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A comparative study of sediments under a marine cage farm at Gran Canaria Island (Spain). Preliminary results
Aquaculture 192 Ž2001. 225–231 www.elsevier.nlrlocateraqua-online
A comparative study of sediments under a marine cage farm at Gran Canaria Island žSpain/ . Preliminary results a,) L. Molina Domınguez , G. Lopez Calero a , ´ ´ J.M. Vergara Martın ´ b, L. Robaina Robaina b a
Instituto Canario de Ciencias Marinas (ICCM), Direccion ´ General de UniÕersidades e InÕestigacion, ´ Consejerıa ´ de Educacion, ´ Cultura y Deportes, Gobierno de Canarias, P.O. Box 56, 35200 Telde, Gran Canaria, Canary Islands, Spain b Departamento de Biologıa, ´ UniÕersidad de Las Palmas de Gran Canaria (ULPGC), P.O. Box 550, 35017 Las Palmas de Gran Canaria, Canary Islands, Spain Received 9 September 1999; received in revised form 2 May 2000; accepted 7 June 2000
Abstract This paper presents the results of a comparative study of sediments beneath an offshore cage farm for on-growing gilthead seabream Ž Sparus aurata L.. over a 1-year period, from the start of farming operations. The cage system was located at Melenara Bay, on the East coast of Gran Canaria Island, Canary Islands. Samples were collected from three zones: underneath the cages Žtwo sampling points., 60 Žfour sampling points. and 200 m Žfour sampling points. from the cages. Sediment samples were analysed for particle size, organic matter, nitrogen, and phosphorus contents. Statistical analyses of all parameters showed no significant differences between samples within the same zones. When different zones were compared, there were no significant differences for organic matter and phosphorus content, and minor differences for particle size. However, the average nitrogen content of the sediments underneath the cages was significantly greater than the other two zones. There were temporal variations in the two nutrients in all zones, with a trend towards the recovering of initial values, except for nitrogen beneath the cages. In general terms,
)
Corresponding author. Tel.: q34-28-132-900; fax: q34-28-132-908. .. E-mail address: [email protected] ŽL. Molina Domınguez ´
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 0 . 0 0 4 5 0 - 6
226
L. Molina Domınguez et al.r Aquaculture 192 (2001) 225–231 ´
the physical and chemical characteristics of sediments studied were little affected by the operation of the farm over the period of this study. This could be explained by the fact that the farm was in its first year of operation, its small size Ž80 Mt output per year., and the high average velocity of water currents Žabout 6 cmrs.. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Environmental impact; Sediments; Cages; Nutrients
1. Introduction The environmental impacts of aquaculture activities have received an increasing amount of attention during recent years Že.g. Gowen and Bradbury, 1987; Iwama, 1991; Wu, 1995.. The research focuses commonly on cyprinids, oysters, mussels, and samonids, and there is a general lack of information with regards to marine finfish species such as seabream and seabass ŽMunday et al., 1994; Karakassis et al., 1998., despite the expanding farming industry of the two species in the southern areas of the northern hemisphere. In addition, in areas such as the Canary Islands, some particularities could be expected as a result of different environmental conditions and the different growth patterns of these species under local conditions. The influence of aquaculture on the sediment and the benthic environment Žspecially from intensive culture systems. is basically due to the deposition of organic wastes. Nutrient loadings Žnitrogen and phosphorus principally. from a fish culture cage system can be separated into a dissolved fraction Žexcreted dissolved and re-dissolved from sediments., and a particulate fraction Žuneaten feed and fecal pellets, plus dead fish.. These nutrients can be finally settled and accumulated on the sediments. The amount of nutrients loaded to the environment has been calculated for different fish species, including gilthead seabream Ž Sparus aurata L.. Že.g. Holby and Hall, 1991; Neori and Krom, 1991; Hall et al., 1992; Schwartz and Boyd, 1994; Molina Domınguez ´ et al., 1997.. However, in most studies, the amount of nutrients released to the environment is theoretically calculated, and only a few of them are based on field nutrients determination. Some authors suggest the need for direct measurements of nutrients, both in solid and dissolved wastes ŽMerican and Phillips, 1985; Ackefors and Enell, 1990.. The aim of this study was to assess the impact of a pilot off shore cage farm for ongrowing marine fish on the sediments beneath and adjacent to the cages by estimating their physico-chemical parameters.
