Differential effects of internal tagging depending on depth treatment in Atlantic salmon:a cautionary tale for aquatic animal tag use

Electronic tags are widespread tools for studying aquatic animal behavior;however,tags risk behavioral manipulation and negative welfare outcomes.During an experiment to test behavioral differences of Atlantic salmon Sal mo salar in different aquaculture cage types,including ones expected to elicit deeper swimming behavior,we found negative tagging effects depending on whether cages were depth-modified.In the experiment,data storage tags implanted in Atlantic salmon tracked their depth behavior and survival in unmodified sea-cages and depth-modified seacages that forced fish below or into a narrow seawater-or freshwater-filled snorkel tube from a 4 m net roof to the surface.All tagged individuals survived in unmodified cages;however,survival was reduced to 62%in depth-modified cages.Survivors in depth-modified cages spent considerably less time above 4 m than those in unmodified cages,and dying individuals in depth-modified cages tended to position in progressively shallower water.The maximum depth that fish in our study could attain neutral buoyancy was estimated at 22 m in seawater.We calculated that the added tag weight in water reduced this to 8 m,and subtracting the tag volume from the peritoneal cavity where the swim bladder reinflates reduced this further to 4 m.We conclude that the internal tag weight and volume affected buoyancy regulation as well as the survival and behavior of tagged fish.Future tagging studies on aquatic animals should carefully consider the buoyancy-related consequences of internal tags with excess weight in water,and the inclusion of data from dying tagged animals when estimating normal depth behaviors.

Influence of prey availability on the movement pattern of breeding saker falcons(Falco cherrug)in Mongolia

As an apex predator in arid steppe,saker falcon plays a crucial role in maintaining ecosystem balance.Understanding their movement patterns concerning conspecific competition and prey availability is important for their conservation.We aim to understand how movement pattern of breeding saker falcons relates to prey availability.Twelve adult sakers were tracked during the breeding seasons,using satellite transmitters.Throughout the breeding season,sakers exhibited territorial behavior,with minimal overlap in range use among neighboring conspecifics.Males occupied larger areas(mean=2,397 ha,median=1,221 ha),compared to females(mean=1,241 ha,median=554 ha),and it remained consistent throughout breeding season.Female home range size increased notably during a late nestling stage and post-fledging dependence periods,indicating release from brooding constraints.Notably,we found a negative relationship between male home range size and prey availability.In high rodent density areas,males occupied smaller areas(mean=1,135 ha,median=1,034 ha),contrasting with low rodent density areas(mean=6,815 ha,median=6,516 ha).However,no significant relationship was observed between female home range size and prey availability.We also noted instances of nest abandonment and early dispersal behavior in some females,potentially influenced by a handicapping effect of tagging.Our findings offer valuable insight into how prey availability shapes saker falcon space use and defines their spatial requirements for successful breeding.This knowledge is crucial for the conservation and management of globally endangered saker falcons,informing targeted habitat protection and resource management strategies.