Javier Sánchez-Hernández

Feeding habits of four sympatric fish species in the Iberian Peninsula: keys to understanding coexistence using prey traits

Trophic interactions are important factors structuring animal communities. We assessed the trophic relations of four fish species that live in sympatry in the River Ladra (NW Spain), and cluster analysis differentiated two feeding strategies: (1) species with omnivorous feeding habits, feeding mainly on detritus and plant material but with aquatic macroinvertebrates as an important complement (Achondrostoma arcasii and Pseudochondrostoma duriense) and (2) species feeding mainly (Salmo trutta) or exclusively (Gasterosteus gymnurus) on aquatic macroinvertebrates. Concerning ingested macroinvertebrates, the trophic overlap was quantified using Schoener’s index and the results obtained revealed a high diet overlap among the species (from 81.3 up to 99.2%). In order to get a deeper insight into mechanisms of fish species coexistence, we used ten biological and ecological traits of macroinvertebrate prey to discriminate feeding preferences. As a result, despite the high similarity among the diets, our analyses suggest that differences in diel activity patterns and drift behaviour of preys, as well as differences in the prey size, are important adaptive features that may reduce the inter-specific competition in the fish community and permit the partitioning of food that allows coexistence.

First feeding diet of young brown trout fry in a temperate area: disentangling constraints and food selection

Diet composition of newly emerged brown trout fry in natural areas remains poorly known, and foraging abilities at this early stage, although presumably reduced, are still under discussion. We have studied gut content composition of brown trout fry in a temperate area (Galicia, NW Spain) and compared it to the benthic macroinvertebrate community. Small prey such as chironomid larvae and baetid nymphs were the most important food items for newborns, some of them still presenting yolk remnants. However, the positive selection observed for Polycentropodidae and Simuliidae and the rejection of Elmidae and Leuctricidae suggest that other factors apart from size, such as locomotor abilities of fish or accessibility and antipredator behaviour of prey play an important role in feeding behaviour. Additionally, analysis of diet changes on the studied fry suggests a dramatic shift in niche breadth at the moment of complete yolk absorption, which might be related to the improvement of swimming and handling ability of fry for capturing and ingesting prey. The presence of aerial imagoes only in the stomachs of fry with no yolk provides further support to this hypothesis. Planning of restoration works on spawning grounds should then allow enough time for complete recolonization by benthic macroinvertebrates, including first instars, as searching for food in newborns is limited to the nest area due to mobility constraints.

Summer differences in behavioural feeding habits and use of feeding habitat among brown trout (Pisces) age classes in a temperate area

Abstract Field studies were conducted to examine intra-specific diet variation in a population of brown trout (Salmo trutta L.) in a temperate area (River Tormes, Central Spain) during the summer. Fuzzy principal component analysis (FPCA) and the modified Costello graphical method were used to illustrate that age classes showed differences in behavioural feeding habits and use of feeding habitat. Despite benthic prey constituting the most important prey in abundance terms in all age classes, our findings suggest that ontogenetic shift among age classes can be related to several factors: (1) terrestrial invertebrates and fishes were more frequently consumed by older trout, (2) resource partitioning among age classes also occurred at the level of prey size and (3) differences among age classes in habitat utilization for feeding were evident. Levins index increases with trout size. However, a noteworthy result of this study is that no differences were found in the evenness, Shannon and Levins indices among age classes. Finally, the dietary analyses showed high values of diet overlap, but the differences in the use of feeding habitat and behavioural feeding habits are important adaptive features that may reduce the intra-specific competition in the population.

Summer food resource partitioning between four sympatric fish species in Central Spain (River Tormes)

Abstract. Knowledge of food resource partitioning between sympatric fish species is critical for understanding the fish communities functioning. Four sympatric fish species (Salmo trutta, Pseudochondrostoma duriense, Squalius carolitertii and Barbus bocagei) were captured in August 2010 in the River Tormes (Ávila, Central Spain) in order to study food resource partitioning between fish species such us as a possible mechanism enabling their coexistence, using a fuzzy principal component analysis (FPCA). Diet comparison among species shows that detritus are present in P. duriense, B. bocagei and S. carolitertii, although in different occurrence, showing that these species present typically omnivorous feeding habits. Regarding prevalent food, in all species, benthic prey constituted the most important prey in abundance terms, and terrestrial invertebrates were only consumed by S. trutta, S. carolitertii and P. duriense. High overlap values (Schoeners index from 0.8 up to 1) were found, but it may not indicate competition, since species can adopt different strategies to overcome competence. Thus, prey traits analyses suggest that differences in macrohabitat use, drift behaviour of prey and prey size are important adaptive features that may reduce the inter-specific competition in the fish community and permit the food partitioning that allows coexistence.

