Temporal and spatial dynamics of hake Merluccius merluccius recruitment in the Tyrrhenian and Ligurian Sea (Mediterranean)

The present doctoral thesis investigated fundamental aspects of temporal and spatial dynamics of hake (Merluccius merluccius) recruitment in the Tyrrhenian and Ligurian Sea (Mediterranean). We identified recruit and post-recruit as two well distinct phases of hake juvenile life stage according to a marked bathymetric segregation. Hake juveniles enter the post-recruit stage at a length range between 13.2 cm and 15.8 cm through a sharp migration from the margin of the shelf-break and upper slope to the shallower continental shelf waters. Spatial models provided accurate identification of the main areas of aggregation of hake recruits before their migration and spreading on the shelf. We found that nurseries in the Tyrrhenian and Ligurian Sea are characterised by some of the highest densities of recruits observed in the Mediterranean, suggesting these areas could play an important role for the dynamics of hake population at a larger scale far beyond the study area. The analysis of time series data for two different periods of the year demonstrated an inter-annual and seasonal stability in the spatial location of the main nursery areas and a strong association for specific habitats of the shelf-break. Together with these main nurseries, a certain number of secondary nurseries were detected only in certain years, but their variability and importance for the population remain to be addressed. Wide fluctuations in the abundance of hake recruits were found to be strongly influenced by both oceanographic and biological processes. Thermal anomalies in summer, characterised by high peaks in water temperature, revealed a negative effect on the abundance of recruits in autumn, possibly due to a reduction in hake egg and larval survival rates. Recruitment was also reduced when elevated sea-surface temperatures in summer were coupled with low levels of water circulation. Enhanced spring primary production related to late winter low temperatures are known to affect water mass productivity in the following months, and according to our results hake spring recruitment. We found a dome-shaped relationship between wind mixing in early spring and recruitment that was interpreted as an “optimal environmental window” in which intermediate water mixing level can play a positive role in phytoplankton displacement, larval feeding rate and appropriate larval drift. The temporal and spatial persistence of hake nurseries represented fundamental features for testing the effects of a spatial management of the population particularly during the highly vulnerable recruit phase. The effect of different levels of protection on recruits were tested through model simulations and compared with a more general reduction of fishing effort, providing unexpected results on the mature portion of the population.