Meeting Abstract
Forage fish, pelagic schooling fish such as sardines and anchovies, are a vital node in ’wasp waist’ food webs, linking lower and higher trophic levels, and are the main food source for a diverse assemblage of predators. Forage fish such as the anchovy, Engraulis mordax, and Pacific sardine, Sardinops sagax, exploit a variety of feeding strategies, with differing energetic consequences. At large relative prey size, they capture by biting, but when prey are small they employ ram filter feeding. Under filter feeding, the mouth is kept wide open, greatly increasing drag in the ultra-dense aquatic environment. These fish switch from highly streamlined, efficient swimming, to highly non-hydrodynamic, energetically costly swimming depending on the food available. A major consideration however, is that school integrity is maintained, a vital strategy against most predators, and so body kinematics must change between swimming modes to maintain speed. We examine the biomechanical challenges of maintaining swimming speed under extremely low and extremely high drag in E. mordax and S. sagax. Using high-speed video, we present detailed kinematic analyses of three swimming modes in these sympatric species; ram, stroke-and-glide and fast escapes. We present data on both behavior in the school, and also detailed midline kinematics of individuals. The dynamics of sardine and anchovy populations follow long-scale, ‘boom-bust’ fluctuations, particularly in the eastern Pacific, which are still not fully understood, but likely include the effects of climate. Based on our analyses, we predict that the energetic consequences of ram filter feeding may also play an important role in the foraging efficiency and fitness of forage fish in different environmental regimes.
