Northern Alboran Sea Research Articles

Spatio-temporal distribution of demersal fish species in the northern Alboran sea and Gulf of Vera

Spatio-temporal distribution of demersal fish species in the northern Alboran sea and Gulf of Vera

Incidence of fisheries impact and purification treatment in the survival rate of wedge clams caught in the northern Alboran Sea

Incidence of fisheries impact and purification treatment in the survival rate of wedge clams caught in the northern Alboran Sea

Effects of mechanized dredging targeting Chamelea gallina, striped venus clams, on the associated discards in the northern Alboran Sea (Western Mediterranean Sea)

Mechanized dredging impact on discards was assessed along the northern Alboran Sea (W Mediterranean Sea). Data from 101 dredging operations were analysed for contrasting spatial and seasonal variability of damage, with the use of a three-level damage scale. 4.5% of discarded individuals displayed intermediate damage, whereas 11.3% displayed severe damage. Echinoderms displayed the highest level of damage (~75% of total collected individuals) and Echinocardium cf. mediterraneum was the most susceptible discarded species (85% with severe damage), followed by bivalves (7.3%) and crustaceans (3.3%). The target Chamelea gallina showed a low proportion of damaged individuals, probably due to their thick protective shell, which promotes the survival of discarded undersized target individuals. Spatial differences in damage level on discards were linked to some gear characteristics, to the higher amount of gravels and to longer tow durations, whereas damage was generally higher in cold months and partly related to higher quantities of hard shelled molluscs, in both cases increasing the abrasion and damage to retained organisms. Data suggest that dredges with a lower number of narrower iron teeth and towed for a shorter time could decrease the damage rate in discards of this fishery. A spatial management plan based on the type of grounds would be useful in order to improve efficiency of these fisheries and minimize their impact to soft bottoms with different commercial catches and biological communities.

Time Variability Patterns of Eutrophication Indicators in the Bay of Algeciras (South Spain)

In the Bay of Algeciras (BA), intensive urban and industrial activityis underway, which is potentially responsible for the release of significant quantities of nutrients. However, the assessment of the impact of these discharges is complex. Nutrient concentration in the surface layer is per se strongly variable due to the variability associated with the upwelling of nutrient-enriched deep Mediterranean water (MW), which in turn is regulated by atmospheric forcing. The aim of this study is to determine the effects of changes in the upwelling intensity on the load of nitrate and phosphate in the BA and to appraise their impact on chlorophyll a variability. Based on this analysis, the possible influence of the nutrients released from land-based sources is indirectly inferred. Data and samples collected during nine research cruises carried out in different seasonal cycle periods between 2010 and 2015 in the BA were analysed. The vertical variation of temperature and salinity indicates that the MW upwelling was favoured in spring, as occurred in other coastal areas of the northern Alboran Sea. However, principal component analysis conducted on physical and chemical data reveals that shifts in nutrients and chlorophyll a in the euphotic layer are poorly explained by changes in the upwelling intensity. Furthermore, during some of these research surveys (particularly in summer), chlorophyll a concentrations were higher in the BA as compared to a nearby coastal area also affected by MW upwelling. Scarce information about land-based pollution sources precludes quantitative analysis of the impact of nutrient loads on water quality; however, the available data suggest that the main source of allochthanous inorganic nitrogen over the period 2010–2015 in the BA was nitrate. Therefore, it is reasonable to hypothesize that the high concentrations of nitrate and chlorophyll a in BA in summer are a consequence of those discharges. Our study highlights the need of more exhaustive inventories of sewage and river discharges to adequately rate their impact in the BA.

Molluscs collected with otter trawl in the northern Alboran Sea: main assemblages, spatial distribution and environmental linkage

