Why did fish catches off the Nile delta decline after the Aswan High Dam was built?

The construction of the Aswan High Dam in 1970 significantly altered the ecological conditions of the Nile delta and the adjacent Mediterranean waters, leading to a decline in sea fish catches. This decline is primarily attributed to two major environmental changes:

• Reduction in Nutrients: Historically, the Nile’s annual flood carried a massive load of waterborne nutrients into the Mediterranean. The dam impounds these waters, causing a serious reduction in the outflow of these essential nutrients, which previously supported the productivity of the coastal ecosystem.123
• Changes in Water Character: The dam stopped the seasonal fluctuations of the Nile's discharge. Before the dam, the influx of fresh water reduced the salinity of coastal waters, which increased their stratification and productivity. The loss of this fresh water flow has modified the hydrological characteristics of the region, making the waters saltier and less conducive to the previous levels of marine life.435

Additionally, the dam has acted as a physical barrier, preventing many migratory fish species from moving through the river system.2