The present study describes for the first time the spatial distribution of five macrourid species throughout the Mediterranean Sea and analyses depth, geographical and time-related trends regarding their abundance, biomass and mean fish weight. The data were collected as part of the MEDITS annual bottom trawl survey carried out by several European Mediterranean countries from 1994 to 2015, using the same standardized gear and sampling protocol. The most represented species in terms of abundance and biomass was
El presente estudio describe por primera vez la distribución espacial de cinco especies de macrúridos a lo largo del Mediterráneo en su vertiente europea, analizando las tendencias batimétricas, geográficas y temporales de la abundancia, la biomasa y el peso medio de las especies. Los datos utilizados provienen de las campañas de arrastre de fondo anuales, MEDITS, desde 1994 a 2015, llevadas a cabo por los países mediterráneos europeos utilizando un arte de arrastre y un protocolo de muestreo estandarizado. La especie más representativa en términos de abundancia y biomasa fue
The Macrouridae are one of the most dominant fish families in deep-sea habitats due to their high number of species and their positive contribution to the global biomass of ecosystems (
Macrourid fisheries occur on the upper and middle continental slopes, either as by-catch (most common) or as target species (
In the Mediterranean Sea, the family Macrouridae includes eight species belonging to five genera
The Mediterranean is a semi-enclosed sea separated from the Atlantic Ocean by a sill in the Strait of Gibraltar, with a high degree of environmental stability below 200 m depth in terms of temperature and salinity (
Comprehension of the spatio-temporal patterns in the distribution of benthopelagic fauna, as well as of the factors controlling them, is a major ecological challenge. Due to its peculiarities, the Mediterranean Sea is an optimal reference for examining the spatio-temporal patterns of species distribution and for testing the influence of possible system drivers. In the Mediterranean Sea, several studies have focused on biological and distributional aspects of macrourids, but only restricted to certain areas (e.g.
Catch data (abundance and biomass) of five Macrouridae species (
For each haul, the number and weight of individuals belonging to the Macrouridae species were standardized to one-hour towing to calculate both species abundance (number of individuals per hour of towing time [ind h−1] and biomass [kg h−1]). The mean fish weight was obtained for each species by dividing biomass by abundance. Data were pooled in a matrix of species abundance, biomass and mean weight according to each haul (species vs haul). Frequency of occurrence of each species (F) was expressed as a percentage and was calculated, for the whole area, as the ratio of the number of occurrences of a species to the total number of hauls and, for each GSA, as the ratio of the number of occurrences of a species to the total number of hauls per GSA. Since the abundance of the macrourids studied was negligible at depths shallower than 200 m, further analysis was confined to bathymetric strata deeper than 200 m.
The centre of gravity (COG) was computed for abundance data in order to describe the mean bathymetric distribution of each species and to indicate the depth interval in which the species reaches its maximum abundance (
Generalized additive models (GAMs) were applied to assess the bathymetric, geographic and temporal effects on abundance, biomass and mean weight of species by haul. Year was considered as a factor. A one-dimensional smoother was used to investigate the bathymetric effect, while a two-dimensional smoother was used to account for the geographic effect, combining latitude and longitude. The logarithmically transformed values (log[x+1]) of abundance and biomass were used in order to ensure a Gaussian distribution of the residuals. For the selection of the best model for each response variable, minimization of the Akaike information criterion (AIC) was applied. The complete applied model for each response variable (RV, log-transformed values of abundance and biomass, and mean weight) and each Macrouridae species was as follows:
RV=factor (Year)+s(Depth)+g(Latitude, Longitude)+ɛ,
with s and g as the univariate and the bi-variate smoothers, respectively, and ɛ denoting the Gaussian error term. The package mgcv in R (
Dynamic factor analysis (DFA) was used to identify common trends in standardized abundance data series among GSAs. DFA is a multivariate analysis belonging to dimension reduction techniques; it is designed for relatives in which a set of time series is modelled as a linear combination of underlying common trends, factor loadings and error terms to explain the temporal variability. Correlation of observation errors was modelled using different error matrices (
