Final answer:
Climate change and overfishing are two plausible reasons for the declining whitefish landings in Sansosi, with climate change altering migration patterns and overfishing leading to population declines.
Step-by-step explanation:
Based on the information provided, two reasons for the decreasing whitefish landings in Sansosi over the past five years could be climate change affecting fish migration and overfishing in the region. Climate change can disrupt the natural migration patterns of fish, leading to changes in their distribution and availability. On the other hand, overfishing is a significant threat to aquatic species and has led to the collapse of many fisheries, including the western Atlantic cod fishery, due to economic and political pressures resulting in overexploitation. Overfishing can reduce the numbers of mature, reproducing fish to levels where the population cannot sustain itself, which will lead to decreased landings.