Gianni F et al. (2017), Threats to large brown algal forests in tempera...

ECB-ART-45641

Sci Rep 2017 Jul 20;71:6012. doi: 10.1038/s41598-017-06394-7.

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Threats to large brown algal forests in temperate seas: the overlooked role of native herbivorous fish.

Gianni F , Bartolini F , Pey A , Laurent M , Martins GM , Airoldi L , Mangialajo L .

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Canopy-forming algae are declining globally due to multiple disturbances. This decline has recently been on the increase due to the spread of some tropical herbivorous fishes. This new phenomenon has drawn attention to the effects of fish herbivory in temperate areas, which have been assumed to be negligible compared to that of invertebrates, such as sea urchins. In this study, the impact of a Mediterranean native herbivorous fish (Sarpa salpa, salema) was assessed on the canopy-forming seaweed Cystoseira amentacea var. stricta. Cystoseira amentacea forms belts in the infralittoral fringe of wave-exposed shores, which has so far been considered a refuge from fish herbivory. To test the effects of salema feeding on natural C. amentacea belts, an innovative herbivore deterrent device was conceived. Salema had a significant effect on C. amentacea by decreasing algal size, biomass and fertility, by up to 97%. The results suggest that the contribution of salema feeding to the loss of Cystoseira forests in the Mediterranean may have been overlooked. In addition, the analysis of temporal and spatial patterns of salema landings in the Mediterranean Sea suggests that salema abundance may have increased recently. Thus, along with invertebrate herbivory and anthropogenic stressors, fish herbivory may also represent a potential threat to algal forests in temperate areas.

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Genes referenced: impact LOC100887844 LOC100893907 LOC115923437 LOC115923516 LOC115925415

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	Figure 1. Study area. The two sampling sites are located in Villefranche Bay, French Riviera, NW Mediterranean Sea. Maps were made by using Adobe® Illustrator® software CS6.
	Figure 2. (a) Cystoseira amentacea belts in the infralittoral fringe; (b) the deterrent device DeFish used to deter fish grazing on C. amentacea; (c) schematic representation of the two vertical zones of the infralittoral fringe characterized by long and short C. amentacea branches; a protected plot with DeFish is also shown on the shore; (d) detail of the primary and secondary C. amentacea branches with fish bites (white arrows). Photos by Bartolini F. Figure 2c attributions: salema and Cystoseira illustrations (Tracey Saxby, Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/)).
	Figure 3. Size class distributions of Sarpa salpa. Percentage frequency of salema individuals per size recorded in the two visual census performed during the experiment.
	Figure 4. Fish herbivory. Number of fish bites/12.5 cm2 (mean plus SE calculated on all plots, n = 4) for each zone (High and Low) and treatment in the different months and at both sites. P: protected; C: control plots; AC: artefact control. The results of the pairwise tests on the factors Treatment and Zone are reported above the graph (see Table 1 and Supplementary Tables S1.1–2 for more details). Letters above the bars in June at Cap du Rubé indicate significant differences of the pairwise-tests on the interaction TrxZo (Table 1 and Supplementary Table S.1.1 for more details).
	Figure 5. Algal size. Algal length (mean plus SE calculated on all plots, n = 4) for each zone (High and Low) and treatment in the different months and at both sites. P: protected; C: control plots; AC: artefact control. The results of the pairwise tests on the factors Treatment and Zone are reported above the graph (see Table 1 and Supplementary Tables S2.1–2 for more details).
	Figure 6. Effect sizes of protection and zones based on the net growth potential. Protection (P: protected, AC: artefact control), Zones (H: High, L: Low). Effect sizes are significant if confidence intervals do not overlap zero. Rt: log-response ratio for each treatment.
	Figure 7. Biomass. Wet weight (g)/12.5 cm2 (mean plus SE calculated on all plots, n = 4) for each Zone (High and Low) and Treatment in June, at the two sites. P: protected; C: control plots; AC: artefact control. Letters above the bars indicate significant differences of the pairwise-test performed on the interaction TrxZo at Pointe du Rubé (see Table 1 and Supplementary Table S3.1 for more details). At Pointe de la Cuisse, the results of the pairwise tests on the factors Treatment and Zone are reported above the graph (see Table 1 and Supplementary Table S3.2 for more details).
	Figure 8. Fertility. Number of receptacles/12.5 cm2 (mean plus SE calculated on all plots, n = 4) for each zone and treatment in June. P: protected; C: control plots; AC: artefact control. Letters above the bars indicate significant differences of the pairwise-tests on the interaction TrxZo (see Table 1 and Supplementary Tables S4.1–2).
	Figure 9. Effect sizes of protection and zones based on the reproductive potential. Protection (P: protected, AC: artefact control), Zones (H: High, L: Low). Rt: log-response ratio for each treatment.
	Figure 10. Patterns of temporal variation in salema landings. Salema catches (in tonnes) are represented for 7 distinct basins (FAO sub-areas) and the Mediterranean as a whole from 1970 to 2014 (x-axes). Trend lines and the r-squared values are reported on each graph. Note different scales among catch axes.

References [+] :

Buonomo, Habitat continuity and stepping-stone oceanographic distances explain population genetic connectivity of the brown alga Cystoseira amentacea. 2017, Pubmed