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Effects of tributyltin compounds from antifoulants on Pacific oysters (Crassostrea gigas) in Scottish sea lochs
Aquuculture, 74 (1988) 319-330 Elsevier Science Publishers B.V., Amsterdam -
319 Printed in The Netherlands
Effects of Tributyltin Compounds from Antifoulants on Pacific Oysters (Crassostrea gigas) in Scottish Sea Lochs I.M. DAVIES, J. DRINKWATER DAFS Marine Laboratory,
and J.C. McKIE
Victoria Road, P.O.B. 101, AberdeenAB98DB
(Great Britain)
(Accepted 7 June 1988)
ABSTRACT Davies, I.M., Drinkwater, J. and McKie, J.C., 1988. Effects of tributyltin compounds from antifoulants on Pacific oysters (Crassostreagigas) in Scottish sea lochs. Aquaculture, 74: 319-330. Before 1987, tributyltin-based antifoulants were used in Scottish sea lochs on both the hulls of vessels and on cage nets at marine salmon farms. In two sea lochs, the leachates from these antifoulants caused accumulation of tin compounds in soft tissue, and had deleterious effects on the shell structure, growth and condition of Pacific oysters (Crassostrea gigas). The extent and intensity of the effects appeared to be dependent upon source size and distribution, and upon hydrographic factors. The use of triorganotin compounds in antifoulants for mariculture applications, and on vessels less than 25 m, has now been prohibited in the U.K.
INTRODUCTION
Tributyltin compounds have been widely used as biocides in antifouling paints. Though greatly reducing the growth of fouling communities, the compounds have also been shown to have undesirable effects on non-target organisms, such as cultivated Pacific (Crassostrea gigus) or flat (Ostrea edulis) oysters (Thain, 1983; Waldock and Thain, 1983) and dogwhelks (Nucella Zupillus) (Bryan et al., 1986). In estuaries in southern England, where oyster cultivation and intense small boat activity coincide with relatively poor water circulation, concentrations of tributyltin in the sea water have been sufficiently high to cause shell thickening and increased mortalities of oysters (Alzieu and Portmann, 1984). In Scotland, in addition to being used on vessels, paints containing tributyltin compounds were, until recently, used on netting enclosures (cages) at marine fish farms. These farms are commonly located in fjordic inlets (sea lochs) on the west coast, which have a restricted water exchange compared to open coastal areas (Milne, 1972). The accumulation of tin compounds in farmed
L
uu--/
I
L
c 1 km
area
1
positions
:
,
Caol Scotnish /!:I
7
of fish farms, oyster farms, boat yards and moorings, and oyster
Main mooring
&
/.
Oyster
*
farm
Fish farms
*
mooring
Oyster
l
Fig. 1. Location of Lochs Sween and Melfort in Argyll, W. Scotland, and positions cage moorings.
r
321
chinook (Short and Thrower, 1986) and Atlantic (Davies and McKie, 1987) salmon has been described, and salmon mortalities in an aquaculture research facility in Alaska have been linked with tributyltin toxicity (Short and Thrower, 1987). Shellfish cultivation (mainly of Pacific oysters) also takes place in sea lochs, and before the recent statutory controls the potential therefore existed for tributyltin compounds leached from fish cages to affect other cultivated organisms. This report describes the results of an experiment carried out to determine the “radius of effect” of tributyltin compounds released from fish farms on accumulation of tin by, and growth of, Pacific oysters (Crussostrea gigas) in two sea lochs. As a consequence of these and other observations and experiments, U.K. legislation (Food and Environment Protection Act, Control of Pesticide Regulations) now does not permit the use of triorganotin compounds in mariculture, or in antifouling paints on vessels less than 25 m, in order to reduce the input of these compounds to coastal waters. MATERIALS AND METHODS
Loch Sween, Argyll (Fig. 1) , is a fjordic sea loch divided into several basins separated by three main sills. It supports two fish farms (in Coal Scotnish and Port Lunna), and an oyster hatchery and on-growing unit at Eilean Mhartan. In 1985 the fish farms were small units, each with approximately eight cages (8 x 8 x 5 m deep ) treated with tributyltin-based antifoulants. The oyster farm was approximately 1.5 km from the nearer fish farm. Water circulation in the inner part of the loch is poor, with a tidal range of about 1 m at springs (Milne, 1972). In 1985 Loch Melfort, Argyll (Fig. 1) , supported a larger fish farm (approximately 30 cages) in Kames Bay, and yacht moorings and a repair and maintenance yard at Kilmelfort at the head of the loch. The entrance to Loch Melfort is not restricted by a sill, and the tidal range at springs is 3 m (Milne, 1972 ). In June 1985 young Pacific oysters of lo-20 g total weight and low tin content were obtained from a nursery unit on the island of Colonsay; an area distant from fish farms and other significant sources of tributyltin. Batches of 120 oysters were deployed in PVC-coated, steel wire mesh trays (60 x 45 x 10 cm, 2 mm gauge, 1 cm mesh) at 13 locations in Loch Sween, and 14 in Loch Melfort (Fig. 1) . The cages were suspended 2 m below surface floats, and were anchored using a simple single-string mooring. Mooring locations were selected approximately 0,200,500,1000,2000 and 5000 m from the fish farms. Samples of 10 oysters were removed from the cages on five occasions between June and November 1985 for analysis. Measurements were made of shell length, total weight, tissue weight, dry shell weight, and thickness of upper valve. Selected pooled and homogenised soft parts were analysed for total tin and tributyltin by graphite furnace atomic absorption spectrophotometry
322
(McKie, 1987), with a detection limit of 0.03 mg/kg of tin. Tributyltin concentrations are expressed as tin, and all data are on a wet weight basis. Additional oysters were stored at the oyster farm in Loch Sween, and used to fill replacement cages at five locations in Loch Sween (3, 5, 6, 9, 11) and two locations (3,7) in Loch Melfort in August after storm losses earlier in the month. RESULTS
Loch Sween
Prior to deployment, the oysters contained < 0.03-0.10 mg/kg total tin. At the end of the experiment, samples from within 200 m of fish cages (positions 7, 8, 13) showed a significant accumulation of total tin. The maximum concentration at this time was 0.78 mg/kg at site 8, although 0.85 mg/kg was recorded earlier at this site in September (Table 1) . There was no consistent trend of total tin concentration with time (Table 1 ), and a similar distribution of tin between mooring positions was shown at each sampling occasion. The tributyltin measurements made (Table 1) emphasise the restriction of significant accumulation of tin compounds to within 200 m of the fish cages (positions 7,8,13). Within this distance, an average of 7580% of the total tin in the oysters was in the tributyl form. A shell thickness index (ratio of shell length to thickness) was calculated TABLE
1
Tow Emand tributyltin concentrations (mg/kg Sn wet weight) and condition index (November) in oysters from cages in Loch Sween
Position
July
Sept.
