Temporal trends of mercury and isotopes in Nile Perch from Winam Gulf, Lake Victoria, Kenya: 1998 vs 2022.

Nile Perch were obtained from Winam Gulf of Lake Victoria in 1998 and 2022. Thirty eight and 43 perch samples ranging in total length from 1.2 - 158 cm and 7.7 to 139.5 cm were processed in 1998 and 2002, respectively. A smaller number of samples of nile tilapia and unionids were co-collected across years. Samples were analyzed for δ¹³C and δ¹⁵N stable isotopes and total Hg. Carbon isotopes were depleted in mussel, tilapia and perch in 2022 compared to 1998 but δ¹³C was independent of body size for both fish species. Nitrogen isotopes were not dependent on size for tilapia (1998) but strongly size dependent for perch (1998 and 2022). For size matched tilapia, δ¹⁵N decreased in 2022 with a similar, but non-significant temporal decrease observed for perch. Total Hg showed a significant decrease in tilapia in 2022 compared to 1998. For perch, the bioaccumulation slopes of Hg vs total length and Hg vs δ¹⁵N were significantly different between years with higher bioaccumulation rates observed in 2022. These shifts were most evident in large perch > 80 cm and highest trophic level fish. Total length was a stronger predictor of Hg (R² of 0.45 and 0.86 for 1998 and 2002 ) compared to δ¹⁵N (R² of 0.07 in 1998 and 0.52 in 2002). Risk analysis on the legal 50 to 85 cm commercial slot size indicates a shift in maximum recommended monthly meals from >32 meal/month in 1998 to 16 meal/month for perch over 70 cm in length for women of childbearing age and children. [ABSTRACT FROM AUTHOR]