The black goby Gobius niger Linnaeus, 1758 in the Marchica lagoon (Alboran Sea, Morocco): Ecological traits and site-related footprint

Fishes belonging to Gobiidae are well represented in the Marchica lagoon on the Moroccan Mediterranean coast, both in terms of species richness and abundance, with the black goby, Gobius niger Linnaeus, 1758, being the dominant species. The present study aims to examine (1) the spatial and temporal distribution of Gobius niger and its environmental drivers in the lagoon and (2) the potential lagoon-related footprint using morphometric, genetic and parasitological proxies. Systematic monthly sampling covering the whole lagoon basin performed between October 2015 and September 2016 revealed year-long presence of G. niger throughout the lagoon with significantly low densities in winter. The higher abundances were recorded in the shallow bottoms of the lagoon inner margins on a variety of substrates (mud, muddy-sand, sandy-mud and fine sand) mostly covered by macroalgae and/or seagrass meadows. Depth, that has to be seen as a variable that acts in concert with other factors such as temperature, vegetation cover and sediment as the Mean Grain Size, seems to be the most important predictor, explaining the distribution of G. niger in the lagoon, with a trend of increasing abundance towards shallower stations. Comparison of black goby populations from the Marchica lagoon with their counterparts from the adjacent Mediterranean coast of Morocco revealed that specimens caught at sea are of a bigger size compared to the lagoon population. Of the 180 gobies investigated, not a single one hosted the parasites we targeted in the parasitological approach, monogenean flatworms. The absence of population structuring, low genetic diversity and presence of common haplotypes indicate no apparent restriction in the gene flow between the two populations. Therefore, the observed morphometric differences seem to be due to the external environmental conditions rather than genetic differences. Gobius niger plays a key eco-trophic role by providing a link between benthic invertebrates and large predators. The shallow beds of the lagoon, where the species is abundant, are key habitats in the Marchica lagoon and need to be considered in all management plans aiming at the conservation of biodiversity and ecological processes.