Tin
TBT
Tin
TBT
1
-
-
0.08
0.17
2 3 4 5 6 7 8 9 10 11 12 13
-
-
0.06
0.40 0.11 0.20 0.32 0.76 0.13 0.14 0.10 0.11 0.54
0.65 1.85
1.25
Nov.
Oct. Tin 0.10 -
TBT
Tin
TBT
Condition %
< 0.07 -
0.08
<0.07 -
25 -
-
0.11
0.12
0.07
0.10
22
0.10 0.04 0.14 0.20 0.85 0.07 -
0.05 0.08 0.08 (0.54) 0.70 (0.26) -
-
0.06 0.11 0.12 0.30 0.78 0.10 -
to.07 to.07 0.17 0.75 1.95 <0.07 -
21 22 22 14 14 22 -
1.08
0.12 0.06 0.48
<0.07 <0.07 1.27
23 21 16
0.09 0.03 0.46
0.40 1.27 -
1.08
0.08 0.06 0.48
0.60 1.75
323
r SHELL THICKNESS
INDEX
Fig. 2. Shell thickness indices of oysters in Loch Sween at the end of the exposure period (November).
40
r
Weeks
Fig. 3. Degrees of progressive thickening of shells in caged oysters from Loch Sween. The shell thickness index used (Alzieu et al., 1982) decreases with increasing thickness of shell. X , Position 1; 0, position 4; n , position 7; A, position 13.
324
% TISSUE GROWTH IN 10 WEEKS TO NOVEMBER
L Fig. 4. Soft tissue growth increments in final 10 weeks of exposure in Loch Sween.
following Alzieu et al. (1982). Data for November (Fig. 2) demonstrate the wide range of values (7-30) found at the end of the experiment. Oyster shells at all sites other than site 1 were significantly (PC 1%) thicker than at deployment, when the shell thickness index was 37.0 (SD 10.9). The shells at most sites had progressively thickened during the experiment (Fig. 3). The mean wet tissue weights (from samples of 10 oysters) were used as indices of growth of soft parts, and were expressed as the tissue weight increments and decrements over a period as a percentage of the inital tissue weight. In the initial 10 weeks no differences in growth occurred between locations except at position 13, immediately adjacent to the fish cages in Caol Scotnish, where there was only 33% growth compared with 104-120% elsewhere in the loch. In general, less growth occurred in the second 10 weeks (Fig. 4). Whilst 48-66% growth occurred at locations 4, 12 and the more distant location 1, rather less was found at $11, and possibly 13, and very little ( - 15% to 19%) at positions 3, and 6-9 inclusive around the Port Lunna fish farm. A condition index (tissue weight as a percentage of the total weight) of the oysters was calculated, and data are presented for the last sampling occasion (Table 1). At deployment the average condition index was 12.5%, and this
325
improved in all cages during the growing season. In November, indices ranged from 21% to 25% at all sites except those adjacent to fish farm nets (locations 8 and 13, condition indices 14%, 16%, respectively) and position 7,200 m from Port Lunna fish farm (condition index 14% ). Loch Melfort In the initial 5 weeks after deployment of oysters in Loch Melfort, animals in all positions accumulated tin (0.10-0.39 mg/kg), with highest concentrations (0.23-0.39 mg/kg) occurring at positions 3-5, i.e., within 500 m of the fish farm (Table 2 ) . Concentrations at positions 3-6 inclusive remained either fairly constant or increased (Fig. 5 ) during the experiment, whilst concentrations at many other locations, particularly those towards the head of the loch, passed through a maximum in August/September, and subsequently decreased (e.g., positions 9, 11, Fig. 5). Measurable accumulation of tributyltin compounds (Table 2) occurred at most locations. In November the highest concentrations (0.47-1.10 mg/kg) were found within 200 m of the fish farm (positions 4-6), and in the area of the yacht moorings at the head of the loch (0.27-0.32 mg/kg at positions 10 and 9). On average, TBT accounted for 72% of the total tin in the oysters in November. There was no consistent trend of tributyltin concentration with time during the experiment at the limited number of positions for which data are available. TABLE 2 Total tin and tributyltin concentrations (mg/kg Sn wet weight) and condition index (October) in oysters from cages in Loch Melfort Position July Sn
1 2 3 4 5 6 7---g---9 10 11 12 13 14
Aug. TBT
0.17 0.19 (0.07 0.23 0.24 0.39 0.67 0.15 0.67
Sn
Sept. TBT
0.16 0.19 (0.07 0.25 0.32 0.25 0.47 0.30 0.40
Sn
Oct. TBT
Sn
Nov. TBT
Condition % Sn
0.10 0.16 23 0.16 0.40 0.10 0.32 24 0.15 0.09 21 0.23 0.26 19 0.41 0.67 0.52 0.82 16 0.31 0.15 0.30 0.42 21 0.16 <0.03 21 --0.17 0.10 0.23 0.26 0.14 20 0.11 0.23 0.25 0.23 18 0.17 0.20 0.23 0.09 24 0.14 0.27 0.16 0.24 23 0.18 0.20 0.16 0.12 0.13 <0.07 0.16 (0.07 24 0.17 0.17 0.18 0.22 25
TBT
0.14 0.15 0.22 (0.07 0.23 0.67 0.50 1.10 0.37 0.47 0.07 0.12 0.14 0.33 0.12 0.27 0.07 0.15 0.03 0.07 0.03 <0.07 -
326
The shell thickness index decreased gradually throughout the experiment at all positions (except position 1). Indices of less than 15 were found (Fig. 6) in the final sample from all positions in the inner part of the loch (cages 3-ll), which includes the areas around the fish farm and yacht moorings. In Loch Melfort, most of the growth of soft tissue took place between June and September. In the outer part of the loch, on the southern side (positions 1, 2, 13), 134-144% growth occurred. Growth was less rapid elsewhere, with 82% (position 5) adjacent to the fish cage, 91% (position 9) close to the yacht
12
18
J 30
24
Weeks
