Evol Ecol Res 20: 487-504 (2019)     Full PDF if your library subscribes.

Contemporary phenotypic divergence of an introduced predatory freshwater fish,
the northern pike (Esox lucius)

Katja I. Berghaus, Joseph R. Spencer & Peter A.H. Westley

College of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, Alaska, USA

Correspondence: K.I. Berghaus, College of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, AK 99775-7220, USA.
email: kiberghaus@alaska.edu


Background: Northern pike (Esox lucius) were introduced to southcentral Alaska in the 1950s and have subsequently colonized multiple lakes and rivers. These populations serve as an opportunistic natural experiment to understand the rate and form of phenotypic divergence.

Question: How have pike diverged in their native and introduced range and how quickly? Do morphological differences correlate with diet-mediated growth or characteristic lake or river habitat attributes?

Hypotheses: Given the known time since colonization, populations that have been separated for longer are likely to have diverged further from their source population than populations that have been separated for less time. Morphology will be at least in part shaped by growth and major (riverine vs. lake) habitat differences.

Analytical methods: We studied phenotypic divergence among nine putative populations of northern pike with a principal component analysis of individual measurements of body dimensions as well as counts of pigmentation (i.e. spotting pattern). Size-at-age was used as a proxy for growth rate and tested for associations with morphology. Evolutionary rates (haldanes) for the phenotypic divergence of each population from the putative source population were calculated and compared to a data set of ~2700 rates from different taxa to put the observed divergence into context.

Results: Morphological differences were detected between native and invasive groups. Invasive populations were significantly deeper bodied and had shorter heads than native populations. Body shape variation could only partially be explained by age, length, and weight (growth proxy), with values of 27% and 24% for PCI and PCII, respectively. Habitat type (river vs. lake) was significantly associated with body shape in native populations, but data limitations preclude a comparison among invasive populations. The detected phenotypic divergence estimates fall near the median of previous evolutionary rate estimates.

Keywords: morphometrics, northern pike, invasion, haldane, phenotypic divergence, shape variation

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