Scientific names for species followed the nomenclature of the World Register of Marine Species (
Among the five macrourids investigated,
GSA | 1 | 2 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 16 | 18 | 19 | 20 | 22 | 23 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Depth range sampled | 200-798 | 304-800 | 210-756 | 200-797 | 213-800 | 260-582 | 200-775 | 200-692 | 200-759 | 200-799 | 200-746 | 200-774 | 236-800 | 200-791 | 200-590 |
Depth range | 215-776 | 304-800 | 215-749 | 218-761 | 213-800 | 263-582 | 245-730 | 235-692 | 210-693 | 228-784 | 246-706 | 218-736 | 271-800 | 200-716 | 217-590 |
F(%) | 77 | 73 | 46 | 45 | 73 | 77 | 38 | 45 | 35 | 65 | 59 | 47 | 59 | 30 | 23 |
Depth range | 245-732 | 358-571 | 376-756 | 275-797 | 341-800 | 377-582 | 302-775 | 241-692 | 245-759 | 204-793 | 246-732 | 240-772 | 301-663 | 261-761 | 300-590 |
F(%) | 13 | 5 | 56 | 45 | 44 | 62 | 61 | 79 | 64 | 81 | 58 | 71 | 37 | 32 | 20 |
Depth range | 235-798 | 319-800 | 354-756 | 279-797 | 659-800 | --- | 315-677 | 382-626 | 396-759 | 380-793 | --- | --- | --- | --- | --- |
F(%) | 63 | 75 | 51 | 36 | 3 | 0 | 8 | 1 | 7 | 5 | 0 | 0 | 0 | 0 | 0 |
Depth range | --- | --- | --- | --- | 361-800 | 323-582 | 298-775 | 304-676 | 222-699 | 316-791 | 220-732 | 240-774 | 326-663 | 205-791 | --- |
F(%) | 0 | 0 | 0 | 0 | 38 | 37 | 34 | 51 | 9 | 55 | 54 | 66 | 21 | 22 | 0 |
Depth range | 326-798 | 509-800 | 511-755 | 386-797 | 315-800 | 400-569 | 297-756 | --- | 326-759 | --- | 248-732 | 246-772 | --- | --- | --- |
F(%) | 41 | 56 | 18 | 31 | 50 | 11 | 16 | 0 | 10 | 0 | 10 | 11 | 0 | 0 | 0 |
According to the COG values,
GAM analysis showed a statistically significant effect of depth and latitude-longitude on log-transformed indices of abundance, biomass and mean weight of the species. The best models, explaining 40.3%, 42.9% and 35% of the deviance (abundance, biomass and mean weight of the species, respectively), included depth, latitude-longitude and year as a factor (
Exp.Dev. (%) | AIC | |
---|---|---|
lnA | 40.3*** | 30524 |
lnB | 42.9*** | 38950 |
Mean weight | 35*** | 37203 |
lnA | 51.3*** | 29226 |
lnB | 54.2*** | 33219 |
Mean weight | 7.4*** | 30690 |
lnA | 53*** | 20131 |
lnB | 51.3*** | 28978 |
Mean weight | 26.8*** | 6920 |
lnA | 54.5*** | 26341 |
lnB | 55.7*** | 34553 |
Mean weight | 17.9*** | 22745 |
lnA | 39.3*** | 19393 |
lnB | 41.2*** | 32752 |
Mean weight | 48.5*** | 11747 |
The bathymetric effect showed a non-linear pattern, with biomass and abundance peaking at 400 to 500 m depth. As depth increases, the mean weight of the species also increases, with a maximum value between 500 and 600 m depth. Abundance and biomass were highest in the northern Alboran Sea, Alboran Island, the Gulf of Lions and southern Sicily (GSAs 1, 2, 7 and 16). Maximum mean weight was registered in the Gulf of Lions, Corsica and the Ligurian Sea (GSAs 7, 8 and part of 9) (
The DFA model for
This species was present in every GSA sampled and recorded on 4934 of the 8289 hauls undertaken on the continental slope (F=60%). The highest F values were observed in the southern and central Tyrrhenian Sea and southern Sicily (GSAs 10 and 16) and the lowest in the northern Alboran Sea and Alboran Island (GSAs 1 and 2) (
The best GAM models explaining 51.3%, 54.2% and 7.4% of the deviance (for abundance, biomass and mean weight of the species, respectively) included depth, latitude-longitude and year as a factor (
Abundance and biomass peaked between 400 and 600 m, whereas the mean weight showed a positive relationship with depth. An increase in abundance and biomass of the species was detected over the central Mediterranean Sea, while no clear geographical effect was observed for mean weight (
For
This species was absent in Corsica, the southern Adriatic, the western and eastern Ionian Sea, Aegean Sea and Crete (GSAs 8, 18, 19, 20, 22 and 23) during the period analysed. It was caught on 842 of the 8289 hauls (F=10%). The highest F values corresponded to Spanish GSAs and were null or very low in the other areas (
The best GAM models explaining 53%, 51.3% and 26.8% of the deviance (for abundance, biomass and mean weight of the species, respectively) included depth, latitude-longitude and year as a factor (
Abundance, biomass and mean weight increased with depth. The abundance and biomass of
For this species, DFA showed the lowest values during the first years and the highest values around 2006. Positive loadings for most western areas suggest a general positive relationship of the abundances with this trend. Negative loadings were obtained for the Sardinian Sea and southern Sicily (GSAs 11 and 16) (
This