Fig. 5. Total tin concentrations (mg/kg wet height) in homogenised soft parts of caged oysters in Loch Melfort. x , Position 5; 0, position 6; n , position 9; A, position 11.
SHELL THICKNESS
INDEX
(November)
1
I 1 km
Fig. 6. Shell thickness indices of oysters in Loch Melfort at the end of the exposure period (November ) .
327 175
r
17.5
I 20.0
Condition
I 22.5
I 25.0
percentage
Fig. 7. Relationship between tissue growth increment and condition index of caged oysters in Loch Melfort in October. Omission of the two anomalous points (see text) yields a linear regression y = -95 + 10.5x, with a correlation coefficient of 0.958.
and 80-86% in the northern outer part of the loch (positions 12, 14). Very little growth ( < 20% ) occurred in any cage between September and November, with significantly (P-C 1% ) better growth (7-19%, mean 13%) occurring along the northern shore (positions g-12)) than on the south side ( - 17 to 8%, mean -3%, positiqns l-6,13). The condition index of the oysters at all times exceeded that at deployment (12.5% ). The range of values in any month was small (e.g., September 18.525.2%, November 15.1-20.3% ). In general, oysters in the inner part of the loch (east of positions 2 and 11) were in poorer condition than those elsewhere. The condition index was strongly correlated with the growth of soft parts (Fig. 7). Oysters at the two positions which depart from this pattern (positions 12, 14) displayed good condition, but relatively poor growth, and were situated in the outer part of the loch on the northern side. moorings,
DISCUSSION
Five parameters (total tin and tributyltin concentrations, shell thickness index, tissue weight increment, and condition index) were used to assess the potential of tributyltin leached from fish cages to affect Pacific oysters. In Loch Sween all parameters showed a gradation of effect towards the fish farms, with the most intense effects occurring close to the farms. The thickening and chambering of the oyster shells observed at most locations in the two lochs is characteristic of exposure of Pacific oysters to organotin compounds (Waldock
328
and Thain, 1983; Alzieu and Portmann, 1984)) and has not, to date, been shown to be caused by any other class of chemical. Taken together with the accumulation of tributyltin compounds in oyster tissue, the presence of thickening indicates that the deleterious effects upon oysters in the cages were caused by tributyltin compounds. The recognised sources of such compounds in the upper part of Loch Sween are the two fish farms, and the yacht anchorage. The distribution of effects strongly indicates that the tributyl compounds affecting the caged oysters had leached from the fish cages in Port Lunna and Caol Scotnish. A survey of the effects of tributyltin compounds on the reproductive system of dogwhelks (induction of imposex) from Scottish sea lochs (Davies et al., 1987) has emphasised the significance of tributyltin leached from fish cages in affecting another species of mollusc, and pointed to areas in Loch Sween where the influence of fish cages could be identified. In Loch Melfort, the situation was complicated by the greater significance of small boat activity as a source of tributyltin. Within 200-500 m of the fish farm marked shell thickening (Fig. 6) and accumulation of tin compounds (Table 2 ) occurred, and the oysters grew less rapidly and were of slightly poorer condition (Table 2); however, similar effects, of perhaps lesser severity, occurred at all positions east of cages 3 and 11. It is not possible to separate clearly the influence of the fish farm from that of the small boats and associated facilities. More intense effects were observed in Loch Sween than in Loch Melfort, but they were confined to a smaller area. The potential source (cages plus boats) was much larger in Loch Melfort than Loch Sween, and the wide area of less intense effects probably reflects the less restricted circulation in Loch Melfort. The parameters used to indicate the effects of the organotin leachates showed considerable differences in sensitivity. Both shell thickening and accumulation of tin are characteristic of exposure to organotin compounds. By November, the shell thickness indices indicated shell deformation, and hence exposure to antifoulant leachates, at least 1000 m (but less than 5000 m) from the fish farm nets in Loch Sween. By contrast, the chemical measurements were not particularly sensitive indicators, with significant accumulation of tin compounds in Loch Sween largely confined to within 200 m of the fish farm nets. A comparison of shell thickness index and tin accumulation in November (Fig. 8) demonstrated a significant accumulation of tin only in oysters with a shell thickness index of less than 14. Below a shell thickness index of 12, total tin and tributyltin concentrations rose sharply. Whilst the shell thickening and chemical measurements are indicative of TBT contamination, the variations in growth parameters (rate of growth and condition) are not of themselves necessarily caused by exposure to tributyltin compounds. Differences in food availability or details of circulation patterns, for example, could alter growth rates of oysters. Of the growth parameters, the condition index was affected only within 200
329 30
x
f$ 24 0 .G I
6
.