Molluscan assemblages of circalittoral and bathyal soft bottoms of the northern Alboran Sea were studied using an experimental otter trawl. Samples of fauna were collected from 190 hauls during four MEDITS surveys carried out in spring between 2012 and 2015 at depths ranging from 30 to 800 m. Measurements of water column variables (temperature and salinity) and sediment samples were taken in the same locations where faunistic sampling was carried out. A total of 101 species grouped in 55 families were recorded. Cephalopods were the most abundant group, with Abralia veranyi dominating in abundance, Octopus vulgaris in biomass and Illex coindetii and Todarodes sagittatus being the most frequently collected species. Multivariate analyses carried out separately with abundance data of demersal species, benthic species, bivalves, gastropods and cephalopods as well as of all molluscs, generally resulted in three main molluscan assemblages corresponding to a shelf assemblage (30-200 m depth), an upper slope assemblage (201-350 m depth), and a middle slope assemblage (351-800 m depth). PERMANOVA test revealed that significant differences of different groups of molluscs in relation to depth were more acute than to geographical sectors of the Alboran Sea (eastern, central, eastern and insular sectors). Abundance, biomass and species richness decreased with depth with a clear dominance of cephalopods in the slope. Significant geographical differences were mainly detected for demersal and benthic species as well as for gastropods and cephalopods between the insular sector (Alboran Island) and the western and eastern continental sectors. Both depth and temperature were the most influencing variables in the different CCA analyses using datasets of molluscs with different life styles and from different classes, but sedimentological variables displayed a more acute significant relationship with the benthic molluscs than with the demersal ones.

Exploitation status and stock assessment of the smooth clam Callista chione (Linnaeus, 1758) in the northern Alboran Sea (GSA01-W Mediterranean Sea)

The smooth clam, Callista chione (Linnaeus, 1758), is a venerid bivalve widely appreciated in southern Spain where it represents the top commercial bivalve species in terms of landings and economic value. In this area, a total of 223 artisanal boats (68% of the artisanal fleet) are involved in shellfishing targeting bivalve molluscs, including the smooth clam. The artisanal mechanised dredging that targets C. chione in the northern Alboran Sea is described and the current exploitation status of its populations is analysed. A surplus-production model was run using ASPIC and used to assess the temporal variation in the levels of fishing for this bivalve throughout the study period (2002-2015), as well as to suggest conservation reference points that could guarantee the sustainable exploitation of this resource. During the study period, the maximum C. chione catch was registered in 2003 (306 t) and the minimum in 2006 (93 t). The ASPIC model for C. chione stock suggests that a Maximum Sustainable Yield (MSY) of 216 t could be produced from a total stock biomass of 983 t (Bmsy) at a fishing mortality rate of total biomass of 0.22 (Fmsy), with B/Bmsy and F/Fmsy values of 1.34 and 0.82, respectively, indicating that the stock is approaching good status.

The Atlantic–Mediterranean ecological connection: a study on decapod larval communities

Larval dispersal mechanisms, although significantly studied, are far from being completely resolved. Local studies are needed to clarify key interactions between individual life cycles and transport processes. The Atlantic-Mediterranean connection through the Strait of Gibraltar is a particularly important area to explore these processes, as a hotspot for detecting species fluxes and/or invasions between several Large Marine Ecosystems. However, the area dynamics and the mechanisms used by larvae to cope with the system hydrological conditions are scantly explored.Summer data on decapod larvae and ocean water masses from an extended area of the southern Portugal, Gulf of Cadiz, Strait of Gibraltar and Alboran Sea were analyzed, and the larval assemblages and their potential retention/dispersal mechanisms were explored. Different larval assemblages were linked to the hydrological conditions of each basin. Shelf width differences and its influence in along/cross-shore transport were the main drivers of decapod distribution, acting as biological barriers. Larvae of mesopelagic species dominated the northern Alboran Sea innershelf, highly influenced by offshore currents. Here, nearshore processes were limited to surface waters, while in the Gulf they extended more deeply and to the outer shelf. Results on species ecological traits can easily be applied to oceanographically similar areas of world coastal regions and could be used for further development of ecological modelling studies.

Populations of Commercial Molluscs within a Highly Biodiverse Marine Protected Area of the Northern Alboran Sea (W Mediterranean): Preferential Habitats, Seasonal Dynamics and Importance for Artisanal Fisheries

Seasonal dynamics and preferential sedimentary habitats of commercial molluscs have been studied in soft bottoms within the limits of the Special Area of Conservation (SAC) “Calahonda” (S Spain). This SAC harbours a high diversity of invertebrate communities, and it is located in a very touristy area of the littoral of Malaga and within the limits of one of the most productive shellfish fishing areas of the NW Alboran Sea. Here, the artisanal fishery targeting molluscs is a locally important activity because some species (e.g. Chamelea gallina, Callista chione, Donax trunculus) are highly demanded by locals and tourists and therefore reaching high commercial value. More than 200 molluscan species inhabit the soft bottoms of the SAC “Calahonda”, from which eight species collected in this study have economic value. The bivalve C. gallina was the most abundant and frequent commercial species, mainly at shallow fine and medium sand bottoms, followed by C. chione at coarse sand bottoms. Some species displayed higher abundance values of large or small size individuals at different depths, and most of them showed maximum abundances in spring and summer. Many invertebrate species associated with these soft bottoms are potentially very sensitive to dredging activities, especially echinoderms, decapods and cnidarians. Further research regarding the potential impact of fisheries on these assemblages would be desirable, especially considering the high biodiversity existing within this SAC and the increase of tourists in the last years.

