Patterns of recruitment are a key factor explaining population dynamics. Nevertheless, recruitment studies are scarce for temperate shallow nearshore waters because research has been hampered in part by the lack of suitable sampling methods. Therefore, there is a poor understanding of the factors affecting recruitment variability in this type of environment, although this knowledge can be of major relevance for fisheries and MPA management. Aiming at understanding the small scale temporal patterns of recruitment of the commercially important Atlantic horse mackerel (Trachurus trachurus) at a nearshore rocky reef, recruits were caught weekly from late July to September 2011 with Standard Monitoring Units for the Recruitment of Reef Fishes (SMURFs) at the Arrábida Marine Park (Portugal). The importance of abiotic variables (SST, chlorophyll concentration, lunar phase) to explain recruitment and back calculated spawning patterns was investigated using Generalised Additive Models (GAM). Furthermore, early life history factors such as early growth rates were examined through otolith microstructure analysis. Our results showed a recruitment pattern with peak arrivals during new and last quarter moon and a decrease over the complete sampling period. Otoliths did not have a settlement mark but secondary growth cores could be observed. The Pelagic Larval Duration (PLD) was estimated to last 21-30 days. The relationship between standard length and age was best explained by the Gompertz growth curve. The estimated hatching dates displayed 3 clear monthly peaks, which occurred around first quarter and new moon. GAM confirmed this association and also revealed a link with temperature and chlorophyll. Between the 3 subcohorts there was a difference in growth rate and the number of secondary cores formed, with early spawned individuals growing faster and having more accessory cores. This study reveals that: I) SMURFs proved an excellent method to monitor recruitment of T. trachurus in nearshore environments; II) moon phase, chlorophyll and temperature seem to be important for the timing of spawning, although the mechanisms behind this are still unknown; III) moon phase is important in regulating the arrival of juveniles to the reef; IV) the shallow reef system is important for the early growth of this species. Hence, management measures in the MPA can potentially protect the juvenile phase of this important commercial fish species, thereby enhancing local fisheries. |
