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Flow reduction, seston depletion, meat content and distribution of diarrhetic shellfish toxins in a long-line blue mussel (Mytilus edulis) farm.

ABSTRACT Seawater velocity, meat content, food availability, and algae toxins were measured in a commercial long-line (200 x 15 m) mussel farm (Mytilus edulis [L]) in Lysefjorden, in southern Norway. The mean current speed decreased rapidly within the farm area during the 4 days current measurements. The current speed 30 m inside the farm was reduced to less than 30% compared with the current speed outside the farm. The reduction was a consequence of friction from the mussels and farm structures. More than 50% of the incoming phytoplankton biomass (chlorophyll a) was depleted within the first 30 m in the mid section of the farm. After this decrease the chlorophyll a concentrations stabilized at approximately 0.6 mg [m.sup.-3] throughout the farm. The reduction in current speed led to food depletion and lower meat content within the farm. The concentration of diarrhetic shellfish toxins (DST) was inversely related to the meat content of the mussels. This relationship can be used to optimize monitoring programs for shellfish toxins. The range of DST in mussels varied from 0.40 to 1.60 mg [kg.sup.-1] steamed meat within the farm. It is suggested that depuration of DST was faster in areas with high food availability.

KEY WORDS: current, deputation, DST, flow reduction, food supply, long-line mussel farm, meat content, Mytilus edulis, seston depletion


Growth of suspension-feeding bivalves is largely controlled by food availability (Winter 1978, Bayne & Newell 1983, Soniat & Ray 1985, Berg & Newell 1986), which is affected by seston concentration and composition as well as seston transport rate (Incze & Lutz 1980, Frechette et al. 1989, Blanco et al. 1996). Food availability is also linked with phytoplankton dynamics (Rosenberg & Loo 1983, Smaal & van Stralen 1991, Heasman et al. 1998). Further, factors associated with overcrowding may have severe effects such as depression of bivalve growth rate and an increase in mortality (Grant et al. 1993). Suspension feeders such as mussels have a remarkable capacity to filter the water column and to deplete the water of seston, and in culture they may be food limited at high density (Navarro et al. 1991). This is especially the case in environments with low concentrations of seston where mussels are dependent on new supply of seston to avoid depletion. Long-line mussel farmers report higher meat content along the edges of the farm than towards the mid section, indicating food limitations in parts of the farms. Boyd and Heasman (1998) studied the water flow pattern through rafts and within a mussel farm. They found that most of the flow diverged around the raft and within the raft and was aligned and retarded. Also for rafts, Navarro et al. (1991) found consistently higher scope for growth in the front of the raft compared with the back. Despite the importance of seston availability to farmed mussels, surprisingly few studies have investigated water movement and seston concentration through long-line farms.

The temporal and geographical occurrence of Dinophysis and DST in mussels in Norway has been recorded for a decade through the national surveillance program. However, little attention has been given to the variability of DST on a farm level. A preliminary study showed large variations in DST in mussel sampled from different parts of a farm in late winter where the mussels were assumed to be in a detoxification situation (A. Svardal unpublished data). One sample may therefore not represent a whole farm during a detoxification period. Blanco et al. (1999) pointed to the possibility of a relationship between food availability and detoxification rates comparing mussel rafts from different areas of Spain. They found that food availability was the most important factor affecting detoxification rates of DST. However, the relationships between food availability and DST concentration have not been studied within a long-line farm.

Farming of the mussel Mytilus edulis in Norway has increased from a level of <500 mt per year during the last two decades to 3000 tons in 2003. During recent years considerable capital (scale of 200-300 mill NOK) has been invested in development of the mussel industry, primarily based on a high market demand in Europe. However, the expected expansion and export volumes have not been realized, mainly because of harvest closures due to DST and post harvest logistic factors. Problems with toxic algae closures and insufficient post harvest logistic capacity have resulted in high-density mussel stocks of low meat content.

The consolidation of the mussel industry development is dependent on targeting the market demands on quality. To meet the constraining factors mentioned above, there is now considerable focus on production management to provide high quality and toxin free mussels at harvest. Fundamental to the success of these strategies is an improved knowledge of ecologic interactions related to mussel farming, primarily on assessment of environmental effects (e.g., seston dynamics) on mussel growth. The objective in this study is to describe the variation in current pattern, food availability, meat content, and distribution of DST within a high-density long-line mussel farm.


