The use of long-term commercial fisheries data as a background for development of fish-based ecological status assessment system of transitional waters

The development of fish-based methods for assessment of the ecological status of transitional waters is challenging due to their unique hydrology, limited reliable historical data, and complex anthropogenic pressures. Nevertheless, long-term commercial fish catch records may serve as a proxy for tracking ecological change. This study analysed commercial catch records from 1953 to 2023. Significant trends of both deterioration and recovery were detected in fish metrics, and the recovery trend was strongly related to declining total phosphorus concentrations in the lagoon, documented since standardized water-quality monitoring began in 1993. Reference values and good/moderate (G/M) status thresholds were defined using metric values from periods preceding and during ecological deterioration. Based on these, an index derived from commercial catch data was developed to assess the lagoon's ecological status. Total phosphorus concentrations corresponding to the reference and GM thresholds were calculated based on the relationship established with the index. These concentrations were then used to calculate the reference and G/M threshold values for the monitoring of fish metrics that were related to total phosphorus, and a monitoring-data-based fish index was developed. Though based on different data sources and metrics, the two indexes significantly correlate ( R = 0.74; P < 0.001). This confirms that fisheries data can help identify long-term trends related to changes in fish communities, can reflect conditions prior to the intensification of human activity, can be linked to monitoring data collected in recent decades, and can serve as a starting point for developing a fish-based method for assessing the ecological status of the lagoon.