Partial diel vertical migration of sympatric vendace (Coregonus albula) and Fontane cisco (Coregonus fontanae) is driven by density dependence

Recent studies have indicated that in fish populations performing diel vertical migrations (DVM), some individuals do not migrate but reflect a resident phenotype, a pattern named as partial DVM. I present data on fish densities and the proportion of residents in Lake Stechlin (Germany) as obtained by annual midwater trawling over four discrete depths during nighttime over 8 years. The lake is inhabited by the sympatric vendace (Coregonus albula) and Fontane cisco (Coregonus fontanae). The proportion of vendace residents increased with the density of vendace, whereas the proportion of Fontane cisco residents declined with increasing density, indicating that density plays a role in the migration patterns for both species, but in opposite directions. There were almost no differences in mean size, size-frequency distributions, or Fulton condition factor between resident or migrant parts of the populations in both species. However, the proportion of dry mass in wet mass, which indicates individual nutritional status, had a tendency to be lower in migrants than in residents in both species in the years 2011,2012, and 2013. These data suggest that density dependence may be an important factor that modifies the proportion of residents in vertically migrating fish populations. In contrast, length-dependent predation vulnerability or systematic individual differences in nutritional status were not strongly supported as potential predictors of the proportion of residents. It needs to be discussed whether partial DVM is conceptually similar to partial seasonal migrations of fish, or whether DVM reflects variants of the ideal free distribution, which are inherently density-dependent. Des etudes recentes indiquent que, dans les populations de poissons caracterisees par des migrations verticales nycthemerales (MVN), certains individus ne migrent pas, refletant un phenotype resident, un motif appele MVN partielle. Je presente des donnees sur la densite de poissons et la proportion de residents dans le lac Stechlin (Allemagne), obtenues par peche annuelle au chalut pelagique a quatre profondeurs differentes durant la nuit, sur une periode de 8 ans. Le lac abrite le coregone blanc (Coregonus albula) et le coregone de Fontane (Coregonus fontanae), des especes sympatriques. La proportion de coregones blancs residents a augmente parallelement a la densite de cette espece, alors que la proportion de coregones de Fontane residents a diminue parallelement a une augmentation de leur densite, ce qui indique que la densite joue un role, bien que de sens opposes, dans les habitudes migratoires des deux especes. Il n'y avait presque aucune difference sur le plan de la taille moyenne, des distributions de frequences de taille ou du facteur de condition de Fulton entre les parties residentes et migratrices des populations des deux especes. Cependant, la proportion de masse seche dans la masse humide, qui indique l'etat nutritionnel individuel, tendait a etre plus faible chez les migrants que chez les residents des deux especes pour les annees 2011, 2012 et 2013. Ces donnees donnent a penser que la dependance a la densite pourrait etre un important facteur de modification de la proportion de residents dans les populations de poissons a migration verticale nycthemerale. En revanche, les donnees n'appuient pas fortement la notion voulant que la vulnerabilite a la predation dependante de la longueur ou des differences individuelles systematiques de l'etat nutritionnel constituent de possibles variables explicatives de la proportion de residents. Il convient d'examiner si la MVN partielle est conceptuellement semblable aux migrations saisonnieres partielles de poissons ou si la MVN reflete des variantes de la distribution libre ideale qui sont intrinsequement dependantes de la densite. [Traduit par la Redaction]
Introduction Diel vertical migration (DVM) is a common behaviour of fish in freshwater and marine habitats. Typically, pelagic fish ascend during dusk from their deeper daytime habitats into shallower layers [...]