Study Site and the Blue Mussel Farm

Lysefjorden is located on the southwest coast of Norway (5[degrees]56'N, 6[degrees]8'E). The fjord is approximately 40 km long and 0.5-2 km wide. The depth range in the fjord is from 13 m at the outer sill and maximum depth is 460 m. The mean tidal range is 0.4 m. The investigated long line mussel farm was located in the outer part of the fjord. The depth under the farm was 28-35 m and the maximum depth in this section of the fjord is 58 m. The farm was situated downstream on the outward tide of two other farms and had one farm between itself and the shore (Fig. 1). The farm was 200 m long, 15 m wide, and had 10 long-lines running lengthwise. The distance between the long-lines was 1.6 m. The mussel ropes attached to the long-lines were 5.5 m long and the spacing between the mussels ropes were 0.5 m. The mussel company workers estimated the biomass in this farm to 60-80 t in May 2002. The investigations were carried out in August and September of 2002.


Water Velocity and Phytoplankton Biomass

Water velocity was measured simultaneously with four current meters (SD 6000, Sensordata AS, Norway), deployed at 4 m depth, and recorded every 10 min. Water velocities were recorded during 2 periods; (1) 18 August from 1400 h to 19 August at 0940 h and (2) 20 August from 1730 h to 23 August at 1240 h. The current meters were placed at station 1, 3, 12, and 21 during period 1 and at station 1, 3, 9, and 12 during period 2 (Fig. 2). Station 1 was a reference to ambient current velocities on in-going tide. Current direction is given for current speeds equal or higher than 3 cm [s.sup-1].


Chlorophyll a (Chl a) was estimated from fluorescence measurements by a STD/CTD instrument (SD 204, SAIV A/S, Norway) from the surface to 10-m depth. The instrument was immersed at a rate of approximately 20-30 cm [s.sup.-1] and measurements were recorded every 1 sec. Fluorescence was measured on inward tide at all stations 1-22 (Fig. 2), within 1 h 30 min. The mean concentration of chl a and confidence intervals were calculated from 3-6 measurements between 3.5 and 4.5 m depth along the mid section of the farm (station 3, 6, 9, 12, 15, 18, and 21).

The fluorescence data were calibrated by chl a measurements from samples of filtered seawater from the reference station. Chl a was analyzed after extraction with 90% acetone using the fluorescence method with correction for acidified measurements (Strickland & Parson 1968). The fluorometer was calibrated with known concentrations of Sigma chl a (Sigma Chemicals, St. Louis, MO., USA) measured spectrophotometrically. Conversion of fluorescent to chl a was calculated by the equation Y = 1.15 x x - 0.33 ([r.sup.2] = 0.91, P < 0.001, StatSoft, Inc. STATISTICA), where Y is the concentration of chl a (mg [m.sup.-3]) and x is the measured value of fluorescent. The range of chl a concentrations from the water samples at the reference station was 0.4 to 3.5 mg [m.sup.-3].

Food availability within the ingoing current (using chl a as a proxy for microalgae biomass) was calculated by the following equation: [Fa.sub.x] = [V.sub.x] x [C.sub.x] where [Fa.sub.x] is the food availability at station x, [V.sub.x] is the mean current speed at station x and [C.sub.x] is the concentration of chl a at station x. Food availability was calculated at station 1, 3, 9, 12, and 21 (Fig. 2).

Meat Content

Mussel samples were collected on September 3 from stations 2-22 (Fig. 2) at 2, 4, and 6 m depth. Each of the 58 samples contained from 32-70 mussels except from 1 sample of 20 mussels. The mussels were cleaned and then steamed. Steaming is the standard preparation for food safety DST analyses in Norway. The meat was removed, weighed, and stored at -20[degrees]C for analysis of DST content. The shells were counted and weighted, and the meat content was calculated as: (weight of steamed meat/shell weight) x 100.

Analysis of Diarrhetic Shellfish Toxins

The total meat from the steamed mussels was used for extraction. A portion of 1 g homogenized total meat was extracted with 9-mL methanol:water (8:2) for 10 min using a Stuart rotator (Bibby Sterilin Ltd, Staffordshire, UK) at maximum velocity. The samples were then centrifuged at 3,000 rpm and subsequently 400 [micro]L of the supernatants were passed through a 0.22[micro]L Spin-X centrifuge filter (Coming Incorporated, NY, USA) by centrifugation at 3000 rpm for 3 min. The filtrate was used for the toxin analysis on LC/MS.

