The NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. The role of density and relatedness in salmon growthThe role of density and relatedness in wild juvenile Atlantic salmon growth Growth is a key life history trait in fishes that is influenced by both abiotic factors (such 18 temperature and water chemistry) and biotic factors (such as density and food availability). 19Investigating how growth performance is influenced by such factors in the wild is important 20 for understanding how population processes influence animals in natural environments and 21 for predicting the response to conservation and management strategies that manipulate these 22conditions. The theory of kin selection predicts that significant growth and survival benefits 23 are conferred upon animals associating with close relatives. However, resource competition 24 may be more intense among close relatives, and little is known about the trade-off between 25 these two processes under different ecological conditions. Here we examine the correlation 26The role of density and relatedness in salmon growth between naturally occurring densities and kin-biased growth rate using a species where kin-27 recognition has a strong impact on behaviour in laboratory studies, but where, paradoxically, 28 field investigations have failed to document predicted kin-biased growth or survival. Intra-29 and inter-family differences in growth rate of juvenile Atlantic salmon (Salmo salar) were 30 studied to examine how relatedness (groups of full-sibling fish and groups of mixed-sibling 31 fish) and sibling group (family/genotype) affects salmon parr growth, and the correlation of 32 growth rate under a range of naturally-occurring densities. Parentage and relatedness of 33 neighbouring fish were assigned using microsatellite and passive integrated transponder (PIT) 34 tags which allowed the growth estimation of individual fish. Results show that growth rate 35 was significantly influenced by both sibling group (family of origin) and also by an 36 interaction between relatedness and density. The latter finding indicates that at higher 37 densities full-sibling groups achieved higher growth rates in comparison to mixed-sibling 38 groups. Thus, the growth benefits of associating with relatives are not conferred under all 39 ecological conditions, but it becomes most apparent at high density when resource 40 competition is greatest. 41 42

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