THE INFLUENCE OF FISH FARMING IN THE ADDITION OF PARTICULATE NITROGEN IN COASTAL ENVIRONMENTS

Transcription

1 Proceedings of the 9 th International Conference on Environmental Science and Technology Rhodes island, Greece, 1 3 September 2005 THE INFLUENCE OF FISH FARMING IN THE ADDITION OF PARTICULATE NITROGEN IN COASTAL ENVIRONMENTS C. BELIAS 1, E. LADAKIS 1, K. PAPAKONSTANTINOU 2, E. DASSENAKIS 1 and M. SCOULLOS 1 1 Laboratory of Environmental Chemistry, Department of Chemistry, University of Athens, Panepistimiopolis, Athens, Greece 2 Laboratory of Mechanical Installations, Department of Mechanical Engineering, Technological Educational Institution of Halkida , Psahna, Evia, Greece mio-ee-env@ath.forthnet.gr EXTENDED ABSTRACT Coastal fish farming of sea bass and sea bream is a significant economic activity in the Mediterranean Sea, and especially in Greece, that presents a continuous development and increase of its productivity. In parallel, over the last decade, an increased scientific interest have been developed for the determination of the effects of fish farming in the neighbouring marine environment and the understanding of the various aspects and mechanisms of this influence. Most of the initial scientific research concerning fish cage farming focused on measurements of environmental parameters in the water column and sediments near the cages, in order to produce a qualitative picture of the environmental conditions in the studied area. The quantification of the impacts of the fish farm is usually left behind because of the difficulty to determine the polluting quantities coming from various non-point sources and the relative mass balances in an open system such as the coastal zone. In the present work the quantification of particulate nitrogen releases from a fish farm was succeeded by the construction and use of prototype sediment traps for the collection of sinking particles bellow the fish cages and near the sea bottom. The research took place at the fish cage farming of the Aquaculture Centre of Acheloos, a Research and Development Enterprise under the General Secretariat of Research and Technology of the Ministry of Development of Greece. The collaboration of the scientists of the Centre was valuable for the success of this research. The nitrogen content of the collected particulate matter was measured by the use of a modification of the Valderama method. The annual particulate nitrogen release to the marine environment under the fish cages, resulting from the unused fish food and the metabolic products of the fishes, was estimated to 3,904Kg N representing 23.8% of the annual nitrogen input to the fish farm. 19Kg of nitrogen per ton of produced fishes are annually released from the fish cages to the sea giving an average daily nitrogen release of 4.5gr N per square meter. At the seabed under the fish cages (depth of about 20m) nepheloid loose sediment is present. 522 Kg of particulate nitrogen was accumulated in this sediment, representing a rather small fraction (13%) of the annual particulate nitrogen release. The rest was rather removed by currents far from the immediate vicinity of the fish farm. Even though, the average annual sedimentation rate of nitrogen under the fish cages was found to be 4 times higher than that at a reference station 1km far from the cages. The influence of annually addition of particulate nitrogen can be high; especially to semi closed coastal areas with low water renewal. It should be regulated near the background s sedimentation levels through the application of proper operational practices such as more effective feeding monitoring and decrease or periodic variation of the productivity of the fish farm units. Keywords: particulate nitrogen; sedimentation rate, fish farm; marine environment; Mediterranean Sea Β-62

