Download Redbait - Complex but interesting

Redbait, SANParks together with Honda Marine – Complex but Interesting
By N. Hanekom (SANParks)
Redbait is apparently the most frequently collected bait species by anglers along the south-eastern Cape coast, where it
is used to target fish, such as galjoe n, bronze bream, white musselcracker and blacktail. It is also an import ant source of
protein for subsistent fishers and in Maputaland approximately 5 tons are gathered annually. Redbait belongs to the
class Ascidiacea (or sea squirts).
The larvae have a primitive backbone and nerve cord, and bec ause of these features, redbait is considered to be a
highly advanced animal and closely relat ed to vertebrates. The backbone is lost once the redbait lar va settles on the
sea bottom and develops into a sessile, sac-like animal. The adult form is roughly barrel -shaped with a pair of tubular
siphons. It is covered in a t hick dark brown wrinkled tunic, which encloses and protects the soft, orange-red flesh
against predation and wave action.
Typically redbait stands about 15 cm high, but it can grow as large as a rugby ball. This species is found on rocky reefs,
from the low shore t o a depth of about 15 m, usually forming dense beds where wave action or currents are strong.
Such beds create a substratum and shelter for numerous other species, and more than 80 different species have been
recorded from redbait beds in KwaZulu-Natal. Like many other filt er-feeding organisms in the sea, redbait relies on
waves and currents to deliver tiny particles of food to it. The particles are sieved from the water, which is drawn in
through an inhalant siphon, filtered through a sieve-like pharynx and discharged through an exhalant siphon.
Mass mortalities of redbait occur occasionally. For exam ple, in 1991 hundreds of redbait pods in the Tsitsikamma
National Park were infected by a thin white microbial growt h and showed signs of decay. During the ensuing t hree
months a huge die-off of redbait occurred. Large proportions of the redbait stocks – amounting to m any thousands of
kilograms – were lost from the coastline. The loss of redbait cover led to a change in t he habitat structure, which in turn
led to a dram atic change in the composition of the fauna and flora. Juvenile redbait, like mussels a nd oysters, prefer to
settle amongst the adults of t heir own species, and the recorded recovery was slow, taking s everal years. Therefore,
this disease-mediated mort ality not only impacted the redbait population, but also, indirectly, the whole community.
No red algal tides were noted in the area during
this die-off of redbait, and past rec ords make no
mention of toxic red-tide events being lethal to
redbait. However, large declines in surface sea
temperatures due t o nutrient-rich upwelling
events, together with high air temperatures may
have created conditions favourable for such a
microbial epidemic. Samples taken from the
outer tunic of decaying redbait specimens were
infected with a bacterial growth and t he colonies
cultured were all comprised Vibrio bacteria. A
Vibrio bacterium was apparently responsible for
substantial mortalities in seven species of
starfish (Asteroidea) in southern California
during the early 1980s.
During a recent survey of the bait stocks at
selected sites in the Garden Rout e National
Park, substantial num bers of infected- and
decaying redbait pods, similar to those noted in
1991, were recorded. The situation is being
monitored, and should a large scale die-off of redbait occur in t he K nysna estuary, then SA NParks, using the inflatable
boat donated by Honda Marine Sout hern A frica in conjunction with Honda M arine Knysna, will investigate the
populations.