The amount of esterified forms of DSP toxins was determined by adding 400 [micro]L of a 1 M sodium hydroxide solution to 2 mL of filtered supernatant. The vial containing this mixture was placed in a Stuart Rotator, which was run at maximal velocity for 1 h. Five hundred micro liters of 1 M HCl was then added to stop the reaction. Two micro liters of hexane was added and the sample extracted for 3 min using the Stuart Rotator. The sample was centrifuged at 3,000 rpm for 3 min for separation of the two phases and the upper phase (hexane) removed and transferred to waste. To the lower phase 1 mL water and 2 mL dichloromethane were added and the sample extracted for 3 min using the Stuart Rotator. Following centrifugation for 3 min at 3,000 rpm the lower phase (dichloromethane) was transferred to a new vial and the upper phase extracted once more with 2 mL dichloromethane. The combined dichloromethane phases were evaporated to dryness using a Turbovap LV (temperature 35[degrees]C), resolved in 2 mL 80% MeOH and filtered through a 0.22 [micro]L. The samples were now ready for LC-MS analysis.

The LC system was an Agilant (Palo Alto, CA) HP1100 with binary pump, variable volume injector, and a thermostated auto sampler. HPLC separation was conducted at 24[degrees]C using an isocratic solvent mixture, methanol: 25 mM ammonium acetate (pH 5.80) (80:20). A Hypersil BDS C-18 column (3 [micro]m, 2.1 x 100 mm); flow rate 0.1 mL/min was used. Ten micro liters of the sample was injected. The mass spectrometer used was a Micromass Quattro LC Tandem MS/MS (Micromass UK Limited, Manchester, UK) equipped with electro spray source. Desolvation temperature was 350[degrees]C and desolvation gas flow rate 650 L/hour. Selected ion monitoring (SIR) for data acquisition and negative ion detection was used (OA: m/z 803.5; DTX-I: m/z 817.5). Analyst software (MassLynx NT) was used for HPLC system control, data acquisition, and data processing.

Okadaic acid (OA) and 35-methylokadaic acid (DTX-I) were purchased from Alexis Biochemicals, Lausen, Switzerland. CRMD-SP-MUS, a certified reference mussel/microalga material (11.0 [micro]g OA and 0.96 [micro]g DTX-1/g homogenate), was obtained from National Research Council, Halifax, Canada. All other chemicals used were analytical grade. STATISTICA, version 6.1 (StatSoft, Inc. 2003) was used for tests of correlation.


Water Velocity and Phytoplankton Biomass

The current direction at the reference station (station 1) on inward (flood) tide was mainly between 0[degrees] and 45[degrees] (Fig. 3), which was along the long axis of the farm. The highest current speed at flood tide was 17 cm [s.sup.-1] and outward (ebb) tide 12 cm [s.sup.-1] (Fig. 4). The mean current speed on the flood-tide was 7.5 cm [s.sup.-1] (n = 214), and on the ebb-tide 3.8 cm [s.sup.-1] (Table 1, n = 186). Because the current speeds on the ebb tide were lower, possibly due to the adjacent upstream farm (Fig. 1), we present the results on current speed and phytoplankton biomass on flood-tide only. The mean current speeds at station 9 (30 m inside the farm) and at station 12 (70 m inside the farm) were 2.0 and 1.4 cm [s.sup.-1] respectively (Table 1). This is a 70% to 80% reduction of the ambient current speed of 7.5 cm [s.sup.-1] recorded at the reference station (see Fig. 6).


The concentration of chl a decreased with increasing distance into the farm (Fig. 5), from 2.4 mg [m.sup.-3] at the entry to 0.6 mg [m.sup.-3] at the outlet of the farm. Along the midsection of the farm the chl a concentration at 30 m and 70 m within the farm were 47% and 40% respectively of the ambient concentration (Fig. 6). From 70 m inside the farm to the end of the farm, at 200 m, only a minor decrease in chl a concentration was recorded. From 140 m to the end of the farm there was no significant change in the chl a concentration (Fig. 7).


As both current speed and the chl a concentration decreased, the calculated food availability was reduced by more than 80% within the first 30 m of the farm and from 30 m the food availability remained low throughout the farm (Fig. 6).

Meat Content and Diarrhetic Shellfish Toxins

Along the midsection of the farm the meat content was highest at the edges (78% and 76%) and decreased rapidly toward the middle of the farm (69 and 66%, Fig. 8). The total DST (i.e., the sum of ocadaic acid [Oa], dinophysis toxin-1 [DTX-1] and their esterified forms) shows an opposite pattern (Fig. 8) with the lowest concentrations at the edges (1.16 and 1.08 mg [kg.sup.-1] mussel meat) and the highest concentrations within the farm (1.39 and 1.33 mg [kg.sup.-1] mussel meat). The meat content was significantly negative correlated with the total DST by [r.sup.2] = 0.63 (P < 0.0001, Fig. 9). Between all stations there were a difference in toxicity of 3.7 times (minimum 0.42 mg [kg.sup.-1] and maximum 1.56 mg [kg.sup.-1]).



