Twenty years of monitoring reveal overfishing of bony fish stocks in the coastal national park Banc d’Arguin, in Mauritania

Along Africa’s western coast, many local communities rely on the ocean for their livelihood. Over the last decades, introductions of new fishing techniques along with globalizing trade have strongly changed local fishing practices. The Parc National du Banc d’Arguin (PNBA) in Mauritania had for centuries been subjected to an artisanal, low-impact, fishery. This fishing was exclusively oriented towards migratory bony fish species, mullet (Mugil cephalus) and meagre (Argyrosomus regius). Since the 1980s, these species have been replaced by illegal catches of internationally traded elasmobranchs (sharks and rays) and by non-migratory and relict species (resident) such as tilapias (Sarotherodon melanotheron) and catfishes (Arius sp.). To date, most monitoring and management efforts have been dedicated to evaluating changes in elasmobranch populations and less focus has been on bony fish species. Data from a fishery monitoring programme are used to analyse the trends in effort, catch and catch per unit of effort of bony fish species by fitting non-parametric generalized additive models to capture changes in the fish community over the last 20 years. Mullet and meagre became overfished early on, and the contribution of resident species (tilapias and catfishes) increased in the catches. Together with a pattern of increased effort on the traditionally targeted species, such a change in the catch could reflect a change in the fish community. These results call for the implementation of sustainable fishing practices within PNBA. We propose the need to implement closures of fisheries during the species’ breeding periods as well as the use of biological reference points such as the size at first capture and maximum sustainable yield targets for resident species.</p

Industrial fishing near West African Marine Protected Areas and its potential effects on mobile marine predators

Marine Protected Areas (MPAs) are increasingly implemented to facilitate the conservation of marine biodiversity and key-habitats. However, these areas are often less effective to conserve mobile marine species like elasmobranchs (i.e. sharks and rays). Industrial fishing near MPA borders possibly impacts vulnerable species utilizing these protected areas. Hence, we aimed to study spatiotemporal patterns of industrial fisheries near MPAs, in relation to the bycatch of elasmobranchs. Specifically, we analyzed the spatiotemporal fishing effort within the West African region, mapped fishing effort in the direct vicinity of the Parc National du Banc d’Arguin (Mauritania) and the Bijagós Archipelago (Guinea Bissau) and compared the seasonal overlap between elasmobranch bycatch and fishing effort near these MPAs. We combined Automatic Identification System data and local fisheries observer data, and determined fishing effort for each gear type and compared this with bycatch of elasmobranchs. We found that industrial fishing effort was dominated by trawling, drifting longlines and fixed gear types. Although no industrial fishing was observed within both MPAs, 72% and 78% of the buffer zones surrounding the MPAs were fished for the Banc d’Arguin and Bijagós respectively. Within the Banc d’Arguin buffer zone, trawling and drifting longlines dominated, with longlines mainly being deployed in fall. In the Bijagós buffer zone, trawling and fixed gears were most prevalent. Fisheries observer data for Mauritania showed that elasmobranch catches increased during the most recent sampling years (2016 to 2018). Elasmobranch catches within the waters of Guinea Bissau peaked in 2016 and decreased in the following two years. Seasonal patterns in elasmobranch bycatch within the waters of both countries are likely caused by increased catches of migratory species. Catches of rays peaked in May and June for Mauritania, and in October for Guinea Bissau. Shark catches were highest in February and July in Mauritanian waters, and in May and October in the waters of Guinea Bissau. Our study indicates that industrial fisheries near the border of ecologically important MPAs may have potentially major implications for ecosystem functioning by the removal of (migratory) predatory species

Fish and fisheries dynamics at Banc d'Arguin, Mauritania:Consequences for the intertidal food webs

The overfishing of marine resources is a global problem. This situation is even more alarming when it concerns the top-predator species and particularly the vulnerable species of elasmobranchs. These species play an important role in the ecological balance of marine ecosystems through their top-down controls of trophic levels. Traditionally, the fishing in the Banc d'Arguin ecosystem mainly targeted migratory species such as Mullet (Mugil cephalus) and Meagre (Argyrosmus regius), and only for subsistence. Since the mid-1980s, fishing shifted focus to elasmobranchs (sharks and rays) as a result of the temptations of the global market. The loss of these species will have had an impact on the structure of the entire food web as well as on the functioning of the ecosystem overall.In this thesis, I combined fishing data obtained from the long-term fisheries monitoring programme and ecological data from field experiments to understand the interplay between fishing activities and aspects of ecosystem functioning. The abundance indices of the main species involved in the Banc d'Arguin fishery have been showing steep declines. Therefore, most fish resources in this ecosystem are overfished as a result of the fishing down of the food web. In contrast to the negative impact of fishing, we laid bare a positive ecological impact of shorebird-probing on the growth of seagrass and hence primary productivity in the Banc d’Arguin ecosystem. I propose new and sincere intentions towards management steps leading to sustainable fishing practices. This includes a complete break with any form of exploitation of elasmobranchs fish within the Banc d'Arguin ecosystem

