The Triglidae (or gurnards) is a large family of fish of the order Scorpaeniformes, which comprises 8 genera and 125 species dwelling in tropical and temperate marine areas (Richards and Jones 2002). In the Mediterranean Sea, the family is represented by 4 genera (Eutrigla, Trigla, Chelidonichthys and Lepidotrigla) and 8 species (E. gurnardus, T. lyra, C. lucerna, C. cuculus, C. obscurus, C. lastoviza, L. cavillone and L. dieuzeidei). These fish are an important component of demersal assemblages in terms of biomass in both the eastern and western Mediterranean basins (Jukic-Peladic et al. 2001, Labropoulou and Papaconstantinou 2004, Massuti and Reñones 2005). Several studies have focused on the life-history traits of these species, such as growth (Papaconstantinou 1981, 1984, Colloca et al. 2003) and spawning (Papaconstantinou 1983, Vallisneri et al. 2011, 2012), feeding (Colloca et al. 1994, Morte et al. 1997, Terrats et al. 2000), as well as on other aspects related to the trophic and habitat partitioning among species (Serena et al. 1990, Tsimenides et al. 1992, Colloca et al. 2010). From an ecological perspective, the 8 gurnard species, along with the closely related African armoured searobin, Peristedion cataphractum, play similar roles in the trophic web, feeding mainly on epibenthic crustaceans (Colloca et al. 1994). Interspecific competition for food is reduced by species segregation across gradients of prey size and habitat type (Morte et al. 1997, Colloca et al. 2010, Montanini et al. 2017).

The commercial importance of the Mediterranean gurnards is not negligible. Indeed, the largest-sized species such as C. lucerna, T. lyra, E. gurnardus, C. cuculus and C. lastoviza are a valuable by-catch of demersal fisheries of many Mediterranean sectors (Ordines et al. 2014). Although the impact of fishing on the gurnard populations has rarely been examined, Ordines et al. (2014) pointed out that some commercially important gurnards, such as C. lastoviza and C. cuculus, have also been affected by the overall overexploitation of commercial stocks in the Mediterranean (Colloca et al. 2013, Cardinale et al. 2017). As a result, their levels of overexploitation are similar to, or even higher than, those detected for some of the most important target species such as hake and red mullet (Ordines et al. 2014). As has been demonstrated for the North Atlantic, the trends in the by-catch species (including gurnards), are quite similar to those of the principal target species, showing high exploitation rates and declining stock biomasses during the late 20th century (Cook and Heath 2018).

In the present study we investigated the abundance and distribution of 9 gurnard species over a large spatial scale covering the northern Mediterranean Sea that corresponds to EU waters. The main objective of the study was to elucidate the temporal and spatial variations in species abundances across the northern Mediterranean and to discuss the possible role of fisheries in the observed pattern.

This study provides an overview of the status of gurnards in the northern Mediterranean Sea using 22 years of MEDITS bottom trawl survey data. Gurnards are an important component of demersal assemblages in Mediterranean ecoregions, inhabiting soft and detrital bottoms from the shallower portion of the continental shelf to the upper slope. Some of them, such as C. lucerna, T. lyra, C. cuculus, C. lastoviza and E. gurnardus, can be important commercial species for the demersal fisheries. The other species mainly constitute commercial or non-commercial trawling by-catch. Results of our study highlighted important differences among Mediterranean ecoregions and sub-areas (GSAs) in terms of species composition and dominance, which are probably related to the different biogeographic affinity of the species. The majority of the Mediterranean gurnards have a sub-tropical affinity, with only C. cuculus, C. obscurus and E. gurnardus being temperate species (http://www.fishbase.org). In particular, the Ionian and western Mediterranean ecoregions appeared very similar in the structure of the gurnard species guild, with C. cuculus being the most abundant species, followed by five others (L. cavillone, L. dieuzeidei, T. lyra, C. lastoviza, P. cataphractum or E. gurnardus) with a similar level of abundance, together accounting for more than 90% of the relative biomass for the examined period. An opposite pattern was observed in the Levantine Sea, where only data for Cyprus are available. Here the k-dominance curve is steep with three species, L. cavillone, C. cuculus and C. lastoviza dominating the species guild.

