Seasonal variation in nutrient content of kelp

AFASA project meeting 2004

• D. V. Robertson-Andersson

Seasonal variation in nutrient content of kelp

What do we know?

• Distribution of kelp is largely dependant on nutrient concentrations (NO3 and PO4).

• There is a difference in kelp bed ecology on west and south coasts

• Kelps obtain all their nutrient and mineral requirement from the water, through the fronds and stipe

• No studies have looked at seasonal variation in nutrient or phenol content

Why is it important?

• Abalone are herbivores

• They obtain all of their protein requirement from their diet

• Their FCR is affected by phenols (an anti-grazer deterrent produced by most seaweeds)

• Kelps can contain very high proportions of phenols

• Abalone obtain all their calcium requirement through their gills, but no phosphate

• The phosphate requirement has to be met by the diet

• Abalone are sold by weight and there can be up to a 30% water loss during transport

West Coast

South Coast / Walker Bay area

Tissue Nitrogen vs. Tissue Phosphorus

Tissue N vs. Tissue P of Ulva lactuca from west coast

0

10

20

30

40

50

Time

Rat

io o

f N:P

w ild stock turbot abalone

Small sea Medium sea

A S O N D J F M A M J J A

Moving forward

• Look at seasonal variations on a long term basis of Tissue N, P and phenol content

• Do this from a variety of kelp beds

Moving forward

How?

• Each farm in an area can take 3 X 500g samples of kelp every two weeks

• Freeze them immediately in labeled black bags until they can be analyzed

• Analysis will occur in batches of not less than 50

• Data will be entered onto a data base and analyzed on a 6 monthly basis

Cost

Materials?

• Selenium tablets most expensive item R1, 00

• Other chemicals work out to 75 c a sample

 THE END

Thank you

ACKNOWLEDGEMENTS

I would like to extend special thanks to the following people and organizations:

I & J Mariculture farm particularly N. Loubser, H. Otto and L. Ansara

JSP Mariculture farm particularly K. Ruck

N R F

Swedish and South African Collaborative Programme