A long-term capture-mark-recapture survey of the European lobster, Homarus gammarus (Linnaeus, 1758) (Decapoda, Nephropidae) at Aguda’s Beach in north Portugal

Abstract The European lobster population on Aguda’s Beach was studied between 2006 and 2023 through an annual capture-mark-recapture survey. The primary objective was to assess the abundance, sex ratio, and growth of lobsters, serving as a basis for future investigations on population density, survival, migration, and site fidelity. In the 18-years sampling period, 5 traps were set on 892 fishing days, from May and September each year, resulting in the capture of 320 lobsters in an experimental fishing area covering approximately 0.4 km 2 , off Aguda’s Beach. The catch rate of the experimental fishery varied from 0.01 to 0.15 lobsters per trap, with an overall mean of 0.07 animals per trap. To augment the number of tagged individuals for the survey, 397 lobsters were obtained over the years from the local commercial fishery, which operated in an area approximately 10 times as large as the experimental fishing area. Out of the total number of 717 lobsters, 512 specimens were marked and released in the experimental fishing area. Between 2009 and 2022, 31 males and 15 females were caught again, resulting in a mean recapture rate of 8.9%. The maximum time a lobster spent at sea before being recaptured was 11.9 years, while the minimum was 2 months. Over the 18-year survey period, the mean absolute growth rate of 46 recaptured lobsters was 467 ± 33 g SE (standard error) and 3.3 ± 0.2 cm CL (carapace length) per year. Males achieved an average rate of 509 ± 38 g and 3.1 ± 0.3 cm CL, while females presented a rate of 379 ± 58 g and 3.6 ± 0.3 cm CL. The findings of this long-term survey did not support a decline in the lobster population at Aguda’s Beach over time, what might be attributed to the decrease of the commercial fishing activity. Additionally, the population also seemed to exhibit a certain mobility, as evidenced by reduced recapture rates in recent years. The range of annual growth rates indicated a diverse composition of the population, with male lobsters exhibiting growth patterns similar to females but revealing greater activity, which led to recapture rates nearly twice as high.