Benthic effects of fish cage farming in the Gulf of Aqaba, Red Sea
By Dror Angel (Israel),
Peter Krost (Germany), and Bill Silvert (Canada)
ABSTRACT
The effects of a net pen fish farm, situated in the northern Gulf of
Aqaba (Red Sea), on the underlying benthos were studied from 1991 to
1994. The flux of particulate organic matter released from the fish
farm (measured by sediment traps) varied between 4.5 and 12.7 g
C.m-2.d-1, and covered approximately
17,000 m2 under the fish farm.
The organically enriched sediments (LOI = 4 - 12%) contained high
concentrations of dissolved nitrogen, phosphorus and hydrogen sulfide
and were covered by microbial mats, consisting mainly of the
chemoautotroph Beggiatoa spp. and an assortment of photoautotrophic
and heterotrophic microorganisms. Rates of organic carbon (OC)
decomposition, calculated from benthic flux chamber measurements and
sediment porewater profiles, ranged between 1 and
5 g C.m-2.d-1.
Despite the fact that OC sedimentation rates exceeded decomposition
rates, there was no sign of OC accumulation in the sediment. The
missing organic material was probably removed by one or a combination
of the following factors: resuspension and removal by occasional
strong bottom currents, decomposition by bioturbation and uptake by
demersal fish and invertebrates. A 3-year time series of 100 logs
summarizing diver observations on the status of the benthic environment
below the fish farm was established in the course of this study. The
initial purpose of the dive logs was to monitor the benthic dynamics
in descriptive terms, since many of the changes that occurred were not
measurable by routine geochemical analyses, yet they provide valuable
information on the status of the benthos. The observations in the
diver logs were mostly qualitative and subjective and there were large
inconsistencies in the type of data recorded making it difficult to
analyse the data by conventional approaches. We decided to apply fuzzy
logic (Zadeh 1965) to the data since this approach allows us to
incorporate ambiguity and non-quantitative data into a classification
scheme to describe the status of the benthos. To the best of our
knowledge, this is the first time fuzzy logic is being used to analyse
environmental data and the preliminary results appear very promising.
We found a strong correspondence between fuzzy scores that match
highly enriched sediments and high levels of sediment organic matter
(loss-on-ignition), porewater nutrients and organic matter decay
rates.