x .
0
I
I
I
0.2
0.4
O-6
Tin concentration
!?I 0.8
I 1.0
(mglkg)
Fig. 8. Relationship between shell thickness and total tin or tributyltin concentration (mg/kg wet weight, expressed as tin) in caged oysters from Lochs Sween and Melfort, sampled in November. x , Tributyltin, Loch Sween; 0, total tin, Loch Sween; A, tributyltin, Loch Melfort; H, total tin, Loch Melfort. The data can be fitted by: Tin = 23.2 Th.-‘.g7 (carrel. coeff. 0.‘756), and TRT = 16’7Thp3.“l (carrel. coeff. 0.750).
m of the fish farms in Loch Sween. The growth of soft parts was initially affected only immediately beside the nets at the Caol Scotnish farm, but later was reduced up to 1000 m away from the Port Lunna farm. The gradients of effect towards the fish farms suggest that the poor performance of the oysters may be related to proximity to the salmon units, but other factors may also be involved. It has been demonstrated that tributyltin compounds leached from antifoulants on fish farm cage nets could affect young Pacific oysters in surrounding waters. In Loch Sween, accumulation of tin compounds in oysters was restricted to within 200 m of the fish farms, and effects on shell structure were observed at 1000 m, but did not extend to 5000 m from the Port Lunna farm. Effects on the growth of soft parts were seen within 100 m of the farm. In Loch Melfort, shell thickening occurred up to 4000 m from the fish farm, and tin accummulation was widespread. The growth of soft parts was reduced over an area of approximately 1.5 x 2.5 km, but the effects of the fish farm and small boat activity could not be clearly separated. The intensity and area of effect in the sea lochs examined appeared to be dependent upon source (treated cages and boats) size and distribution, and hydrographic factors. REFERENCES Alzieu, C. and Portmann, J.E., 1984. The effect of tributyltin on the culture of Crussostrea gigas and other species. Proc. 15th Annu. Shellfish Conf., 15-16 May 1984. Shellfish Assoc., London, pp. 87-100.
Alzieu, C., Heral, M., Thibaud, Y., Dardignac, M.J. and Feuillet, M., 1982. Influences des peinteuses antisalissures a base organostanniques sur la calcification de la coquille de l’huitre, Crassostreagigas. Rev. Trav. Inst. Peches Marit., 45: 100-116. Bryan, G.W., Gibbs, P.E., Hummerstone, L.G. and Burt, G.R., 1986. The decline of the gastropod Nucella lapillus around south west England: evidence for the effect of tributyltin from antifouling paints. J. Mar. Biol. Assoc. U.K., 66: 611-640. Davies, I.M. and McKie, J.C., 1987. Accumulation of total tin and tributyltin in muscle tissue of farmed Atlantic salmon. Mar. Pollut. Bull. 18 (7): 405-407. Davies, I.M., Bailey, S.K. and Moore, D.C., 1987. Tributyltin in Scottish sea lochs, as indicated by degree of imposex in the dogwhelk, Nucella lapillus (L.). Mar. Pollut. Bull., 18 (7): 400404. McKie, J., 1987. The determination of total tin and tributyltin in marine biological material using graphite furnace atomic absorption spectrophotometry. Anal. Chim. Acta, 197: 303-308. Milne, P.H., 1972. Hydrography of Scottish west coast sea lochs. Marine Research, 3. HMSO, Edinburgh, 50 pp. Short, J.W. and Thrower, F.P., 1986. Accumulation of butyltins in muscle tissue of Chinook salmon reared in sea pens treated with tri-n-butyltin. Mar. Pollut. Bull., 17 (12): 542-545. Short, J.W. and Thrower, F.P., 1987. Toxicity of tri-n-butyltin to Chinook salmon, Oncorhynchus tshawytscha, adapted to seawater. Aquaculture, 61: 193-200. Thain, J., 1983. The acute toxicity of bis (tributyltin) oxide to adults and larvae of some marine organisms. Int. Count. Explor. Sea, CM 1983/E: 13,5 pp. Waldock, M.J. and Thain, J., 1983. Shell thickening in Crassostrea gigas: organotin antifouling or sediment induced? Mar. Pollut. Bull., 14: 411-415.
