Conflicts between cormorant species and fishery activities are a worldwide concern (Carss et al. 1997). The interactions are site-specific, ranging from serious damage in breeding and wintering areas where commercial and recreational fisheries take place to unusually high or uncommon effects (Carss 1993, 2002, Lilliendahl and Solmundsson 2006). In some places on the Mediterranean coast, the European shag Phalacrocorax aristotelis spp. desmarestii (hereafter referred as the shag) and the great cormorant P. carbo spp. sinensis (hereafter referred as the cormorant) coexist during the breeding season of the former and the wintering period of the latter, i.e. they are syntopic species. One of these places is in Murcia (SE Spain; Fig. 1): Grosa Island holds a local shag breeding population of 12 to 18 pairs (Eguía-Martínez et al. 2015), and together with Redonda islet inside the nearby Mar Menor coastal lagoon, it also holds a wintering population of cormorant of about 5000 individuals (Molina 2013). During the breeding season, shag foraging areas are located near nesting colonies (Aguado-Giménez, pers. obs.), just as occurs around most breeding grounds (Wanless et al. 1991a). Conversely, cormorant mobility for feeding is greater during wintering, including both coastal and inland areas (Farinós and Robledano 2010). Although both bird species are opportunistic piscivorous predators (Carss 2002, Morat et al. 2014), we do not expect competition between them during syntopy, in line with the findings of Lilliendahl and Solmundsson (2006). Nevertheless, the large difference in their population sizes could suggest some interaction. Also, foraging of both species could increase pressure on prey populations (Lilliendahl and Solmundsson 2006), also interacting with the local artisanal fisheries occurring near the breeding-wintering area. Local fishermen are concerned with the increasing abundance of cormorants, though this interaction has not yet been evaluated.

Dietary studies of birds include regurgitated pellet analyses to characterize diet composition and estimate food intake through fish otolith identification (Hillersøy and Lorentsen 2012, Morat et al. 2014). Pellet collection is fairly easy, involving little or no disturbance to birds and analysis requires minimal laboratory facilities (Carss et al. 1997). Inferring diet composition after analysing regurgitated pellets is recognized as an appropriate proxy method for studying the diet of cormorant species (Duffy and Laurenson 1983, Barrett et al. 1990). In this work, we aimed to study the diet of syntopic shags and cormorants in the vicinity of Grosa Island and the Mar Menor coastal lagoon to determine whether there is potential competition with artisanal fishery and to check for any competition for food resources between the two bird species. To this end, freshly regurgitated pellets and fish otolith identification were used to explore shag and cormorant diet qualitatively and quantitatively. However, since there are criteria for (Johansen et al. 2001) and against (Carss et al. 1997) the utilization of pellets to infer total intake, the results obtained should be considered as a proxy.

A total of 18 shag pellets were collected at nests on Grosa Island, and 78 cormorant pellets were removed from their roosting places on Grosa Island and Redonda Islet. Altogether, a total of 2130 otoliths were obtained, of which 10.4% were too eroded and were recorded as unidentified. A total of 1065 fish were identified, belonging to 14 families and 23 species. Mugilidae were identified only up to family level. Table 1 shows the species composition of shag and cormorant diet in terms of abundance and biomass, as well as the assignation of fish species to functional groups and categories of economic importance.

Shag diet was mainly based on small pelagic fish species, but demersal fish are also an important resource. The shag diet is more varied and focused on smaller prey than the cormorant diet during syntopy in the study area, but the size range of cormorant preys is larger. Cormorants fed mainly on demersal fish, so there is no sign of competition with shags throughout their syntopic period. Likewise, there is no suspicion of competition between shags and local fishermen. However, cormorants may compete with local fishermen for D. labrax, Mugilidae and, to a lesser extent, for D. annularis.

Composition of shag diet has been studied at different places and seasons in the Atlantic and the Mediterranean, mainly from otoliths in regurgitated pellets (Velando and Freire 1999, Al-Ismail et al. 2013, Morat et al. 2014), but also by assessing stomach content (Lilliendahl and Solmundsson 2006), or using combined methods (Harris and Wanless 1993). These studies agree on the variety in shag diet, revealing geographical differences in diet composition and biomass intake, and variations over time related to the life-cycle phase and resource availability. The diet of the Atlantic shag subspecies (P. a. aristotelis) is mainly composed of sand-eels (Ammodytes spp.), and some authors have therefore suggested some degree of specialization (Harris and Wanless 1993, Velando and Freire 1999). Shag diet composition depends on habitat diversity in the foraging area, which will largely determine prey diversity and availability. Local environmental conditions strongly affect fish recruitment, which also influences species composition and abundance (Myers 1997). Availability of local resources at a given time seems to be the main driver of diet composition for shags (Carss 1993, Lilliendahl and Solmundsson 2006). In our case, prey availability was unknown, so we hypothesize that for the studied season shag must feed on the most abundant species. Therefore, they could be considered as truly adaptable opportunists. Not surprisingly, some authors suggest the possibility of studying local resource availability based on the diet composition of this bird (Barrett et al. 1990, Barrett 1991). Some studies suggest that Atlantic shag is a predominantly benthic feeder (Wanless et al. 1998, Watanaki et al. 2008). Our results revealed that in the study area shags fed predominantly on small-size pelagic fish (B. boops, Trisopterus sp.), though demersal resources (C. julis, D. annularis) were also important in their diet. These results agree with those of Al-Ismail et al. (2013) studying shag diet in Mallorca (Balearic Islands) regardless of differences in the submerged habitat. In our study, the shag foraging area includes a vast Posidonia oceanica meadow, wide shallow sandy bottoms and, to a lesser extent, rocky reefs (MAPAMA 2017). Most fish species identified here are totally or partially linked to the meadow, even small pelagic species (Cetinić et al. 2011). Small-size B. boops, Trisopterus sp. and Spicara smaris form large shoals at different depths in the water column over the meadow canopy, where they find food and some shelter from predators. Therefore, these species may behave as purely pelagic but also as demersal (epibenthic) occasionally, and then shags may capture them both in the water column and near the bottom. Assignation of fish to a particular functional group is somewhat controversial, increasing discrepancies with regard to considering shags pelagic, demersal or benthic feeders. Nevertheless, most authors agree on the importance of the seabed in provisioning this bird, and also its foraging flexibility (Wanless et al. 1991b, Grémillet et al. 1998, Cosolo et al. 2011).