The best GAM model explaining 54.5%, 55.7% and 17.9% of the deviance (for abundance, biomass and mean weight, respectively) included depth, latitude-longitude and year as a factor (
Abundance and biomass displayed sharp increases from 500 m depth, with maximum values around 700 m and a further decrease to 800 m. The species mean weight increased slightly from 300 to 700 m and sharply below this depth range. The species increased its abundance in the central and eastern Mediterranean areas, but showed an opposite pattern when the mean weight was analysed (
The DFA showed one non-linear but general increasing trend, with relatively stable values from 1998 to 2006. All areas showed positive factor loadings, with the exception of Corsica (GSA 8) (
The species was not recorded in the southern and central Tyrrhenian Sea, southern Sicily, the eastern Ionian Sea, the Aegean Sea and Crete (GSAs 10, 16, 20, 22 and 23). It was recorded on 1035 of the 8289 hauls (F=13%). The highest F values were reached in Alboran Island, the Gulf of Lions and the northern Alboran Sea (GSAs 2, 7 and 1) (
The best model explaining 39.3%, 41.2% and 48.5% of the deviance (for abundance, biomass and mean weight, respectively) included latitude-longitude, depth and year as a factor (
Species abundance and biomass rose from 300 to 700 m depth, with a noteworthy increase from 700 to 800 m depth. The mean weight remained stable to 500 m and increased continuously beyond this depth to 800 m. This species was mainly present in the western Mediterranean, particularly in the northern Alboran Sea, Alboran Island and the Gulf of Lions (GSAs 1, 2 and 7), with a consistent pattern of highest mean weight of the species in these areas (
Overall, increasing trends were detected by DFA models, the highest values being reached in 2006. After this year, the lowest values were reached in 2008, but they increased again thereafter. All areas showed positive factor loadings, with the exception of the southern Adriatic Sea (GSA 18), with the highest values recorded in the westernmost Mediterranean areas (GSAs 1, 2 and 5) (
Depth and geographical distribution trends in abundance, biomass or mean weight differed among the five macrourid species. Abundance and biomass of
In marine environments, depth is one of the main factors influencing biological distributions and is generally coupled with the combination of the gradients that co-occur with depth, affecting the biology and physiology of marine organisms and the ecological interactions between taxa (e.g.
The five species studied showed a general pattern of mean weight increasing with depth. The bigger-deeper phenomenon has been previously described for Mediterranean macrourids as a well-defined rule (
Different relationships were detected between the abundance and biomass of the five macrourid species and different geographical areas of the Mediterranean basins.
Large differences between the western and eastern Mediterranean basins were found for
According to our results,
Overall, in this study, the high recorded values of abundance, biomass and mean weight were recurrent for some species in the Alboran Sea and Gulf of Lions. Local hydrographic features and topographic differences greatly influence the spatial variability of the environmental parameters within each sub-basin (
Regarding inter-annual trends, abundance of macrourid species showed no decreasing trends over the 22 years analysed, despite the fact that these species constitute a large fraction of the discards from the deep-water bottom trawl fishery (
Inter-annual abundance increasing trends were detected for
The Mediterranean Sea is a complex ecosystem with contrasting regions in terms of productivity (
This study was carried out within the framework of the MEDITS survey programme and the Data Collection Framework. The European Union and the Mediterranean Member States involved in this framework are thankfully acknowledged. Fernandez-Arcaya was funded by a post-doctoral grant co-funded by the Regional Government of the Balearic Islands and the European Social Fund 2014-2020. The authors would like to thank all participants involved in the MEDITS surveys. We really appreciate the positive comments and corrections given by two anonymous referees.
The following supplementary material is available through the online version of this article and at the following link:
Table S1. – DFA results for each species analysed from the Mediterranean basin. For each model, the model number, error matrix structure (R), number of common trends (m) and corrected Aikake information criterion (AICc) are recorded.
Fig. S1. – Model fits (black lines) to the best models obtained by DFA on standardized abundance time series for the five Macrouridae species. Data are logarithmically transformed (log[x+1]).