Abstract
Migration is an important event in many animal life histories, but the degree to which individual animals participate in seasonal migrations often varies within populations. The powerful ecological and evolutionary consequences of such partial migration are now well documented, but the underlying mechanisms are still heavily debated. One potential mechanism of partial migration is between-individual variation in body condition, where animals in poor condition cannot pay the costs of migration and hence adopt a resident strategy. However, underlying intrinsic traits may overrule such environmental influence, dictating individual consistency in migratory patterns. Unfortunately, field tests of individual consistency compared to the importance of individual condition on migratory propensity are rare. Here we analyse 6 years of field data on roach migration, gathered by tagging almost 3000 individual fish and monitoring their seasonal migrations over extended periods of time. Our aims were to provide a field test of the role of condition in wild fish for migratory decisions, and also to assess individual consistency in migratory tendency. Our analyses reveal that (1) migratory strategy, in terms of migration/residency, is highly consistent within individuals over time and (2) there is a positive relationship between condition and the probability of migration, but only in individuals that adopt a migratory strategy at some point during their lives. However, life-long residents do not differ in condition to migrants, hence body condition is only a good predictor of migratory tendency in fish with migratory phenotypes and not a more general determinant of migratory tendency for the population. As resident individuals can achieve very high body condition and still remain resident, we suggest that our data provides some of the first field evidence to show that both facultative and obligate strategies can co-exist within populations of migratory animals.
Highlights
Migration is a central part in the life history of many animals [1,2,3], where migrants can benefit from temporal variation in the profitability of different habitats [4,5]
Whereas previous studies have often focused on the heritability of migratory propensity to show a genetic influence upon partial migration [24] or on experimental manipulation of the environment to show influence of environmental factors on partial migration [15,25], we here take an observational approach, where we study natural variation in somatic condition and its effect on individual migration in combination with individual consistency in migration/residency
Individual consistency in migratory propensity Fish migrating the second year after tagging were significantly more likely to have migrated in the previous year as compared to the whole population (x2 = 160.7; p,0.001)
Summary
Migration is a central part in the life history of many animals [1,2,3], where migrants can benefit from temporal variation in the profitability of different habitats [4,5]. Partial migration is widespread among animals [7,8], and can influence both trophic dynamics [9,10] and population divergence [11,12], suggesting that knowledge on the underlying mechanisms is much needed [7,13]. A quarter of a century ago Lundberg [14] suggested that partial migration is controlled by a combination of fixed and variable factors. More recent theory has suggested, as Lundberg did, that both environmental and genetic influences have to be taken into account when considering mechanisms of partial migration [18]. Irrespective of the relative importance of genetic and environmental factors, migration is generally considered to be caused by seasonal or ontogenetic changes in habitat-specific costbenefit tradeoffs. Environmental variability may drive partial migration through relative success of residents and migrants dependent on their somatic condition [15]
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