Dear Isogeochems,

I have just started my PhD on the environmental impacts of cage aquaculture. As part of my PhD I would like to trace the waste from aquaculture cages using diets enriched in 15N.

The idea is as follows:
- As part of a separate nutritional experiment, 15N will be incorporated into the diet by feeding fish 15N enriched Ulva (sea lettuce). The 15N labelled fish will be converted into fishmeal, and the isotope labelled fishmeal incorporated into a pelleted diet.
- The end result of the nutrition trial will be a lot of 15N labelled fishmeal based pellets. As I am already investigating the impacts of cage aquaculture, my idea is to use this 15N enriched diet to feed fish in marine cages for a short period of time.
- The N-waste from the cages will include uneaten feed, faeces and dissolved N (ammonia and urea).
- The environmental fate of the waste will be traced by analysing the sediments, the benthic invertebrate community, macrophytes and seagrasses nearby, phyto- and zooplankton for 15N.

The experimental cages are situated in shallow depth (5m) on the edge of a harbour, in a low-energy environment. The harbour is at the mouth of an estuary where there is considerable anthropogenic influence. The particulate waste from the cages will spread over a maximum area of approximately 20m x 30m. Tracing the dissolved waste will presumably be more difficult, as it will spread over a larger area. Nearby the cages are piers and other structures, which are covered in mussels, barnacles and macro algae. The sediment is a layer of varying thickness of fine sand overlying limestone bedrock. All samples will be taken by divers.

Approximately 50% of the N fed to the fish will be excreted as ammonium and urea. About 15-20% will be incorporated into faecal matter, and 30% will be retained in the fish. The percentage of uneaten pellets will probably be quite low (max. 5%). The total amount of feed fed will depend on the density of fish and their size.

My questions are:
- Does anybody know of 15N enrichment tracer studies in the marine environment? I have encountered projects tracing sewage and groundwater seepage in coastal zones, but have yet not heard of anybody enriching a substance for subsequent tracing?
- What is the minimum detectable level of enriched 15N in the natural environment? How much 15N am I likely to have to feed the fish to be able to detect where the waste is going, i.e. how much particulate 15N will I have to add to an area of approximately 20m x 30m to ensure that I can trace it? This would allow me to ¿work backwardsÀ to establish whether I have enough enriched diet to be able to use it for a tracer study.
- How long would I have to feed the fish the enriched diet for?
- What is the best method for establishing the baseline level of 15N in the area Ä analyse samples of the different plants and animals, or of the sediment, or both?

I will be grateful for any advice, comments, and suggestions.

Thanks,


Malene Felsing
Fisheries Western Australia
1 Fleet Street
Fremantle, W.A. 6160
Australia
Tel: +61 (08) 9239 8104
Fax: +61 (08) 9239 8105
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