319 Printed in The Netherlands
Effects of Tributyltin Compounds from Antifoulants on Pacific Oysters (Crassostrea gigas) in Scottish Sea Lochs I.M. DAVIES, J. DRINKWATER DAFS Marine Laboratory,
and J.C. McKIE
Victoria Road, P.O.B. 101, AberdeenAB98DB
(Great Britain)
(Accepted 7 June 1988)
ABSTRACT Davies, I.M., Drinkwater, J. and McKie, J.C., 1988. Effects of tributyltin compounds from antifoulants on Pacific oysters (Crassostreagigas) in Scottish sea lochs. Aquaculture, 74: 319-330. Before 1987, tributyltin-based antifoulants were used in Scottish sea lochs on both the hulls of vessels and on cage nets at marine salmon farms. In two sea lochs, the leachates from these antifoulants caused accumulation of tin compounds in soft tissue, and had deleterious effects on the shell structure, growth and condition of Pacific oysters (Crassostrea gigas). The extent and intensity of the effects appeared to be dependent upon source size and distribution, and upon hydrographic factors. The use of triorganotin compounds in antifoulants for mariculture applications, and on vessels less than 25 m, has now been prohibited in the U.K.
INTRODUCTION
Tributyltin compounds have been widely used as biocides in antifouling paints. Though greatly reducing the growth of fouling communities, the compounds have also been shown to have undesirable effects on non-target organisms, such as cultivated Pacific (Crassostrea gigus) or flat (Ostrea edulis) oysters (Thain, 1983; Waldock and Thain, 1983) and dogwhelks (Nucella Zupillus) (Bryan et al., 1986). In estuaries in southern England, where oyster cultivation and intense small boat activity coincide with relatively poor water circulation, concentrations of tributyltin in the sea water have been sufficiently high to cause shell thickening and increased mortalities of oysters (Alzieu and Portmann, 1984). In Scotland, in addition to being used on vessels, paints containing tributyltin compounds were, until recently, used on netting enclosures (cages) at marine fish farms. These farms are commonly located in fjordic inlets (sea lochs) on the west coast, which have a restricted water exchange compared to open coastal areas (Milne, 1972). The accumulation of tin compounds in farmed
L
uu--/
I
L
c 1 km
area
1
positions
:
,
Caol Scotnish /!:I
7
of fish farms, oyster farms, boat yards and moorings, and oyster
Main mooring
&
/.
Oyster
*
farm
Fish farms
*
mooring
Oyster
l
Fig. 1. Location of Lochs Sween and Melfort in Argyll, W. Scotland, and positions cage moorings.
r
321
chinook (Short and Thrower, 1986) and Atlantic (Davies and McKie, 1987) salmon has been described, and salmon mortalities in an aquaculture research facility in Alaska have been linked with tributyltin toxicity (Short and Thrower, 1987). Shellfish cultivation (mainly of Pacific oysters) also takes place in sea lochs, and before the recent statutory controls the potential therefore existed for tributyltin compounds leached from fish cages to affect other cultivated organisms. This report describes the results of an experiment carried out to determine the “radius of effect” of tributyltin compounds released from fish farms on accumulation of tin by, and growth of, Pacific oysters (Crussostrea gigas) in two sea lochs. As a consequence of these and other observations and experiments, U.K. legislation (Food and Environment Protection Act, Control of Pesticide Regulations) now does not permit the use of triorganotin compounds in mariculture, or in antifouling paints on vessels less than 25 m, in order to reduce the input of these compounds to coastal waters. MATERIALS AND METHODS
Loch Sween, Argyll (Fig. 1) , is a fjordic sea loch divided into several basins separated by three main sills. It supports two fish farms (in Coal Scotnish and Port Lunna), and an oyster hatchery and on-growing unit at Eilean Mhartan. In 1985 the fish farms were small units, each with approximately eight cages (8 x 8 x 5 m deep ) treated with tributyltin-based antifoulants. The oyster farm was approximately 1.5 km from the nearer fish farm. Water circulation in the inner part of the loch is poor, with a tidal range of about 1 m at springs (Milne, 1972). In 1985 Loch Melfort, Argyll (Fig. 1) , supported a larger fish farm (approximately 30 cages) in Kames Bay, and yacht moorings and a repair and maintenance yard at Kilmelfort at the head of the loch. The entrance to Loch Melfort is not restricted by a sill, and the tidal range at springs is 3 m (Milne, 1972 ). In June 1985 young Pacific oysters of lo-20 g total weight and low tin content were obtained from a nursery unit on the island of Colonsay; an area distant from fish farms and other significant sources of tributyltin. Batches of 120 oysters were deployed in PVC-coated, steel wire mesh trays (60 x 45 x 10 cm, 2 mm gauge, 1 cm mesh) at 13 locations in Loch Sween, and 14 in Loch Melfort (Fig. 1) . The cages were suspended 2 m below surface floats, and were anchored using a simple single-string mooring. Mooring locations were selected approximately 0,200,500,1000,2000 and 5000 m from the fish farms. Samples of 10 oysters were removed from the cages on five occasions between June and November 1985 for analysis. Measurements were made of shell length, total weight, tissue weight, dry shell weight, and thickness of upper valve. Selected pooled and homogenised soft parts were analysed for total tin and tributyltin by graphite furnace atomic absorption spectrophotometry
322
(McKie, 1987), with a detection limit of 0.03 mg/kg of tin. Tributyltin concentrations are expressed as tin, and all data are on a wet weight basis. Additional oysters were stored at the oyster farm in Loch Sween, and used to fill replacement cages at five locations in Loch Sween (3, 5, 6, 9, 11) and two locations (3,7) in Loch Melfort in August after storm losses earlier in the month. RESULTS
Loch Sween
Prior to deployment, the oysters contained < 0.03-0.10 mg/kg total tin. At the end of the experiment, samples from within 200 m of fish cages (positions 7, 8, 13) showed a significant accumulation of total tin. The maximum concentration at this time was 0.78 mg/kg at site 8, although 0.85 mg/kg was recorded earlier at this site in September (Table 1) . There was no consistent trend of total tin concentration with time (Table 1 ), and a similar distribution of tin between mooring positions was shown at each sampling occasion. The tributyltin measurements made (Table 1) emphasise the restriction of significant accumulation of tin compounds to within 200 m of the fish cages (positions 7,8,13). Within this distance, an average of 7580% of the total tin in the oysters was in the tributyl form. A shell thickness index (ratio of shell length to thickness) was calculated TABLE
1
Tow Emand tributyltin concentrations (mg/kg Sn wet weight) and condition index (November) in oysters from cages in Loch Sween
Position
July
Sept.
