Migraciones

When investigating the prevalence of K. thyrsites per season for the SA sardine samples, the P- value (P = 0.000) for the Chi-square test indicated that the prevalence of K. thyrsites was dependent on season (Table 5.9). The prevalence was lowest during spring (82.14%) while highest during autumn (100%), winter (98.28%) and summer (97.22%). However, in this study, spring was represented by only 28 SA sardine samples, as compared to more than half as many samples for summer, autumn and winter. When the data for spring was removed, the Chi-square test showed no significant difference (P = 0.496) in prevalence between summer, autumn and winter. These findings are in contrast to studies from the Pacific Ocean. In a study by Moran, Whitaker et al. (1999), where Atlantic salmon (Salmo salar) were held in seawater net-pens in the Pacific Ocean (British Columbia) to investigate the seasonality of the infective stage of K. thyrsites, it was found that higher prevalence occurred during the summer and autumn months compared to the winter and early spring months. Studies by Munday et al. (1998), St-Hilaire et al. (1998) and Moran, Whitaker et al. (1999) also reported that the level of infection tend to be higher during summer than during winter months.

Area K. thyrsites prevalence (%) Number of samples

East coast 100 70

South coast 98.21 56

West coast 94.33 141

104

However, results from a similar study (Jones et al., 2016) to that of Moran, Whitaker et al. (1999), indicated that infections in Atlantic salmon occurred throughout the year. Jones et al. (2016) suggested that warmer water temperatures during the winter months of 2013-2014 compared to 1995-1996 may explain the differences in results between these two similar studies.

The South African coast is flanked by two different oceanic currents; the cold Benguela Current from the South Atlantic Ocean along the west coast, and the warm Agulhas Currents from the Indian Ocean along the east coast (Teske et al., 2011). The water temperature of the warm Agulhas Currents range between 22 and 27°C, while that of the cold Benguela Currents is typically 4 to 7°C colder throughout the year (Teske et al., 2011; Forbes, 2017; Anon., 2017b). In contrast to the South African coast, the difference in water temperatures between winter and summer months for the Pacific Ocean along British Columbia range between 5 and 14°C (Jones et al., 2016; Anon., 2017c; Anon., 2017d). The larger difference in water temperatures between winter and summer months for British Columbia compared to that along the South African coast, may explain in part the difference in prevalence of K. thyrsites in marine fish species from those parts of the world compared to the South African coast. The infection of SA sardines and Cape hake with K. thyrsites may therefore not differ between warmer summer and colder winter months.

Table 5.9 Prevalence (%) of K. thyrsites for SA sardine samples per season: summer, autumn, winter and spring

*P-value of Chi-square test at confidence level 0.05.

** _{Total number of samples differs here from the total number of samples indicated in Table 5.3 (qPCR: n = 296) since}

only 2015 qPCR data was used, and all samples with any missing data (including sex, area, size, and season) were excluded from statistical analysis.

For the Cape hake (P = 0.055) and SA kingklip (P = 0.366) samples, no significant relationships were found between K. thyrsites prevalence and season (Table 5.10). However, with the limited number of samples for Cape hake and SA kingklip, in addition to the limitation in that only two seasons were represented within each fish species, it was difficult to make any conclusion about the relationship between K. thyrsites prevalence and season for these two species of fish. It would be worthwhile for future studies to collect equal number of samples per fish species per season over a one- or two-year period in order to investigate whether there are seasonal changes for the prevalence of K. thyrsites. It is further suggested to also monitor the seasonal abundance of the parasite in the environment between different locations of capture and spawning grounds along the South African coast. Knowledge of the distribution and onset of K. thyrsites infection in economically important marine fish species, such as SA sardines and Cape hake, may be of value in developing means of control during annual catches by fisheries.

Season K. thyrsites prevalence (%) Number of samples

Summer 97.22 72

Autumn 100 51

Winter 98.28 116

Spring 82.14 28

105 Table 5.10 Prevalence (%) of K. thyrsites for Cape hake and SA kingklip samples, respectively, per season

Cape hake SA kingklip

Season K. thyrsites prevalence (%) Number of samples (n) Season K. thyrsites prevalence (%) Number of samples (n) Autumn 82.05 39 Spring 46.66 30 Winter 100 18 Winter 35.90 39 Total n 57 69 *P-value 0.055 0.366

*P-value of Chi-square test at confidence level 0.05.

5.3.2.3 Sex (male and female)

The prevalence of K. thyrsites was independent on sex of fish (Table 5.11) for SA sardine (P = 0.133), Cape hake (P = 0.117) and SA kingklip (P = 0.874). Morado and Sparks (1986) also reported no difference in K. thyrsites prevalence between male and female samples for Pacific hake.

Table 5.11 Prevalence of (%) K. thyrsites in female and male samples for SA sardine, Cape hake and SA kingklip, respectively

K. thyrsites prevalence (%) Number of samples**

Sex SA sardines Cape hake SA kingklip SA sardines Cape hake SA kingklip

Male 95.07 93.75 42.11 142 32 19

Female 98.40 80.00 40.00 125 25 50

*P-value 0.133 0.117 0.874 Total n = 267 Total n = 57 Total n = 69 *P-value of Chi-square test at confidence level 0.05

** _{The numbers of samples for SA sardines, Cape hake and SAS kingklip differ here from the number of samples for}

qPCR as indicated in Table 5.3 since all samples with any missing data (including sex, area, size, and season) were excluded from statistical analysis.