The Alboran Sea is featured by the lowest relative biomass of gurnards; the two Lepidotrigla species are the most common gurnards and the grey gurnard is absent. This peculiarity can be related to the influence of Atlantic inputs and the semi-permanent Almería–Oran hydrographic front, which plays a key role in defining the Alboran Sea as a specific fish fauna area within the northern Mediterranean Sea (Gaertner et al. 2005).

In the Adriatic region, the guild is dominated by L. cavillone, A. cuculus and C. lucerna. Here gurnards were found to be more abundant on the east than on the west side of the basin, probably also due to the eastward ontogenetic migration of the juveniles towards the Croatian coasts, as observed for C. cuculus (Vallisneri et al. 2014). Interesting is the absence in this region of the longfin gurnard (C. obscurus), which was also absent in Crete (GSA 23), as reported also by Tsimenides et al. (1992), and in Cyprus (GSA 25). The longfin gurnard also appears rare in the Aegean Sea, where it was not found in a study carried out in the 1970s (Papaconstantinou 1983). In this region, the four most abundant gurnards, in order of importance, were found to be C. cuculus, C. lastoviza, C. lucerna and L. cavillone.

The species displaying the highest geographical differences are E. gurnardus and P. cataphractum. The first is abundant only in the Gulf of Lions, rare in most of the GSAs and absent in the southern Tyrrhenian (GSA 10), Crete and Cyprus. The dominance of E. gurnardus on the continental shelf of the Gulf of Lions was already reported by Campillo et al. (1992). P. cataphractum is consistently more abundant in the Maltese waters than in all the other GSAs.

In terms of overall abundance, the Balearic Islands (GSA 5), Maltese waters (GSA 15), the Gulf of Lions (GSA 7), eastern Corsica (GSA 8) and the eastern Ionian Sea (GSA 20) are the areas with the highest relative biomass of gurnards, while the lowest biomasses are observed in the Alboran and Tyrrhenian seas (GSAs 9 and 10), on the Italian side of the Adriatic (GSA 17a) and in the western Ionian Sea (GSA 19). At first glance this pattern in gurnard abundance could likely be the result of the current distribution of trawl fleets and fishing effort in the Mediterranean (see Colloca et al. 2017, Ferrà et al. 2018). The areas with the highest gurnard relative biomass are those that have the lowest trawl pressure on the continental shelf, while the opposite is observed in areas that have a high effort. Trawl catchability of gurnards is probably high since they live close to the sea bottom, mostly on soft and detrital sediments that are easily exploitable by trawlers. Some local studies support this hypothesis. For example, gurnards were the species showing the highest rate of abundance increase (about 500 fold) on the outer shelf (50-100 m) of northwest Sicily after some years of trawling ban (Pipitone et al. 2000). In the Ionian Sea species like T. lyra, L. dieuzeidei and P. cataphractum were several times more abundant on the poorly exploited continental slope of Greece than on the intensively exploited Italian fishing grounds (D’Onghia et al. 2003). Similarly, in the northern sector of the Strait of Sicily the abundance of gurnards on poorly exploited fishing grounds on the continental slope was several time higher than in the traditionally exploited areas (Gristina et al. 2006). In the same region, Dimech et al. (2008) found higher relative biomass of C. cuculus and L. cavillone inside the less exploited Fisheries Management Zone (FMZ) of Malta than in the areas outside the FMZ. This evidence supports the hypothesis that the observed geographical differences in gurnard abundance may result from the different levels of fishing pressure, with areas less impacted by fishing hosting a higher gurnard biomass. Although more investigation is required on the subject, the results of our study point to a possible future use of gurnards as indicators of the fishing pressure exerted on the ecosystem and, as such, a useful tool for monitoring the fishing impact on the demersal fish communities. A summary of the trend observed since the mid-1990s by species and areas is provided in Figure 14 using a traffic light approach. In the western Mediterranean, a general decrease was found for the shallowest species (C. obscurus, C. lastoviza and C. lucerna), while a “green” status was more common for the upper slope species (T. lyra and P. cataphractum). A better situation was observed in the Ionian ecoregion (GSAs 15, 16, 19 and 20), where the “green” status was common for the deep-shelf (e.g. Lepidotrigla spp., C. cuculus) and upper-slope species. The only significant linear decrease in this region was found for C. obscurus and C. lastoviza in the south of Sicily.

