Page 39 - Research & Innovation Report 2020

SCIENCE

Prof Nadine Strydom Nurdle spill

of Science titled “Nurdle drifters around South Africa as indicators “It appears that the transport of eggs and fish larvae run a wind-
of ocean structures and dispersion”, and the other in 2020 in driven gauntlet of either entrainment in southward edge flow that
Estuarine, Coastal and Shelf Science, titled “Nurdle drifters links to coast nurseries or dispersal via the Agulhas Current core
as indicators of egg and early-stage larval fish dispersion and into unfavourable Southern Ocean waters.”
transport on the south-east coast of South Africa”.
The main migrant species are our most popular angling species,
Describing the rationale for their research, Prof Strydom says: such as spotted grunter, dusky cob, elf/shad and common breams.
“Many of our fish species migrate to KwaZulu-Natal to spawn and Other migrants are species like sardine.
it has always been a conundrum for scientists about what happens
to the eggs and larvae in the Agulhas Current as there is very little As a rule of thumb, only 0.1% of a spawning batch of most fish
information about this.” Over the years, scientists have released species will survive to adulthood. There is also excessive fishing
drift cards from various places with telephone numbers for citizens in the nursery areas – shallow water areas and estuaries – that are
along the coast who pick them up to call in the location. targeted by recreational fishers and anglers.

In the South African Journal of Science paper the authors write: South Africa is facing a situation where the populations of many of
“Using known oceanographic current structures, satellite imagery, our important coastal fishes have collapsed, meaning there is less
wave data and surface wind drift values of between 5% and 8% of than 20% left from what scientists back-calculated as being pristine
wind speed, good agreement was found between the modelled stock before major fishing took place.
dispersion and nurdle sightings. In particular, it was found that
nurdles remained in specific sections of the coast for long periods, The African Journal of Aquatic Science published a paper of Prof
and that sporadic wind events were required to move them into Strydom’s in April 2020, titled: “Review of fish life history strategies
new coastal areas.” associated with warm temperate South African estuaries and a call
for effective integrative management”.
“What we didn’t know is how fast this can happen if the winds
are favourable. The timing was confirmed by this free drifter “There is huge value in establishing Marine Protected Routes in
experiment from the pollution spill,” says Prof Strydom. addition to Marine Protected Areas to create a series of corridors
to protect the spawning migrations of various fish species, as
The modelling showed that anything spawning in Durban harbour many are caught during their migrations,” Prof Strydom explains,
could be in the Agulhas Current off Durban within three days if in reference to her article.
the wind is blowing offshore. Within a week, if the winds turn
to onshore, surface water moving from the Agulhas Current can “What we also need to look at is the manner in which various fish
return to shore off the Eastern Cape carrying the fish larvae, which species gather before migrating, often in estuaries. The social
can then swim into estuaries. behaviour of fish has not been taken into consideration with regard
to migration and it’s essential. What is also key is that the older the
In the Estuarine, Coastal and Shelf Science article, the authors fish get the more eggs they produce – they are the best spawners
surmise: – and protecting the mature fish is absolutely critical.”

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