2 1. INTRODUCTION In the Mediterranean region over the last decade it was observed an exponential growth of marine culture for gilthead sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax). The production of those species reached a total of 115,500 tn in 1999 [1]. This impressive development was neither fully programmed, nor followed by rigorous environmental monitoring and control. As a result serious environmental impacts immerged. The main of them is the accumulation of organic matter in the sediment leading to permanent or intermittent anoxic conditions and downgrading of the benthic fauna [2]; [3] with regard to marine enrichment with nutrients [4], heavy metals [5], geochemistry of sediment and benthos [6]. This study attempts to quantify the particulate nitrogen addition from a marine fish cage farm in a typical Mediterranean coastal embayment, as the addition of nitrogen and phosphorus compounds into the marine environment is a matter of great significance. The study took place at the University of Athens in collaboration with the Aquaculture Center of Acheloos SA (ACEA SA), a Research and Development Enterprise working on the development of aquaculture production in Western Greece under the supervision of the General Secretariat of Research and Technology of the Ministry of Development of Greece. 2. MATERIALS AND METHODS 2.1 The study area The Aquaculture Center of Acheloos is situated in a coastal marine area of the Ionian Sea, in the western Greece near the Acheloos river estuary. The farm was established in 1986 and produces sea bream and sea bass. The farm has 52 cages and occupies a surface sea area of 2,425 m 2 in total. The depth under the farm varies from 18 to 21 m. Despite the strong water circulation, an approximately 30 cm layer of loose organic matter is observed just above the surface of the sediment under the fish cages. 2.2 Sample collection and processing A variety of samples were collected and pre-treated as it is described herewith: Fish Food Three sets of samples from four diameter clusters (total 12 samples) of the fish food type used in the farm were homogenised into fine particles. Particulate release (P.R.) experiment Four prototype sediment traps [7] were constructed and used to collect sinking particles under the cages. Traps were positioned below the bottom of different fish cages (sea bream/sea bass, small size/big size) and kept horizontally using hanging weights. Each sediment trap had a surface area of m 2. Three homogenized samples of the suspended matter were received from each trap. The experiment was carried out in May 2003 and the sampling period was 15 days. Sedimentation (SN) experiment Sedimentation was measured in May 2003 by using two prototype sediment traps [7] that were set 5 meters above the sea bottom in order to avoid particulate re-suspension. Sediment traps were kept in place using a system of counterbalance. One trap was placed exactly below the fish cages and another was placed far from the sea farm as a reference sampling point. Three homogenized samples were received from each of the traps. Each sediment trap had a surface area of m 2. The sampling period was 21 days. All samples were freeze dried and the water content was determined from the weight loss. Concentration of particulate nitrogen in fish food samples and sedimentation material, were determined following a slight modification of Valderrama [8] persulfate Β-63

3 oxidation method [9]. The produced nitrates were determined with standard spectrophotometric methods [10]. 3. RESULTS and DISCUSSION The average water content of the fish food was 8.3% (wet w/w) and its nitrogen concentration 52.9 g/kg. The annual fish food supply was 336,956 kg (data provided by the fish culture) giving thus 308,854 Kg (dry w/w) of fish food containing 16,348 Kg N. For the determination of annual release rates we used the methodology of splitting the annual breading season into two periods, based on water temperature: a high rated breading period of 7 months with water temperature above 18 o C and a low rated breading period of 5 months with water temperature below 18 o C. While the water temperature is below 18 o C the fish food supply is 50% reduced. Using the available data of the environmental losses (Table 1) four particulate matter s (PM) release rates were calculated and their average value represented the high season PM release rate. For the low season PM we considered a 50% reduced value of release rate. Having a net cage surface of 2,425 m 2 we estimated an annual PM release of 158,009 Kg (dry) giving a PM release rate of 181 Kg / m 2 d. The annual N release was found 3,904 Kg that gives a Nitrogen release rate of g N/ m 2 d (Table 2). The estimation of PM sedimentation rate to the seabed was based on the previous methodology, but in order to find the net fish farm PM sedimentation rate we had to subtract the background sedimentation rate of the coastal area using a reference sediment trap. With a distinctive blanket of loose sediment present over an area of approximately 18,000 m 2 (180m x m) we estimated a net annual PM sedimentation release of 60,768 Kg (dry) giving a PM sedimentation rate of 9.4 g / m 2 d. The annual net N sedimentation release was found 522 Kg that gives a net Nitrogen sedimentation rate of g N/ m 2 d (Table 2). Table 1: Data concerning suspended matter collected in the sediment traps Particulate Particulate Matter % Water Nitrogen (PN) (PM) Content Concentration (wet Kg) (wet w/w) (dry g/kg) P.R. Trap - Cage P.R.Trap - Cage P.R.Trap - Cage P.R.Trap - Cage SN Trap Under the cages SN Trap - Open sea In the area of the fish farm (near the estuaries of the Acheloos River) there is a rather high annually addition of PM, which was found 42,769 Kg giving a PM reference sedimentation rate of 6.6 g / m 2 d. The annual N reference sedimentation release was found 113 Kg leading to a Nitrogen reference sedimentation rate of g N/ m 2 d. The results are shown in summary in Table 2. Table 2: Data concerning particulate material and particulate nitrogen (Produced kilos and release rates) due to the annual operation of the fish farm unit. PM (Kg) PN (Kg) PM Rate (g / m 2 d) PN Rate (g / m 2 d) Fish food 308,854 16, Fish cage Release 158,009 3, Net Sedimentation 60, RP Sedimentation 42, Β-64

