Fish nursery value of algae habitats in temperate coastal reefs

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 contain more prey independent of season compared to simpler structured morphotypes such as many 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.

Typology of fish nurseries in shallow Mediterranean coastal zones: all habitats matter

“…Habitat and seascape tri-dimensional structure can be qualified by its heterogeneity and complexity 83,84 . Generally high quality habitats for juvenile fishes are recognized to be associated with high degrees of three-dimensional structuration 85 , in terms of both complexity 46,48,86,87 and/or heterogeneity 36 . Natural rocky habitats (RS) presented a high structural and biological complexity due to different macrophytes assemblages, and indeed supported the highest mean species richness and abundance of juvenile fishes whatever the period.…”

“…However, the higher abundance of juveniles in seagrass bed habitats and rocky substrates during the warm period could be related to the greater protection and food resources provided by the greater canopy height of Posidonia oceanica and macroalgae communities 45,46,48 . The role of highly complex Cystoseira forest canopies with regard to the composition of juvenile fish assemblage was well studied by Cheminée et al 45,46 , Cuadros et al 47 and Hinz et al 48 , who demonstrated that Symphodus spp., Labrus spp. and Serranus spp.…”

“…Numerous studies have been undertaken on the role of particular shallow coastal habitats as fish nursery sites in the Mediterranean Sea. One approach was to focus on one type of habitat, such as coastal lagoons 29,30 , soft bottoms 31,32 , Cymodocea nodosa meadows [33][34][35][36] , Posidonia oceanica beds [37][38][39][40][41] , shallow rocky reefs more or less colonized by macrophytes assemblages [42][43][44][45][46][47][48][49] and shallow heterogeneous rocky substrates 50,51 . Another approach was to focus on specific fish species such as the gilthead seabream Sparus aurata 52 , sparids of the genus Diplodus 25,53-57 , flatfishes such as Solea solea 58,59 , the dusky grouper [60][61][62][63] , or labrid species [64][65][66] .…”

Using diver-operated stereo-video to monitor juvenile fish assemblages in Mediterranean coastal habitats formed by macrophytes

“…Macroalgal forests and seagrass meadows provide numerous ecosystem services and have been proved to be key nursery habitats, essential for the life cycle maintenance of many coastal fishes in the Mediterranean Sea [3] [13]. Several studies suggest a positive relationship between macrophyte three-dimensional structural complexity and its key role in fish recruitment, by providing increased potential prey and shelter from predators [4] [10] [14] [18].…”

Ecosystem functioning of canopy and turf-forming algae: potential supply of invertebrate prey to pelagic consumers

“…Canopy-forming macroalgae are also colonized by a diverse and abundant fauna of mobile benthic invertebrates (Wikstrom & Kautsky 2007, normally dominated by amphipods, isopods, gastropods and polychaetes (Tano et al 2016), and constitute both reproduction sites and nursery habitats for many other larger species (Hinz et al 2019), where juveniles find effective shelter from predators and adequate foraging grounds (Edgar et al 2004). As such, canopy-forming algae constitute a semi-closed biodiversity loop, containing a self-sustained food-web, which include the main algal host and their epiphytic algal assemblages as a basal trophic level, direct mesoherbivore grazers and detritivores as intermediate levels and small fish and invertebrate predators as high within-canopy trophic levels.…”