Our study reveals that shags prey preferably on small pelagic fish and cormorants on demersal fish. Similar results were obtained by Lillendahl and Solmundsson (2006) in Iceland. Conversely, shag diet was more diverse than cormorant diet during their period of syntopy around Grosa Island than in Iceland. However, both species show a noteworthy dietary variability at different spatial and temporal scales. We estimated that shag daily biomass intake was slightly less (but the difference was not statistically significant) than that of cormorant, in agreement with the results of Grémillet et al. (1996, 1998). However, shag mean prey size was smaller than that of cormorant, so shags had to compensate for the small size of preys caught by intensifying catch effort and number of prey per feeding event to obtain a comparable biomass. Barrett et al. (1990) and Lillendahl and Solmundsson (2006) also found that cormorant preys tend to be larger than those of shag. Consistently, cormorants from our study area needed less than half the prey numbers of shags to achieve their daily intake, due to the ability of cormorant to hunt larger fish. Differences in mean prey size between shag and cormorant could be related to differences in bill size. Though mean prey size of cormorant is larger than that of shag, the former also hunted small prey frequently, mainly A. boyeri. This meant that their prey modal size was even smaller than that of shags. However, the contribution of those small fish to the diet of cormorant was very low in terms of biomass intake.

The differences in diet composition between shag and cormorant during syntopy were notable, and the species most represented in their diets also differed. Therefore, it seems that trophic competition, as we expected, did not occur between these two coexisting bird species around Grosa Island. Lillendahl and Solmundsson (2006) also noticed segregation in diets of Icelandic shags and cormorants in syntopy. A partial spatial segregation in our study area between the two coexisting birds with regard to their preferred fishing zones must also explain the lack of competition. While the shag is a strictly marine bird, cormorants forage both in the open sea and inside the coastal lagoon, and even in fresh water irrigation ponds and reservoirs on the mainland tens of kilometres inland (Farinós and Robledano 2010), where they prey on C. carpio. This freshwater fish represented a considerable fraction in their diet (about 10% in biomass). In Spain, C. carpio is considered an exotic invasive species included in eradication programmes (BOE 2013), so its consumption by cormorants can be considered an ecosystem service (Constanza et al. 1997). This pattern agrees with Mizutani et al. (1990), who observed that large groups of great cormorants (>2000 individuals) tend to segregate spatially to forage over an environmental gradient, in our case alternating between marine waters, the coastal lagoon and inland freshwater bodies. Conversely, shags rarely enter the coastal lagoon and never forage in inland ponds, but forage in the open sea most of the time.

Considering the low shag population density, their low predation on FASP target species and the facts that most fishing effort is focused inside the lagoon where shags rarely forage, there is virtually no interaction between shag and fishermen in the study area. Morat et al. (2014) did not observe major competition between shags and fisheries on the French Mediterranean coast either. However, in other places where shag population density is higher, they might affect commercially important fish stocks (Barrett et al. 1990, Lillendahl and Solmundsson 2006). Unlike shag, cormorants enter and leave the Mar Menor frequently, investing considerable fishing effort inside the lagoon during autumn and winter months. Taking into account the FASP fishing effort for those months and the estimated cormorant biomass intake, our results reveal that cormorants may compete for some commercially targeted fish such as D. labrax, and Mugilidae species and, to a lesser extent, for D. annularis. The magnitude of this interaction might change depending on fish availability, fishing effort and variations in cormorant population density. However, such a high consumption of S. aurata by cormorants does not automatically mean there is competition over fish resources, in accordance with Östman et al. (2013), especially if we consider that FASP beat the previous catch record for this species in the Mar Menor during the studied season (FASP, pers. comm). Regarding D. annularis, it seems that cormorants are more efficient than the recruitment to the fishing gear. A side-effect of the intense interaction between cormorants and fisheries is the high mortality of cormorants (about 10% of the wintering population in 2014), most of them young and sub-adult specimens trapped in fishing nets deployed in the Mar Menor (Zamora 2015).

Geographical and temporal variability in diet composition of shags and cormorants may cause varied intensities of competition with fisheries (Östman et al. 2013), from minimal (Liordos and Goutner 2007) to strong competition (Steffens 2011). Measures to prevent economic impact on fisheries caused by cormorants are mostly focused on the control of nesting at breeding grounds through egg-oiling or nest destruction, and of population density at nesting and wintering places through harassment or culling. However, the effectiveness of all these measures is in question (Carss 2002, Taylor and Dorr 2003). At Grosa Island and in the Mar Menor area, no management actions are implemented to deal with this interaction. To the complaints of fishermen regarding cormorant catches, we must add those of conservationist organizations about the high fishing effort and accidental bird by-catches which include not only cormorant but also ducks, gulls and grebes (Zamora 2015). Therefore, it seems obvious that the sustainability of natural resources will not have to focus only on controlling cormorant wintering population, but also on preventing fishing side-effects and managing fishing effort of both fishermen and cormorants.