Analysis of the daily catch and effort data of the bluefin (Thunnus thynnus) Algarve trap fishery during the years 1898-1900

摘要

This paper is based on recently recovered daily catch and effort data from the Algarve traps (south of Portugal) during the 18981900 period. These 18 traps targeted the two migrations of bluefin tuna, entering and leaving the Mediterranean Sea, each year during a period between mid April and the end of August. Their bluefin catches were significant, corresponding to average yearly catches of 61 000 bluefin, i.e., approximately 7900 t. Migration dates and durations towards and away from the Mediterranean Sea were estimated based on the daily catch per unit effort (CPUE) by the traps. Migration patterns were very stable during these three years. The entry migration took place over an average of 42 days, and the return lasted 53 days. Daily catches show large waves of tunas passing during the two migratory phases. It can be hypothesised that each of these waves of tuna spent about 52 days in the Mediterranean Sea, and that each group was stable and consistent during their arrival and return migrations. Migration trajectories along the Algarve coast can be estimated from the daily CPUEs of the different traps. They appear to be different between the arrival and return migrations. The daily data per trap show that most traps caught entering bluefin at quite low CPUE, whereas only a few traps were efficient for catching the returning bluefin, but with much larger catches and CPUEs. The analysis of catch and CPUEs during the entering and leaving phases allowed us to conclude that the catchability and fishing mortality of the bluefin stock in the Algarve trap fishery was much larger during the return migration. Additional daily data from other traps, both historical and contemporary, should be collected and analyzed in order to provide more comprehensive results on the patterns and changes in migration, stock structure and stock exploitation rate.