Growth and population structure of bloody cockles Senilia senilis at Banc d’Arguin and Bijagós with different environmental conditions and harvesting regimes

In addition to density dependent (i.e., food availability) and independent (i.e., abiotic gradients) forces, size-selective harvests of organisms often induce changes on its life history, demography, and reproduction. Shellfish are increasingly overexploited worldwide and the effect of size-selective harvest on their populations is unclear. The African bloody cockle Senilia senilis is a dominant bivalve in most West African coastal systems where it is an important source of protein. Here, we studied its population structure (abundance, biomass, and size) and life history traits (age, annual growth rate, and size for age) under contrasting harvesting conditions at the two largest intertidal systems of the region: Bijagós archipelago in Guinea Bissau (harvested) and Banc d’Arguin in Mauritania (unexploited). To account for local ecological context, we assessed environmental variables that could also affect the abundance and size of Senilia at these sites. We found that the Senilia population of Bijagós is significantly denser, older, with lower standing biomass and smaller-sized shells than the one from Banc d’Arguin. In the Bijagós, the annual growth is a threefold lower and the size for age is half of that at Banc d’Arguin. At both sites, Senilia densities and sizes were correlated with habitat quality indicators: mangrove and mudflat productivity at Bijagós and seagrass and sediment characteristics at Banc d’Arguin. The smaller-sized mature individuals, denser populations, and lower standing biomass at Bijagós compared to Banc d’Arguin are indications of a considerable size-selection pressure at Bijagós

A genetic cline shapes population structure of the round sardinella along its West African distribution

Small pelagic fishes play an important role in human-food security and marine food webs relationships. Among them, the round sardinella Sardinella aurita is an extremely valuable fish resource in West Africa. A suite of twelve microsatellite loci was developed ad hoc for this species using ddRADseq technology, and genotyped on some 1,300 individuals collected in the coastal area spanning from Morocco to Angola. The significant differentiation detected between the northernmost samples (from Morocco to Guinea) and the southernmost ones (from Liberia to Angola) was due to one single locus, which was strong and consistently flagged as a candidate outlier to positive selection, thus suggesting that local adaptation might be one of the drivers of the observed pattern. Furthermore, the signal due to this locus resided uniquely in the frequency of allele SauMS002_131, which displayed a remarkable latitudinal cline with the centre situated in the coastal waters off Sierra Leone. This allele frequency, starting around 47-63% in Morocco-Guinea and dropping to 0% in Gabon and southwards, accounts for the aforementioned North-South declining trend, and can be used as a meaningful tool to assist the outline of the population structure for this species in the West coast of Africa for management purposes.The information deposited here consists of two excel files: 1. Sardinella aurita_Microsatellites.xlsx. A set of twelve microsatellites was isolated for Sardinella aurita through the analysis of ddRADsequencing fragments. Markers were developed ad hoc using individuals from Morocco, Gabon and Angola. The file contains two spreadsheets. a. Optimized microsatellites: Refers to the twelve microsatellites genotyped in the current study. Detailed information about forward and reverse primers, annealing temperature, MgCl2 concentration and arrangement in multiplex reactions is provided. b. Sequences for microsatellites: Consists of 49 sequences containing motif repeats that would allow the eventual isolation of some extra markers. 2. Sardinella aurita_Genotypes.xlsx: Genotypic data obtained from the mentioned twelve microsatellite loci, which were genotyped on 1294 individuals sampled along the NW coast of Africa, from Morocco to Angola. Individuals were collected by trawl onboard the R.V. “Dr. Fridtjof Nansen”. a. Geographic & genetic information: Contains: ID, sampling year, survey code, sampling station with geographic coordinates, name of the sample, country it belongs to, and genotypic information at 12 microsatellites per individual. b. GenAlEx_Format: File ready for exportation into different formats