. Another set of trials was designed to combine one of the crab species (1 individual) with all the prey species within the same enclosure (multiple-choice experiment). Crabs were not fed for a 24-h period before their introduction into the enclosures. A total of five different prey treatments were used. Three different predator treatments, including controls (no predator), were also used. All trials were replicated four times for a total of 60 experimental enclosures (3 predator treatments x 5 prey treatments x 4 replicates). Three individuals per size-class were used in enclosures bearing all the prey species for a total of 36 individuals. Predator-prey combinations were randomly assigned to the experimental enclosures. Green and rock crabs weighted 61.43 [+ or -] 10.51 g and 103.89 [+ or -] 29.27 g (mean [+ or -] SD, n = 20), respectively. All enclosures were sieved 4 d after the introduction of crabs and the mortality rate of bivalves was determined.
Predation Trials Between Green Crabs, Common Starfish and Moon Snails
The experimental design in experiment 2 was similar to the one described in experiment 1. Trials were designed to have single- and multiple -choice experiments. Prey densities were the same as described in experiment 1. Five different prey treatments were used. Four different predator treatments, including controls (no predator), were also used. All trials were replicated three times for a total of 60 experimental enclosures (4 predator treatments x 5 prey treatments x 3 replicates). Individuals, within each predator group, had a similar weight (mean [+ or -] SD, n = 15): green crab: 59.47 [+ or -] 9.14 g; common starfish: 8.93 [+ or -] 3.69 g; moon snail: 15.13 [+ or -] 2.26 g. Again, all enclosures were sieved 4 d after the introduction of predators and the mortality rate of bivalves was determined.
The data were analyzed with SPSS 11.5 for Windows. All analyses were calculated at a confidence level of P < 0.05. Mortality rates for each prey species were arcsin transformed to meet normality and homoscedasticity requirements and analyzed with a 3-way ANOVA in each experiment. The factors were predator species, prey species and size-class. Control data were not included in the ANOVA models because of the numerous zero values (high survival rates of prey in the absence of predators). Tukey multiple comparisons were applied when significant differences were found on factors.
Predation Trials Between Green Crabs and Rock Crabs
Single-choice Experiment The green crab preyed on all species and all size-classes except for large eastern oysters (25-40 mm size-class) (Fig. 1A). Mean mortality rates for each species were, however, all under the 50% level. Blue mussels and soft-shell clams were the preferred prey species. For all prey species, small individuals (0-15 mm) were preyed on more heavily. Overall, results showed that about 21% of all prey available was eaten by the green crabs (all trials, all species and all sizes confounded), from which 35% and 27% of mussels and clams, respectively, were eaten (all trials and all sizes confounded). The overall numbers also showed (all trials and all species confounded) that 35% of the 0-15 mm individuals were eaten compared with 20% for the 15-25 mm individuals and 8% for the 25-40 mm individuals. Mortality patterns were similar for rock crabs in the single-choice experiment (Fig. 1A). Again, results showed that rock crabs preferred clams and mussels and that small size individuals were preyed more heavily. For instance, almost 80% of the small clams (0-15 mm) were eaten. Except for this particular case, mean mortality rates were, however, all under the 40% level. Rock crabs did not prey on quahogs. Overall, results showed that 22% of all prey was eaten by the rock crabs (all trials, all species and all sizes confounded), from which 47% and 28% of clams and mussels, respectively, were eaten (all trials and all sizes confounded). The overall numbers also showed (all trials and all species confounded) that 38% of the 0-15 mm individuals were eaten compared with 14% for the 15-25 mm individuals and 13% for the 25-40 mm individuals. A low number of individuals died in the control treatments during the single-choice experiment. All clams survived. The ANOVA analysis (Table 1) confirmed that both crab species preferred small individuals of soft-shell clams and blue mussels. No interaction effect was observed.
[FIGURE 1 OMITTED]
Multiple-choice Experiment As observed in the single-choice experiment, green crabs preyed on all species and all size-classes (Fig, 1B). Mean mortality rates were over 50% in various cases: mussels and oysters <25 mm and small (0-15 mm) clams. Mussels and oysters were the preferred prey species. Overall, results showed that 38% of all prey available was eaten by the green crabs (all trials, all species and all sizes confounded) compare with 21% in the single-choice experiment. About 64% and 50% of mussels and oysters, respectively, were eaten (all trials and all sizes confounded). The overall numbers also showed (all trials and all species confounded) that 50% of the 0-15 mm individuals were eaten compared with 52% for the 15-25 mm individuals and 13% for the 25-40 mm individuals. In other words, about 83% of mussels and 75% of oysters <25 mm were eaten by the green crabs compared with 58% of the small clams (0-15 mm) in the multiple-choice experiment. Rock crabs preyed more on mussels and oysters as well, except for large oysters (Fig. 1B). No quahogs were preyed on. In contrast to results observed in the single-choice experiment, clams were not preyed on by rock crabs. Overall, results showed that only 11% of all prey available was eaten by rock crabs (all trials, all species and all sizes confounded), from which 33% and 8% of mussels and oysters, respectively, were eaten (all trials and all sizes confounded). Only 1 soft-shell clam of the 27 specimens exposed was eaten. This is in contrast to results observed in the single-choice experiment where about 47% of clams were eaten by rock crabs. All individuals survived in the control treatments. The ANOVA analysis (Table 2) confirmed two interaction effects. Overall, the low mortality rates observed for the clams in rock crab treatments probably explained a large proportion of the variability related to prey species x size-class and predator species x size-class interactions as well as the significant effect from the predation species.
