Kapenta
At the inception of fishing in the 1970s the catch per unit was high and has been decreasing since then. Of note was the sharp decrease in 1978-79 (figure 1). From 1986 to 2020 the decrease has been more gradual. Total catch peaked in 1990 and has been declining ever since.
Figure 1: Kapenta catch trends 1974 to 2020
Kapenta length and weight monitoring
The samples collected in 2020 had the highest mean weight for the three-year period (table 1). The samples collected in 2018 had the highest mean weight.
Table 1: Kapenta length and weight for 2018 to 2020
Year | Average Length (mm) | Average Weight (g) |
2018 | 48.77 ± 7.07 (n=18799) | 0.94 + 0.43 |
2019 | 35.50 + 6.80 (n=2273) | 0.49 + 0.29 |
2020 | 37.08 + 7.70 (n= 27582) | 1.94 + 0.85 |
*fewer samples were collected and analysed in 2019 hence the smaller sample size
Inshore fishery
In the year 2020, in the inshore fishery, O. niloticus comprised of 77% of the total catch by number (figure 2). The rest of the species which were harvested by the fishers did not individually make a significant contribution to the total catch.
Figure 2: Inshore Species Composition by number for 2020
Experimental gillnetting
In the year 2020, S. zambezensis was the dominant species being 62% of the total catch by numbers, followed by H. vittatus which was 25% (figure 3). The species with 0% did contribute to the total catch, but their individual contribution was so low that the % contribution when calculated is recorded as 0.
Figure 3: Species composition by number in lakeside gillnetting 2020
Tiger fish tournament
In the year 2018 there were males than females caught during the tournament (figure 4). The years 2019 and 2020 saw more females than males being caught. This may have implications on the breeding since the females produce eggs.
Figure 4: Tiger fish sex distribution
*sample sizes differ depending in the number of teams at the tournament and the cooperation of participating teams
The year 2020 had the least values for weight and length compared to the previous 2 years (table 2).
Table 2: Average length and weight for tiger fish from 2018 to 2020
Year | Average Length | Average weight |
2018 | 43,23 ± 7.199 (n=628) | 1,77 ±1.015 |
2019 | 42,56 ± 7.052 (n=328) | 1,74 ± 1.100 |
2020 | 40,56 ± 5.015 (n=131) | 1,54 ± 0.840 |
The growth of fish can be categorized into three which are isometric growth, negative allometric growth and positive allometric growth. In isometric growth, there is no change in body shape and weight according to the increase in length. A fish in good condition exhibits positive allometric growth; it increases in weight at faster rate in relation to its increase in length. Insufficient availability of food and environmental changes are the major reasons for negative allometric growth in fish. Samples from 2016 to 2019 reflected negative allometric growth while 2020 reflected positive allometric growth (table 3).
Table 3: Length and weight relationship for tiger fish
YEAR
|
LENGTH-WEIGHT RELATIONSHIP |
FULTON CONDITION FACTOR
|
GROWTH TYPE
|
||
a (intercept) | b (slope) | r2 (regression coefficient) | |||
2016 | 0.091 | 2.61 | 0.66 | 0.041 | Negative allometric |
2017 | 0.11 | 2.53 | 0.40 | 0.022 | Negative allometric |
2018 | 0.11 | 2.55 | 0.66 | 0.06 | Negative allometric |
2019 | 0.065 | 2.69 | 0.58 | 0.028 | Negative allometric |
2020 | 0.009 | 3.23 | 0.86 | 0.012 | Positive allometric |