[Cod. PGC2018-096256-B-I00 PROFISH]

Coastal habitats offer a variety of ecosystem services, and shallow areas in particular stand out for their value as nursery areas both for the commercial as well as the species targeted by recreational fisheries. However, the characteristics which determine why a habitat acts as a quality nursery area are largely unknown. It is widely assumed that the quality increases with the structural complexity, since it is related to an increased food availability and shelter. If true, human activity would reduce the value of the coastal zone as nursery, since it tends to reduce the structural complexity of habitats. Nonetheless, the most complex habitats can also accommodate more predators, thus the relationship between habitat complexity and its value to the nursery is unclear. In the coastal area of the Mediterranean, macroalgae of the genus Cystoseira have a high primary productivity, high morphological complexity and harbour rich invertebrate fauna. Accordingly, they are considered as high value nursery areas. However, unlike in the case of the seagrass beds, the data on the actual role of Cystoseira forests as nursery areas in such regions is largely absent. The lack of knowledge regarding this type of habitat is surprising considering the degree of its loss during the 1970s and 1980s. The few available studies have evaluated the quality of Cystoseria forests based on the abundance of juvenile fish, however without comparing it with those observed in other habitats and without taking into account other parameters such as the availability of prey, predator density or survival and growth of juveniles. Thus, it is necessary to implement other methodologies to assess the actual survival of juveniles in those habitats. Assessing the health of organisms is an increasingly used approach when assessing the habitat quality, but still little applied to the study of marine nursery habitats. This project aims to: a) evaluate the quality of different types of algal communities with varying structural complexity (ranging from barrens to Cystoseira forests) as nursery habitats, and b) determine the role of predator density on the nursery habitat quality. In order to achieve the aims, we will use a combination of: i) abundance indices, ii) behavioural observations and iii) estimates of the condition and growth of juveniles of several species in coastal fish habitats of different structural complexity, also manipulating the densities of predators as a result of different levels of protection from fishing (marine reserve vs. open areas). In the first phase, the response of growth rates, condition and health of juveniles will be determined in different habitat types as a result of the predator abundance. Those habitat parameters that positively correlate with the indices used will be assessed in the second phase under laboratory conditions. The effect of habitat quality will be evaluated in the absence/presence of predators to determine the value of the indices of fish health in conditions where interspecific relationships are controlled. This will be vital for the use of health indicators proposed as indicators of the quality of habitats.