Predation Trials Between Green Crabs, Common Starfish and Moon Snails
Single-choice Experiment Results from this experiment confirmed that green crabs preyed on all the studied prey species (Fig. 2A). Blue mussels and soft-shell clams were again the first and second preferred prey species, respectively. The relation with the size of prey was however not as clear as in Experiment 1. This was particularly true with mussels and clams. The common starfish was also very active during the trials (Fig. 2A). Mussels, as observed in green crab treatments, were the preferred prey species. The eastern oyster, however, represented the second preferred prey species. The starfish did not prey on other prey species. Again, small individuals were preyed on more heavily. The moon snail was the least active predator (Fig. 2A). No predation was observed on quahogs and oysters. Mean mortality rates for mussels and clams were below 10%. Mortality rates in control treatments were 0% regardless of the size-class for all prey species, except for clams (Fig. 2A). Clams suffered low mortality in all size-classes (ca 5%). This means there could be a slight overestimation of mortality rates for clams. The ANOVA analysis (Table 3) underlined a strong predator species x prey species x size-class interaction.
[FIGURE 2 OMITTED]
Multiple-choice Experiment Trials where all prey species were challenged with a single predator showed different results from what was observed in the previous single-choice experiment. The green crab preyed on all prey species except for quahogs, whereas the common starfish preyed on all prey species including small-size quahogs (Fig. 2B). The moon snail was again the least active predator. Large-size quahogs were, however, preyed on in the moon snail trials. Low mortality rates were observed in small-size oysters and mussels and midsize clams (<5%) in the control treatments. The overall effect of size-class was not as clear as observed in the other experiments. The ANOVA analysis (Table 4) failed to show any significant effect from predator species, prey species and size-class. No interaction was observed.
The present laboratory experiments suggest that the green crab, rock crab and common starfish may be important predators in bivalve aquaculture sites off PEI shores. Single-choice experiments showed that blue mussels were selected more often by these predators compared with moon snails. This was particularly true for small-size individuals (<25 mm). O'Neill et al. (1983) and ap Rheinalt (1986) documented similar results with seastars and crabs (including green crabs), respectively, versus cultivated blue mussels. This suggests that large mussels probably attained a partial prey refuge in size (at ~25 mm) from predators. Several authors came to the same conclusions while studying the relationship between various species of crabs and bivalve species, including the ones between the green crab and the blue mussel (e.g., Elner 1980, Townsend & Hughes 1981, Navarrete & Castilla 1988, Eggleston 1990, Juanes 1992, MacNair, personal observation) and soft-shell clam (e.g., Welch 1968, Cohen et al. 1995, Beal et al. 2001, Floyd & Williams 2004). Large-size prey may be more difficult to handle by crabs (manipulation with the chela) as well as being more difficult to open by seastars (large prey have stronger adductor muscles). This response may, however, vary in relation to the size of the predator. Eggleston (1990), for instance, observed that large-size crabs feed on large eastern oysters (>45 mm) because larger individuals display an increased crushing strength. Furthermore, male crabs exhibit a larger crusher chela height, and in turn, more strength, than a female of the same year class (Hines et al. 1990).
Soft-shell clams and quahogs are both infaunal species. The fact that both species bury themselves in the sediments may provide them a spatial refuge from some predators. However, our results suggest that crabs were able to detect and handle soft-shell clams without any difficulty. The shape and hardness of quahogs may offer some protection from predators compared with softshell clams that have a weaker shell (Blundon & Kennedy 1982a, Boulding 1984) as well as exposed siphon and pedal gapes (Dare & Edwards 1981, Boulding 1984). Burial depth, for soft-shell clams, is apparently the only refuge from epibenthic predators (Blundon & Kennedy 1982b, Whitlow et al. 2003). Because the green crab may dig pits up to 15 cm deep (Ropes 1968, Lindsay & Savage 1978), the predation level observed during this study may be higher than in nature where burial depth is not limited. This may be particularly true for the large individuals. Large soft-shell clams have the tendency to be found deeper (ca 15 cm) in the sediment compare with small individuals (Zaklan & Ydenburg 1997).
