Multiple lines and levels of evidence for avian zoochory promoting fish colonization of artificial lakes

Abstract

Understanding how obligate freshwater organisms colonize seemingly isolated ecosystems has long fascinated ecologists. While recent investigations reveal that fish eggs can survive the digestive tract of birds and successfully hatch once deposited, evidence for avian zoochory in natura is still lacking. Here, we used a 'multiple lines and levels of evidence' approach to demonstrate possible bird-mediated colonization of lakes by the European perch (Perca fluviatilis). We studied a set of newly-formed and isolated artificial lakes that the public is either prohibited to access because of gravel extraction or allowed to access (mainly for angling). The motivating observation is that a large proportion of prohibited-access lakes (greater than 80%) were colonized by European perch even though stocking by anglers and managers never occurred. Three supplementary lines of evidence supported avian zoochory. First, European perch spawning occurs when waterfowl abundance is very high. Second, European perch lays sticky eggs at shallow depths where they can be eaten by waterfowls or attached to their bodies. Third, genetic analyses suggested that European perch actually migrate among lakes, and that distances moved match with daily flight range of foraging waterfowl. Together, multiple lines of evidence point to avian zoochory as a probable pathway for fish colonizing remote or newly-formed freshwater ecosystems.

Keywords: Perca fluviatilis; bird-mediated colonization; freshwater.

Figure 1.

Hypothesized colonization pathways of European perch, Perca fluviatilis, in artificial gravel pit lakes: (H1) active waterborne colonization; (H2) legal stocking by lake managers; (H3) illegal ‘bucket release’ by anglers; (H4) colonization through avian zoochory.

Figure 2.

(a) Waterfowl density (individuals · km⁻²; left y-axis) between 1996 and 1998 [16] and daily water temperature measured in 2020–2021 (in degrees Celsius; right y-axis). Red square indicates the onset of European perch spawning period (8–10°C). (b) Waterfowl density (individuals · km⁻², log-transformed) of common waterfowl species observed in February, with species with dabbling tendencies displayed in blue and species with diving tendencies displayed in yellow. (c) Histogram of straight-line pairwise distances between the study lakes (bars) with migration distances inferred for potential first-generation migrants (blue dots) and the maximum distance putatively travelled by genetic-based potential first-generation migrants of European perch (blue-dotted lines), and the maximum reported foraging distance for mallard ducks (red-dotted line [19]).