Tin
TBT
Tin
TBT
1
-
-
0.08
0.17
2 3 4 5 6 7 8 9 10 11 12 13
-
-
0.06
0.40 0.11 0.20 0.32 0.76 0.13 0.14 0.10 0.11 0.54
0.65 1.85
1.25
Nov.
Oct. Tin 0.10 -
TBT
Tin
TBT
Condition %
< 0.07 -
0.08
<0.07 -
25 -
-
0.11
0.12
0.07
0.10
22
0.10 0.04 0.14 0.20 0.85 0.07 -
0.05 0.08 0.08 (0.54) 0.70 (0.26) -
-
0.06 0.11 0.12 0.30 0.78 0.10 -
to.07 to.07 0.17 0.75 1.95 <0.07 -
21 22 22 14 14 22 -
1.08
0.12 0.06 0.48
<0.07 <0.07 1.27
23 21 16
0.09 0.03 0.46
0.40 1.27 -
1.08
0.08 0.06 0.48
0.60 1.75
323
r SHELL THICKNESS
INDEX
Fig. 2. Shell thickness indices of oysters in Loch Sween at the end of the exposure period (November).
40
r
Weeks
Fig. 3. Degrees of progressive thickening of shells in caged oysters from Loch Sween. The shell thickness index used (Alzieu et al., 1982) decreases with increasing thickness of shell. X , Position 1; 0, position 4; n , position 7; A, position 13.
324
% TISSUE GROWTH IN 10 WEEKS TO NOVEMBER
L Fig. 4. Soft tissue growth increments in final 10 weeks of exposure in Loch Sween.
following Alzieu et al. (1982). Data for November (Fig. 2) demonstrate the wide range of values (7-30) found at the end of the experiment. Oyster shells at all sites other than site 1 were significantly (PC 1%) thicker than at deployment, when the shell thickness index was 37.0 (SD 10.9). The shells at most sites had progressively thickened during the experiment (Fig. 3). The mean wet tissue weights (from samples of 10 oysters) were used as indices of growth of soft parts, and were expressed as the tissue weight increments and decrements over a period as a percentage of the inital tissue weight. In the initial 10 weeks no differences in growth occurred between locations except at position 13, immediately adjacent to the fish cages in Caol Scotnish, where there was only 33% growth compared with 104-120% elsewhere in the loch. In general, less growth occurred in the second 10 weeks (Fig. 4). Whilst 48-66% growth occurred at locations 4, 12 and the more distant location 1, rather less was found at $11, and possibly 13, and very little ( - 15% to 19%) at positions 3, and 6-9 inclusive around the Port Lunna fish farm. A condition index (tissue weight as a percentage of the total weight) of the oysters was calculated, and data are presented for the last sampling occasion (Table 1). At deployment the average condition index was 12.5%, and this
325
improved in all cages during the growing season. In November, indices ranged from 21% to 25% at all sites except those adjacent to fish farm nets (locations 8 and 13, condition indices 14%, 16%, respectively) and position 7,200 m from Port Lunna fish farm (condition index 14% ). Loch Melfort In the initial 5 weeks after deployment of oysters in Loch Melfort, animals in all positions accumulated tin (0.10-0.39 mg/kg), with highest concentrations (0.23-0.39 mg/kg) occurring at positions 3-5, i.e., within 500 m of the fish farm (Table 2 ) . Concentrations at positions 3-6 inclusive remained either fairly constant or increased (Fig. 5 ) during the experiment, whilst concentrations at many other locations, particularly those towards the head of the loch, passed through a maximum in August/September, and subsequently decreased (e.g., positions 9, 11, Fig. 5). Measurable accumulation of tributyltin compounds (Table 2) occurred at most locations. In November the highest concentrations (0.47-1.10 mg/kg) were found within 200 m of the fish farm (positions 4-6), and in the area of the yacht moorings at the head of the loch (0.27-0.32 mg/kg at positions 10 and 9). On average, TBT accounted for 72% of the total tin in the oysters in November. There was no consistent trend of tributyltin concentration with time during the experiment at the limited number of positions for which data are available. TABLE 2 Total tin and tributyltin concentrations (mg/kg Sn wet weight) and condition index (October) in oysters from cages in Loch Melfort Position July Sn
1 2 3 4 5 6 7---g---9 10 11 12 13 14
Aug. TBT
0.17 0.19 (0.07 0.23 0.24 0.39 0.67 0.15 0.67
Sn
Sept. TBT
0.16 0.19 (0.07 0.25 0.32 0.25 0.47 0.30 0.40
Sn
Oct. TBT
Sn
Nov. TBT
Condition % Sn
0.10 0.16 23 0.16 0.40 0.10 0.32 24 0.15 0.09 21 0.23 0.26 19 0.41 0.67 0.52 0.82 16 0.31 0.15 0.30 0.42 21 0.16 <0.03 21 --0.17 0.10 0.23 0.26 0.14 20 0.11 0.23 0.25 0.23 18 0.17 0.20 0.23 0.09 24 0.14 0.27 0.16 0.24 23 0.18 0.20 0.16 0.12 0.13 <0.07 0.16 (0.07 24 0.17 0.17 0.18 0.22 25
TBT
0.14 0.15 0.22 (0.07 0.23 0.67 0.50 1.10 0.37 0.47 0.07 0.12 0.14 0.33 0.12 0.27 0.07 0.15 0.03 0.07 0.03 <0.07 -
326
The shell thickness index decreased gradually throughout the experiment at all positions (except position 1). Indices of less than 15 were found (Fig. 6) in the final sample from all positions in the inner part of the loch (cages 3-ll), which includes the areas around the fish farm and yacht moorings. In Loch Melfort, most of the growth of soft tissue took place between June and September. In the outer part of the loch, on the southern side (positions 1, 2, 13), 134-144% growth occurred. Growth was less rapid elsewhere, with 82% (position 5) adjacent to the fish cage, 91% (position 9) close to the yacht
12
18
J 30
24
Weeks