The multiple-choice experiment suggests that the predation behavior of green crab and common starfish may be more general when several prey species are challenged at the same time. Results showed that prey species were more homogeneously selected compared with single-choice trials. This was particularly true for starfish that also preyed on small-size quahogs. Various feeding studies showed that green crabs might feed on various taxonomic groups including polychaetes, gastropods and barnacles (e.g., Le Calvez 1984, Williams 1984). Field trials conducted in 2000 by the Department of Fisheries, Aquaculture and Environment of PEI (DFAE) showed, however, that in multiple-choice experiments with green crabs, mussels were the preferred food over soft-shell clams and oysters (MacNair, pers. comm.). Though they were able to find and eat buried soft-shell clams in the single-choice experiment, rock crabs preferred mussels and oysters in the multiple-choice experiment. Our results also showed that rock crabs might not be as efficient when facing multiple prey species at the same time. This suggests that the rock crab might spend more time searching for certain prey species compared with the green crab, which seems to exhibit opportunistic behavior.
Single-choice and multiple-choice experiments suggest that moon snails were the least active predator. Observed predation rates were generally low for all prey species. This was a surprising result because soft-shell clams and blue mussels are reported to be heavily preyed upon by this gastropod (Vencile 1997). The percentage of live clams demonstrating moon snail drill holes, however, was high in this study. This was also the case in the DFAE field study carried in 2000 (MacNair, pers. comm.). A longer experimental period probably would give a higher mortality rate than that observed in the laboratory.
Conclusion and Recommendations
Our results confirmed that the native rock crab and common starfish, as well as the invasive green crab, are potential threat for various PEI commercial bivalves, particularly for small-size individuals (<25 mm). The moon snail did not appear to be an important predator except for large-size quahogs (25-40 ram) and midsize clams (15-25 mm). Our results suggest that the status of the green crab around the coasts of PEI should continue to be monitored yearly and that PEI shellfishery operations should direct their effort in protecting small-size individuals from predators until they reach a size refuge. This is particularly true for ventures that harvest commercial species in a natural setting where predators have a direct access to their prey. Laboratory and field experiments should be designed to test antipredation techniques adapted to specific aquaculture operations. Predator exclusion by fencing is a possibility in the intertidal habitat, The use of certain prey species to redirect predators away from a given species may also bear interesting results for infaunal species harvested in tidal flats as well as for species suspended above the sea floor. For instance, our results tend to show that mussels could be used by farmers to attract predators and reduce oysters or soft-shell clam mortalities. The development of a fishery for green crabs could also help to diminish economic losses as well as to generate direct revenues. Small newly-molted green crabs, for instance, are in high demand in many Mediterranean countries (Peter J. W. Olive, pers. comm.).
The similar feeding diet between green crabs and rock crabs as underlined in the single choice-experiment show that green crabs may represent a competing threat to rock crabs. Though green crabs are usually found in more brackish tidal environments, green crabs may be also found in other types of environment including the ones of the rock crabs (Neal MacNair, pers. comm., Dominique Audet, personal observation). Their ubiquitous distribution, diversified feeding diet, tolerance to extreme field conditions and their more aggressive behavior may give them an advantage over the rock crabs. Studies should now look more precisely at the relationship between these two species in the context of a boreal environment (e.g., feeding and agonistic behavior in relation to water temperature). The development of a green crab fishery could also help the native rock crab.
The authors thank Daniel Bourque, Angeline LeBlanc and Bruno Frenette for their help in the field and in the laboratory; also Roy Winn and particularly Paul Burleigh who greatly facilitated their lives at the hatchery. Comments provided by two anonymous referees helped improve the clarity of the text. Funding for this research was provided by a research contract from the PEI Aquaculture & Fisheries Research Initiative (AFRI) to G. M. and T. L. and a Universite de Moncton (Faculte des etudes superieures et de la recherche) grant to G. M. The Department of Fisheries and Oceans and Universite de Moncton provided internships to D. A.