Water Velocity and Phytoplankton Biomass

The mean current speed decreased rapidly towards and into the farm. Already 30 m inside the farm the current speed was reduced to less than 30% of the current speed at the reference station outside the farm. An ideal control to the current retardation in the farm would be a similar arrangement of current meters situated on the outside of the farm; but because the farm length is short compared with the scale of the currents in this site and the depth and bottom topography were similar under the farm and outside the farm area, we assume the reduction in current speed to be a consequence of friction from the mussels and the farm structure. The water was probably forced underneath, above, and around rather than through the farm (Boyd & Heasman 1998). Therefore, additional farms on the same location should not be placed nearby and downstream, because the downstream farm will face even slower moving water. In addition, the downstream farm will regularly receive seston-depleted water from the upstream farm, which is obviously a disadvantage (Grant et al. 1993). This type of seston-depletion is readily demonstrated in raft culture (Navarro et al. 1991).

Blanco et al. (1996) found strong lateral flow and not always a clear front rear flow through mussel rafts in Galicia, Spain. We do not have reliable data on lateral flow into the farm because the current meter used inside the farm cannot determine current direction when current speed is below 3 cm [s.sup.-1]. Pilditch et al. (2001) conducted a field and modeling study to examine factors affecting seston supply to culture of suspended sea scallops. Their current direction results indicate minimal lateral flow for long-line farms oriented parallel to the current direction and situated in a tidal driven current area. Low lateral input is also supported by this study, because the meat content in the mid section and in the middle of the farm was low. In high density long-line farms the narrow arrangement of mussel ropes may lead to a "canal effect" that directs the flow lengthwise. In addition there will be a pressure difference from the front inflowing side to the rear and out flowing end of the farm. The higher pressure at the front should force water out of the farm and thereby avoid lateral flow into the farm. This scenario should not be the case for farms that are not aligned parallel to the current direction, such farms are situated in wind driven current areas. The contribution of lateral input to long-line farms (according to orientation) should be addressed in future studies.

The importance of lateral advection in supplying bivalves with new seston is widely recognized (Wildish & Kristmanson 1979, Frechette & Bourget 1985, Gibbs et al. 1991, Navarro et al. 1991, Dame 1996), and the limiting nature of advected food delivery to bivalves relative to their consumption is well founded in the literature (Wildish & Kristmanson 1979, Incze et al. 1981, Rosenberg & Loo 1983) and also documented in field observations (Blanton et al. 1987, Grizzle & Lutz 1989, Frechette et al. 1989, Smaal & van Stralen 1990, Dolmer 2000b). It is important that the design and situation of mussel farms take into account site specific water advection characteristics to optimize the availability of seston to the mussels. This is particularly important on the west coast of Norway where the biomass of phytoplankton is low (e.g., 1-3 mg [m.sup.-3] chl a, this study) compared with regions where large scale mussel farming is practiced, such as Ria de Arousa (chl a = 4-12 mg [m.sup.-3], Figueiras et al. 2002), Benguela Bay (chl a = 8 mg [m.sup.-3], Pitcher & Calder 1998), Oosterschelde (chl a = 7.5 mg [m.sup.-3]), Marennes-Oleron Bay (chl a = 4-22 mg [m.sup.-3]), and Chesapeake bay (chl a = 6.9 mg [m.sup.-3]) (Dame & Prins 1998).

Along the mid section of the farm more than 50% of the incoming phytoplankton biomass was depleted within the first 30 m. Throughout the rest of the 200-m long farm only a further minor amount of phytoplankton biomass was extracted. Perez-Camacho et al. (1995) demonstrated that 98% of the variance of the growth rates in length and dry weight of cultured mussels in the Rias Baixas is explained by the current velocity and chl a concentration. The current velocity explained 66% to 79% and the chl a concentration explained 33% to 19% of the variance in growth rates. The relative contribution of current speed and chl a concentration determining the actual amount of phytoplankton available to mussels in long-line farms is not known.

