Aquacult Int (2011) 19:381–394 DOI 10.1007/s10499-010-9380-1
Development of larval culture techniques for the shore crab, Carcinus maenas (L) Thomas H. Galley • Benjamin C. Green • Lloyd Watkins Lewis Le Vay
•
Received: 13 April 2010 / Accepted: 15 September 2010 / Published online: 2 October 2010 Ó Springer Science+Business Media B.V. 2010
Abstract The shore crab Carcinus maenas is a commercially important species, utilised as sea angling bait as well as supporting a European-wide fishery. Hatchery production could provide an alternative source of bait crabs, alleviating potential competition between these sectors and environmental concerns regarding bait collection practices. A series of experiments were carried to investigate the potential for hatchery production, focusing on effects of dietary regimes and stocking densities through the zoeal stages and the influence of tank substrates and stocking density during the megalopa stage. Inclusion of the rotifer Brachionus plicatilis as live food for early larval stages conveyed no advantage in terms of survival or rate of development compared to a diet of Artemia nauplii. Increasing zoea stocking densities (from 94 to 557 l-1) had a significantly negative effect upon survival to the megalopa stage (from 75% down to 47%), although this was off-set by a significant increase in production, with 260 megalopae-1 produced from an initial density of 557 zoeae l-1. The inclusion of substrates for megalopa stages had no impact on production or development rate, compared to tanks with no substrate. The completely benthic behaviour of megalopae indicates that tank floor area will be a limiting factor for crab production. Increasing stocking density of megalopae was found to significantly and negatively influence survival, although above 10,000 megalopae m-2 the rate of decline in survival stabilised and maximum production (3,114 juveniles m-2) of juvenile crabs could be achieved at the highest stocking densities tested (40,000 m-2). Keywords Hatchery
Shore crab Carcinus maenas Larvae Stocking density Substrate
T. H. Galley (&) B. C. Green L. Le Vay School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey LL59 5EY, UK e-mail:
[email protected] L. Watkins JW Aquaculture Ltd, 128 Gnoll Park Road, Neath, Port Talbot, Wales, UK
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Introduction Sea angling is a highly popular activity in the UK, with an estimated 1 million people participating in England and Wales. The sport is heavily reliant on wild caught intertidal marine animals to serve as fishing bait (Fowler 1999; Drew Associates 2004), with an overall market for bait species estimated to be worth between £25 and £30 million annually (Fowler 1999). The green shore crab, Carcinus maenas, is commonly used as bait, being collected just prior to or just after ecdysis. Supply is completely reliant on the collection of animals from the wild and so is seasonal and weather dependent. Traditionally, shore crabs have been collected from beneath boulders near the low tide line on rocky shores, where moulting crabs hide from predators (Cryer et al. 1987; Fowler 1999). Collection has also extended to soft sediment shores, with thousands of artificial crab shelters, such as tiles and pipes, being laid and routinely searched, especially in estuarine areas of southwest England (Cryer et al. 1987; Fowler 1999). However, these methods of collection of shore crabs for bait have raised concerns over their environmental impacts, including disruption and damage caused by boulder turning on rocky shores (Cryer et al. 1987; Fowler 1999), crab shelters on muddy shores altering the natural habitat and causing hazards to other shoreline users, disturbance to bird populations, as well as impacts upon the visual environment (Fowler 1999). One approach to address concerns surrounding bait collection, whilst also increasing reliability of supply, is to develop aquaculture production of bait species. In Europe, this has already been achieved for polychaete species such as the rag worm Nereis virens, and the lug worm Arenicola marina (Olive 1999); whilst in the USA hatchery production of penaeid shrimp as angling bait is well established (Davis and Arnold 1998). The shore crab is also exploited in a small European-wide fishery with annual catches of approximately 700–1,400 tons, with the UK supplying up to a third of the total catch (FAO 2010). It is anticipated that development of hatchery production of bait crabs could provide a safeguard for the future of this fishery and help alleviate conflict between bait collecting and commercial fishing sectors. Larval culture techniques for larval C. maenas are well documented at the laboratory scale (Williams 1968; Dawirs 1982, 1985; Dawirs and Dietrich 1986; Dawirs et al. 1986; Mohamedeen and Hartnoll 1989; Harms et al. 1994). However, many of these studies have been concerned with physiology and developmental biology and hence have focused on the culture of individual animals in isolation. Hence, there is still a need to establish commercially relevant techniques for the mass production of crab larvae. As a first step towards this objective, the present study comprised a series of experiments that investigated the effect of diet and stocking density on the development of zoeae and also the influence of stocking density and the use of substrates in the development of megalopae to metamorphosis.