Ontogenetic Dietary Shifts in a Predatory Freshwater Fish Species: The Brown Trout as an Example of a Dynamic Fish Species

The brown trout (Salmo trutta) is a species of Eurasian origin, but at has become naturalized in many other parts of the world. It has an outstanding socio-economic importance, both in commercial and sport fisheries, and it is frequently used as tourist attraction [1,2]. The case of brown trout is a clear example of a ‘dynamic’ fish species, as its diet and feeding behaviour can vary greatly among individuals, age classes, seasons and rivers. The composition of brown trout diet is strongly influenced by environmental and biotic factors. For example, water temperature plays an important role, as it influences food intake and the activity of fishes [3], but also the emergence and activity of aquatic insects or other potential prey items. Also water flow rate can be extremely important for drifting feeders such as brown trout, as they regulate food availability. The are many abiotic factors that influence feeding behaviour, but in general, biotic factors such as the locomotor ability of fishes, accessibility, abundance and antipredator behaviour of prey are thought to be the most important factors in the determination of the diet and feeding strategies in fishes. Usually not all the available prey is consumed by the predator, a feature that allows biologists to distinguish between trophic base and trophic niche (Figure 1). The trophic base consists of all potential prey items that the brown trout is able to consume and it is determined by the feeding habits of the fish, the size of the mouth and the anatomical characteristics of its digestive tract. However, the trophic niche is the variety of organisms that are really consumed by the predator, which depends on different factors that play an important role when choosing criteria prey items as, for example, prey abundance, including site-specific

Prey trait analysis shows differences in summer feeding habitat use between wild YOY Atlantic salmon and brown trout

Abstract We evaluated the vertical use of the water column for feeding by young of the year (YOY) of brown trout (Salmo trutta) and Atlantic salmon (S. salar) in a natural river during the summer. Prey trait analysis and diet composition demonstrated that brown trout showed a higher ability to feed on prey in the surface of the water column than Atlantic salmon. Hence, the differences in the ability to feed at different depths of the water column (surface vs. bottom) are important adaptive features that may reduce interspecific competition between the two sympatric fish species. Finally, results derived from the prey trait analysis provide useful information on resource partitioning between sympatric salmonid species.

Food resource partitioning between stream-dwelling Arctic charr Salvelinus alpinus (L.), Atlantic salmon Salmo salar L. and alpine bullhead Cottus poecilopus Heckel, 1836: an example of water column segregation

We assessed the food resource partitioning of three fish species (Arctic charr, Atlantic salmon and alpine bullhead) living in sympatry in a subarctic river. Fish were sampled monthly during the ice-free season (May–October), and dietary overlap among the species was calculated according to Schoener’s index. In October, the diet overlap among all three species was high (>70%). In contrast, large to modest food resource partitioning occurred among Arctic charr and the other two species from May to September (27–59% overlap), whereas there was a distinct diet overlap between Atlantic salmon and alpine bullhead in May, August and September (>64%), but not in July (53%). Surface prey (terrestrial and emerged aquatic insects), probably caught at the surface, were important for Arctic charr in August and September (24.9 and 46.6%, respectively), whereas the other fish species mainly fed on Apatania stigmatella, Mystrophora intermedia and Ephemerella aurivilli. Alpine bullhead seemed to feed close to the bottom, Atlantic salmon used both the bottom and water of various depths, whereas Arctic charr showed the greatest capacity to forage at the water surface. This vertical segregation may be important for fish assemblages in subarctic rivers, allowing food resource partitioning and coexistence of sympatric species.

