2.4 Sampling procedure
Information on the infection of E. sardella fish hosts by the
parasite L. intestinalis was collected over a period of 10 years,
from 2005 to 2013 and then in 2015 in Wissman Bay. In the period
2005-2013 data was generated from fish caught by local fishermen from
sites of Matema, Mwaya, Kafyofyo and Kiwira within Lake Nyasa. E.
sardella were caught using an open water seine net, locally known as
‘Ndaturu’, with 10 mm mesh size at a depth of about 100 m during the
dark moon phase days. In 2015, fish were caught by our research team,
using the same traditional fishing method as was used in the period
2005-2013.
The fishing procedure involved nine crew members using two dug-out
canoes and one large plank-boat. On the fishing ground, one of the
dug-out canoes was equipped with pressurized paraffin lamps (between one
to three) and was stationed with one crew member away from the remaining
vessels. The artificial light was used to concentrate the fish into the
given area. This process took several hours. After a sufficient amount
of fish had been attracted, the other unlit fishing vessels
simultaneously deployed a net in a semicircular shape around the
concentrated fish, and this was hauled by hand into the plank-boat.
Sampling was carried out in both lake habitats (i.e. littoral and
pelagic zones) as well as throughout different seasons (i.e. dry and wet
seasons) and a total of 3,488 E. sardella were sampled (Table 1).
Upon landing, the total length and weight of each E. sardellawere measured to the nearest 5 mm and 0.01 g respectively. The fish were
later dissected for parasite determination. L. intestinalis was
identified according to the protocol by Dobben (1952). The sex ofE. sardella was determined using a stereomicroscope (Wild
Heerbrugg M5) at 6.4X magnification. Gonad maturity was assessed on a
seven-stage maturity scale (Table 2), modified from Holden and Raitt
(1974). For seven years of the ten years (i.e. 2005, 2006, 2010, 2011,
2012, 2013 and 2015) the maturity stages of E. sardella was
determined and recorded by the same investigator (N.P.Gabagambi).
Therefore, we were able to maintain a good level of consistency and
accuracy in the determination of maturity stage across our sampling
period. In 2007, 2008 and 2009 maturity determination was carried out by
trained research technicians (E.J.Magesa and J.M.Masore), following the
same seven-stage maturity scale as was applied in all other sampling
years.
Gonads were weighed to the nearest 0.01 g (wet weight) using sensitive
precision balances (vwrTM-model ECN 611-2315 and
EndelTM- model WPS) and fecundity for infected and
non-infected female E. sardella was determined through
gravimetric methods (Holden and Raitt, 1974) by counting the advanced
yolked oocytes present in ripe and gravid E. sardella . The
complete ovary was taken out and preserved in modified Gilson’s fluid
(100 ml 60% alcohol, 800 ml water, 15 ml 80% nitric acid, 18 ml
glacial acetic acid, 20 g mercuric chloride) for 24 hours. Thereafter,
the ovaries were shaken periodically to help loosen the eggs from
connecting ovarian tissues. After the eggs were liberated from the
ovarian tissues, they were washed thoroughly, spread on blotting paper,
and allowed to dry at ambient temperature ranging between 25 and 30 °C.
Thereafter, the total numbers of eggs were weighed to the nearest 0.01 g
using sensitive precision balance to have a total weight of eggs.
Afterwards we collected a random sub-sample of the eggs, which were
weighed and counted out on petri dish sub-sections using a
stereomicroscope (Wild Heerbrugg M5) at 6.4X magnification. The total
number of eggs (i.e. fecundity) in the ovaries was calculated following
the formula given by Holden and Raitt (1974) as follows; F= nG/g where;
n=number of eggs in sub-sample, G=total weight of eggs from the ovary,
g= weight of the sub-sample. Fish somatic weight was determined by
subtracting the gonad weight from the total weight of the fish.