Materials and method Broodstock maturation and hatching Ovigerous female Carcinus maenas, with egg masses at varying stages of maturation, were collected from intertidal and sub-littoral areas of the Menai Strait, North Wales, UK, during the winter of 2004/2005 and the spring of 2005. Crabs were held in individual flowthrough aquaria at a temperature of 17–22°C and salinity of 32%, as supplied from the
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Menai Strait. The 8-litre aquaria (33 9 17 9 15 cm) were provided with constant gentle aeration and illumination. They were fed every other day with fresh and frozen mussel, Mytilus edulis. Waste material and shed eggs were removed daily from aquaria. Prior to hatching the aquaria were converted into a static system, with daily water changes. Hatched larvae were concentrated by phototaxis and gently siphoned into a submerged 300–500-lm mesh sieve. Estimates of the number of collected larvae were made from replicate sub-samples. The carapace width (mm) and wet weight (g) of each broodstock crab were recorded prior to release. Live food production The rotifer Brachionus plicatilis (L-strain) was cultured indoors in 26 l vessels fed on the algae Tetraselmis chuii. Cultures were operated on a semi-continuous mode. Prior to feeding to larvae, rotifers were harvested through an 80-lm-mesh sieve and rinsed with filtered and UV-sterilised seawater. Artemia nauplii (Vinh Chau strain; Aquaculture and Fisheries Science Institute, College of Agriculture, Can Tho University, Vietnam) were hatched based upon the method described by Lavens and Sorgeloos (1996). Nauplii were rinsed in filtered and UV-sterilised seawater prior to use in feeding. Zoeae rearing experiments Experiment 1: Artemia versus rotifers Recently hatched larvae obtained from a single parent female were stocked at a density of 100 l-1 in triplicate 2 l round-bottom glass flasks containing seawater at a salinity of 32%. Culture flasks were provided with gentle aeration and constant illumination. Zoeae were fed using three different dietary treatments shown in Table 1. All diets were provided at a concentration of 10 organisms ml-1 day-1. All culture flasks were held in a temperaturecontrolled water bath maintained at 20 ± 1°C. Flasks were operated as static environments with 100% water changes conducted daily. Experiment 2: Zoeae stocking density Recently hatched larvae released from a single female were reared at nine different stocking densities: 94, 162, 219, 276, 344, 449, 497 and 557 zoeae l-1. Culture vessels Table 1 Dietary treatments used to culture Carcinus maenas larvae from zoea 1 to megalopa
Artemia nauplii = newly hatched Artemia nauplii
Day
Dietary treatment 1
2
3
1
Rotifer
Rotifer
Artemia nauplii
2
Rotifer and Artemia nauplii (ratio 1:1)
Rotifer
Artemia nauplii
3
Artemia nauplii
Rotifer
Artemia nauplii
4
Artemia nauplii
Rotifer and Artemia nauplii
Artemia nauplii
5
Artemia nauplii
Artemia nauplii
Artemia nauplii
6?
Artemia nauplii
Rotifer = Brachionus plicatilis
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were 2 l round-bottom flasks as described above. All density treatments were fed daily with newly hatched Artemia nauplii at a concentration of 10 nauplii ml-1. Counts were made of survival at the megalopa stage. All other conditions and practices were identical to experiment 1.
Megalopa stage rearing experiments Experiment 3: Effect of culture substrate Megalopae were stocked at a density of 926 m-2 in 8 l polypropylene aquaria lined with three different substrates. The substrate treatments were cockle shells, mesh pad and ceramic tile. A control treatment had no substrate added. All treatments were conducted in triplicate. Aquaria were attached to a recirculation system providing a gentle inflow of filtered seawater, and they were also provided with gentle aeration, constant illumination and maintained at 20°C. Stocked animals were fed daily with frozen adult Artemia biomass (Gamma Foods) equivalent to 20 gm-2, supplemented with live Artemia nauplii at a concentration of 10 nauplii ml-1 for the first 3 days of the experiment. Experiment 4: Megalopae stocking density Megalopae were stocked in 1 l glass beakers with a base area of 86.7 cm-2 at densities equivalent to 1,250, 2,500, 5,000, 10,000, 15,000, 20,000, 30,000 and 40,000 m-2. All beakers were aerated and connected to a recirculation system providing constant renewal with filtered seawater. The feeding regime was as described for experiment 3.