Community structure affects trophic ontogeny in a predatory fish

Abstract While most studies have focused on the timing and nature of ontogenetic niche shifts, information is scarce about the effects of community structure on trophic ontogeny of top predators. We investigated how community structure affects ontogenetic niche shifts (i.e., relationships between body length, trophic position, and individual dietary specialization) of a predatory fish, brown trout (Salmo trutta). We used stable isotope and stomach content analyses to test how functional characteristics of lake fish community compositions (competition and prey availability) modulate niche shifts in terms of (i) piscivorous behavior, (ii) trophic position, and (iii) individual dietary specialization. Northern Scandinavian freshwater fish communities were used as a study system, including nine subarctic lakes with contrasting fish community configurations: (i) trout‐only systems, (ii) two‐species systems (brown trout and Arctic charr [Salvelinus alpinus] coexisting), and (iii) three‐species systems (brown trout, Arctic charr, and three‐spined sticklebacks [Gasterosteus aculeatus] coexisting). We expected that the presence of profitable small prey (stickleback) and mixed competitor–prey fish species (charr) supports early piscivory and high individual dietary specialization among trout in multispecies communities, whereas minor ontogenetic shifts were expected in trout‐only systems. From logistic regression models, the presence of a suitable prey fish species (stickleback) emerged as the principal variable determining the size at ontogenetic niche shifts. Generalized additive mixed models indicated that fish community structure shaped ontogenetic niche shifts in trout, with the strongest positive relationships between body length, trophic position, and individual dietary specialization being observed in three‐species communities. Our findings revealed that the presence of a small‐sized prey fish species (stickleback) rather than a mixed competitor–prey fish species (charr) was an important factor affecting the ontogenetic niche‐shift processes of trout. The study demonstrates that community structure may modulate the ontogenetic diet trajectories of and individual niche specialization within a top predator.

Food Web Topology in High Mountain Lakes

Although diversity and limnology of alpine lake systems are well studied, their food web structure and properties have rarely been addressed. Here, the topological food webs of three high mountain lakes in Central Spain were examined. We first addressed the pelagic networks of the lakes, and then we explored how food web topology changed when benthic biota was included to establish complete trophic networks. We conducted a literature search to compare our alpine lacustrine food webs and their structural metrics with those of 18 published lentic webs using a meta-analytic approach. The comparison revealed that the food webs in alpine lakes are relatively simple, in terms of structural network properties (linkage density and connectance), in comparison with lowland lakes, but no great differences were found among pelagic networks. The studied high mountain food webs were dominated by a high proportion of omnivores and species at intermediate trophic levels. Omnivores can exploit resources at multiple trophic levels, and this characteristic might reduce competition among interacting species. Accordingly, the trophic overlap, measured as trophic similarity, was very low in all three systems. Thus, these alpine networks are characterized by many omnivorous consumers with numerous prey species and few consumers with a single or few prey and with low competitive interactions among species. The present study emphasizes the ecological significance of omnivores in high mountain lakes as promoters of network stability and as central players in energy flow pathways via food partitioning and enabling energy mobility among trophic levels.

Ontogenetic dietary shifts and food selection of endemic Squalius carolitertii (Actinopterygii: Cypriniformes: Cyprinidae) in River Tormes, Central Spain, in summer

In the Iberian Peninsula, the freshwater fish fauna is dominated by cyprinids and is characterized by a high level of endemism (Doadrio 2001), as around 45% of Iberia’s native fish species are endemic (Gómez and Lunt 2007). Recently, Iberian populations of the genus Leuciscus were transferred into the genus Squalius (see Sanjur et al. 2003, Kottelat and Freyhof 2007), and the majority of species are endemic at drainage level (Leunda et al. 2009). The development of effective conservation programmes for endemic fish species requires a clear understanding of the ecological requirements of these species, and a better knowledge of their feeding habits is essential for this objective. The Northern Iberian chub, Squalius carolitertii (Doadrio, 1988), is a small endemic cyprinid inhabiting the rivers of the Iberian Peninsula across a large area, including the Douro, Mondego, Lima, Minho, and Lérez basins (Doadrio 1988, 2001, Carmona and Doadrio 2000). Recently Perea et al. (2011) reported this species for the first time from the upper reaches of the Alberche River (a tributary of the Tagus basin in central Spain) and in the ACTA ICHTHYOLOGICA ET PISCATORIA (2012) 42 (2): 101–111 DOI: 10.3750/AIP2011.42.2.03