All shallow coastal habitats matter as nurseries for Mediterranean juvenile fish

Abstract

Coastal zones are ecosystems of high economic value but exposed to numerous disturbances, while they represent nurseries for many fish species, raising the issue of the preservation of their functions and services. In this context, the juvenile fish assemblages of all types of habitats present in shallow coastal zones were studied on the south-east coast of France using underwater visual censuses in warm (June-July 2014) and cold (April 2015) periods. A total of fourteen habitat types were characterized, which could be grouped into three broad categories, rocky substrates (natural and artificial), sedimentary bottoms with all levels of granulometry, and seagrass beds including Cymodocea nodosa and Posidonia oceanica meadows; the ecotones or interfaces between the three broad habitat categories were individualized as particular habitat types. The abiotic and biotic descriptors of the 14 habitat types individualized did not vary with time, except for a higher cover percentage and canopy height of macrophytes in the warm period, which increased the three-dimensional structure of some habitats. The taxonomic composition and density of juvenile fish assemblages were analyzed using both multivariate and univariate descriptors, after grouping the 57 fish species recorded into 41 well-individualized taxa. Juvenile fishes were recorded in all habitat types, with higher mean species richness and abundance during the warm than the cold period. The richest habitats in terms of both fish species richness and abundance were the natural rocky substrates and the interfaces between Posidonia beds and the other habitats. Although juvenile fish assemblage composition differed among habitat types and between periods, the most abundant fish species were Atherina sp., Sarpa salpa, Gobiidae, Symphodus spp., Pagellus spp. and several Diplodus species, which colonized 7 up to 14 different habitat types (depending on taxa) during their juvenile life. Most species settled in one or a few specific habitats but rapidly colonized adjacent habitats when growing. This study provided evidence of the role of all types of shallow coastal habitats as fish nurseries and their varying pattern of occupation in space and time by the different juvenile stages. It highlighted the importance of the mosaic of habitats and interfaces for the complete development of all juvenile life stages of fishes, and for the preservation of a high diversity of coastal fish assemblages and fisheries resources in the Mediterranean Sea.

Figure 1

Map of the studied area: 42 stations were sampled along the 100 km stretch of the studied portion of the French Provence coastline (black rectangle). The map was drawn using free and open source software Inkscape 0.91 (https://inkscape.org/en/) and QGIS 2.14 (http://www.qgis.org/). Map was drawn by authors using online Standard tile layer from OpenStreetMap data as background model (© OpenStreetMap contributors), available under ODbL licence (CC-BY-SA) at http://www.openstreetmap.org/.

Figure 2

Principal coordinate analysis (PCoA) ordination plot of centroids of habitat descriptor assemblages according to habitat types. Correlation vectors (Spearman) of descriptors are plotted (correlations > 0.2). RS: Rocky substrates; AR: Artificial rocky reefs; SB: soft bottoms; CY: Cymodocea nodosa beds; PO: Posidonia oceanica beds; POBR: Posidonia barrier reef flat; POEX: Posidonia barrier reef outer slope; POIN: Posidonia barrier reef inner slope; POCY: Barrier reef lagoon with Cymodocea; PODM: Posidonia dead matte; IPR: Interface Posidonia/Rocky substrates; IPS: Interface Posidonia/Soft bottoms; IPM: Interface Posidonia/Dead matte; IRS: Interface Rocky substrates/Soft bottoms.

Figure 3

Proportion of each observed taxa in the total abundance of juvenile fishes recorded in all samples and habitats combined; E. = Epinephelus, L. = Lithognathus, S. = Spondyliosoma. Period(s) of observation of each taxa and total number of taxa observed per period are indicated with colored boxes.

Figure 4

Mean taxa richness (± SE) of juvenile fishes per 10 m² in shallow coastal juvenile habitats for both periods (Cold and Warm). RS: Rocky substrates; AR: Artificial rocky reefs; SB: soft bottoms; CY: Cymodocea beds; PO: Posidonia oceanica beds; POBR: Posidonia barrier reef flat; POEX: Posidonia barrier reef outer slope; POIN: Posidonia barrier reef inner slope; POCY: Barrier reef lagoon with Cymodocea; PODM: Posidonia dead matte; IPR: Interface Posidonia/Rocky substrates; IPS: Interface Posidonia/Soft bottoms; IPM: Interface Posidonia/Dead matte; IRS: Interface Rocky substrates/Soft bottoms. Main habitat categories are indicated in grey.

Figure 5

Mean (± SE) total density (without Atherinidae and larvae) of juvenile fishes among habitats for both periods (Cold and Warm). RS: Rocky substrates; AR: Artificial rocky reefs; SB: soft bottoms; CY: Cymodocea beds; PO: Posidonia oceanica beds; POBR: Posidonia barrier reef flat; POEX: Posidonia barrier reef outer slope; POIN: Posidonia barrier reef inner slope; POCY: Barrier reef lagoon with Cymodocea; PODM: Posidonia dead matte; IPR: Interface Posidonia/Rocky substrates; IPS: Interface Posidonia/Soft bottoms; IPM: Interface Posidonia/Dead matte; IRS: Interface Rocky substrates/Soft bottoms. Main habitat categories are indicated in grey; “nd” = no data available.

Figure 6

Mean (± SE) juvenile density of each taxa in juvenile habitats for both periods (Cold and Warm). Atherinidae and larvae, as well as the 15 least abundant taxa, were removed for a clearer representation. Note that vertical axes display different scales. S. cinereus = Symphodus cinereus; S. viridensis = Sphyraena viridensis; D. labrax = Dicentrarchus labrax; Details of taxa are given in Table S2. For each period, comparisons of juvenile fish assemblages between juvenile habitats (pairwise tests results) are given (treatments that share at least one lower case character do not significantly differ).

Figure 7

Mean size (TL cm ± SD) of some fish species juveniles settling in Mediterranean shallow coastal habitats. D. = Diplodus; Oblada = Oblada melanura; Pagellus = Pagellus spp.; Sarpa = Sarpa salpa. Warm: warm period (summer 2014); Cold: cold period (spring 2015). Results of t-test for difference in mean size of a given species according to period: *P < 0.05; **P < 0.01; ***P < 0.001.