Kob (salmon, kabeljou, croaker, drum) belong to the scientific group of fish named the Sciaenidae. They are common throughout the world and are highly valued as table fish. About nine different species of Sciaenids are found off South Africa; the most common are the squaretail kob (Argyrosomus thorpei), silver kob (A. inodorus), dusky kob or dagga salmon (A. japonicus) and the snapper kob (Otolithes ruber). The geelbek and the tasselfish (baardman/bellman) are also members of this family. Kobs are usually large, predatory fish that can tolerate marine and brackish waters. They are, therefore, frequently found in estuaries and lagoons, as well as in shallow waters close to the shore and in deeper waters. Deep river mouths are often home to large kobs.
Kobs have a coppery sheen, are fairly robust with an elongate body and a rounded tail fin. Their lateral line is easy to see and the dorsal fin is distinctly divided into two sections. The kob family is internationally known as croakers or drums, because of the sound which most of the species produce using specialised drumming muscles. This pair of muscles, often found only in the males, rubs against the side of the swim bladder, which acts as an amplifier. It appears that this phenomenon is linked to courtship behaviour or to territorial displays; it may even be a form of underwater communication.
Various kob species are often superficially very similar, making it difficult for non-scientists to distinguish between them. In fact, scientists described the silver kob as a new species as recently as the early 1990s, where previously it was thought to be the same species as the dusky kob.
This factsheet will focus on the three most commonly caught kob species - the dusky, squaretail and silver kob. However, it should be noted that smaller kob species, such as the mini-kob and the snapper kob, are also found along the KwaZulu-Natal coast.
COMMON NAME: kobs
Dusky kob reach sexual maturity at a length of 900cm - 1m at about 5-6 years of age. Most adults migrate from the Cape to KwaZulu-Natal to spawn between August and November. Spawning generally occurs inshore in 10 - 15 m of water. Juveniles enter the upper reaches of estuaries where they remain until they about 15cm. They then move into the lower reaches of estuaries and the nearshore marine environment. Silver kob reach sexual maturity at about 30cm in length and spawn between August and December in inshore waters. The juveniles prefer the sandy or muddy substrates in shallow embayments, while adults prefer low profile reefs in 20 - 120m of water.
Squaretail kob reach sexual maturity at about 33cm in length and spawn between June and September on the Tugela Banks and over other muddy substrates.
Most kob species are voracious, shoaling predators and some species have become highly specialised for feeding in their muddy, murky environment. Their lateral line system (a sensory system found in all fish that enables them to detect vibrations and pressure changes in the water) is very well developed and this, in conjunction with the sensory barbles which some have on their snouts, makes the kob less reliant on sight when feeding. Small fish, crustaceans such as prawns and crabs, and molluscs such as squid and cuttlefish are all eaten by the various kob species.
All three kob species are slow growing and long-lived. It is relatively easy to determine the age of kobs, as their ear bones, or otoliths are large. (Scientists can tell the age of fish by counting the growth rings in their otoliths and relating that to the size of the fish).
The dusky kob can reach 1.8m in length at an age of 42 years and a weight of 75kg. The dusky kob is widespread and is found on the eastern seaboard of southern Africa, off southern Australia, and in the northern Indian and northern Pacific Oceans.
The silver kob reaches 1.4m in length at about 25 years of age and a weight of 36kg. This species occurs from Northern Namibia to the Eastern Cape in depths of less than 150m.
The squaretail kob reaches 120cm at about 13 years of age and a weight of about 13kg. This fish has a limited distribution and is endemic to southern Africa, from Mozambique to Port Elizabeth. It is usually found north of Durban, where it appears to be fairly resident, congregating in large shoals around deep reefs and pinnacles. Unlike the dusky kob, it rarely ventures near the shore or into estuaries.
All three large kob species are highly sought after by both recreational and commercial fishers as the flesh of these fish makes for excellent eating.
Dusky kob are caught by recreational estuarine and shore anglers and by recreational and commercial skiboat fishers, as well as by commercial beach seine netters.
Silver kob are an important component of the catches of shore anglers in the Western Cape and are caught by commercial and recreational skiboat fishers. They are also caught in beach seine nets in False Bay and are an important by-catch taken by inshore trawlers.
Squaretail kob are caught by commercial and recreational skiboat fishers and are caught as by-catch by the prawn trawlers operating on the Tugela Banks
WHAT YOU CAN DO TO HELP
There are a limited number of kobs in the ocean. If anglers catch more than can be replaced by their breeding, over-fishing results. This results in fewer and fewer kobs being caught and their average size becoming smaller and smaller. To prevent this there are regulations to control the number of kob that are caught. These regulations ensure that everyone catches their fair share and that kobs can continue to be caught in the future.
Obey the fishing regulations
- Minimum size limits give fish a chance to breed at least once before they are caught and protect the fish when they are growing at their fastest.
- Bag limits restrict daily catches so that there will be enough fish for everyone. Scientists work out how many fish can be harvested safely. This information is used to set a bag limit that restricts the number of fish caught per day. This prevents more successful anglers from catching great numbers of fish, so leaving some behind for less successful anglers.
Join the tagging programme and tag and release your fish.
Tagged fish can provide scientists with useful information about the seasonal movements of fish, their growth rates and in some cases, the size of the stock. They also give anglers an opportunity to become involved in an exciting research programme; taggers receive information about their tagged fish, if they are recaptured. If you catch a fish with tag in it, read the tag number or remove the tag from the fish and measure the fish (from the tip of the mouth to the fork of the tail). Send the tag number (or tag), the type of fish, where it was caught (try to give a specific location), the date caught, the length and/or weight of the fish and your name, address and telephone number to: The Tagging Officer, Oceanographic Research Institute, P.O. Box 736, Durban, 4000.
van der Elst, R.P. 1988. A Guide to the Common Sea Fishes of Southern Africa (2nd ed). Struik Publishers, Cape Town.