Stable isotopes: can they serve as tracers of fish farm effluents in all environments? Sonja Lojen Department of Environmental Sciences, Jožef Stefan Institute, Ljubljana, Slovenia, with great support of all Biofaqsers prepared by

Podgorica Piran Ljubljana

Why stable isotopes of C and N? = tracers for material flow = enrichment in the food-chain “You are what you eat plus 3‰” (1984) δ 15 N organism = δ 15 N food + ε Prof. Wada

What we wanted to do: = Trace the debris from fish cages in the sediment (δ 13 C) = Estimate the amount of debris from the cages in POM (δ 13 C, δ 15 N) = Estimate the amount of debris from cages recycled by organisms colonising biofilters (δ 15 N) Xδ 13 C A + Yδ 13 C B = δ 13 C A+B, X+Y=1 δ 15 N organism = δ 15 N food + ε Xδ 15 N A + Yδ 15 N B = δ 15 N food, X+Y=1

Sampling, sample preparation, analyses Stable C and N isotope analyses of sedimentary organic matter, faecal material, suspended particulate organic matter (POM, collected in traps), fish food and fouling organisms at the fish cages and control sites Number of replicate analyses: 1 -12

POM collected at the cages = fish food + faeces + “background” POM X faeces + Y pellets Assumed the same as at the control site Go to library Ye et al., 1991, salmonid farm: 55% faeces, 45% pellets Lefebvre et al., 2001, sea bass farm: 37% faeces, 63% pellets X + Y = 1; X, Y = ?

Results I.: suspended material

Theoretical isotopic composition of POM deriving from fish cages

Who has DATA on F:P ratio in debris released from fish farms?

Results II. : sediment influence of terrestrial debris transported by the river with  15 N about +1‰ Influence of debris and effluents from fish cages

Whom we wish to have on biofilters = Active suspension feeders = Sessile or strongly sedentary in habit = Able to ingest and retain particles in size range released in aquaculture effluents = Able to survive and grow on a diet of non-living organic detritus = High pumping and clearance rates

What we got: = Piran: predominantly bryozoa = Crete: predominantly bryozoa = Oban: predominantly tunicates = Eilat: tunicates, mussels, worms, sponge, sea anemone, Thyroscopus fructisosus

Results: fouling organisms, Piran Influence of 13 C depleted fish food, terrestrial input? Influence of 15 N depleted riverine input of POM indirectly

Stable isotope composition of bryozoans collected in Piran

Temporal variations of  13 C of particulate organic matter and bryozoans collected in Piran

Temporal variations of  15 N of particulate organic matter and bryozoans collected in Piran

Results: fouling organisms, Crete Indirect influence of fish farm effluents

Results: fouling organisms, Oban No significant differences due to turbulent environment

Similar effect as in Piran Results: fouling organisms, Eilat enrichment in average Sensitive to dissolved nitrogen

 13 C vs.  15 N of Pteria aegiptiaca, collected in Eilat in April 2002

 15 N of Pteria aegiptiaca collected in April 2002 in Eilat vs. its size

Estimation of retained N from fish cages ε = δ 15 N organism, control - δ 15 N POM, control δ 15 N food, cage = δ 15 N organism, cage - ε

Conclusions - particulate organic matter = controversial data from the literature = it was not possible to determine the source of POM only from stable C and N isotopic composition, or to quantitatively estimate the amount of organic debris originating from the fish farm = seasonal variation in carbon and nitrogen isotopic composition of phytoplankton  common scheme of production regime for modern environments: nitrate based in the spring and nitrogen fixation in summer, resulting in lower δ 15 N = large isotopic fractionation during degradation processes of particulate nitrogen = the suspended material is thoroughly mixed and the influence of a fish farm on the concentration and isotopic composition of suspended material is spatially very limited due to dispersion and dilution

Conclusions- sediment = the degree of remineralisation of organic particulates during sedimentation depends upon many biotic and abiotic factors - temperature, turbulence, stratification, biotic community composition and food conditions = no systematic variations in δ 13 C, systematic enrichment in 15 N (except in Piran) = rapid remineralisation of fine POM in turbulent environments = rapid sedimentation of large particles  in environments with low organic matter content and where oxic conditions prevail during the year, the  15 N of sedimentary organic matter can undergo considerable changes toward more positive  15 N values with respect to the primary signal = findings consistent with data from literature

Conclusions - fouling organisms = questionable effect of bryozoans = effective consumption of farm-POM by sponge and worms = partially effective: tunicates, mussels

= Need data on average isotopic composition of POM for each growing period = then we could estimate the amount of debris retained by biofilters = need data on increase of biomass in each growing period

Thank you!