Evaluation criteria All experiments, with the exception of experiment 4, were concluded once all stocked larvae had reached either the megalopa or juvenile C1 crab stages. The survival of crab larvae was determined in all experiments from total counts made in all treatment replicates. In experiments 1 and 3, surviving number of megalopae and juvenile crabs were used to calculate production per l-1 and m-2, respectively. The occurrence of metamorphosis into megalopae and into juvenile crabs, respectively, was also monitored daily to identify the mean length of time for the development of 50% of stocked animals. In experiments 2 and 4, the number of megalopae and juvenile crabs respectively at the end of the experiments was used to calculate final survival (%) and estimate production, allowing analysis of the relationship with animal stocking density. In experiment 4, counts of surviving numbers of megalopae and juvenile crabs were made after 7 days, equivalent to a point where [75% of crabs had reached metamorphosis in experiment 3, with no removal of juvenile crabs during the course of the experiment.
Statistical analysis Probit analysis was applied to determine the time at which 50% of stocked animals developed to the experimental endpoint (megalopae or juvenile), followed by one-way
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analysis of variance (ANOVA) to compare 50% values across treatments, which was also used to compare mean production rates. Linear regression analysis was applied to relationships between stocking density and survival and production of animals in experiments 2 and 4. In experiment 4, a semi-logarithmic transformation was applied to stocking density before analysis of survival rate of juvenile crabs in order to meet the assumptions for analysis. The Anderson–Darling test was used to investigate departure from normality and Bartlett’s test was used to assess heteroscedasticity before applying any test of comparison (Sokal and Rohlf 1995). All data analysis was carried out using the MinitabÒ software package.
Results Broodstock maturation Laboratory incubation of wild female shore crabs carrying egg masses provided a reliable supply of zoeal stage crab larvae. Figure 1 shows the relationship between number of zoeae released by female crabs with increasing size (wet weight, g) (correlation coefficient 0.675, P = 0.001). Regression analysis indicates that this relationship is significant (F = 16.77, P = 0.001, df = 1) and the linear relationship between zoeae production (y) and wet weight in grams (x) is best described by the equation: y = 2,606x ? 2,871.5.
Zoeae rearing experiments Experiment 1: Artemia versus rotifers There was no significant difference in the mean production of megalopae between the three dietary treatments (ANOVA: F = 0.33, P = 0.731) (Table 2).
Number of zoeae produced
250,000
200,000
150,000
100,000
50,000
0 20
25
30
35
40
45
50
55
Wet weight (g) Fig. 1 Relationship between wet weight (g) of female crabs and production of zoeae larvae in laboratoryincubated Carcinus maenas
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Table 2 Mean production of megalopae (l-1) and mean time to 50% metamorphosis (days post hatch) in Carcinus maenas larvae reared from Z1 to the megalopa stage on three different dietary treatments (± standard deviation) Treatment
Production (megalopae l-1)
Time to 50% metamorphosis (days)
1
62.0 (±4.0)
15.1 (±0.5)
2
64.2 (±15.0)
15.1 (±0.6)
3
69.2 (±11.3)
14.6 (±0.2)
No significant difference between treatments
Mean Number of Megalopae
80 70
Treatment 1 Treatment 2
60
Treatment 3
50 40 30 20 10 0 13
14
15
16
17
18
19
20
21
22
23
Days Post Hatching Fig. 2 Occurrence of megalopae in the zoeal stages over time (days post hatching) in Carcinus maenas reared on three dietary regimes (Table 1). Values are means ± standard deviation
Megalopae appeared after 13 days, with peak development between 14 and 15 days after hatching, although some larvae took up to 23 days to reach the megalopa stage (Fig. 2). There was no significant difference between time at which 50% of larvae became megalopae between the three treatments (ANOVA: F = 1.21, P = 0.362). Experiment 2: Zoeae stocking density Figure 3 shows the relationships of both survival (%) and production (l-1) of megalopae with initial stocking density. With increasing density, the survival of zoeae to the megalopa stage declines significantly (regression analysis: F = 7.17, P = 0.037) from 75% at 94 megalopae l-1 to 47% at 557 megalopae l-1. In comparison, megalopae production l-1 increases significantly as stocking concentration increases (regression analysis: F = 70.03, P = \ 0.001). At the highest stocking density, 260 megalopae were produced l-1. The relationship between survival % (y) and stocking density l-1 (x) is best described by the equation: y = -0.046x ? 70.34, while the relationship between production of megalopae (y) and stocking density l-1 (x) is best described by the equation: y = 0.41x ? 34.9.