4 It is calculated that half the quantity of the particulate material that is induced via the food, is lost with the form of excess food and excrements, enriching the marine environment with important quantities of organic material. It is appeared in Figure 1. From this particulate material about 20% results on the seabed whereas the rest is probably consumed by the wild fishes, transported by currents or remains in suspension for a longer time. In comparison, the sedimentation originating from the region s background corresponds in the one seventh of the initial flow of particulate material (14%). One quarter of the provided, via the fish food, particulate nitrogen (24%) is released in the environment (Figure 1). From the initial particulate nitrogen only the 3% reaches the seabed whereas the rest is transported with currents or dissolved while it settles down. In comparison, the sedimentation originating from the region s background corresponds in 1% of the initial flow of particulate nitrogen % % 3 1 Fish food Fish Cage Net Sedimentation RP Sedimentation PM Release PN Release Figure 1: % Comparison of Release Rates of particulate material and particulate nitrogen. Comparing these sedimentation rates, we found out that the sedimentation rate of material originating from the fish farm is 43% higher than the region s background rate, while the sedimentation rate of fish farm s particulate nitrogen is almost four times higher than that of the region background (Figure 2). Having in mind a fish production of 208 tn/y (wet w/w), it was found that 760 Kg of particulate material are released from the cages (dry weight) for each tone of produced fish while only 292 Kg of them (38%) ends to the seabed. In other words, while the particulate material release under the cages reaches the 181 g/m 2 d, only 70 g/m 2 d settles down to the immediate surroundings of the fish farm, the rest is probably consumed by wild flocks of fishes or mainly transported by sea currents Fishfarm Background 20 0 (%) (%) 22 PM Sedimentation Rate N Sedimentation Rate Figure 2: Comparison of sedimentation rates concerning particulate material and particulate nitrogen from the fish culture and the background of the installation region. Β-65