2. Material and methods Melenara Bay is located on the East coast of Gran Canaria Island Ž27859X N, 15822X W.. Depths at the site range from 18 to 22 m. Average current velocity was 6 cmrs and NE–SW directions predominated Žmeasured with an automatic current-meter ŽAanderaa Instruments, Mod. RCM-7, Norway. located near the cages at an average
L. Molina Domınguez et al.r Aquaculture 192 (2001) 225–231 ´
227
depth of 4 m.. Water temperature ranged between 178C and 248C, and dissolved oxygen values in surface water always exceeded 6.2 mgrl. Six flexible, polystyrene off shore cages, 12 m in diameter and 8 m mesh depth Ž900 m3 volume per cage., were anchored on a sandy bottom. Gilthead seabream were stocked at an average 15 kgrm3 density, with a total output of 80 Mt at the end of the year of study. Values for seabream stocking in commercial operations in the area range between 10 and 25 kgrm3. Fish were fed both by hand and automatically using commercial, pelleted and extruded diets with average nutrient content Žon a dry weight basis.: protein 49%, fat 18.5%, carbohydrate 22%, nitrogen 7.9% and phosphorus 1.08%. The range of pellet size employed was 2–7 mm, and fish were fed at an average rate Ž% of biomass per day. of 1.95%, with resulting Food Conversion Ratio of 1.6. Sampling stations were selected after considering reports from different studies. The extent of the seabed area impacted by fish farming activities varies in general with current velocity and depth ŽGowen and Bradbury, 1987.. Influences of cages on the sediment have been reported as far as 15 m ŽBrown et al., 1987., 25 m ŽHansen et al., 1991., 60 m ŽYe et al., 1991., or 90 m ŽWeston, 1990. from the facilities. Johannessen et al. Ž1994. reported no noticeable effects 250 m from the facilities. In most cases, the impact appears as a highly localized phenomenon not exceeding 20 to 50 m from cages ŽBeveridge, 1996.. The area studied in this work can be defined as Atransportation bottomB ŽLauren-Maatta ´ ¨¨ ¨ et al., 1991., where solid wastes tend to be spread over a large area, and have slightly lower effects than in other bottom types. Sampling stations were established in three zones: underneath the cages, and 60 and 200 m from the cages Žzones 1, 2 and 3, respectively., the last one being considered as a reference zone. Sediment samples were collected by divers prior to fish stocking Žinitial sample. and then at 2-month intervals using PVC cores of 25 cm length and 8 cm internal diameter Žsample depth of the sediment core was at least 12 cm.. Replicate samples were taken in order to determine variability between samples. Ten different sampling points were established: underneath the cages Žtwo points., and 60 Žfour points. and 200 m Žfour points. from the cages. In order to minimise the effects of pseudoreplication, replicate cores were collected randomly further apart from each other in the control zone than in the other sampling zones ŽKarakassis et al., 1998.. On arriving at the laboratory Žless than 1 h., cores were frozen Žat y208C. until analysis. Weighed sediment samples were dried in an oven at 1058C until constant weight Žapproximately 24 to 36 h.. Particle-size analysis Ždry sieving. was then performed by using a sieve ŽCISA, Mod. 211, Spain. for 15 min and subsequent weighing of resulting fractions. Organic matter content was estimated by heating dried and homogenized samples in a muffle furnace at 6508C for 12 h ŽAOAC, 1985.. Total nitrogen was determined in the dried and homogenized samples using the Micro-Kjedhal technique with a TecatorrKjeltec System ŽMod. 1002. distilling unit ŽAOAC, 1985.. The concentration of total phosphorus was determined in dried samples by spectrophotometry as molibdate reactive phosphorus ŽStrickland and Parsons, 1972., after digestion with nitric and perchloric acids ŽBurton and Riley, 1956; Sturgeon et al., 1982.. Analysis of variance ŽANOVA. was used to determine the significance between data and Tukey test for multiple comparisons between groups. Two-way ANOVA Žzone by sampling time. was used to test for differences in the full data set.