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(a) Survival
80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00 0.00 0
100
200
300
400
500
600
500
600
-1
(b) Production
300
-1
Production (megalopae l )
Initial stocking density (larvae l )
250 200 150 100 50 0 0
100
200
300
400 -1
Initial stocking density (larvae l ) Fig. 3 Relationship between initial stocking density of Carcinus maenas zoea 1 larvae and a survival of larvae to the megalopa stage; and b production of megalopae
Megalopae rearing experiments Experiment 3: Effect of culture substrate Table 3 shows the estimated production of juvenile crabs (m-2) and the mean time taken for 50% of stocked animals to reach metamorphosis. Production ranged from 277.8 ± 56.6 to 500.0 crabs ± 103.1 m-2, and time taken to achieve 50% metamorphosis ranged from 5.7 ± 0.3 to 6.6 ± 0.8 days. Figure 4 shows the pattern of juvenile development over the course of the experiment, with metamorphosis starting from 5 days post stocking. There was no significant difference between treatments for either production (ANOVA: F = 3.05, P = 0.092) or development rate (ANOVA: F = 3.05, P = 0.092). Experiment 4: Megalopae stocking density Survival of megalopae larvae to metamorphosis declined rapidly with increasing stocking density (Fig. 5). The lowest stocking density, 1,250 megalopae m-2, exhibited the highest survival (67.5% ± 4.6), which was 8.7 times higher than the survival at the highest
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Table 3 Mean estimated production of juvenile crabs (m-2) and mean time to 50% metamorphosis (days post stocking) for Carcinus maenas reared from megalopa to crab stage 1 in the presence of different substrates (±standard deviation) Treatment
Production (juveniles m-2)
Time to 50% metamorphosis (days)
No substrate
500.0 (±103.1)
6.0 (±0.5)
Mesh
413.6 (±140.2)
5.9 (±0.2)
Cockle shell
277.8 (±74.1)
6.6 (±0.8)
Tile
475.3 (±56.6)
5.7 (±0.3)
No significant differences between treatments
stocking density of 40,000 megalopae (7.8%). The relationship between survival %, (y) and stocking density m-2, (x) is best described by the equation: y = 140–13.3 logx. In contrast, production of juvenile crabs increased significantly with increasing megalopae stocking density (regression analysis: F = 24.59, P \ 0.001) with 3,114 juvenile crabs produced at a stocking density of 40,000 m-2. The relationship between production of juveniles m-2 (y) and stocking density m-2 (x) is best described by the equation: y = 0.0715x ? 468.
Discussion Under the laboratory conditions applied here, a reliable supply of C. maenas larvae was achieved with a clear positive relationship between weight of female crabs and production of crab larvae. A strong correlation between reproductive output and body size has been previously demonstrated in many brachyuran crabs (Hines 1982; Shields 1991; Haddon 1994; Hamasaki et al. 2006; Tallack 2007) including C. maenas (Audet et al. 2008). The relationship between fecundity and female size observed in the present study is consistent with previous data for wild C. maenas. Audet et al. (2008) observed a minimum size of females carrying egg masses in natural populations to be 38.74 mm (carapace width, CW), which is similar to the size of the smallest female with an egg mass in our study (34.0 mm carapace width, 10.1 g wet weight). In the present study, females between 40 and 55 mm carapace width released fewer (50,000–100,000) viable zoeae than expected from previous data (Audet et al. 2008). As ovigerous female crabs were sourced from the wild at varying stages of embryo development, this may in part be due to egg losses due to handling and transfer to the laboratory in addition to expected egg loss during incubation, as reported in a range of portunid species (Kuris 1991; Hamasaki et al. 2006; Audet et al. 2008). In this respect, closure of the life cycle through rearing of captive broodstock would be beneficial, providing increased control over larval supply as well as further reducing the potential for conflict arising between different users of this resource. Mating and spawning in C. maenas are readily achieved in captivity (pers. obs), but further work is required on the optimisation of husbandry conditions during maturation, particularly the influence of broodstock nutrition on larval quality, for which there is very little previous data compared to some other crab species. For comparison, see Quinitio (in press, this volume) for review of the process of domestication of the portunid Scylla serrata, Larval dietary regime has been shown to influence both survival and rate of development in the early larval stages of a number of crab species cultured under laboratory conditions, including Scylla serrata (Ruscoe et al. 2004), Callinectes sapidus (Sulkin 1975; Sulkin 1978) Maja brachydactyla (Andre´s et al. 2007) and Cancer anthonyi (Anderson and
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Juvenile crabs (m-2)
250
(a) Cockle shell
200 150 100 50 0 3
5
7
9
11
Experimental period (days)
Juvenile crabs (m-2)
160
(b) Mesh pad
140 120 100 80 60 40 20 0 3
5
7
9
11
Experimental period (days)
Juvenile crabs (m-2)
250
(c) Tile
200 150 100 50 0 3
5
7
9
11
Experimental period (days) 300
Juvenile crabs (m-2)
Fig. 4 Occurrence of metamorphosis of megalopae to juvenile crab over time for Carcinus maenas cultured in the presence of four substrates: a cockle shell, b mesh pad, c tiles and d no substrate. Values are mean ± standard deviation
389
(d) No substrate
250 200 150 100 50 0 3
5
7
9
11
Experimental period (days)
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(a) Survival
Juvenile crab survival (%)
90 80 70 60 50 40 30 20 10 0 0
5000
10000 15000
20000
25000
30000 35000
40000 45000
-2
No. of juvenile crabs produced
Initial stocking density (megalopae m )
(b) Production
4500 4000 3500 3000 2500 2000 1500 1000 500 0 0
5000
10000 15000
20000 25000 30000
35000 40000
45000
-2
