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Marine Ecology Progress Series

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MEPS 650:175-189 (2020)  -  DOI:

Settlement processes induce differences in daily growth rates between two co-existing ecotypes of juvenile cod Gadus morhua

Kris-Emil M. Jørgensen1, Anna B. Neuheimer1,2,3, Per Erik Jorde4,5, Halvor Knutsen4,5, Peter Grønkjær1,*

1Department of Bioscience, Aquatic Biology, Aarhus University, 8000 Aarhus C, Denmark
2Aarhus Institute of Advanced Studies (AIAS), Aarhus University, 8000 Aarhus C, Denmark
3Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
4Institute for Marine Research, Flødevigen, 4817 His, Norway
5Centre for Coastal Research, University of Agder, 4604 Kristiansand, Norway
*Corresponding author:

ABSTRACT: Co-existence of sympatric populations of a fish species is a common phenomenon. In the fjords of southern Norway, 2 ecotypes of Atlantic cod Gadus morhua co-exist during their juvenile life stage. Cod of the North Sea ecotype are on average 2 cm larger than the fjord ecotype at the end of their first growing season in October, suggesting either differences in spawning time or differences in growth during the larval and early juvenile phase. We analysed 24352 daily otolith increments from 145 cod sampled in September 2000, 2003, 2008 and 2015 at 2 locations to estimate individual hatching times and daily growth rates. There was no significant difference in hatching time between ecotypes, locations or years. Population-specific models of the effect of year, temperature and fish length on growth rates showed that the North Sea ecotype grew significantly faster than the fjord ecotype, but growth rate differences were small until the juveniles reached ~40 mm and disappeared again in juveniles larger than 110 mm. The size range (40-110 mm) corresponds to the period following settling. This study documents how vital rates (e.g. growth rates) may change rapidly at ontogenetic transition points, leading to different phenotypic trajectories in co-existing ecotypes. It also highlights the importance of settlement as a key life-history transition that may amplify small existing differences in growth rates through positive feedback of size on competitive ability and eventually recruitment.

KEY WORDS: Otoliths · Growth · Nursery · Temperature · Competition · Local adaptation

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Cite this article as: Jørgensen KEM, Neuheimer AB, Jorde PE, Knutsen H, Grønkjær P (2020) Settlement processes induce differences in daily growth rates between two co-existing ecotypes of juvenile cod Gadus morhua. Mar Ecol Prog Ser 650:175-189.

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