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3. Results and discussion Only minor differences were observed for particle size between the different zones during the period of study. There was a slight decrease of the finest fraction in zones 1 and 2 ŽTable 1.. This effect has also been reported for cages in freshwater lakes ŽGuiral, 1986.. The presence of floating cages can produce a decrease in water current velocity, leading to settlement of bigger particles close to the system. This may be reflected in the increased component of 200–500 mm particles in zones 1 and 2, but the major component of the sediment Ž105–200 mm particles. did not vary. Similarly, temporal variations in particle size were little, suggesting that the current patterns in the area was enough to spread solid wastes. Sediment analysis including determination of silt–clay fraction would have provided a more complete set of data. Two-way ANOVA showed significant differences between zones and sampling times for nitrogen and organic matter contents in sediments, while significant differences were found for sampling time for phosphorus concentrations but not for zone ŽTable 2.. The organic matter content of the initial samples was approximately 3.5%, while the year average values for all the zones ranged between 5% and 6%. Larger values have been reported for a sea bream farm in the Gulf of Aqaba ŽAngel et al., 1995. and for salmonids farms ŽHansen et al., 1991.. Temporal variations were observed for organic matter in all three zones; however, they were less apparent in the zone directly underneath the cages. Maximum values occurred in summer, with a decrease during autumn until minimum values were reached in winter ŽTable 2.. The nitrogen content of the initial samples, approximately 11 mgr100 g, was slightly lower than the average value for zone 1 Ž14.55 mgr100 g., and higher than those for zones 2 and 3 ŽTable 3.. Nitrogen values for zones 2 and 3 showed a similar seasonal pattern, with maximum and minimum values in summer and winter, respectively. However, there was no pattern in zone 1, where there was no significant differences between samples at different times ŽTable 2.. The phosphorus content of the initial samples, approximately 10.5 mgr100 g, was only slightly lower than the average values for all the zones ŽTable 3.. Higher values for phosphorus concentrations in sediments impacted by fish farms have been reported by others authors such as Karakassis et al. Ž1998. and Holby and Hall Ž1991..
Table 1 Percentage of particle size distribution for the sediment of three zones. Mean values"standard deviation, on a dry weight basis, for the study period. Zones 1, 2 and 3, see text Particle size Žmm.
Zone 1
Zone 2
Zone 3 Žreference.
-105 105–250 250–500 500–1000 1000–2000 ) 2000
5.66"1.41a 85.29"2.65a 8.58"3.10a 0.25"0.15a 0.16"0.24a 0.06"0.08a
5.72"2.28a 85.94"2.97a 7.79"1.50a 0.22"0.21a 0.10"0.11a 0.12"0.25a
7.44"2.48b 85.95"2.53a 6.01"1.50b 0.32"0.28a 0.12"0.15a 0.14"0.22a
Values in the same row with similar letter are not significantly different ŽTukey test, P - 0.05..
L. Molina Domınguez et al.r Aquaculture 192 (2001) 225–231 ´
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Table 2 Organic matter Ž%OM., nitrogen ŽN mgr100 g. and phosphorus ŽP mgr100 g. content for the different sediment zones. Mean values, on a dry weight basis, for each month. Zones 1, 2 and 3, see text Zone 1
June August October December February April June
Zone 2
Zone 3
N
P
%OM
N
P
%OM
N
P
%OM
11.23a 15.63a 11.03a 14.67a 16.73a 16.38a 14.94a
9.09ab 13.41cd 10.91bc 12.39bcd 10.83bc 15.18d 9.09a
3.5a 8.03b 6.88c 6.04cd 6.32cd 5.92cd 5.73d
11.09abc 16.08c 9.32ab 5.98a 9.85ab 12.37bc 9.5ab
10.69ab 11.46ab 9.49a 10.39ab 13.98b 14.16b 9.65a
3.43a 8.71b 7.76bc 5.61bcd 6.34cd 4.58ad 6.66bcd
11.62ab 14.17b 5.94a 6.17a 9.67ab 7.41a 9.46ab
11.68ab 11.28ab 10.51ab 10.82ab 7.78a 15.14b 7.82a
3.35a 8.11b 5.66ab 3.46a 3.33a 4.88a 5.63ab
Values in the same column with similar letter are not significantly different ŽTukey test, P - 0.05..
The phosphorus content of the sediment also showed a seasonal pattern, although different to that for other parameters. Maximum values were found in spring and late summer, and minimum values in autumn and winter, showing in all zones a trend to recover initial values ŽTable 2.. No differences were found in phosphorus concentrations between the three sampling zones ŽTable 3.. On the whole, no significant differences were found for average organic matter and phosporus content between the different influence zones studied. However, the average nitrogen content of the sediments underneath the cages was significantly different from the other zones sampled ŽTable 3.. Similar seasonal variations for the two nutrients and the organic matter content were observed in all zones, with a repeated trend to recover initial values in the three zones, except for nitrogen content beneath the cages. Similar seasonal trends were found by Karakassis et al. Ž1998., although a different one has been reported by Gilbert et al. Ž1997.. These differences may be explained by different management of the farms. In this study as in Karakassis et al. Ž1998., the fish farm depended on exogenous input of fish feed, whereas Gilbert et al. Ž1997. dealt with a shellfish farm depending on natural production of phytoplankton. The results of our study suggest that no noticeable accumulation of solid particulate wastes from the farm had taken place after the first year of operation, even when predominant water current directions ŽNE–SW. are considered. It is possible that the
Table 3 Organic matter, nitrogen and phosphorus contents for the different sediment zones. Mean values"standard deviation, on a dry weight basis, for the study period. Zones 1, 2 and 3, see text
Organic matter Ž%. Nitrogen Žmgr100 g. Phosphorus Žmgr100 g.
