Density dependence in post-recruitment processes of juvenile sparids in the littoral of the Mediterranean Sea

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While variability in recruitment of young reef fish is now recognised, the question remains how the variation in recruitment will contribute to determining assemblage structure. This has been the basis for an active debate about whether patterns in abundance are determined by the supply of planktonic larvae or whether they result from competition (for space, food, etc.) between potential recruits leading to density-dependent recruitment. We tested population regulation of newly recruited juveniles of Sparidae in the north-western Mediterranean Sea. Recruitment of three species of Sparidae was followed over a three-month period in each of two years to estimate the peak density of new recruits settling from the ocean, the rate of mortality occurring during the first three months after settlement, and the final density after three months. Sites and species were carefully chosen in order to avoid migration during the survey.

Initial density of recruited larvae of Diplodus spp. varied among species and sites from 0.045 to 3.8 individuals per metre of coastline. Mortality estimates also varied enormously. Between 24.3% and 99.4% of the initial population disappeared within three months. Our survey demonstrated that both post-settlement mortality (r = 0.847; p < 0.0001; n = 18) and final density (r = 0.818; p < 0.0001; n = 18) were correlated with density of recruitment and consequently that both pre-recruitment (larval phase) and post-recruitment processes regulated sparid populations of the Mediterranean Sea.

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