Fig. 5. Total tin concentrations (mg/kg wet height) in homogenised soft parts of caged oysters in Loch Melfort. x , Position 5; 0, position 6; n , position 9; A, position 11.
SHELL THICKNESS
INDEX
(November)
1
I 1 km
Fig. 6. Shell thickness indices of oysters in Loch Melfort at the end of the exposure period (November ) .
327 175
r
17.5
I 20.0
Condition
I 22.5
I 25.0
percentage
Fig. 7. Relationship between tissue growth increment and condition index of caged oysters in Loch Melfort in October. Omission of the two anomalous points (see text) yields a linear regression y = -95 + 10.5x, with a correlation coefficient of 0.958.
and 80-86% in the northern outer part of the loch (positions 12, 14). Very little growth ( < 20% ) occurred in any cage between September and November, with significantly (P-C 1% ) better growth (7-19%, mean 13%) occurring along the northern shore (positions g-12)) than on the south side ( - 17 to 8%, mean -3%, positiqns l-6,13). The condition index of the oysters at all times exceeded that at deployment (12.5% ). The range of values in any month was small (e.g., September 18.525.2%, November 15.1-20.3% ). In general, oysters in the inner part of the loch (east of positions 2 and 11) were in poorer condition than those elsewhere. The condition index was strongly correlated with the growth of soft parts (Fig. 7). Oysters at the two positions which depart from this pattern (positions 12, 14) displayed good condition, but relatively poor growth, and were situated in the outer part of the loch on the northern side. moorings,
DISCUSSION
Five parameters (total tin and tributyltin concentrations, shell thickness index, tissue weight increment, and condition index) were used to assess the potential of tributyltin leached from fish cages to affect Pacific oysters. In Loch Sween all parameters showed a gradation of effect towards the fish farms, with the most intense effects occurring close to the farms. The thickening and chambering of the oyster shells observed at most locations in the two lochs is characteristic of exposure of Pacific oysters to organotin compounds (Waldock
328
and Thain, 1983; Alzieu and Portmann, 1984)) and has not, to date, been shown to be caused by any other class of chemical. Taken together with the accumulation of tributyltin compounds in oyster tissue, the presence of thickening indicates that the deleterious effects upon oysters in the cages were caused by tributyltin compounds. The recognised sources of such compounds in the upper part of Loch Sween are the two fish farms, and the yacht anchorage. The distribution of effects strongly indicates that the tributyl compounds affecting the caged oysters had leached from the fish cages in Port Lunna and Caol Scotnish. A survey of the effects of tributyltin compounds on the reproductive system of dogwhelks (induction of imposex) from Scottish sea lochs (Davies et al., 1987) has emphasised the significance of tributyltin leached from fish cages in affecting another species of mollusc, and pointed to areas in Loch Sween where the influence of fish cages could be identified. In Loch Melfort, the situation was complicated by the greater significance of small boat activity as a source of tributyltin. Within 200-500 m of the fish farm marked shell thickening (Fig. 6) and accumulation of tin compounds (Table 2 ) occurred, and the oysters grew less rapidly and were of slightly poorer condition (Table 2); however, similar effects, of perhaps lesser severity, occurred at all positions east of cages 3 and 11. It is not possible to separate clearly the influence of the fish farm from that of the small boats and associated facilities. More intense effects were observed in Loch Sween than in Loch Melfort, but they were confined to a smaller area. The potential source (cages plus boats) was much larger in Loch Melfort than Loch Sween, and the wide area of less intense effects probably reflects the less restricted circulation in Loch Melfort. The parameters used to indicate the effects of the organotin leachates showed considerable differences in sensitivity. Both shell thickening and accumulation of tin are characteristic of exposure to organotin compounds. By November, the shell thickness indices indicated shell deformation, and hence exposure to antifoulant leachates, at least 1000 m (but less than 5000 m) from the fish farm nets in Loch Sween. By contrast, the chemical measurements were not particularly sensitive indicators, with significant accumulation of tin compounds in Loch Sween largely confined to within 200 m of the fish farm nets. A comparison of shell thickness index and tin accumulation in November (Fig. 8) demonstrated a significant accumulation of tin only in oysters with a shell thickness index of less than 14. Below a shell thickness index of 12, total tin and tributyltin concentrations rose sharply. Whilst the shell thickening and chemical measurements are indicative of TBT contamination, the variations in growth parameters (rate of growth and condition) are not of themselves necessarily caused by exposure to tributyltin compounds. Differences in food availability or details of circulation patterns, for example, could alter growth rates of oysters. Of the growth parameters, the condition index was affected only within 200
329 30
x
f$ 24 0 .G I
6
.