Zone 1
Zone 2
Zone 3 Žreference.
6.06"1.34a 14.55"3.01a 11.08"6.15a
6.16"1.84a 10.60"3.36b 11.41"3.87a
4.92"1.88a 9.38"3.14b 10.72"5.05a
Values in the same row with similar letter are not significantly different ŽTukey test, P - 0.05..
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average water current velocity Žabout 6 cmrs. was sufficient to disperse solid wastes, thus avoiding the undesirable effects of organic sediment accumulation. Also of importance was the small size of the facility under study, and the fact that the farm was in its first year of operation. In time, the farm may show greater influence on the environment. A more prolonged and detailed sampling program, including continuous measurements of current velocity and direction, dissolved nutrients, the use of sediment traps, and ecological surveys on the nearby coasts, in addition to the above studies, is being carried out at the present time. We hope this will provide as a more complete picture on the nutrients load model; that is, the interactions between this particular farm and the environment.
Acknowledgements The authors wish to thank the staff of the farm Alevines y Doradas for their valuable cooperation and also to the divers Pablo Camara and Borja Garbizu for their help in the ´ collection of sediment samples.
References Ackefors, H., Enell, M., 1990. Discharge of nutrients from Swedish fish farming to adjacent sea areas. Ambio 19 Ž1., 28–35, Feb. Angel, D.L., Krost, P., Gordin, H., 1995. Benthic implications of the net cage aquaculture in the oligotrofic Gulf of Aqaba. In: Rosenthal, H., Moav, B., Gordin, H. ŽEds.., Improving the knowledge base in modern aquaculture. Eur. Aquacult. Soc. Spec. Publ., vol. 25, pp. 129–173. AOAC, 1985. Official Methods of Analysis of the Association of Analytical Chemist. 14th edn., Washington, 1018 pp. Beveridge, M.C.M., 1996. Cage aquaculture, Fishing News Books. Blackwell, Oxford, 131 pp. Brown, J.R., Gowen, R.J., McLusky, D.S., 1987. The effect of salmon farming on the benthos of a Scottish sea loch. J. Exp. Mar. Biol. Ecol. 109, 39–51. Burton, J.D., Riley, J.P., 1956. Determination of soluble phosphate, and total phosphorus in sea waters and of total in marine muds. Microchim. Acta 9, 1350–1365. Gilbert, F., Souchy, P., Bianchi, M., Bonin, P., 1997. Influence of shellfish farming activities on nitrification, nitrate reduction to ammonium and denitrification at the water–sediment interface of the Thau lagoon, France. Mar. Ecol.: Prog. Ser. 151, 143–153. Gowen, R.J., Bradbury, N.B., 1987. The ecological impact of salmonid farming in coastal waters: a review. Oceanogr. Mar. Biol. 25, 563–575. Guiral, D., 1986. Modifications et transformations des ecosystems sedimentaires par del elevages piscicoles en ´ lagune Ebrie ŽCote ˆ d’Ivoire.. Aquaculture 52, 287–302. Hall, P.O.J., Holby, O., Kollberg, S., Samuelsson, M.-O., 1992. Chemical fluxes and mass balances in a marine fish cage farm: IV. Nitrogen Mar. Ecol. Prog. Ser. 89, 81–91. Hansen, P.K., Pittman, K., Ervik, A., 1991. Organic waste from marine fish farms — effects on the seabed. In: Makinen, T. ŽEd.., Marine Aquaculture and Environment Nordic Council of Ministers, Copenhagen, ¨ pp. 105–119. Holby, O., Hall, P.O.J., 1991. Chemical fluxes and mass balances in a marine fish cage farm: II. Phosphorus Mar. Ecol. Prog. 70, 263–272.