Modelling trawling discards of the Alboran fisheries in the Mediterranean Sea

The ecosystem processes underlying the discard rates of the demersal otter-trawl fishery in the Alboran Sea, Western Mediterranean are investigated. We evaluate whether environmental and climatic variability, measured from operational oceanographic data, allow assessment of discarded species abundances. Monthly data collected by onboard fishery observers during 2011 and 2012 in the Northern Alboran Sea served as a case study of the trawling fishery’s dynamics. The study area was split into west and east zones to account for differences in the Atlantic and Mediterranean Sea water influences, for distinct geomorphology and for species composition of catches. Sixteen species divided in three groups were analysed: species without discards, those partially discarded and those completely discarded. Applying Gaussian Generalized Additive Models (GAM), we tested the relationships of standard oceanographic data products and climatic variability with discard abundances. These variables were sea surface temperature (SST) and chlorophyll a (chl-a) from satellite imagery, as indicators of environmental conditions, and the North Atlantic Oscillation index (NAO) as a climatic indicator. Models were applied for target and bycatch species partially discarded. These were the following species: gadoid like European hake (Merluccius merluccius), Blue Whiting (Micromesistius poutassou), Seabreams (Pagellus acarne, and Pagellus erythrinus), and Mackerels (Trachurus mediterraneus, Trachurus picturatus and Trachurus trachurus). Surface seawater temperature was a relevant variable in coastal and eastern, more haline Mediterranean waters. Climatic variability represented by NAO was useful for assessing discard rates in the more productive western zone. Chlorophyll a showed less relevance for discard prediction models, but it was more related in the coastal zones. Including oceanographic data products and climatic patterns in GAM models of fishery discards provides a new perspective that may improve our predictive capabilities for management decisions.

Contrasting evolutionary patterns in populations of demersal sharks throughout the western Mediterranean

The spotted catshark (Scyliorhinus canicula) and the blackmouth catshark (Galeus melastomus) are demersal sharks showing a distinct bathymetric distribution in the western Mediterranean. Together, both species represent more than 85% of the total abundance of demersal chondrichthyans in this Mediterranean basin. Our study provides a complete analysis of the genetic population structure, connectivity and demographic history of both species. Sampling was performed across four geographical subareas (GSAs) established by the General Fisheries Commission for the Mediterranean in the western Mediterranean: the northern Alboran Sea (GSA01), Alboran Island (GSA02), Balearic Islands (GSA05) and northern Spain (GSA06). Three mitochondrial fragments were analyzed for both species, and 12 microsatellite loci for S. canicula. We found contrasting patterns of population structure and connectivity in both species. Scyliorhinus canicula displayed significant genetic differences and low connectivity between some GSAs corresponding to different sub-basins (Alboran vs. Balearic). In contrast, G. melastomus showed absence of a population structure and high connectivity between GSAs. These findings are in accordance with the fact that both species exhibit different dispersal behaviors, which leads to distinct bathymetric distributions. Contrasting demographic histories were also identified: Scyliorhinus canicula revealed a recent stable population, with evidence of bottlenecks in the past, which may be related to Pleistocene glacial periods; whereas G. melastomus showed a recent population expansion. Altogether, our findings indicate a mismatch between fishery subareas and population structure for both sharks, which must be considered for fisheries management purposes.

Short comment about the octopus life cycle in the northern Alboran Sea (western Mediterranean Sea)

Octopus vulgaris is a plastic species capable of adapting to different environmental conditions. The different phases of its life cycle seem to be highly influenced by such environmental variables. Therefore the reproductive season and the length of its embryonic and planktonic phases can differ substantially for different geographical areas. The Mediterranean Sea is one of the regions were the octopus life cycle is not yet well known. Biological data in the Alboran Sea (Western Mediterranean) in the present work show the existence of migration movements to coastal waters for the breeding season during spring and mainly during summer. Water temperature measurements show that the length of the embryonic and paralarval phases are around 120 days and new individuals incorporate to fisheries from autumn to winter. These results coincide with landings time series allowing the full description of the O. vulgaris life cycle.