x .
0
I
I
I
0.2
0.4
O-6
Tin concentration
!?I 0.8
I 1.0
(mglkg)
Fig. 8. Relationship between shell thickness and total tin or tributyltin concentration (mg/kg wet weight, expressed as tin) in caged oysters from Lochs Sween and Melfort, sampled in November. x , Tributyltin, Loch Sween; 0, total tin, Loch Sween; A, tributyltin, Loch Melfort; H, total tin, Loch Melfort. The data can be fitted by: Tin = 23.2 Th.-‘.g7 (carrel. coeff. 0.‘756), and TRT = 16’7Thp3.“l (carrel. coeff. 0.750).
m of the fish farms in Loch Sween. The growth of soft parts was initially affected only immediately beside the nets at the Caol Scotnish farm, but later was reduced up to 1000 m away from the Port Lunna farm. The gradients of effect towards the fish farms suggest that the poor performance of the oysters may be related to proximity to the salmon units, but other factors may also be involved. It has been demonstrated that tributyltin compounds leached from antifoulants on fish farm cage nets could affect young Pacific oysters in surrounding waters. In Loch Sween, accumulation of tin compounds in oysters was restricted to within 200 m of the fish farms, and effects on shell structure were observed at 1000 m, but did not extend to 5000 m from the Port Lunna farm. Effects on the growth of soft parts were seen within 100 m of the farm. In Loch Melfort, shell thickening occurred up to 4000 m from the fish farm, and tin accummulation was widespread. The growth of soft parts was reduced over an area of approximately 1.5 x 2.5 km, but the effects of the fish farm and small boat activity could not be clearly separated. The intensity and area of effect in the sea lochs examined appeared to be dependent upon source (treated cages and boats) size and distribution, and hydrographic factors. REFERENCES Alzieu, C. and Portmann, J.E., 1984. The effect of tributyltin on the culture of Crussostrea gigas and other species. Proc. 15th Annu. Shellfish Conf., 15-16 May 1984. Shellfish Assoc., London, pp. 87-100.
Alzieu, C., Heral, M., Thibaud, Y., Dardignac, M.J. and Feuillet, M., 1982. Influences des peinteuses antisalissures a base organostanniques sur la calcification de la coquille de l’huitre, Crassostreagigas. Rev. Trav. Inst. Peches Marit., 45: 100-116. Bryan, G.W., Gibbs, P.E., Hummerstone, L.G. and Burt, G.R., 1986. The decline of the gastropod Nucella lapillus around south west England: evidence for the effect of tributyltin from antifouling paints. J. Mar. Biol. Assoc. U.K., 66: 611-640. Davies, I.M. and McKie, J.C., 1987. Accumulation of total tin and tributyltin in muscle tissue of farmed Atlantic salmon. Mar. Pollut. Bull. 18 (7): 405-407. Davies, I.M., Bailey, S.K. and Moore, D.C., 1987. Tributyltin in Scottish sea lochs, as indicated by degree of imposex in the dogwhelk, Nucella lapillus (L.). Mar. Pollut. Bull., 18 (7): 400404. McKie, J., 1987. The determination of total tin and tributyltin in marine biological material using graphite furnace atomic absorption spectrophotometry. Anal. Chim. Acta, 197: 303-308. Milne, P.H., 1972. Hydrography of Scottish west coast sea lochs. Marine Research, 3. HMSO, Edinburgh, 50 pp. Short, J.W. and Thrower, F.P., 1986. Accumulation of butyltins in muscle tissue of Chinook salmon reared in sea pens treated with tri-n-butyltin. Mar. Pollut. Bull., 17 (12): 542-545. Short, J.W. and Thrower, F.P., 1987. Toxicity of tri-n-butyltin to Chinook salmon, Oncorhynchus tshawytscha, adapted to seawater. Aquaculture, 61: 193-200. Thain, J., 1983. The acute toxicity of bis (tributyltin) oxide to adults and larvae of some marine organisms. Int. Count. Explor. Sea, CM 1983/E: 13,5 pp. Waldock, M.J. and Thain, J., 1983. Shell thickening in Crassostrea gigas: organotin antifouling or sediment induced? Mar. Pollut. Bull., 14: 411-415.