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Iwama, G.I., 1991. Interactions between aquaculture and the environment. Crit. Rev. Environ. Control 21, 177–216. Johannessen, P.J., Botnen, H.B., Tvedten, O.F., 1994. Macrobenthos: before, during and after a fish farm. Aquacult. Fish. Manage. 25, 55–66. Karakassis, I., Tsapakis, M., Hatziyanni, E., 1998. Seasonal variability in sediment profiles beneath fish farm cages in the Mediterranean. Mar. Ecol. Prog. Ser. 162, 243–252. Lauren-Maatta, ´ ¨¨ ¨ C., Granlid, M., Henriksson, S.H., Koivisto, V., 1991. Effects of fish farming on the macrobenthos of different bottom types. In: Makinen, T. ŽEd.., Marine Aquaculture and Environment ¨ Nordic Council of Ministers, Copenhagen, pp. 57–84. Merican, Z.O., Phillips, M.J., 1985. Solid waste production from rainbow trout, Salmo gairdneri Richardson, cage culture. Aquacult. Fish. Manage. 1, 55–69. Molina Domınguez, L., Lopez Calero, G., Vergara Martın, ´ ´ ´ J.M., Robaina Robaina, L., Fdez.-Palacios, H., 1997. Retention and discharge of nutrients in a marine cage farm at Canary Islands. Preliminary results. Proceedings of the workshop of the CIHEAM Network on Technology of Aquaculture in the Mediterranean ŽTECAM., Cahiers Options mediterraneennes. Feeding Tomorrow Fish, vol. 22, pp. 291–300. ´ Munday, B., Eleftheriou, A., Kentouri, M., Divanach, P., 1994. Quantitative statistical analysis of the literature concerning the interaction of the environment and aquaculture — identification of gap and lacks. J. Appl. Ichthyol. 10, 319–325. Neori, A., Krom, M.D., 1991. Nitrogen and phosphorus budgets in an intensive marine fishpond: the importance of microplankton. In: Cowey, C.B., Cho, C.Y. ŽEds.., Proceedings of the first International Symposium on Nutritional Strategies in management of Aquaculture waste ŽNSMAW.,University of Guelp, Guelp, Ontario, pp. 223–230. Schwartz, M.F., Boyd, C.E., 1994. Channel catfish pond effluents. Prog. Fish-Cult. 56, 273–281. Strickland, J., Parsons, T., 1972. A practical handbook of sea water analysis. Bull. Fish. Res. Bd. Can., 167, 312 pp. Sturgeon, R.E., Desaulniers, J.A.H., Berman, S.S., Russell, D.S., 1982. Determination of trace metal in estuarines sediments by graphite-furnace atomic absorption spectrometry. Anal. Chim. Acta 134, 283–291. Weston, D.P., 1990. Quantitative examination of macrobenthic community changes along an organic enrichment gradient. Mar. Ecol. Prog. Ser. 61, 233–244. Wu, R.S.S., 1995. The environmental impact of marine fish culture: towards a sustainable future. Mar. Pollut. Bull. 31, 159–166. Ye, L.X., Ritz, D.A., Fenton, G.E., Lewis, M.E., 1991. Tracing the influence on sediments of organic waste from a salmonid farm using stable isotope analysis. J. Exp. Mar. Biol. Ecol. 145 Ž2., 161–174.
A comparative study of sediments under a marine cage farm at Gran Canaria Island žSpain/ . Preliminary results a,) L. Molina Domınguez , G. Lopez Calero a , ´ ´ J.M. Vergara Martın ´ b, L. Robaina Robaina b a
Instituto Canario de Ciencias Marinas (ICCM), Direccion ´ General de UniÕersidades e InÕestigacion, ´ Consejerıa ´ de Educacion, ´ Cultura y Deportes, Gobierno de Canarias, P.O. Box 56, 35200 Telde, Gran Canaria, Canary Islands, Spain b Departamento de Biologıa, ´ UniÕersidad de Las Palmas de Gran Canaria (ULPGC), P.O. Box 550, 35017 Las Palmas de Gran Canaria, Canary Islands, Spain Received 9 September 1999; received in revised form 2 May 2000; accepted 7 June 2000
Abstract This paper presents the results of a comparative study of sediments beneath an offshore cage farm for on-growing gilthead seabream Ž Sparus aurata L.. over a 1-year period, from the start of farming operations. The cage system was located at Melenara Bay, on the East coast of Gran Canaria Island, Canary Islands. Samples were collected from three zones: underneath the cages Žtwo sampling points., 60 Žfour sampling points. and 200 m Žfour sampling points. from the cages. Sediment samples were analysed for particle size, organic matter, nitrogen, and phosphorus contents. Statistical analyses of all parameters showed no significant differences between samples within the same zones. When different zones were compared, there were no significant differences for organic matter and phosphorus content, and minor differences for particle size. However, the average nitrogen content of the sediments underneath the cages was significantly greater than the other two zones. There were temporal variations in the two nutrients in all zones, with a trend towards the recovering of initial values, except for nitrogen beneath the cages. In general terms,
)
Corresponding author. Tel.: q34-28-132-900; fax: q34-28-132-908. .. E-mail address: [email protected] ŽL. Molina Domınguez ´
