Abstract
The nursery function of coastal habitats is one of the most frequently mentioned and recognized ecosystem services in the valuation of coastal ecosystems. Despite its importance our understanding of the precise habitat parameters and mechanisms that make a habitat important as a nursery area is still limited for many species. The study aimed to establish the importance of different algae morphotypes in providing shelter and food for juvenile coastal fish during the main settlement peaks, in early spring and late summer, in littoral rocky reef systems in the Northwestern Mediterranean. The results of our study showed strong seasonal differences in algae cover, composition and height between the two sampling periods. Overall, during spring the algae were well developed, while in late summer, both density and height, of most algae decreased considerably. Equally, prey biomass, in form of suitable sized invertebrate fauna associated to the algae, decreased. Accordingly, the shelter and food for the fish settling in this habitat during late summer were less abundant, indicating a mismatch between the observed presence of juvenile fish and optimal habitat conditions. Differences in prey densities were detected between algae morphotypes, with structurally more complex algae, such as Cystoseira spp. and Halopteris spp. consistently containing more prey, independent of season, compared to simpler structured morphotypes such as Dictoytales. The study furthermore related juvenile fish density to habitats dominated by different algae morphotypes. Out of the three-study species (Diplodus vulgaris, Symphodus ocellatus, Coris julis) only S. ocellatus showed a significant association with an algae habitat. S. ocellatus related positively to habitats dominated by Dictoytales which provided the highest cover during late summer but had the lowest prey densities. A strong association of this species with Cystoseira, as reported by other studies, could not be confirmed. Cystoseira was abundant within the study area but in a state of dieback, showing loss and reduced height of foliage, typical for the time of year within the study area. It is therefore likely that algae-fish associations are context-dependent and that several algae species may fulfil similar functions. We also discovered that prey biomass did not appear to have an important effect on juvenile abundances. Nevertheless, the availability of prey may influence juvenile fish condition, growth performance and ultimately long-term survival. We therefore suggest that future studies on habitat quality should also include, besides abundance, indicators related to the condition and growth of juveniles.
Highlights
The provision and value of nursery habitats by the coastal zone is one of the most frequently mentioned and recognized ecosystem services in the valuation of coastal marine ecosystems (Duarte, 2000; Jackson et al, 2001; Jackson et al, 2015)
Seagrass meadows and estuarine systems have been the focus of marine nursery habitat research (e.g., Heck, Hays & Orth, 2003; Seitz et al, 2013; Woodland et al, 2012; Ruiz-Frau et al, 2017), while studies that focus on littoral rocky reef systems appear less frequently in the literature
Algae morphotype composition and height across sampling seasons and islands Algae morphotype composition within sampling areas and season were compared by Principal Component Analysis (PCA) ordination of the dive transects sampled
Summary
The provision and value of nursery habitats by the coastal zone is one of the most frequently mentioned and recognized ecosystem services in the valuation of coastal marine ecosystems (Duarte, 2000; Jackson et al, 2001; Jackson et al, 2015). Spatially less extensive compared to seagrass meadows, littoral rocky reef systems harbour a wide variety of different algae species, providing shelter and food, in the form of associated fauna, to juvenile fish (Harmelin-Vivien, Harmelin & Leboulleux, 1995; Cheminée et al, 2013; Félix-Hackradt et al, 2014). In the Mediterranean, littoral rocky reef habitats are used by a variety of commercial and non-commercial species for part, or their entire life cycle (Harmelin-Vivien, Harmelin & Leboulleux, 1995; Guidetti, 2000; La Mesa et al, 2011; Félix-Hackradt et al, 2014; Cheminée et al, 2017).
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