• Welcome to the DeeperBlue.com Forums, the largest online community dedicated to Freediving, Scuba Diving and Spearfishing. To gain full access to the DeeperBlue.com Forums you must register for a free account. As a registered member you will be able to:

    • Join over 44,280+ fellow diving enthusiasts from around the world on this forum
    • Participate in and browse from over 516,210+ posts.
    • Communicate privately with other divers from around the world.
    • Post your own photos or view from 7,441+ user submitted images.
    • All this and much more...

    You can gain access to all this absolutely free when you register for an account, so sign up today!

Interesting study on fish

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.


Desk Driver
Dec 6, 2001
We all know about reef fish and their territories ... this is an interesting study going on in SA

Roman Polanski is a complete ninny, only once during a full 14 months venturing further than 50m from his home - and that was just a quick dash of about 100m before returning.

No, it's not the famous film director who has made such classics as The Pianist, Rosemary's Baby and Tess.

Rather, this Polanksi is a Red Roman fish, and he lives in a marine cave about 200m offshore of Miller's Point.

The movements of Polanski, and 13 other Romans - and a lone rock lobster - are being closely monitored by PhD student Sven Kerwath, thanks to some state-of-the-art technology valued at more than half-a-million rand.

The indication at Goukamma is that the marine reserve is working
Marine scientist Kerwath, who did his undergraduate studies in his native Germany before completing his master's in the West Indies, met up with South African fish specialist Paul Cowley while visiting a friend at Port Alfred in the Eastern Cape.

Here he was quickly hooked (so to speak) into studying for a doctorate at Rhodes University's Department of Ichthyology and Fisheries Science.

Kerwath's particular research topic is the movement behaviour of the Red Roman - one of South Africa's heavily exploited reef fishes.

His findings are being closely watched by, among others, scientists in the Marine and Coastal Management (MCM) branch of the department of environmental affairs, in aid of the extremely important question of how and where to proclaim marine reserves for optimum conservation efficiency.

Kerwath's research at Miller's Point, which is within an existing marine reserve, is being supervised by MCM's Colin Attwood.

Kerwath also has a second site in the Goukamma Nature Reserve between Sedgefield and Knysna.

Part of his study technique involves catching fish, surgically implanting tiny radio transmitters into their body cavities, releasing them and tracking their movements.

The tracking can be done using a hand-held radio telemetry system, which confirms the presence of the fish and its general direction, but this is a somewhat primitive system that gives limited data.

Kerwath has installed a much more sophisticated system, developed by a Canadian company, which gives highly accurate and complex data and can be read from the comfort of a computer screen in an onshore office.

This system involves three buoys, anchored in a triangle 70m apart and a couple of hundred metres offshore.

The buoys pick up the pulsed signal from the fish and send these to a computer, which uses the "triangulated" data to calculate continuously the exact position of each fish.

"My main objective is to design a population model of Red Roman inside and around a marine reserve, to see whether the reserve works in terms of benefiting the fish population," Kerwath explained.

Red Roman, like some other fish species, change sex - they are female for the first part of their adult life, then become males when they reach a length of 28cm to 32cm.

Once they have established a home range, they move very little.

The problem in the past is that fishermen targeted the bigger fish, which were almost all males.

And because of the Romans' lack of mobility, heavy fishing quickly removes all or most the males, effectively wiping out that population.

But their lack of mobility also means that even a very small marine reserve - like the Miller's Point reserve, which is only about 2km by 4km - can be highly beneficial in conserving these fish.

"Already the indication at Goukamma is that the marine reserve is working - we're catching more males there - and it seems to be the same here as well," said Kerwath.

Kerwath's tracking system at Miller's Point, called V-rap (from Venco, the company's name, and "radio acoustic positioning"), can also give readings for tagged fish swimming outside the triangle of buoys.

"If conditions are really calm, you can still get a reading from a range of as much as 300m, but the further you go out of the triangle, the bigger the error."

There are different size transmitters that can be fitted to the fish, and their lifespan depends on the quality of the battery - some last for just 30 days, while others transmit for several years.

And it's not only the fish's position that can be recorded.

"There are lots of applications - for example, we can get temperature recorders and depth recorders."

But the transmitters are expensive - about R2 000 each. "And it was a real mission-and-a-half to set it up - first the revenue service didn't want to let them into the country, then we had to get navigational lights for the buoys, and we really struggled to get insurance, and it took four days of diving work to position the buoys, which each have 500kg anchors."

Now, however, the system is working and producing excellent data - and will probably help ensure that Roman Polanski and his mates are still around in the years to come.

This article was originally published on page 3 of The Cape Argus on November 13, 2003
Awesome, how can I get a grant to do something like that? maybe I can start a cultured abalone pearl study :hmm
Hi SAspearo,

Went past the millers point reserve on sunday and saw the bouys in the article. I think i must chat to Kerwath and see whether i can be of assistance. Always enjoyed diving in the reserve, plenty of fish. I will have to take a camera along sometime!!