Size-dependent growth tactics of a partially migratory fish before migration.

Ryo Futamura,Osamu Kishida,Hiroshi Sugiyama,Yoichiro Kanno,Hiroyuki Takahashi,Shoji Kumikawa,Atsushi Okuda,Yuichi Matsuoka,Jiro Uchida,Kentaro Morita

doi:10.1007/s00442-022-05111-0

Abstract

In many migratory species, smaller migrants suffer higher mortality rates during the risky migration. To minimize the size-selective mortality, migrants with smaller body sizes would need to accelerate growth rates or delay migration timing to attain a large enough body size prior to migration. To test these predictions, we investigated size-dependent patterns of growth rates and migration timing of juvenile masu salmon (Oncorhynchus masou) before their oceanic migration. We tracked uniquely marked individuals in a study population consisted of oceanic migrants and river-dwelling residents using mark-recapture surveys and PIT-tag antenna-reader system. Data supported our predictions about size-dependent growth rates and migration timing. For approximately 6months before outmigration (i.e., between the decision of migration and the start of migration), eventual migrants grew more than residents if their initial size was smaller, but such a difference in growth rate diminished for fish with larger initial sizes. In addition, smaller eventual migrants delayed the timing of outmigration compared to larger individuals, to attain a larger body size in the river prior to migration. These results suggest that size-selective mortality during migration has shaped size-dependent patterns of the pre-migration growth in migratory masu salmon. Size-conditional changes in growth rate and duration of pre-migration period may be an adaptive tactic for the migratory animals.

Highlights

Some animals definitely or conditionally migrate to another regions in particular life stages for the success of their life (Dingle & Drake, 2007; Gross et al, 1988; Newton, 2010)
Smaller prospective migrants descend the river later in the migratory period than larger ones to grow more in the river before the ocean entry. To test these operational predictions, (1) we compared size-dependent patterns of growth rates between prospective migrants and residents during 6 months leading up to migration, and (2) we investigated whether body size of prospective migrants at early spring just prior to migration explains their timing of seaward migration
Size-selective mortality during migration selects for larger body size, the costs associated with growth require individuals to optimize but not maximize growth before migration (Arendt, 1997; Dmitriew, 2011)

Summary

Introduction

Some animals definitely or conditionally migrate to another regions in particular life stages for the success of their life (Dingle & Drake, 2007; Gross et al, 1988; Newton, 2010). Migratory animals suffer varying degrees of moralities en route due to energetic demands and risk of predation Migrants begin their preparations for the long journey well before migration by adopting strategies to cope with the costs of migration. Attaining sufficiently large body size prior to migration is critical for completing the risky migration because smaller individuals are more vulnerable to a multitude of stressors during migration (Koenings et al, 1993; Sogard, 1997; Zabel et al, 2002) This suggests that pre-migration growth is under natural selection, and it leads us to expect different growth patterns between migrants and non-migrants (Gillanders et al, 2015). Because intensive growth inflicts physiological and ecological costs (Hector & Nakagawa, 2012), the faster growth of migrants before migration than residents represents an adaptive tactic of migratory individuals (Arendt, 1997; Roff, 1991)

Methods

Results

Conclusion