Diet partitioning in sympatric Atlantic salmon and brown trout in streams with contrasting riparian vegetation

Abstract

Prey intake by Atlantic salmon Salmo salar and brown trout Salmo trutta was measured across different riparian vegetation types: grassland, open canopy deciduous and closed canopy deciduous, in upland streams in County Mayo, Western Ireland. Fishes were collected by electrofishing while invertebrates were sampled from the benthos using a Surber sampler and drifting invertebrates collected in drift traps. Aquatic invertebrates dominated prey numbers in the diets of 0þ year Atlantic salmon and brown trout and 1þ year Atlantic salmon, whereas terrestrial invertebrates were of greater importance for diets of 1þ and 2þ year brown trout. Terrestrial prey biomass was generally greater than aquatic prey for 1þ and 2þ year brown trout across seasons and riparian types. Prey intake was greatest in spring and summer and least in autumn apart from 2þ year brown trout that sustained feeding into autumn. Total prey numbers captured tended to be greater for all age classes in streams with deciduous riparian canopy. Atlantic salmon consumed more aquatic prey and brown trout more terrestrial prey with an ontogenetic increase in prey species richness and diversity. Atlantic salmon and brown trout diets were most similar in summer. Terrestrial invertebrates provided an important energy subsidy particularly for brown trout. In grassland streams, each fish age class was strongly associated with aquatic, mainly benthic invertebrates. In streams with deciduous riparian canopy cover, diet composition partitioned between conspecifics with older brown trout associated with surface drifting terrestrial invertebrates and older Atlantic salmon associated with aquatic invertebrates with a high drift propensity in the water column and 0þ year fish feeding on benthic aquatic invertebrates. Deciduous riparian canopy cover may therefore facilitate vertical partitioning of feeding position within the water column between sympatric Atlantic salmon and brown trout. Implications for riparian management are discussed.

Ontogenetic shifts in terrestrial reliance of stream-dwelling brown trout

“…Noteworthy, our findings can exemplify the interface between terrestrial and aquatic ecosystems through the feeding of top predators in riverine food webs (here brown trout). As noted in the Introduction, other workers have found that terrestrial invertebrates may enhance fish production, food resource partitioning among competing species and stream benthic community (Edwards and Huryn, 1995;Nakano et al 1999a;Dineen et al, 2007). That said, planning of restoration works or land-use practices on river basins should take into account the input of terrestrial invertebrates from the riparian canopy as a key factor for food supply of streamdwelling salmonids species, as well as other advantages in ecosystem processes such as resource partitioning or trophic cascades.…”

“…In fact, inputs of terrestrial invertebrates from riparian canopy cover may represent an important food resource for stream-dwelling fish species (Kawaguchi and Nakano, 2001;Utz and Hartman, 2007;Syrjänen et al, 2011). Therefore, the importance of terrestrial invertebrates in riverine systems is unquestionable, such importance has been identified in different ways; that is, in terms of energy subsidy for fish species (Edwards and Huryn, 1996;Nakano et al, 1999b;Utz and Hartman, 2007), annual fish production (Edwards and Huryn, 1995), food resource partitioning between sympatric fish species (Dineen et al, 2007; or ecosystem functioning (Nakano et al, 1999c). That said, it should be kept in mind that the terrestrial invertebrate input to the riverine systems greatly depends on riparian canopy cover (Edwards and Huryn, 1996;Kawaguchi and Nakano, 2001;Ryan and KellyQuinn, 2015), but this allochthonous prey supply occurs primarily during summer, when aquatic invertebrate biomass is usually low (Nakano and Murakami, 2001).…”

Feeding habits of the alien brook trout Salvelinus fontinalis and the native brown trout Salmo trutta in Czech mountain streams

“…Terrestrial invertebrates may largely cover the energy intake of streamdwelling salmonids in summer (Sánchez-Hernández and Cobo, 2016). This reduces interspecific competitive interactions for benthic invertebrates through resource partitioning (Dineen et al, 2007), and becomes an important mechanism promoting the coexistence of related fish species (Miyasaka et al, 2003). Therefore, it is possible that the observed shift in brook trout diet from our study resulted from competitive interactions with brown trout.…”

“…An important mechanism allowing the species to reduce competition is food resource partitioning, including both spatial and temporal differentiation and changes in feeding habits (Pianka 1969;Schoener, 1974). Indeed, it has been formerly shown that the interaction between related salmonid species magnifies the differences in the use of habitat or food resources (Blanchet et al, 2007;Dineen et al, 2007;.…”

“…Some other studies described the coexistence of brown trout or brook trout with other related taxa. For example, in coexistence with Atlantic salmon Salmo salar Linnaeus, 1758, brown trout was shown to feed more on terrestrial resources (Dineen et al, 2007;Sánchez-Hernández et al, 2013), while brook trout similarly increased intake of terrestrial invertebrate prey and exhibited wider diet breadth (Mookerji et al, 2004). Such results suggest that the partitioning of food resources is an important mechanism enabling the coexistence of related salmonids.…”