Science in Hong Kong: Volume 1, Spring 2006, p1
‘Red tides’
by Stockard W.Y. Leung
What is ‘red tide’?
‘…and all the waters that were in the river were turned to blood. And the fish that was in the river
died…’ The curtain-raiser of the10 plagues in Egypt recorded in Exodus in the Bible was probably a
serious red tide. Although it has the name ‘red tide’, it is not necessarily red, and has
nothing to do with tides (GM n.d.). Instead, according to the Hong Kong Red Tide Information Network,
‘red tide’ is a type of unicellular micro algal (phytoplankton) bloom, mostly made up of dinoflagellates,
a group of weak-swimming algae which swims with the
help of their two flagella (Garrison 2005). In many cases,
a large portion of surface water will turn red, brown, pink
(Figs. 1a & b) or sometimes deep green (HKRTIN n.d.)
as a result of discoloration by the large amount of algae
in the water. Although most algal blooms are not harmful,
some can cause adverse effects on marine organisms
by toxin production or oxygen depletion (Wong 2003).
This phenomenon is known as harmful algal bloom
(HAB) (HKRTIN n.d.)
Figs. 1a & b: Red tide in Tolo Harbour
(causative species: Noctiluca scintillans)
How ‘red tide’ affects Hong Kong
Thousands of years after Exodus, the ‘red tide’ is still a common global natural phenomenon in water
including the subtropical waters of Hong Kong. HABs are usually related to massive fish kills
regardless of whether the kills were of caged or natural coral fishes (Yang and Hodgkiss, 2004).
According to Wong (2003), 20 out of 26 Fish Culture Zones (FCZ) were affected during the worst red
tide in 1998. Fish farmers claimed a total loss of more than 3000 tonnes at a cost of HK $315 million.
Science in Hong Kong: Volume 1, Spring 2006, p2
Although it is unusual for Hong Kong waters to be affected by intense HABs as the one that occurred
in 1998, there are periodically about 10 to 40 ‘red tide’ reports every year (Wong 2003). This
demonstrates the need to understand red tides in Hong Kong in order to help construct an efficient
prediction system and better management plans in fish farms and conservation.
Research on red tides in Hong Kong
Research on red tides in Hong Kong started after the first report in 1971 (Wong 2003). Although the
understanding of HABs is still limited, Hong Kong researchers such as ecologists, civil engineers and
mathematicians, have paid a lot of effort to understand ‘red tides’ and their patterns.
With the help of the present equipment, Lee, Hodgkiss, Wong and Lam (2005) have developed a more
advanced continuous observation system which functions in real time to monitor the occurrences of
algal blooms. This system has provided a large amount of data to investigate important changes in
factors that caused algal blooms such as water turbulence, temperature, wind speed and water current.
In their study, correlations between the types of algal bloom and the amount of dissolved oxygen (DO)
and some other factors such as the Nitrogen to Phosphorus ratio (N:P ratio) were found (Lee et al.
2005).
According to Lee et al. (2005), the findings have helped in the understanding of the dynamics of micro
algae, hydrodynamics and water quality, which can be used to construct models on algal bloom
mechanisms. With appropriate models and field monitoring systems, accurate and efficient predictions
and monitoring of algal blooms will be possible. When an algal bloom is detected, manual causative
species’ identification can be carried out to determine whether what is detected is a HAB (Lee et al.
2005). Observing changes in the DO in a bloom may give an idea on the type of the causative algae
Science in Hong Kong: Volume 1, Spring 2006, p3
even before the species’ identification, and therefore provide a quick and rough estimation of possible
HABs.
A sensitive real time monitoring system can increase the efficiency of detecting possible occurrences
of red tides than regular manual water sampling and as such sensible measures can be taken in
advance to minimize their effects. For example, the Agriculture, Fisheries and Conservation
Department can provide advice on raft relocation for toxin producing algal bloom which leads to fish
mortality, to fish farmers (Wong 2003). Also, as the duration of HABs is usually very short (Lee et al.
2005), the regular water sampling method used may not be sensitive enough to detect the occurrence
of a red tide before a negative impact on fish farms. In addition, labour costs of manual water sampling
are high when the sampling frequency is high. If a possible HAB can be predicted before an algal
bloom, then measures can be enforced before any harmful effects occur. In this way the loss to the
fish farmers and any possible harm to customers can be minimized.
Conclusion
The occurrence of red tides can lead to massive killings of marine organisms which can cause huge
economic losses in mariculture. Therefore, the understanding of red tides is critical as it can help to
construct a representative model which can provide sufficient information to implement efficient
management plans in mariculture. Efficient measures can then be enforced so that the loss to the fish
farmers can be reduced.
Science in Hong Kong: Volume 1, Spring 2006, p4
References
Chinese and English Bible Online. (n.d.) Exodus. Available: http://www.obible.com/cgibin/ob.cgi?version=kjv&book=exo&chapter=7. Assessed 2006 Feb 21.
Garrison T. 2005. Oceanography: an invitation to marine science. California: Thomson-Brooks, Cole.
Glossary of Meteorology (GM). (n.d.) Available:
http://amsglossary.allenpress.com/glossary/browse?s=r&p=32. Accessed 2006 Feb 21.
Hong Kong Red Tide Information Network (HKRTIN). (n.d.). What is Red Tide / Harmful Algal Bloom
(HAB?) Available: http://www.hkredtide.org/eng/red.html. Accessed 2006 Apr. 5.
Lee J.H.W., Hodgkiss I.J., Wong K.T.M. and Lam I.H.Y. 2005. Real time observations of coastal algal
blooms by an early warning system. Estuarine, Coastal and Shelf Science 65 (1-2): 172-190.
Available: http://www.sciencedirect.com/science/article/B6WDV-4GSJRBP1/2/1df6afe65966634c1537a42416769113. Accessed 2006 Feb 21.
Wong P.P.S. 2003. Red tide research and management in Hong Kong. In P. K. S. Shin, editors.
Turning the Tides. Hong Kong: The Marine Biological Association of Hong Kong; p 299.
Yang Z.B. and Hodgkiss I.J. 2004. Hong Kong’s worst "red tide"-causative factors reflected in a
phytoplankton study at Port Shelter station in 1998. Harmful Algae 3 (2): 149-161. Available:
http://www.sciencedirect.com/science/article/B73D7-4B722GK1/2/ee5f993c5e000d92311f568d904f6749. Accessed 2006 Feb 21.