Spatio-temporal patterns and environmental controls of small pelagic fish body condition from contrasted Mediterranean areas

Small pelagic fish are among the most ecologically and economically important marine fish species and are characterized by large fluctuations all over the world. In the Mediterranean Sea, low catches and biomass of anchovies and sardines have been described in some areas during the last decade, resulting in important fisheries crises. Therefore, we studied anchovy and sardine body condition variability, a key index of population health and its response to environmental and anthropogenic changes. Wide temporal and spatial patterns were investigated by analyzing separately data from scientific surveys and fisheries in eight Mediterranean areas between 1975 and 2015.Results showed that anchovy and sardine body condition as well as maximum size in some areas sharply decreased in most Mediterranean areas along years (except in the Northern Alboran Sea). Despite this general pattern, well-marked environmental differences between sub-regions were highlighted by several analyses and variations in body condition were not found to be homogeneous over all the Mediterranean Sea. Further, other analyses revealed that except for the Adriatic where major changes towards a lower body condition were concomitant with a decrease in river runoffs and chl-a concentration, no concomitant environmental regime shift was detected in other areas.Together, these analyses highlighted the current poor body condition of almost all small pelagic fish populations in the Mediterranean. Yet, global environmental indices could not explain the observed changes and the general decrease in condition might more likely come from regional environmental and/or anthropogenic (fishing) effects. A prolonged state of poor fish body condition, together with an observed reduced size and early age-at-maturity may have strong ecological, economic and social consequences all around the Mediterranean Sea.

The first record of Drymonema dalmatinum from the northern Alboran Sea (western Mediterranean)

The first record of Drymonema dalmatinum from the northern Alboran Sea (western Mediterranean)

Discard analysis and damage assessment in the wedge clam mechanized dredging fisheries of the northern Alboran Sea (W Mediterranean Sea)

The composition and structure of discard, as well as the damage caused to target and non-target species, was analyzed in 95 commercial fishing hauls carried out between March 2013 and March 2014 by the mechanized dredging fleet targeting wedge clam (Donax trunculus) in the northern Alboran Sea (southern Spain). A large proportion of the catch (ca. 42% weight) is discarded, mainly wedge clam undersized individuals and non-commercial molluscs, decapods and echinoderms. Total discards within the study period showed a high species richness, with a total of 84 taxa discarded, and biomasses ranging from 337.1 to 8532.5g haul−1 (1579.9±157.9g haul−1, mean±SE), with maxima in spring. The composition, structure and biomass of discards displayed significant differences among study sites due to differences in the benthic communities of the fishing grounds. Individuals displaying intermediate (intermediate survival chance) and severe (low survival chance) damage in the discard displayed higher values when gravel and bioclastic material was more abundant in the catch. Overall, 15% of the discarded individuals displayed any type of damage, from which 12% were severely damaged, and with Echinoderms displaying the highest mean proportion of damaged individuals (79.5±2.9% of individuals). The target species displayed a high proportion of undamaged individuals to fishing impacts, which may represent an important factor for the maintenance of populations of this commercial species on the fishing grounds.

Exploring diversity and distribution of demersal fish species from the Northern Alboran Sea and Gulf of Vera (Western Mediterranean Sea)

Exploring diversity and distribution of demersal fish species from the Northern Alboran Sea and Gulf of Vera (Western Mediterranean Sea)

Biodiversity and biogeographical patterns of molluscan assemblages in vegetated and unvegetated habitats in the northern Alboran Sea (W Mediterranean Sea)

This paper evaluates the influence of habitat heterogeneity, inter-habitat connectivity, and biogeograhical patterns in the context of a biodiversity hotspot in an Atlantic-Mediterranean marine transition area. We analyzed some biological and ecological aspects of molluscan assemblages inhabiting different habitats located in the northwestern Alboran Sea, in order to elucidate the inter-habitat connectivity among coastal habitats, and to understand the contribution of each habitat to the integrity of the infralittoral ecosystem. The assessment of molluscan assemblages was performed by combining information regarding (1) feeding guilds, to analyze trophic strategies; (2) microhabitat preference, to study the link between habitat heterogeneity with biodiversity; and (3) biogeographical affinities, to identify distributional patterns of molluscs. A total of 354 molluscs were identified, with vegetated habitats harbouring the highest number of species. A wide variety of trophic strategies were observed, with infaunal filter feeders, specialized carnivores living on sessile invertebrates and microalgal grazers dominating the assemblages. Most of the species have a wide distribution while few strictly Mediterranean species have been collected. The presence of subtropical species highlights the uniqueness of this area in European seas. The high β-diversity value obtained is indicative of high habitat heterogeneity and a high amount of uncommon species, some of them displaying their sole European populations. The different assemblages enrich the global taxocoenosis through complementarity, with an important inter-habitat connectivity reflected by the different temporal ordination of samples according to both micro-habitat affinities and feeding guilds. This work highlights the balanced contribution of soft and vegetated bottoms as reservoirs of biodiversity.