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 0 . 0 0 4 5 0 - 6
226
L. Molina Domınguez et al.r Aquaculture 192 (2001) 225–231 ´
the physical and chemical characteristics of sediments studied were little affected by the operation of the farm over the period of this study. This could be explained by the fact that the farm was in its first year of operation, its small size Ž80 Mt output per year., and the high average velocity of water currents Žabout 6 cmrs.. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Environmental impact; Sediments; Cages; Nutrients
1. Introduction The environmental impacts of aquaculture activities have received an increasing amount of attention during recent years Že.g. Gowen and Bradbury, 1987; Iwama, 1991; Wu, 1995.. The research focuses commonly on cyprinids, oysters, mussels, and samonids, and there is a general lack of information with regards to marine finfish species such as seabream and seabass ŽMunday et al., 1994; Karakassis et al., 1998., despite the expanding farming industry of the two species in the southern areas of the northern hemisphere. In addition, in areas such as the Canary Islands, some particularities could be expected as a result of different environmental conditions and the different growth patterns of these species under local conditions. The influence of aquaculture on the sediment and the benthic environment Žspecially from intensive culture systems. is basically due to the deposition of organic wastes. Nutrient loadings Žnitrogen and phosphorus principally. from a fish culture cage system can be separated into a dissolved fraction Žexcreted dissolved and re-dissolved from sediments., and a particulate fraction Žuneaten feed and fecal pellets, plus dead fish.. These nutrients can be finally settled and accumulated on the sediments. The amount of nutrients loaded to the environment has been calculated for different fish species, including gilthead seabream Ž Sparus aurata L.. Že.g. Holby and Hall, 1991; Neori and Krom, 1991; Hall et al., 1992; Schwartz and Boyd, 1994; Molina Domınguez ´ et al., 1997.. However, in most studies, the amount of nutrients released to the environment is theoretically calculated, and only a few of them are based on field nutrients determination. Some authors suggest the need for direct measurements of nutrients, both in solid and dissolved wastes ŽMerican and Phillips, 1985; Ackefors and Enell, 1990.. The aim of this study was to assess the impact of a pilot off shore cage farm for ongrowing marine fish on the sediments beneath and adjacent to the cages by estimating their physico-chemical parameters.
2. Material and methods Melenara Bay is located on the East coast of Gran Canaria Island Ž27859X N, 15822X W.. Depths at the site range from 18 to 22 m. Average current velocity was 6 cmrs and NE–SW directions predominated Žmeasured with an automatic current-meter ŽAanderaa Instruments, Mod. RCM-7, Norway. located near the cages at an average
L. Molina Domınguez et al.r Aquaculture 192 (2001) 225–231 ´
227
depth of 4 m.. Water temperature ranged between 178C and 248C, and dissolved oxygen values in surface water always exceeded 6.2 mgrl. Six flexible, polystyrene off shore cages, 12 m in diameter and 8 m mesh depth Ž900 m3 volume per cage., were anchored on a sandy bottom. Gilthead seabream were stocked at an average 15 kgrm3 density, with a total output of 80 Mt at the end of the year of study. Values for seabream stocking in commercial operations in the area range between 10 and 25 kgrm3. Fish were fed both by hand and automatically using commercial, pelleted and extruded diets with average nutrient content Žon a dry weight basis.: protein 49%, fat 18.5%, carbohydrate 22%, nitrogen 7.9% and phosphorus 1.08%. The range of pellet size employed was 2–7 mm, and fish were fed at an average rate Ž% of biomass per day. of 1.95%, with resulting Food Conversion Ratio of 1.6. Sampling stations were selected after considering reports from different studies. The extent of the seabed area impacted by fish farming activities varies in general with current velocity and depth ŽGowen and Bradbury, 1987.. Influences of cages on the sediment have been reported as far as 15 m ŽBrown et al., 1987., 25 m ŽHansen et al., 1991., 60 m ŽYe et al., 1991., or 90 m ŽWeston, 1990. from the facilities. Johannessen et al. Ž1994. reported no noticeable effects 250 m from the facilities. In most cases, the impact appears as a highly localized phenomenon not exceeding 20 to 50 m from cages ŽBeveridge, 1996.. The area studied in this work can be defined as Atransportation bottomB ŽLauren-Maatta ´ ¨¨ ¨ et al., 1991., where solid wastes tend to be spread over a large area, and have slightly lower effects than in other bottom