5 The particulate nitrogen load was found 0.05 g per kilo of produced fish (fresh weight) per day and the particulate matter load 2.1 g/ Kg d. Other studies report particulate load varying from 0.9 to 7.1 g N/ d Kg of fish produced (fresh weight) depending on the cultivation method, the species and the size of fish produced [11]; [12]. The amount of released particulate nitrogen was found 18.8 Kg N/tn of fish produced which is smaller than that of 28.4 Kg N/tn and 24.1 Kg N/tn reported in other studies, [13], [14]. 4. CONCLUSIONS This is one of the few studies in the Mediterranean which attempts to quantify nitrogen flows between the fish farm and the marine environment around the unit. Our purpose was to estimate how much of the nitrogen supplied through fish food is lost to the environment in particulate form. Furthermore we are interested to find out how much of the particulate form is suspended and how much is accumulated in the sediment under or near the cages. To make these determinations reliable a series of new prototype devices were constructed and successfully used. Although no eutrophication incidents have been reported in the study area till now, the cumulative effect of nitrogen addition to the surrounding marine area should be regularly monitored. Particulate nitrogen release represents 24% of the total N input. A small particulate nitrogen fraction (13%) is further accumulated in the nepheloid loose sediment formed under the fish farm, while the rest seems to be removed far from the immediate vicinity of the fish farm and it may accumulate when the geomorphological conditions are favourable. The annual waste per ton of fish production (fresh weight) contains 18.8 Kg of particulate nitrogen, while the environmental particulate matter load per kilo of fish produced (fresh weight) per day corresponds to 2.1 g. The results of this work are important in designing the locations of existing fish farms and, of course, they can be used to optimise operational practices of existing fish farms units meeting specific environmental criteria. Furthermore direct measurements of material fluxes in fish cage farming are essential for determining the environmental carrying capacity of the specific sites. Benefits such as decrease of potential dangers for the production and improvement of the applied environmental management practices are of great importance. REFERENCES 1. Dassenakis, M., & Belias, C., (2003). Environmental problems from marine aquacultures In the Mediterranean Sea. International Conference: Scientific and policy challenges towards an effective management of the marine environment. Varna, pp Belias, C., Bikas V., Dassenakis, M., & Scoullos, M., (2003). Environmental Impacts of Coastal Aquaculture in Eastern Mediterranan Bays. The case of Astakos Gulf, Greece. ESPR- Environ. Sci. & Pollut. Res. 10 (5), pp Karakassis, I., Tsapakis, M., Hatziyanni, E., Papadopoulou, K.N., & Plaiti, W., (2000). Impact of bass and bream farming in cages on the seabed in three Mediterranean coastal areas. ICES J. Mar. Sci. 57, pp Belias, C., Bikas, Β., Dassenakis, Μ., & Scoullos, M., (2000). Environmental effects from intensive fish cage farming of sea bass and sea bream at Astakos Gulf, Part I: Nutrients. 6th Hellenic symposium of Fisheries and aquaculture, May 2000, Vol. B, pp (In Greek) 5. Belias, C., Bikas, Β., Dassenakis, Μ., & Scoullos M., (2000). Environmental effects from intensive fish cage farming of sea bass and sea bream at Astakos Gulf, Part II: Heavy Metals. 6th Hellenic symposium of Fisheries and aquaculture, May 2000, Vol. A, pp (In Greek) 6. Mazzola, A., Mirto, S., Danovaro, R., & Fabiano, M., (2000). Fish farming effects on benthic community structure in coastal sediments: analysis of meiofaunal resilience. ICES J. Mar. Sci. 57, pp Β-66

6 7. Belias C., PhD Thesis (2004) Study of fish farming effects on the marine environment: Mass balances of nitrogen, phosphorus and silica in a marine fish cage farm in Western Greece. 8. Valderama J.C., (1981). The simultaneous analysis of total nitrogen and total phosphorus in natural waters. Mar. Chem. 10: Ladakis M., Belias C., Dassenakis M., Scoullos M., Salta F., (2003): An effective oxidation method for the simultaneous determination of nitrogen and phosphorus in marine sediments. 3rd International Conference IMA, September 2003 Thesalloniki, Greece, pp Stricland J.D.H. & Parsons T.R. (1968): Fisheries Research Board of Canada. A Practical Handbook of Seawater Analysis. pp , 65-70, 71-76, Bergheim A., Siversten A. & Selmer-Olsen A.R. (1982): Estimated pollution loadings from Norwegian fish farms. I. Investigations Aquaculture 28(3-4): Conides A., Anastasopoulou K. & Fotis G. (1993): Application of standard procedure for forecating the maximum possible environmental impact of sea bream cage culture in Greece.Envir. Educ. Inform., 12(1): Hall, P.O.J., Holby, O., Kolberg, S., & Samuelson, M.O., (1992). Chemical fluxes and mass balances in a marine fish cage farm. 4. Nitrogen. Marine Ecology Progress Series 89, pp Enell M. (1987): Environmental Impact of cage fish farming with special reference to phosphorus and nitrogen loading. Comm. Meet. Int. Coun. Explor. Sea, C.M.- ICES1987/F:44, Ref. MEQC. 13p. Β-67