Fishery changes and implications - need for a wider approach to address socioeconomical effects in fisheries management
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Original versionThis report is not to be quoted without prior consultation with the General Secretary.
Probabilistic maturation reaction norms (PMRN) were used to investigate changes in the maturation of sardine cohorts and to discuss whether genetic effects, environmental effects or both are plausible explanations of maturation changes. Long‐term changes in maturation were described using samples collected from landings off the western Portuguese coast since 1947. Estimates of the length at 50% maturity (L50) were calculated for 44 years of the study period and were shown to be good proxies of PMRN midpoints of recruit spawners (Lp50 for age 0–1 fish). Data collected during acoustic surveys were used to explore density‐dependent and environmental effects on the maturation probability of recruit spawners since 1996. Sardine probability of maturing at a given length declined from the early 1950s to the late 1960s, corresponding to an increase of ca. 2 cm in both L50 and Lp50. This trend reversed abruptly in the early 1970s but became shallower or even halted in the early–mid‐1990s. Long‐term changes in maturation probability showed a direct relationship with fish condition in the growing season preceding maturation. The maturation trend also agrees with reported warming of Portuguese coastal waters since the 1970s while survey data support the relationship between Lp50 and SST in the growing season. The hypothesis of a genetic change, possibly induced by high fishing exploitation, could explain the positive trend in maturation probability since the 1970s but is not consistent with the earlier reversal of that trend. On the other hand, the consistency of trends in L50/Lp, condition, and temperature provides support to the hypothesis of direct environmental effects on sardine maturation probability.