A new regional algorithm for estimating chlorophyll-a in the Alboran Sea (Mediterranean Sea) from MODIS-Aqua satellite imagery

ABSTRACTThe utility of three different algorithms for retrieving surface chlorophyll-a values from satellite images of MODIS-Aqua is tested in the northern Alboran Sea. The available global algorithm to calculate chlorophyll-a from reflectance of MODIS-Aqua (OC3M) overestimates the surface chlorophyll-a in the study area. Another regional algorithm specifically developed for the Mediterranean Sea (MedOC3) improves the estimates although the best outcome is obtained with OC5, which was developed for Atlantic coastal waters. The three tested algorithms perform worse at in situ chlorophyll-a concentrations higher than 1 mg m−3 and exhibit uncertainty levels higher than 35% for this range of concentrations. A new algorithm (ALBOC3) is proposed which produces a good estimation of the in situ chlorophyll-a for the whole range of concentrations normally registered in the study area (0.1–3.5 mg m−3). We hypothesize that the particular bio-optical features of the northern Alboran Sea phytoplankton explain the poor functioning of the published algorithms that have been tested in this work.

Controls of picophytoplankton abundance and composition in a highly dynamic marine system, the Northern Alboran Sea (Western Mediterranean)

The Alboran Sea is a highly dynamic basin which exhibits a high spatio-temporal variability of hydrographic structures (e.g. fronts, gyres, coastal upwellings). This work compares the abundance and composition of picophytoplankton observed across the northern Alboran Sea among eleven cruises between 2008 and 2012 using flow cytometry. We evaluate the seasonal and longitudinal variability of picophytoplankton on the basis of the circulation regimes at a regional scale and explore the presence of cyanobacteria ecotypes in the basin. The maximal abundances obtained for Prochlorococcus, Synechococcus and picoeukaryotes (12.7×104, 13.9×104 and 8.6×104cellsmL−1 respectively) were consistent with those reported for other adjacent marine areas. Seasonal changes in the abundance of the three picophytoplankton groups were highly significant although they did not match the patterns described for other coastal waters. Higher abundances of Prochlorococcus were obtained in autumn-winter while Synechococcus and picoeukaryotes exhibited a different seasonal abundance pattern depending on the sector (e.g. Synechococcus showed higher abundance in summer in the west sector and during winter in the eastern study area). Additionally, conspicuous longitudinal gradients were observed for Prochlorococcus and Synechococcus, with Prochlorococcus decreasing from west to east and Synechococcus following the opposite pattern. The analysis of environmental variables (i.e. temperature, salinity and inorganic nutrients) and cell abundances indicates that Prochlorococcus preferred high salinity and nitrate to phosphate ratio. On the contrary, temperature did not seem to play a role in Prochlorococcus distribution as it was numerically important during the whole seasonal cycle. Variability in Synechococcus abundance could not be explained by changes in any environmental variable suggesting that different ecotypes were sampled during the surveys. In particular, our data would indicate the presence of at least two ecotypes of Synechococcus: a summer ecotype widely distributed in the whole Alboran Sea and a winter ecotype adapted to lower temperature and higher nutrient concentration whose growth is favoured in the eastern sector.

Exploring diversity and distribution of demersal fish species from the Northern Alboran Sea and Gulf of Vera (Western Mediterranean Sea)

Exploring diversity and distribution of demersal fish species from the Northern Alboran Sea and Gulf of Vera (Western Mediterranean Sea)

Preliminary characterization of echinoderm assemblages in circalittoral and bathyal soft bottoms of the northern Alboran Sea

Preliminary characterization of echinoderm assemblages in circalittoral and bathyal soft bottoms of the northern Alboran Sea