types. Sampling stations were established in three zones: underneath the cages, and 60 and 200 m from the cages Žzones 1, 2 and 3, respectively., the last one being considered as a reference zone. Sediment samples were collected by divers prior to fish stocking Žinitial sample. and then at 2-month intervals using PVC cores of 25 cm length and 8 cm internal diameter Žsample depth of the sediment core was at least 12 cm.. Replicate samples were taken in order to determine variability between samples. Ten different sampling points were established: underneath the cages Žtwo points., and 60 Žfour points. and 200 m Žfour points. from the cages. In order to minimise the effects of pseudoreplication, replicate cores were collected randomly further apart from each other in the control zone than in the other sampling zones ŽKarakassis et al., 1998.. On arriving at the laboratory Žless than 1 h., cores were frozen Žat y208C. until analysis. Weighed sediment samples were dried in an oven at 1058C until constant weight Žapproximately 24 to 36 h.. Particle-size analysis Ždry sieving. was then performed by using a sieve ŽCISA, Mod. 211, Spain. for 15 min and subsequent weighing of resulting fractions. Organic matter content was estimated by heating dried and homogenized samples in a muffle furnace at 6508C for 12 h ŽAOAC, 1985.. Total nitrogen was determined in the dried and homogenized samples using the Micro-Kjedhal technique with a TecatorrKjeltec System ŽMod. 1002. distilling unit ŽAOAC, 1985.. The concentration of total phosphorus was determined in dried samples by spectrophotometry as molibdate reactive phosphorus ŽStrickland and Parsons, 1972., after digestion with nitric and perchloric acids ŽBurton and Riley, 1956; Sturgeon et al., 1982.. Analysis of variance ŽANOVA. was used to determine the significance between data and Tukey test for multiple comparisons between groups. Two-way ANOVA Žzone by sampling time. was used to test for differences in the full data set.
228
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3. Results and discussion Only minor differences were observed for particle size between the different zones during the period of study. There was a slight decrease of the finest fraction in zones 1 and 2 ŽTable 1.. This effect has also been reported for cages in freshwater lakes ŽGuiral, 1986.. The presence of floating cages can produce a decrease in water current velocity, leading to settlement of bigger particles close to the system. This may be reflected in the increased component of 200–500 mm particles in zones 1 and 2, but the major component of the sediment Ž105–200 mm particles. did not vary. Similarly, temporal variations in particle size were little, suggesting that the current patterns in the area was enough to spread solid wastes. Sediment analysis including determination of silt–clay fraction would have provided a more complete set of data. Two-way ANOVA showed significant differences between zones and sampling times for nitrogen and organic matter contents in sediments, while significant differences were found for sampling time for phosphorus concentrations but not for zone ŽTable 2.. The organic matter content of the initial samples was approximately 3.5%, while the year average values for all the zones ranged between 5% and 6%. Larger values have been reported for a sea bream farm in the Gulf of Aqaba ŽAngel et al., 1995. and for salmonids farms ŽHansen et al., 1991.. Temporal variations were observed for organic matter in all three zones; however, they were less apparent in the zone directly underneath the cages. Maximum values occurred in summer, with a decrease during autumn until minimum values were reached in winter ŽTable 2.. The nitrogen content of the initial samples, approximately 11 mgr100 g, was slightly lower than the average value for zone 1 Ž14.55 mgr100 g., and higher than those for zones 2 and 3 ŽTable 3.. Nitrogen values for zones 2 and 3 showed a similar seasonal pattern, with maximum and minimum values in summer and winter, respectively. However, there was no pattern in zone 1, where there was no significant differences between samples at different times ŽTable 2.. The phosphorus content of the initial samples, approximately 10.5 mgr100 g, was only slightly lower than the average values for all the zones ŽTable 3.. Higher values for phosphorus concentrations in sediments impacted by fish farms have been reported by others authors such as Karakassis et al. Ž1998. and Holby and Hall Ž1991..
Table 1 Percentage of particle size distribution for the sediment of three zones. Mean values"standard deviation, on a dry weight basis, for the study period. Zones 1, 2 and 3, see text Particle size Žmm.
Zone 1
Zone 2
Zone 3 Žreference.
