## 3 Results
### Change Detection Analysis  
#### 3.5.1 Spatio-Temporal Mangrove Distribution in Chwaka Bay  
The 1973-2020 period shows that mangrove cover reduced from 3988.0 ha to 1706.6 ha, i.e., a 57.2% decrease. This is equivalent to a deforestation rate of 1.2% per annum. The 1973-1990 period shows that mangrove forests reduced from 3988.0 ha to 3529.9 ha, i.e., an 11.4% decrease. This is equivalent to a deforestation rate of 26.9 ha per annum. The mangrove forest cover lost 783.5 ha to sea water, probably due to rising sea levels, 473.3 ha were converted into bare land, 291.3 ha into agricultural land use, and 303.4 ha into other forests. During this period, mangroves gained 6.7 ha initially occupied by sea water, 508.6 ha from bare land, 420.5 ha from agriculture, and 1373.8 ha from other forests. From 1990 to 1995, mangrove forests decreased from 3529.9 ha to 3222.0 ha, i.e., an 8.7% decrease, a decrease of, c., 62 ha per annum. About 2.7 ha of the mangrove forest cover was covered by sea water, 327.8 ha were converted into agricultural land, 1369.8 ha were converted into other forests, and 426.7 ha were degraded to bare land. Mangroves gained 774.2 ha from the area formerly occupied by the sea, 529.0 ha from bare land, 602.7 ha from agricultural land, and 528.9 ha from other forests. Between 1995 and 2000, mangrove forests decreased from 3222.0 ha to 3092.0 ha, i.e., c., a 4% decrease, translating to a loss of about 26 ha per annum. The mangroves lost 768.4 ha to water, 149.5 ha to bare land, 556.2 ha to agriculture, and 676.3 ha to other forests. During this period, mangrove forests gained 9.5 ha from water, 585.7 ha from bare land, 465.1 ha from agricultural land, and 1220.2 ha from other forests. From 2000 to 2009, the mangroves decreased from 3092.0 ha to 2449.2 ha, i.e., a 28.2% decrease, translating to a rate of change of 71.4 ha per annum. About 3.0 ha of the mangrove forest cover was covered by sea water, 134.4 ha were converted into agricultural land use, 343.3 ha were converted into other forests, and 620.6 ha were degraded to bare land. The mangroves gained an area of about 766.4 ha from the sea, 147.4 ha from bare land, 490.4 ha from agricultural land, and 340 ha from other forests. In the 2009 to 2020 period, there was a decrease of mangrove forests by about 32.5%, at a deforestation rate of 67.5 ha per annum. The mangrove area was turned into other land use, i.e., 23.6 ha were covered by sea water, 115.2 ha were transformed into bare land, 193.6 ha were transformed into agricultural land use, and 227.8 ha were converted into other forests. The mangrove forest cover gained about 82.0 ha from the sea, 165.8 ha from bare land, 343.6 ha from agricultural land, and 711.1 ha from other forests. Figure 7 shows the mangrove forest gain and loss (in ha) from 1973 to 2020. The amount of mangrove change differed from one period to another. The green color shows the area (in ha) that the mangrove forest cover gained from the other LULC classes, while \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Chwaka Bay**} & \multicolumn{4}{c}{**Menai Bay**} \\ \hline  & **Ground** & **Classification** & **Overall** & **Kappa** & **Ground** & **Classification** & **Overall** & **Kappa** \\  & **Data** & **Results** & **Accuracy** & **Coefficients** & **Data** & **Results** & **Accuracy** & **Coefficients** \\ \hline
1973 & 160 & 142 & 82.5\% & 0.72 & 1973 & 160 & 138 & 85.5\% & 0.77 \\
1990 & 160 & 136 & 85.4\% & 0.76 & 1990 & 160 & 141 & 85.5\% & 0.77 \\
1995 & 160 & 136 & 86.6\% & 0.79 & 1995 & 160 & 134 & 85.4\% & 0.78 \\
2000 & 160 & 146 & 89.3\% & 0.81 & 2000 & 160 & 149 & 93.1\% & 0.87 \\
2009 & 160 & 139 & 91.7\% & 0.86 & 2009 & 160 & 141 & 91.0\% & 0.88 \\
2020 & 160 & 153 & 92.7\% & 0.89 & 2020 & 160 & 156 & 94.5\% & 0.90 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The overall accuracies and kappa coefficients for Chwaka Bay and Menai Bay in Unguja, Zanzibar.