I ANNALES • Ser. hist. nal. • 13 • 2003 • 1 • Supplement short scientific article UDK 63933:574.6(262.3-18} received: 2003-09-05 DISTRIBUTION OF THE FOOD SURPLUS AND FAECAL PARTICLES ON THE SEABED BELOW A FISH FARM IN THE BAY OF PIRAN Viudo MALACIC & Janez FORTE National institute of Biology, Marine Biology Station, 51-6330 Piran, Fornale -11 E-mail: maiacic@mbss.org ABSTRACT Simulations of the spread of food pellets and faecal particles beneath a fish farm were performed on the basis of field measurements of currents and laboratory measurements of settlement velocities. The simulations showed that the patch of the food surplus at the sea floor covered an area smaller than 30 x 15 m, while faecal pellets should have covered an area smaller than 300 x ¡50 m. Key words: nwiculture, currents, environment, Bay of Piran DISTRIBUZIONE DI ECCESSO DI MAN GIME E PARTICELLE FECALI SUL FONDO MARINO SOTTOSTANTE UN ALLEVAMENTO DI PESCI (BAIA DI PÍRANO) SINTESI L'articolo riporta simulazioni della dispersione di pallottoline di mangime e particelle fecali al di sotto di un alle-vamento di pesci, ottenute da misurazioni di correnti sul campo e misurazioni in laboratorio delle velocitb di affon-damento del materiale orgánico. E stato dimostrato che i'eccesso di mangime va a ricoprire un'area minore di 30 x 15 m, mentre le particelle tecali restaño confínate in un'area di 300 x 150 m. Parole chiave: rnaricoltura, correnti, ambiente, Bata di Pirano 41 ANNALES - Ser. hist. nat. - 13 • 2003 • 1 • Supplement Vbito MAlAf.lv X |.>iM>jr'i;'Cii!E: «5TRlEl.il ionOf T'lriOi >D «'.«Pit AND FAFCAÍ PAKT1CI l:S ON Ti lt SI-ABITO BC OW A PISH f ARM .. J.s INTRODUCTION Fish farming, in comparison to fishing of the wild stocks, has economic advantages, but it also presents a threat to the ecological equilibrium in the marine environment. Different influences are coming from the mariculture in a form of the food supply surplus, solid and dissolved excrements, and as a wide range of different pharmaceutical drugs. There are two types of particles that enter the ecosystem from the fish farm: the fish food surplus, that is the par; of the food supply that is not consumed by fish and passes through the nets of the cage, and fish excrements. This process increases the consumption of dissolved oxygen (short-term impact) and the accumulation of decayed material in the upper layer of the sediment. The latter process is considered as a long-term impact since this material al the surface film of the seabed is later regenerated. This means that proper planning of the fish farm location should consider the amount of panicles deriving from the farm and their spreading around the cages. The paper presents the simulation of the distribution of food surplus particles and faecal pellets on the seabed due to the horizontal advection of the water mass during their sinking below the fish farm. MATERIALS AND METHODS The shallow Bay of Piran (Fig. 1) is part of I lie north- ern Adriatic Sea and is opened to the Gulf of Trieste. I he depth beneath the cages is 13 ± 1 m; depth variations are caused mainly by tides. The sea floor rs smooth and muddy, with a moderate declination towards the open sea The water column structure resembles thai of the Gulf of Trieste (Malacic, 1991) with seasonal temperature variations. However, at. the fish farm site the influence of the river Dragonja modifies the vertical stratification in the shallow water column, in particular the distribution of salinity. Records of monthly data collected at the two nearby stations that are less than 1.5 km away (Tab. 1) show that during the spring-summer time (Forte, 2001) temperature in the upper part of the water column varies between 18-26 DC, and between 12-17 °C in its lower part. Salinity also shows some seasonal variations, although less pronounced, for it varies between 28.0 38.5 PSU in the upper part of the water column. Currents were measured on the fish farm site (45°29.226' N, 13°34.838' E) with the acoustic Doppler current-meter profiler of Nortek AD company (NDPi mounted on the sea floor. The instrument scans the water column above with three beams and outputs the ve locity of layers that are 1 m (hick. The average period of currents was 600 s, while the sampling period was .5600 s. Measurements were performed in the spring-summer period, from 19 April to 4 July and from 31 July to 31 August 2000. i T t ^ •• \ 'V - \ Gv • Tx ML s v S; "i', 0 .....S-C v PI.RAN ■t ■■ IZÓLA .MA lj¡gs*x SLOVENIA ¿P í.l'í O,'.. 