-105 105–250 250–500 500–1000 1000–2000 ) 2000
5.66"1.41a 85.29"2.65a 8.58"3.10a 0.25"0.15a 0.16"0.24a 0.06"0.08a
5.72"2.28a 85.94"2.97a 7.79"1.50a 0.22"0.21a 0.10"0.11a 0.12"0.25a
7.44"2.48b 85.95"2.53a 6.01"1.50b 0.32"0.28a 0.12"0.15a 0.14"0.22a
Values in the same row with similar letter are not significantly different ŽTukey test, P - 0.05..
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229
Table 2 Organic matter Ž%OM., nitrogen ŽN mgr100 g. and phosphorus ŽP mgr100 g. content for the different sediment zones. Mean values, on a dry weight basis, for each month. Zones 1, 2 and 3, see text Zone 1
June August October December February April June
Zone 2
Zone 3
N
P
%OM
N
P
%OM
N
P
%OM
11.23a 15.63a 11.03a 14.67a 16.73a 16.38a 14.94a
9.09ab 13.41cd 10.91bc 12.39bcd 10.83bc 15.18d 9.09a
3.5a 8.03b 6.88c 6.04cd 6.32cd 5.92cd 5.73d
11.09abc 16.08c 9.32ab 5.98a 9.85ab 12.37bc 9.5ab
10.69ab 11.46ab 9.49a 10.39ab 13.98b 14.16b 9.65a
3.43a 8.71b 7.76bc 5.61bcd 6.34cd 4.58ad 6.66bcd
11.62ab 14.17b 5.94a 6.17a 9.67ab 7.41a 9.46ab
11.68ab 11.28ab 10.51ab 10.82ab 7.78a 15.14b 7.82a
3.35a 8.11b 5.66ab 3.46a 3.33a 4.88a 5.63ab
Values in the same column with similar letter are not significantly different ŽTukey test, P - 0.05..
The phosphorus content of the sediment also showed a seasonal pattern, although different to that for other parameters. Maximum values were found in spring and late summer, and minimum values in autumn and winter, showing in all zones a trend to recover initial values ŽTable 2.. No differences were found in phosphorus concentrations between the three sampling zones ŽTable 3.. On the whole, no significant differences were found for average organic matter and phosporus content between the different influence zones studied. However, the average nitrogen content of the sediments underneath the cages was significantly different from the other zones sampled ŽTable 3.. Similar seasonal variations for the two nutrients and the organic matter content were observed in all zones, with a repeated trend to recover initial values in the three zones, except for nitrogen content beneath the cages. Similar seasonal trends were found by Karakassis et al. Ž1998., although a different one has been reported by Gilbert et al. Ž1997.. These differences may be explained by different management of the farms. In this study as in Karakassis et al. Ž1998., the fish farm depended on exogenous input of fish feed, whereas Gilbert et al. Ž1997. dealt with a shellfish farm depending on natural production of phytoplankton. The results of our study suggest that no noticeable accumulation of solid particulate wastes from the farm had taken place after the first year of operation, even when predominant water current directions ŽNE–SW. are considered. It is possible that the
Table 3 Organic matter, nitrogen and phosphorus contents for the different sediment zones. Mean values"standard deviation, on a dry weight basis, for the study period. Zones 1, 2 and 3, see text
Organic matter Ž%. Nitrogen Žmgr100 g. Phosphorus Žmgr100 g.
Zone 1
Zone 2
Zone 3 Žreference.
6.06"1.34a 14.55"3.01a 11.08"6.15a
6.16"1.84a 10.60"3.36b 11.41"3.87a
4.92"1.88a 9.38"3.14b 10.72"5.05a
Values in the same row with similar letter are not significantly different ŽTukey test, P - 0.05..
230
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average water current velocity Žabout 6 cmrs. was sufficient to disperse solid wastes, thus avoiding the undesirable effects of organic sediment accumulation. Also of importance was the small size of the facility under study, and the fact that the farm was in its first year of operation. In time, the farm may show greater influence on the environment. A more prolonged and detailed sampling program, including continuous measurements of current velocity and direction, dissolved nutrients, the use of sediment traps, and ecological surveys on the nearby coasts, in addition to the above studies, is being carried out at the present time. We hope this will provide as a more complete picture on the nutrients load model; that is, the interactions between this particular farm and the environment.
Acknowledgements The authors wish to thank the staff of the farm Alevines y Doradas for their valuable cooperation and also to the divers Pablo Camara and Borja Garbizu for their help in the ´ collection of sediment samples.
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