The potential influence of Atlantic salmonSalmo salarand brown troutSalmo truttaon density and breeding of the white‐throated dipperCinclus cinclus

Abstract

Interactions between birds and fish are often overlooked in aquatic ecosystems. We studied the influence of Atlantic salmon and brown trout on the breeding population size and reproductive output of the white‐throated dipper in a Norwegian river. Acidic precipitation led to the extinction of salmon, but salmon recolonized after liming was initiated in 1991. We compared the dipper population size and reproductive output before (1978–1992) and after (1993–2014) salmon recolonization. Despite a rapid and substantial increase in juvenile salmon, the breeding dipper population size and reproductive output were not influenced by juvenile salmon, trout, or total salmonid density. This might be due to different feeding strategies in salmonids and dippers, where salmonids are mainly feeding on drift, while the dipper is a benthic feeder. The correlation between the size of the dipper population upstream and downstream of a salmonid migratory barrier was similar before and after recolonization, indicating that the downstream territories were not less attractive after the recolonization of salmon. Upstream dipper breeding success rates declined before the recolonization event and increased after, indicating improved water quality due to liming, and increasing invertebrate prey abundances and biodiversity. Surprisingly, upstream the migratory barrier, juvenile trout had a weak positive effect on the dipper population size, indicating that dippers may prey upon small trout. It is possible that wider downstream reaches might have higher abundances of alternative food, rending juvenile trout unimportant as prey. Abiotic factors such as winter temperatures and acidic precipitation with subsequent liming, potentially mediated by prey abundance, seem to play the most important role in the life history of the dipper.

Relationship between salmon egg subsidy and the distribution of an avian predator

“…For example, the American dipper Cinclus mexicanus can achieve higher reproductive success (as measured by fecundity and juvenile growth) in reaches where Oncorhynchus spawn than in those where it does not (Obermeyer et al, 2006; Tonra et al, 2016). The population size of the white‐throated dipper C. cinclus in Norway may also benefit from eating salmon fry because it was correlated with the annual density of salmon fry (Nilsson et al, 2018). Because dippers, which are not scavengers, are not affected by the amount of carcasses—in addition to their well‐studied relationship with salmon, as noted above—they are a suitable model species for examining the effect of salmon egg subsidies on the abundance and distribution of terrestrial animals.…”

Relationship between salmon egg subsidy and the distribution of an avian predator

“…For example, the American dipper Cinclus mexicanus can achieve higher reproductive success (as measured by fecundity and juvenile growth) in reaches where Oncorhynchus swim upstream than in reaches where it does not (Obermeyer et al, 2006; Tonra et al, 2016). The population size of the white-throated dipper C. cinclus in Norway may also benefit from eating salmon fry because it was correlated with the annual density of salmon fry (Nilsson et al, 2018). Because dippers, which are not scavengers, are not affected by the amount of carcasses – in addition to their well-studied relationship with salmon, as noted above – they are a suitable model species for examining the effect of salmon egg subsidies on the abundance and distribution of terrestrial animals.…”

Location Is Everything, but Climate Gets a Share: Analyzing Small-Scale Environmental Influences on Breeding Success in the White-Throated Dipper

“…Given the effect of future climate predictions on population dynamics (Gamelon et al, 2017), attributing reproductive success to variation in specific local variables is of particular interest. The lower parts of the river system have been limed to mitigate the effects of acidic precipitation, because such acidification has negative impacts on dipper reproductive performance (Ormerod et al, 1991;Nybø et al, 1997;Nilsson et al, 2018). We therefore also included information on liming status among the explanatory variables.…”