1 —fj-^y^- •T::;. C R 0 A T 1A . ■ .... Fig. 1: Location map of the fish farm and the nearby measurement stations MA (45°30.20' N, 1.3°34.20' E, depth 16 m) and 35 (45°29.40' N, 13"35.00' £, depth 12 m) from where the long-term temperature and salinity records were applied for the estimate of density on the farm site. SI. 1: Skica lokacije ribogojnice in bližnjih merilnih postaj MA (45°30.20' N, 13"34.20' E, 10 m) in 35 (45°29.40' N, 13"35-00' E, 12 m), kjer so bile opravljene dolgoročne meritve temperatur in slanosti za kasnejši izračun gostote morske vode. ANNALES - Ser. hist. nat. - 13 • 2003 • 1 • Supplement vlado MAI AflC & Inter rORTf:: DISTRIBUTION OF iHf FOOD SURPLUS AND FA0CA1 PARTICLC5 ON THE 51 ABED BELOW A FISH FARM 3-8 Tab. 1: Simple statistics of 1362 temperature and salinity data obtained in 25 years in the Ray of Piran (19752000). The average value is denoted with < >, and deviation with SD. Sampling was performed at stations 35 and MA and at four different depths: on the surface, at depths ranging from 5 to 10 m, and just above the sea floor (Ruttner sampling). Tab. 1: Osnovni statistični podatki o temperaturah in slanostih, izračunani iz 1362 meritev, pridobljenih v 25 letih (v obdobju 1975-2000) v Piranskem zalivu. Znak < > označuje srednjo vrednost, S D pa standardno de-viacijo. Meritve so bile napravljene na postajah 35 in MA na štirih globinah (površina, 5 m, 10 m in tik nad dnom - Ruttner je v vzorčevalnik). T(çC) S (PSU} <ï7 (kg/m3) < > 1 5.67 36.88 27.16 SD 5.42 1.02 1.73 Min 5.85 28.77 20.69 Max 27.63 39.83 30.89 Sinking velocity of fish food and fish excrements was measured in the lab. Measurements of the sinking velocity of fish food were performed in a plastic cylinder filled with seawater with salinity of 37.5 FSU and temperature of 22.5 °C. This seawater density is thereafter calculated as 25.9 kg/m3. From twelve throws of fish pellets, the highest and the lowest measured velocity were eliminated; from the remaining ten values the average and the standard deviation (SD) were calculated. We combined the sinking velocity of fish food with the current-meter data to predict the distribution of uncon-sumed fish food on the seabed below fish cages. This idea of simulating the distribution of uncon-sumed fish food was extended to faecal particles. For this reason, faeces were collected from the fish cage using a particular trap of polyethylene foil. The foil covered an area of 1 rrr at the bottom of the cage, with a plastic bottle attached in its centre as a recipient of the sinking material. In the laboratory', a sample of collected particles with a volume of 1.5 I was diluted to 5 I and carefully mixed to achieve more homogeneous distribution of excrement. From this mixture, a sub sample of 5 ml was taken and released into another plastic cylinder, in which the sinking velocity was measured. This second cylinder was filled with seawater collected a few miles away, with temperature of 22.0 °C, salinity of 38.5 PSU, and the calculated density of 26.9 kg/m3. RESULTS AND DISCUSSION in the year 2000, currents at the fish farm were, measured twice, in the late spring (19 April - 5 July) and summer (31 July - 31 August 2000) periods. Frequencies of the directions of currents show (Forte, 2001) that in late spring the prevailing currents over the whole water column were directed towards NE and E (30% - 40%}, while they were more evenly distributed during the summer, with more frequent directions towards NW, W and SW (J5% - 20%). In both cases, the frequency of directions towards the south was not higher than 10%, indicating that over the entire water column the currents were mostly oriented away from the mouth of the Dragonja river (Fig. 1) during the spring-summer period. The distribution of velocity magnitude in different directions (eight sectors) show higher values of currents in NE and E directions during late spring, while during the summer period currents had evenly distributed magnitude in all directions. Currents in the surface layer 11 m above the NDP and about 2 m below the sea-surface deviated from the rest of the water column during the spring period. The most frequent directions then were to the E or W, and their velocities reached values of 0.17 m/s in the W direction and of 0.15 m/s in the i direction. During the summer period, however, surface currents had similar directions as the rest of the water column. This indicates that the surface layer is dominantly wind-driven, since during the spring period the wind is more frequent than during the summer (Ogrin, 1995), while in the rest of the water column tidal and density driven currents prevail. Tab. 2: Sinking velocities (mean+SD) of fish food pellets and faecal particles, measured in the fab. Seawater density for the experiment with