The opposite was observed in the Aegean Sea (GSAs 22-23) and in Cyprus, where the trend was generally positive or stable for the shelf species, with most of the red lights found for deep-shelf and slope gurnard (Fig. 14). A shifting of fishing activities of Greek vessels towards slope resources has recently been postulated as an effect of the implementation of EC regulation 1967/2006, which bans bottom trawl activities within 1.5 nautical miles off the coast (Tserpes et al. 2016).

Similarly, deep-water gurnards in Sardinian waters showed the lowest relative biomasses in recent years, with the exception of L. dieuzeidei. P. cataphractum in particular showed a significant linear decrease in abundance across years, in strong contrast with all other ecoregions and GSAs. This temporal pattern may, besides ecological factors, reflect the modernization of the trawling fishing fleet that took place in the Sardinian fisheries (Marongiu et al. 2017), allowing fishing pressure to shift towards deeper habitats such as those inhabited by P. cataphractum.

While there is clear evidence that the excess fishing pressure is reducing the sizes of the gurnard populations in many Mediterranean sectors, there are still important aspects related to the ecology of this guild of species that would be worth exploring in order to understand the factors driving species abundance and distribution. One key feature is related to the habitat preferences of the species. Gurnards seem to prefer detrital sediments on both the continental shelf and slope. L. cavillone and C. cuculus off the western coasts of Italy appear more abundant on the detrital bottoms over the continental shelf break (Serena et al. 1990, Colloca et al. 1997, Damalas et al. 2010). L. diuzeidei was found to be more common on the detrital and muddy bottoms of the shallow part of the upper slope (Voliani et al. 2000), while on the coastal shelf, C. lucerna was concentrated on the coastal and detrital sandy bottoms and C. lastoviza on the coastal detrital sediments (Serena et al. 1990, Ordines et al. 2014). In this study, we found that the areas showing the highest gurnard abundances are those characterized by the prevalence or the wide extension of detrital bottoms, such as Malta, the Balearic Islands, the Greek GSAs and the south of Sicily, the latter characterized by wide offshore banks (Di Lorenzo et al. 2018). Another important aspect is connected with the occurrence of “refugia”, where large spawners of species like C. lucerna can find protection from trawling, as was observed in the western Ionian Sea (D’Onghia et al. 2017).

The rapid ongoing warming of the Mediterranean is another key factor that, in combination with fishing pressure, might explain the species temporal trends observed. The increase in water temperature can be potentially beneficial for sub-tropical species such as the harmoured searobin but have a negative effect on the temperate ones such as E. gurnardus). The overall effect may be a progressive shift in gurnard distribution and abundance as effects of climate forcing. The NAO index, for example, was found to have an indirect effect on triglids by shaping their prey’s availability (López-López et al. 2011). In Portuguese waters the landings of the large-scaled gurnard, a species with a sub-tropical affinity, decreased unexpectedly during the warm years (Teixeira et al. 2014). On the other hand, the congeneric sub-tropical L. dieuzeidei has expanded its distribution range in the Atlantic as an effect of the water temperature increase (Punzón et al. 2016).

Future studies on Mediterranean gurnards should focus on the above aspects, in order to understand specifically how fishing pressure, climate forcing and habitat suitability interact to determine the trends observed in the present study. Other important aspects that would be worth exploring are related to the compensatory effects that may occur within the gurnards guild. The nine species of gurnards inhabiting the Mediterranean basin exploit similar trophic niches, preying on the same groups of crustaceans. Therefore, a decrease in the abundance of one or more of these species could be compensated by the increase and the expansion of the more tolerant species, thus avoiding a functional diversity loss in the ecosystem.