Initial stocking density (megalopae m ) Fig. 5 Relationship between initial stocking density of Carcinus maenas megalopa and a survival (%) of larvae to the C1 juvenile crab stage; and b production of C1 juvenile crab stages. Values are mean ± standard deviation
Ford 1976). The culture of C. maenas has primarily been used as a tool to study growth and development of brachyuran crab larvae and not the optimisation of culture technology. Early efforts to culture C. maenas determined that Artemia nauplii were a suitable larval food source for the development of the zoeal and megalopa stages through to the first crab stage, provided there was inclusion of a smaller food source during development from zoeae stage I to zoeae stage II (Williams 1968). Consequently, Artemia nauplii have been the most commonly used diet for the culture of C. maenas larvae, either as the sole diet or supplemented with the rotifer Brachionus plicatilis (Dawirs 1982; Dawirs 1985; Dawirs et al. 1986; Mohamedeen and Hartnoll 1989; Harms et al. 1994). Dawirs (1985) achieved a survival rate of 70% to the megalopa stage on just Artemia nauplii in individually reared larvae, and Dawirs (1982) achieved a similar survival rate on a diet of Artemia nauplii and rotifers (70%), whilst Mohamedeen and Hartnoll (1989) achieved a survival rate of approximately 90% using the same diet, also in individually reared larvae. The difference in survival between Mohamedeen and Hartnoll (1989) and Dawirs (1982) could be attributable to the rearing methods used; Mohamedeen and Hartnoll (1989) held larvae in a larger water volume that was increased as the larvae developed.
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The incorporation of rotifers in diets during larval crab culture has proved to be beneficial during early zoeal stages, with increased growth and survival in species such as S. serrata when fed up to zoeae stage 2 and then fed Artemia nauplii, compared to larvae fed a diet solely on Artemia nauplii (Ruscoe et al. 2004). In the present study, the benefit of including a prey organism smaller than Artemia nauplii during the initial stages of culture under mass-rearing conditions applicable to hatchery operations was evaluated. The results show that a diet of Artemia nauplii alone can be sufficient to sustain development to the megalopae stage, equivalent to that achieved with a combined rotifer-Artemia dietary regime, with no detrimental effects on either production or development rate. The survival rate in all three treatments under mass rearing conditions is an improvement on the 10% achieved by Dawirs (1982) but is comparable to the 68% achieved by Rice and Ingle (1975) under mass rearing conditions, although the latter study made use of antibiotics. The duration of larval development in all treatments of 20–23 days is comparable to 23.8–24.8 days reported by Dawirs (1982). For hatchery operations, this means that use of Artemia as a single and convenient food source is possible. However, the strain of Artemia used here is known to be of high quality particularly in terms of lipids (Nhgia et al. 2007), and further work is needed to investigate effects of fatty acid composition of live foods on larval performance in C. maenas. Recent studies have shown that several crab species can be cultured under intensive rearing conditions, including the blue crab C. sapidus, successfully reared at densities as high as 95 larvae litre-1 (Zmora et al. 2005), and the spider crab Maja brachydactyla, reared at densities as high as 100 larvae litre-1 (Andre´s et al. 2007). Past efforts to rear C. maenas larvae in mass culture conditions have met with varied success. Dawirs (1982) achieved survival to the megalopae stage of approximately 10% in larvae reared at 80 larvae l-1, Williams (1968) achieved a survival of up to 42.5% at 100 larvae l-1, whilst Rice and Ingle (1975) achieved up to 68% survival to the first crab stage from initial zoeae densities of 600–800 larvae l-1. In the present study, the best results in terms of survival to the megalopae stage were achieved at the lowest stocking density (94 larvae litre-1) which was comparable to survival at approximately this density in experiment 1. A decline in survival with increasing stocking density was also identified by Andre´s et al. (2007) in M. brachydactyla at densities between 50 and 100 larvae l-1, whilst Zmora et al. (2005) also found a similar, although not significant, relationship between larval density and survival in C. sapidus reared between 38 and 138 larvae l-1. However, in the present study, this decline is off set by the rate of megalopae production. Andre´s et al. (2007) recognised that although low stocking densities prove optimal for survival, they are not necessarily optimal for production. The optimum stocking density for mass production should be evaluated based upon the production of megalopae per unit volume rather than survival (Zmora et al. 2005). It is clear that over the densities tested, production of megalopae is maximised by using as high a zoeae stocking density as possible, assuming that supply of hatching larvae is not limiting. Present results indicate that larvae of C. maenas can be effectively cultured under intensive rearing conditions at densities of up to at 550 larvae l-1, at which point a survival rate of 50% can be achieved. Survival maybe boosted further through the use of antibiotics, as applied by Rice and Ingle (1975) at similar high densities; however, this is neither desirable nor necessary, as survival rates can be improved by reducing stocking densities. It has been demonstrated that portunid crab megalopae are able to differentiate between habitats, favouring habitats such as mussel beds, eelgrass and filamentous green algae and will actively avoid open habitat where they are at risk of predation (Hedvall et al. 1998; Moksnes et al. 1998). In laboratory rearing conditions, cannibalism has been identified as the main cause of mortality during the early megalopa and post-larval stages (Zmora et al.
