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The Sardine Run, what makes it happen?

The Sardine Run, what makes it happen?

As I wait for the sardines to make their appearance this year, I started thinking more about the sardine run and what it is all about. Each year during July, we generally get a migration of these tasty baitfish making their way up the KZN South Coast, accompanied by masses of sharks, gannets, dolphins, whales and game fish. The sardines themselves are in fact Southern African pilchards, Sardinops sagax, which form dense shoals and often come in close inshore, and sometimes even wash up on our beaches, much to the delight of fishermen and beach users along the KZN coast. Some years, however, they do not appear and we are left disappointed and wondering what has gone wrong.

There are websites, facebook pages, phone in centres and threads on fishing forums all dedicated to providing the latest information on any movements of sardines along our coastline, and yet still the rumours fly and solid information seems scarce at times. Sources of this information include the KZN Sharks board, dive operators, recreational and commercial fishermen and the general public. Planes and microlights fly along the KZN and Eastern Cape coastline looking for sardine shoals on a daily basis during the peak season. Along with all of the above I have the advantage of being in touch with some of the inshore seine netters, who follow the sardines and make every effort to be on the spot should they make an appearance anywhere along the KZN coastline. Even with all of the above information sources, I have yet to hear of any action along our inshore KZN coast this year at the time of writing, apart from some mixed baitfish off Port Edward in early July and some offshore activity off the Transkei coast. Some years the run just doesn’t seem to happen, and it is interesting to look into what the causes are behind a good year and a bad year with regards to sardines.

I decided to research the sardine run and find out more about this phenomenon in order to better understand it. I spent some time digging around and came away better informed, yet still curious, as there are many contradictory opinions, even amongst respected marine scientists on some aspects of the run. I thought that I would share what I did find out with readers, as it is actually pretty interesting and helps to put some of the myths to bed.

The first mystery is why the sardines undertake this migration at all. It is documented that the Southern African pilchards do their spawning off the Aghulas banks in the Southern Cape and so for many years it was thought that the sardine run had nothing to do with spawning. This made the migration a complete mystery as environmental conditions along the KZN coast are at best marginal for the sardines. More recent studies have however shown that eggs are in the plankton mass of the current s coming down the East Coast, and therefore it is concluded that the sardines which undertake the migration up the coast are in fact doing so for spawning purposes. The probability is that part of the population of Southern African pilchards undertake this spawning run and spawn off the East Coast of Africa, as opposed to the rest of the biomass, which spawn off the Aghulas Banks.

The next question is how the sardines, which are comfortable in temperate waters are able to migrate into sub-tropical areas.

Firstly there appears to be a cyclonic eddy of cool water off the Waterfall Bluff area in the Transkei, and this is why the shoals of sardines often gather at this point and wait for favourable environmental conditions to take on the next part of the journey up the coast.

The reason that the sardines pass along the KZN coastline so close to the shore is basically because they need to move in cool water and cannot swim against a strong current. With the narrow continental shelf, and the strong, warm Aghulas offshore current, the sardines are forced to use the narrow band of cooler inshore water along the coast, where counter currents and Natal Pulses make it easier for them to swim northwards. These cool north flowing currents usually only last a few days, with a maximum of ten days of uninterrupted flow having been recorded in the past.  The return migration after spawning is undetected, as it occurs at depth, where the fish use cool, deep, south running currents to ease their journey back down the coast.

The theory is that in years when there appears to be no sardine run at all, they either pass by further out and deeper down, due to an unusual deep, cool counter current, or they do not come up the coast at all due to some environmental barrier, such as water being too warm, or not enough northward inshore current.

It is clearly evident that thousands of gannets, dolphins, whales and sharks follow the sardine run and gorge themselves on this easily available food source. History has also proved that a number of predatory species of fish are around at the same time and also often take advantage of this feeding opportunity. Shoals or pockets of sardines are not necessarily accompanied by hordes of feeding game fish, although this can be the case at times. It is most likely that a number of species of predatory fish favour the same environmental conditions as the sardines for their own spawning migratory purposes. Fish such as shad, kob, Garrick, king mackerel and some kingfish species are definitely more prevalent along the KZN coast during the same period as the sardine run, and do take advantage of feeding opportunities when the sardines appear inshore. Scientists however, feel that it is not clear that this is purely a feeding migration by these predators, and could also be a reproductive migration for some of them and also them simply making the best of the same environmental conditions which favour the sardines.

One of the reasons for rumours, misinformation and confusion during the sardine run is the fact that at the same time of year there are a number of other species of baitfish which also undertake migrations up the East Coast. Red eye sardines, anchovies, sauries, round herring and mackerel also gather in shoals and move along the coastline at this time, causing much confusion amongst the public and many false reports of sardine shoals. Dark patches can be seen out at sea, with birds and dolphins feeding on them, but they are not necessarily sardines.

An interesting point is that researchers have found that the biomass of sardines on the KZN coast during winter has remained pretty consistent for years, regardless of the overall South African sardine population size. Another interesting point is that sardine catches made by beach-seine netters in KZN have been sampled since 1951 and a record of biological characteristics of these fish now exists. It appears that the fish caught off KZN generally have a lower body mass to length ration than sardines found elsewhere in South Africa. This can be partly attributed to the fact that they are not feeding much during the migration, but it has also lead scientists to speculate that this may possibly “represent a distinct stock or a functionally discrete adult assemblage.” Which basically means that they think our Natal Sards are a separate group to the rest.

Basically the bottom line to all of this is the fact that a no show by the sardines in KZN does not necessarily indicate that the South African sardine stocks are in trouble. This has happened over the years and if it is becoming more common for the sardines to not run close inshore in recent years, then this could be attributed more to changing climatic conditions, than to depletions in fish biomass. If we get favourable conditions for sardine runs in the future such as calm currents, light north-westerly land breezes and stable atmospheric conditions during winter there is no reason why we shouldn’t enjoy a bumper sardine run along with the great fishing that comes with it!

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