Behaviors in groups: Measuring aggression between native bluegill (Lepomis macrochirus) and invasive mosquitofish (Gambusia affinis)

Background/Question/Methods

As an invasive species mosquitofish (Gambusia spp.) are known to have negative impacts on several native species including fish and amphibians.  However little is known about the interactions between mosquitofish and bluegill sunfish (Lepomis macrochirus).  These species are both zooplanktivorous fish that may consume similar prey and have overlapping habitat use in ponds.  This overlap is especially true of juvenile bluegill living in vegetation in shallower waters along the shore.  In addition to influencing other species, mosquitofish also show intraspecific aggression, and in particular male harassment of females.  Mosquitofish are often introduced to ponds as a mosquito control agent, but their introduction may come at a cost to native species like bluegill that may be affected by mosquitofish aggressive or foraging behaviors.  The results of this study shed light on the aggressive behavior of the mosquitofish (G. affinis) and the potential impacts that may exist as a consequence.  We quantified aggression by mosquitofish and bluegill in experimental trials consisting of different fish assemblages (e.g., bluegill alone, or bluegill with male and female mosquitofish).  Groups of fish were filmed and aggressive behaviors were recorded in the presence and absence of food. 

Results/Conclusions

Male mosquitofish nipped, chased, and displayed intention movements towards other males more often when only male mosquitofish were present, and the presence of female mosquitofish or bluegill depressed these aggressive behaviors.  Male mosquitofish nipped females less often when bluegill were present, indicating that the presence of the bluegill dampened the amount of aggression that male mosquitofish exhibit toward female mosquitofish.  Male mosquitofish showed significantly less aggression toward bluegill when food was present.  In contrast, aggressive behaviors by female mosquitofish were not influenced by the presence of other fish, but in the presence of food females displayed more total aggressive actions.  Bluegill showed fewer aggressive behaviors in the presence of food, regardless of the fish assemblage.  They also had fewer intention movements when female mosquitofish were present in the assemblage.  When females were absent, bluegill showed a strong trend of more total aggressive actions towards male mosquitofish.  Our experiment suggests that the presence of food reduces aggressive interactions in these fish assemblages, and that the structure of the assemblage can be important in moderating aggressive behaviors between the three groups.