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2005; Ut et al. 2007). In systems where cannibalism can be prevented by rearing larvae in isolation, survival rates of 59–90% to the first juvenile crab stage can be achieved from the first zoeae stage in C. maenas (Dawirs 1982; Mohamedeen and Hartnoll 1989). However, individual rearing is unrealistic for hatchery production. In recent studies, increased yield and survival has been attributed to the use of substrates within culture tanks for C. sapidus (Zmora et al. 2005), Paralithodes camtschaticus (Daly et al. 2009) and Scylla paramamosain (Ut et al. 2007), as a means of providing refuge to reduce cannibalism. For C. maenas, the incorporation of filamentous algae at low densities (105 megalopae m-2) has proved effective at reducing cannibalism to just 14% (Moksnes et al. 1998). In contrast, in the present study at a density of almost 1,000 megalopae m-2, the presence of a substrate did not improve production, with 54% of megalopae metamorphosising into juvenile crabs in bare tanks. The surfaces of the mesh pads used may have been too complex, as they were the most intricate substrate tested; C. maenas megalopae are known to avoid settlement on excessively complex structures because of potential for entrapment and predation (Moksnes et al. 1998; Hedvall et al. 1998). In the current study, megalopae were seen burrowing into the mesh matrix, possibly accounting for the lower survival observed. The development rate of megalopae was also not influenced by substrate, with 50% of megalopae metamorphosising between 5.7 and 6.6 days at 22°C. This is a shorter duration of the megalopae stage than reported by either Mohamedeen and Hartnoll (1989) (9.6 days at 20°C) or Dawirs (1982) (9.3 days at 18°C). Based on present results, the inclusion of a substrate in hatchery culture, at least in the range of stocking densities investigated, is unnecessary. However, the influence of substrates on production and development rate may alter at higher stocking densities than used in the present study, as Zmora et al. (2005), Ut et al. (2007) and Daly et al. (2009) all indicate the effectiveness of substrates at increasing survival in crab megalopae. As observed in the megalopae of C. sapidus by Zmora et al. (2005), the megalopae of C. maenas immediately became benthic, under the laboratory conditions applied in the present study. In the wild, C. maenas megalopae have an exogenously controlled vertical swimming rhythm, with active upward migration synchronised with the tidal cycle (Queiroga 1998). However, without the influence of natural tidal and day cycles, megalopa behaviour changes (Zeng and Naylor 1996). In the present study, under continuous lighting, megalopae remained on the floor of tanks throughout the day and therefore the benthic surface area of culture systems becomes critical from this phase. Our results demonstrate that stocking density of megalopae significantly affects survival and production of juvenile C. maenas, especially over the lower end of the range of densities tested. In some crab species, small increases at relatively low stocking densities have been found to have no effect upon survival (S. paramamosain, Ut et al. 2007), but present results are consistent with the negative effects of increasing stocking density in early post-larval and juvenile stages of other species (Zmora et al. 2005; Penha-Lopes et al. 2006; Daly et al. 2009). However, whilst higher stocking densities may result in lower percentage survival of megalopae, they can lead to substantially increased production of juvenile crabs per unit area. In a hatchery setting, accepting lower survival rates may be a compromise required to achieve maximised production (Zmora et al. 2005). In the present study, once megalopae stocking densities reached 10,000 m-2, survival plateaus at approximately 10%. If this holds true in larger scale systems, then crab production will become dependent upon availability of megalopae rather than survival from that point on. The results of the experiments reported here represent preliminary steps towards development of efficient mass-rearing techniques for the shore crab, C. maenas under hatchery conditions using separate systems for zoeal and megalopa stages. They