The food availability was reduced to <20% within the first 30 m of the farm. This was mainly due to a strong decrease in current speed, caused by friction forces. In addition, mussels may increase the consumption of phytoplankton by compensating for low food availability by altering their feeding behavior (Bayne et al. 1989, Bayne et al. 1993). The phytoplankton biomass reached a plateau 140 m into the farm and this suggests that the mussels could not use a phytoplankton concentration below ~0.6 mg chl a [m.sup.-3]. It has been demonstrated that low algal concentrations may lead to reduced valve gape and reduction of the filtration rate (Riisgard & Randlov 1981, Riisgard 1991, Dolmer 2000 b), although the level reported is generally <0.5 mg chl a [m.sup.-3] (Riisgard 2001). Based on the rapid decrease of the chl a concentration within the first 30 m and the following stabilization of chl a concentration throughout the farm, a minimum "chl a threshold concentration" is suggested. This threshold represents the low limit of chl a concentration that is not utilizable for mussels. Our estimation of this threshold level is only valid for the period of the chl a measurements, because numerous factors may influence the food availability and threshold level. Phytoplankton fragments and feces can also bias fluorescence measurements within the farm. However, the concept of a threshold level is strengthened by the high meat content in mussels situated at the edges of the farm and decreases in meat content inside the farm. Fuentes et al. (2000) also reported decreasing weight of Mytilus galloprovincialis through rafts in Ria de Arusa. As a consequence, food depletion in parts of the farm will clearly reduce the overall production of the farm.

Meat Content and Shellfish Toxins

Phytoplankton and DST sampled from the farm area prior to this investigation show that the concentration of Dinophysis acuta peaked at 680 cells [L.sup.-1] in early August (Fig. 10). After mid August it was below 100 cells [L.sup.-1]. From August 22 to October the DST concentration decreased from 0.88 mg [kg.sup.-1] to a level of 0.20 mg [kg.sup.-1] in samples from different farms situated in the area (Fig. 10). Therefore the mussels in this study were in a situation of detoxification.


There was a decrease in meat content of the mussels and an increase in toxin levels from the edges towards the middle of the farm. This corresponded with a strong decrease in food availability. The most likely explanation for the observed negative relationship between toxin and meat content is that mussels from areas of the farm with higher food availability and higher meat content had faster deputation of the toxins, that is, decline in toxin concentrations, than mussels from areas with lower food availability and lower meat content. These differences in concentrations could be partly due to size-related loss of toxins during steaming of the mussels that were not tested for these toxins. However, it is more likely that steaming gives an increase in toxin concentrations of these lipid soluble toxins, accompanying loss of water. The differences in toxin concentrations could also be due to different extents of dilution of the toxins due to different rates of soft tissue growth. Time series with toxin burden of the mussels would be required to evaluate if the different feeding conditions would give physiologically based differences in the actual elimination rates. Nevertheless, the differences reflect the true concentrations that would meet the consumers, ingesting cooked mussels. Differences in concentrations within a farm, as in the present study, should obviously affect sampling strategies for food safety toxin analyses. Further studies should be made during periods with toxic algae in the water and during detoxification.

It may be argued that the negative relationship between meat content and toxin levels is due to a negative impact of the toxins on somatic growth. However, the mussels are probably unaffected by the toxins (Landsberg 2002, Svensson 2003). They appear to feed readily on and even select the DST producing Dinophysis algae (Sidari et al. 1998). Svensson and Forlin (1998) and Svensson et al. (2003) suggested that the mussels have protective mechanisms against the DSP toxins. Hence, these toxins probably have no effect on the meat content of the mussels.


Long and narrow mussel production units reduce current speed by increasing friction. When such farms are situated in a low-seston environment the risk of food depletion within farm is high and may lead to uneven meat content and DST throughout the farm. The correlation between meat content and DST during the depurating period suggests that high food availability gives faster depuration of the toxins.
Average current speed independent of current direction (Va),
average current speed at ingoing tide (V_in) and average current
speed at outgoing tide (V_out). Standard deviation is in parenthesis.

 Distance into Va (cm [s. V_in (cm [s. V_out (cm [s.
Station Farm (m) sup.-1]) sup.-1]) sup.-1])

 1 -20 5.2 (4.4) 7.5 (4.7) 3.8 (2.7)
 3 0 3.5 (3.5) 5.4 (4.1) 1.6 (0.6)
 9 30 1.7 (0.8) 2.0 (1.0) 1.5 (0.5)
 12 70 1.2 (0.5) 1.4 (0.6) 1.2 (0.5)
 21 200 2.2 (1.2) 2.0 (0.5) 2.8 (1.8)


The authors thank Torunn Eide for excellent technical assistance and Coastshell A/S for providing the data on toxicity history of the mussels prior to our sampling (see data in Fig. 10). The Research Council of Norway (project no. 139593/140) and the Directorate of Fisheries supported this work. The manuscript was considerably improved by comments from two anonymous reviewers.


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(1) Institute of Marine Research, Nordnes gt. 50 5024, Bergen Norway; (2) National Institute of Nutrition and Seafood Research, Norway

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Article Details
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Author:Strand, O.
Publication:Journal of Shellfish Research
Geographic Code:1USA
Date:Jan 1, 2005
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