pellets was 25.9 kg/m3 and density for the experiment with faecal particles 26.9 kg/m3. Tab. 2: Hitrosti padajočih delcev ribje hrane in delcev ribjih iztrebkov (srednja vrednost±SD). Meritve so bile opravljene v laboratoriju. Gostota morske vode med poizkusom z ribjo hrano je bila 25,9 kg/m ', medtem ko je bila gostota med poizkusom z delci iztrebkov 26,9 kg/m3. Sinking velocity (cm/s) Pellets Faeces < > 5.72 0.64 SD 0.15 0.05 Sinking velocities of food pellets were estimated in the lab as 5.7 ± 0.15 cm/s (Tab. 2). In comparison to food pellets, the sinking of faecal particles was significantly slower. Their sinking velocity was estimated at 0.64 ± 0.05 cm/s, being roughly 8.9 times smaller than that of food pellets. This means that through the water column of the same density as the one of the seawater in lab cylinder, food pellets would pass vertically through a layer of thickness of I m in less than 16 s, and would reach the sea floor from the surface in less than 3.8 minutes. Faecal particles would travel through a ANNALL-S • Ser. hist. nat. • 13 • 2003 ■ 1 • Supplement Vlado M MAC if & Janez FORTE: OSTRiBUHON OF THf. FOOD SURPLUS AND FAlCAI. PARTICLES ON IHt SEARED 6ELOW A f iSli f AF,M .... >8 iayer of 1 m on average in less than 157 s, and would reach the sea floor in less than 34 minutes. Since currents were measured with a sampling rate of 5 hour, we reasonably assumed that both types of particles were sinking through the water column during the single measurement cycle, and that the sampling period of 10 minutes represents currents during the sinking period of hail an hour. It is estimated that the range of sinking speed is larger due to the unknown densities of the water column, food pellets and faecal particles, and due to the unknown geometry of the latter during the entire measurement period. We may reasonably suppose that food pellets and faeca! particles of density p., are sinking familiarly with a constant speed within a layer of a constant density p. In the balance of forces that act on sinking particles, a problem arises with the friction force, since the geometry of food pellets and faecal particles is not sufficiently known. However, we may qualitatively describe the linear friction law (Kundu, 1990) that holds for the low Reynolds numbers (Re = vd/tj, where v is the sinking speed, dthe typical dimension across the particle, and i] = 10"f' m; s is the kinematic viscosity at 20 "O. It is estimated that Re is smaller than 300 for food pellets (v = 0.06 m/s and d< 5 10'3 m), and that Re is below 13 (v = 6.4 10'3 m/s and d < 2 10"m) for faecal particles. This indicates that the linear dependence of friction force on the sinking velocity is applicable and the friction force is parameterised as Kp0 1} a v, where K is the dimension-less constant that accounts for geometry {K - 6 v. for sphere-like particles). The balance between the buoyancy fcioe (pp - p)gV{g - 9.81 m/s' is the gravity acceleration and V the volume of a particle), and the friction force yields the sinking velocity as: ,.....(vph Ki]pd This means that if the density of a sinking panicle p., is known, the sinking speed is decreasing with the ambient density in an inverse linear way, proportional to Mx -pjpp where x ~ pjp is the ratio of densities, with p0 being the density at which the sinking speed was measured, The density is related to the density excess ey as p = 1000.0 + N I Hi SLAULD litlOVV A FISK FARM . .i-". RAZPRŠITEV RIBJE HRANE IN IZTREBKOV NA DNO POD RIBOGOJNICO V PIRANSKEM ZALIVU Vlado MALAČIČ & Janez FORTE Nacionalni inštitut 2a biologijo, Morski bioloSka postaja, SI-633& Piran, (ornače 41 E-mail: malacic®mbss.org POVZETEK !z meritev fokov in laboratorijskih meritev hitrosti Lonjenja ribje hrane in delcev ribjih iztrebkov je bila narejena simulacija njihove razpršitve. Pokazalo se je, da višek hrane pokrije površino, manjšo cd 30 .v 75 m, medtem ko iztrebki ostanejo znotraj meja 300 x 150 m. Ključne besede: marikultura, tokovi, okolje, Piranski zaliv REFERENCES Forte, J. (2001): The influence of a fish farm to ecological processes in the inner part of the Bay of Piran (Vpliv gojišča rib na ekološke razmere notranjega dela Piranskega zaliva). B.Sc. Thesis. University of Ljubljana, Biotechnical Faculty, 50 pp.(in Slov.) Kundu, P. K. (1990): Fluid Mechanics. Academic Press, 638 pp. Malaetč, V. (1991): Estimation of the vertical eddy diffusion coefficient of heat in the Gulf of Trieste (Northern Adriatic). Oceanol. Acta, 14(1), 23-32, Ogrin, D. (1995): The Climate of Slovenian Istria (Podnebje slovenske Istre). Knjižnica Annales, Zgodovinsko društvo za južno Primorsko, 381 pp. 41