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demonstrate that a high level of efficiency can be achieved in the planktonic zoeal stages, with high survival rates on a simple feeding regime at very high larval densities. At the more benthic megalopa stage, very high stocking densities can be utilised to maximise crab production, but at the cost of very low survival. The economic viability of high-density megalopa production in a two-stage hatchery system will depend on the relative costs of the zoeal and megalopal phases of larval rearing. In the current study, megalopa production efficiency was based upon stocking densities per unit tank floor area, reflecting the benthic behaviour of the larvae. In contrast, Rice and Ingle (1975) reared C. maenas larvae at stocking densities of 600–800 larval l-1 to the first crab stage, achieving survival rates of up to 68% in a system designed to maintain larvae in suspension. Further comparison of these two approaches in scaled-up production would be worthwhile. Refinement of production techniques, such as optimisation of nutrition requirements, may also lead to improvements and the relatively low survival at high megalopa stocking densities may be alleviated through further investigation of the use of substrates and comparison with the use of kreisel tank designs to maintain megalopae larvae in suspension. Acknowledgments The authors would like to thank Berwyn Roberts and Gwyn Hughes of the School of Ocean Science, Bangor University and Trevor Jones (Extra Mussels Ltd) who supplied broodstock crabs for this work. This research was supported in part by a grant to JW Aquaculture Ltd from the European Union, Financial Instruments for Fisheries Guidance, project number 54298, administered by the Welsh European Funding Office.
References Anderson WR, Ford RF (1976) Early development, growth and survival of the yellow crab Cancer anthonyi Rathbun (Decapoda, Brachyura) in the laboratory. Aquaculture 7:267–279 Andre´s M, Este´vez A, Rotllant G (2007) Growth, survival and biochemical composition of spider crab Maja brachydactyla (Balss, 1922) (Decapoda: Majidae) larvae reared under different stocking densities, prey: larvae ratios and diets. Aquaculture 273:494–502 Audet D, Miron G, Moriyasu M (2008) Biological characteristics of a newly established green crab (Carcinus maenas) population in the Southern Gulf of St. Lawrence, Canada. J Shell Res 27:427–441 Cryer M, Whittle GN, Williams R (1987) The impact of bait collection by anglers on marine intertidal invertebrates. Biol Cons 42:83–93 Daly B, Swingle JS, Eckert GL (2009) Effects of diet, stocking density, and substrate on survival and growth of hatchery-cultured red king crab (Paralithodes camtschaticus) juveniles in Alaska, USA. Aquaculture 293:68–73 Davis DA, Arnold CR (1998) The design, management and production of a recirculating raceway system for the production of marine shrimp. Aquacult Eng 17:193–211 Dawirs RR (1982) Methodical aspects of rearing decapod larvae, Pagurus bernhardus (Paguridae) and Carcinus maenas (Portunidae). Helgola¨nder Meeresunters 35:439–464 Dawirs RR (1985) Temperature and larval development of Carcinus maenas (Decapoda) in the laboratory; predictions of larval dynamics in the sea. Mar Ecol Prog Ser 24:297–302 Dawirs RR, Dietrich A (1986) Temperature and laboratory feeding rates in Carcinus maenas L. (Decapoda: Portunidae) larvae from hatching through metamorphosis. J Exp Mar Biol Ecol 99:133–147 Dawirs RR, Pu¨schel C, Schorn F (1986) Temperature and growth in Carcinus maenas L. (Decapoda: Portunidae) larvae reared in the laboratory from hatching through metamorphosis. J Exp Mar Biol Ecol 100:47–74 Drew Associates (2004) Research into the economic contribution of sea angling. Final report to the Department for Environment, Food and Rural Affairs, 71 pp. http://statistics.defra.gov.uk/esg/reports/ SeaAngling/default.asp. Cited 10 March 2009 FAO (2010) Global aquaculture production 1950–2007. FIGIS, fisheries global information system. http://www.fao.org/ Fowler SL (1999) Guidelines for managing the collection of bait and other shoreline animals within UK European marine sites. English Nature (UK Marine SACs Project). 132 p