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GILLES MIRON, (1) * DOMINIQUE AUDET, (1) THOMAS LANDRY (2) AND MIKIO MORIYASU (2)
(1) Departement de biologie, Universite de Moncton, Moncton, Nouveau-Brunswick, Canada E1A 3E9;
(2) Centre des Peches du Golfe, Peches et Oceans Canada, C. P. 5030, Moncton, Nouveau-Brunswick, Canada E1C 9B6
* Corresponding author. E-mail: email@example.com
TABLE 1. Three-way ANOVA examining the effect of predator species, prey species and prey size-classes on mortality rates in single-choice experiments. Data were arcsin transformed. Multiple comparisons were carried out with Tukey multiple comparisons test. Prey species and size-classes are presented in increasing order of mortality rates. Non significant differences among prey species and size-classes are underlined. Source of Variation SS DF MS F P Predator 4.27 1 4.27 0.10 0.922 Prey 14902.88 3 4967.63 11.31 <0.001 Size 7796.27 2 3898.14 8.88 <0.001 Predator x Prey 1586.71 3 528.90 1.21 0.314 Predator x Size 536.10 2 268.05 0.61 0.546 Prey x Size 2700.77 6 450.13 1.03 0.416 Predator x Prey x Size 964.13 6 160.69 0.37 0.898 Error 31614.08 72 439.08 Total 60105.21 95 Multiple comparisons: Prey species Quahaugs Eastern oysters Mussels Soft-shell clams Size-classes (mm) 25-40 15-25 0-15 TABLE 2. Three-way ANOVA examining the effect of predator species, prey species and prey size-classes on mortality rates in multiple-choice experiments. Data were arcsin transformed. The interaction effects were examined using a Tukey test. Prey species and size-classes are presented in increasing order of mortality rates. Non significant differences among prey species and size-classes are underlined. Source of Variation SS DF MS F P Predator 13887.91 1 13887.91 35.33 <0.001 Prey 21094.30 3 7031.43 17.89 <0.001 Size 6879.88 2 3439.94 8.75 <0.001 Predator x Prey 2182.37 3 727.46 1.85 0.146 Predator x Size 4246.70 2 2123.35 5.40 0.007 Prey x Size 8490.98 6 1415.16 3.60 0.004 Predator x Prey x Size 4627.27 6 771.21 1.96 0.082 Error 28303.57 72 393.11 Total 89712.98 95 Multiple comparisons Predator species Prey size-classe(mm) Green crab: 25-40 0-15 15-25 Rock crab: 25-40 0-15 15-25 Prey species Prey size-(m m) Blue mussels: 25-40 0-15 15-25 Eastern oysters: 25-40 0-15 15-25 Soft-shell clams: 25-40 15-25 0-15 Quahaugs: 25-40 15-25 0-15 TABLE 3. Three-way ANOVA examining the effect of predator species, prey species and prey size-classes on mortality rates in single-choice experiments. Data were arcsin transformed. The interaction effects were examined using a Tukey test. Prey size-classes are presented in increasing order of mortality rates. Non significant differences among prey species and size-classes are underlined. Source of Variation SS DF MS F P Predator 1663.25 2 831.63 9.02 <0.001 Prey 1606.60 3 535.54 5.81 0.001 Size 681.40 2 340.70 3.70 0.030 Predator x Prey 2499.09 6 416.52 4.52 0.001 Predator x Size 404.69 4 101.17 1.10 0.365 Prey x Size 2100.85 6 350.14 3.80 0.002 Predator x Prey x Size 4412.41 12 367.70 3.99 <0.001 Error 6639.38 72 92.21 Total 20007.67 108 Multiple comparisons Green crab Prey species Prey size-classes (mm) Blue mussels: 25-40 0-15 15-25 Eastern oysters: 25-40 15-15 0-15 Soft-shell clams: 0-15 15-25 25-40 Quahaugs: 15-15 25-40 0-15 Common seastar Prey species Prey size-classes (mm) Blue mussels: 25-40 15-25 0-15 Eastern oysters: 25-40 15-25 0-15 Soft-shell clams: 25-40 15-25 0-15 Quahaugs: 25-40 15-25 0-15 Moon snail Prey species Prey size-classes (mm) Blue mussels: 25-40 0-15 15-25 Eastern oysters: 25-40 15-15 0-15 Soft-shell clams: 15-25 25-40 0-15 Quahaugs: 25-40 15-25 0-15 TABLE 4. Three-way ANOVA examining the effect of predator species, prey species and prey size-classes on mortality rates in multiple-choice experiments. Data were arcsin transformed. Source of Variation SS DF MS F P Predator 1102.59 2 551.30 2.19 0.119 Prey 193.56 3 64.52 0.26 0.856 Size 39.51 2 19.76 0.08 0.924 Predator x Prey 681.26 6 113.54 0.45 0.841 Predator x Size 292.20 4 73.05 0.29 0.883 Prev x Size 996.10 6 166.02 0.66 0.682 Predator x Prey x Size 2974.92 12 247.91 0.99 0.470 Error 18097.34 72 251.35 Total 24377.47 108
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|Publication:||Journal of Shellfish Research|
|Date:||Aug 1, 2005|
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