Egging on aggression—Poor salmon returns increase aggression among egg-predating fishes
A field study developed by members of the Moore Lab stationed at the Keogh River on Vancouver Island suggests that low returns of salmon can result in an increase in aggressive behavior among fish species that prey upon their eggs.
Over the past 40 years, pink salmon returns to the Keogh River have fluctuated from fewer than 1,000 to more than 100,000 spawning individuals.
From late September through October, pink salmon eggs offer a nutrient-rich food subsidy for many fish species in the Keogh. Salmon eggs are made available when they are unsuccessfully buried during the spawning process.
The availability of these eggs may generate not only competition between species (interspecific competition), but also competition within a species (intraspecific competition).
To mimic the natural fluctuations in food availability, the researchers conducted a field experiment where they deployed varying quantities of eggs (ranging from 6 to 3,575) and recorded underwater videos of the competitive interactions between species.
The study found that when food availability was low (fewer eggs available), aggressive behavior increased. In addition, they found a distinct hierarchy among species, with cutthroat trout—the largest species—exerting dominance over smaller species such as juvenile steelhead trout.
Larger individuals of a single species were found to regularly attack smaller individuals of the same species. The most frequent aggressive behaviors were observed between coho salmon.
While aggressive behavior can result in higher consumption of energy-rich eggs, which improves growth and survival; it also comes with costs. Aggressive interactions are energetically expensive to the aggressor, leaving less energy for growth, and results in physical damage to the victim, which can negatively impact its survival.
The Keogh River has been the focus for long-term salmonid research by the Moore lab for many years, in collaboration with Instream Fisheries, Provincial Scientists, and First Nations.
This study concludes that when there are few available salmon eggs, such as in years with poor salmon returns, local fish will not only have less to eat, but will increase fighting activities, and expend more energy.
Bailey, C.J., L.C. Andersson, M. Arbeider, K. Bradford, and J.W. Moore. 2019. Salmon egg subsidies and interference competition among stream fishes. Environmental Biology of Fishes. 1-12. (PDF)