123
394
Aquacult Int (2011) 19:381–394
Haddon M (1994) Size-fecundity relationships, mating behaviour, and larval release in the New Zealand paddle crab, Ovalipes catharus (White 1843) (Brachyura: Portunidae). N Z J Mar Freshw Res 28:329–334 Hamasaki K, Fukunaga K, Kitada S (2006) Batch fecundity of the swimming crab Portunus trituberculatus (Brachyura: Portunidae). Aquaculture 253:359–365 Harms J, Meyer-Harms B, Dawirs RR, Anger K (1994) Growth and physiology of Carcinus maenas (Decapoda, Portunidae) larvae in the field and in laboratory experiments. Mar Ecol Prog Ser 108:107–118 Hedvall O, Moksnes P-O, Pihl L (1998) Active habitat selection by megalopae and juvenile shore crabs Carcinus maenas: a laboratory study in an annular flume. Hydrobiologia 375(376):89–100 Hines AH (1982) Allometric constraints and variables of reproductive effort in brachyuran crabs. Mar Biol 69:309–320 Kuris AM (1991) A review of patterns and causes of crustacean brood mortality. In: Schram FR (ed) Crustacean issues, vol 7. Balkema, Rotterdam, pp 117–141 Lavens P, Sorgeloos P (eds) (1996) Manual on the production and use of live food for aquaculture. FAO Fisheries Technical Paper No. 361 Mohamedeen H, Hartnoll RG (1989) Larval and postlarval growth of individually reared specimens of the common shore crab Carcinus maenas (L.). J Exp Mar Biol Ecol 134:1–24 Moksnes P-O, Pihl L, van Montfrans J (1998) Predation on postlarval and juveniles of the shore crab Carcinus maenas: importance of shelter, size and cannibalism. Mar Ecol Prog Ser 166:211–225 Nghia TT, Wille M, Binh TC, Thanh HP, Danh NV, Sorgeloos P (2007) Improved techniques for rearing mud crab Scylla paramamosain (Estampador 1949) larvae. Aquacult Res 38:1539–1553 Olive PJW (1999) Polychaete aquaculture and polychaete science: a mutual synergism. Hydrobiologia 402:175–183 Penha-Lopes G, Rhyne AL, Lin J, Narciso L (2006) Effects of temperature, stocking density and diet on the growth and survival of juvenile Mithraculus forceps (A. Milne Edwards, 1875) (Decapoda: Brachyura: Majidae). Aquacult Res 37:398–408 Queiroga H (1998) Vertical migration and selective tidal stream transport in the megalopa of the crab Carcinus maenas. Hydrobiologia 375(376):137–149 Quinitio ET (in press) Domestication of the mud crab Scylla serrata. Aquaculture International, this volume Rice AL, Ingle RW (1975) The larval development of Carcinus maenas (L.) and C. mediterraneus Czerniasky (crustacea, brachyura, portunidae) reared in the laboratory. Bull Br Mus Nat Hist (Zool) 28:103–119 Ruscoe IM, Williams GR, Shelley CC (2004) Limiting the use of rotifers to the first zoeal stage in mud crab (Scylla serrata Forska˚l) larval rearing. Aquaculture 231:517–527 Shields JD (1991) The reproductive ecology and fecundity of Cancer crabs. In: Wenner A, Kuris A (eds) Crustacean egg production. Balkema, Rotterdam Sokal RR, Rohlf FJ (1995) Biometry: the principles and practice of statistics in biological research. N.W. Freeman, New York Sulkin SD (1975) The significance of diet in the growth and development of the larvae of the blue crab, Callinectes sapidus Rathbun, under laboratory conditions. J Exp Mar Biol Ecol 20:119–135 Sulkin SD (1978) Nutritional requirements during larval development of the portunid crab, Callinectes sapidus Rathbun. J Exp Mar Biol Ecol 34:29–41 Tallack SML (2007) Size-fecundity relationships for Cancer pagurus and Necora puber in the Shetland Islands, Scotland: how is reproductive capacity facilitated? J Mar Biol Ass UK 87:507–515 Ut VN, Le Vay L, Nghia TT, Hanh TTH (2007) Development of nursery culture techniques for the mud crab Scylla paramamosain (Estampador). Aquacult Res 38:1563–1568 Williams BG (1968) Laboratory rearing of the larval stages of Carcinus maenas (L.) [Crustacea: Decapoda]. J Nat Hist 2:121–126 Zeng C, Naylor E (1996) Occurrence in coastal waters and endogenous tidal swimming rhythms of late megalopae of the shore crab Carcinus maenas: implications for onshore recruitment. Mar Eco Prog Ser 136:69–79 Zmora O, Findiesen A, Stubblefield J, Frenkel V, Zohar Y (2005) Large-scale juvenile production of the blue crab Callinectes sapidus. Aquaculture 244:129–139
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