Ontogenetic Shifts in Diets of Juvenile and Subadult Coho and Chinook Salmon in Coastal Marine Waters: Important for Marine Survival?

Abstract

Successfully shifting to a more piscivorous diet may be an important factor in the growth and survival of juvenile coho salmon Oncorhynchus kisutch and Chinook salmon O. tshawytscha during their first summer in the northern California Current. Nonmetric multidimensional scaling and cluster analysis of diets by size showed several distinct groupings as the salmon grew during their first marine summer. These sizebased diet differences were clearly driven by increased rates of piscivory for both species. Fish prey composition, feeding intensity, and fish prey-predator length ratios all significantly increased for coho salmon at approximately 240 mm fork length when they changed from diets dominated by juvenile rockfishes Sebastes spp., the larvae of crabs Cancer spp., and adult euphausiids to one of predominantly juvenile forage fish. As Chinook salmon grew, they gradually increased the proportional contribution (by weight) of fish prey in their diets-from 55% in the smallest length-class examined (80-100 mm) to 95% in the largest one (.375 mm). Chinook salmon fed in the same marine environments as coho salmon and consistently ate more and longer fish prey at a given size than coho salmon but had lower overall feeding intensity, perhaps owing to a higher level of prey selection. Relating subsequent interannual adult salmon returns to juvenile diets showed mixed results. During lower-survival years, coho salmon ate fewer and smaller fish prey, while subyearling Chinook salmon had less total food and more empty stomachs. We did not find consistent trophic patterns for yearling Chinook salmon in relation to their ultimate survival.

Winter ichthyoplankton biomass as a predictor of early summer prey fields and survival of juvenile salmon in the northern California Current

“…Soon after ocean entry, the majority of the diets of juvenile salmon are made up of 0-age fish prey (Daly et al 2009). In addition to the juvenile fishes eaten, juvenile salmon also eat modest amounts of crab mega-ABSTRACT: Diets of juvenile coho Oncorhynchus kisutch and Chinook O. tshawytscha salmon are made up primarily of winter-spawning fish taxa in the late-larval and early juvenile stages that are undersampled in plankton and larger trawl nets.…”

“…Larval fishes appeared to be an early and cost-effective indicator of ocean ecosystem conditions and future juvenile salmon survival. lopae and adult euphausiids (Peterson et al 1982, Brodeur 1991, Daly et al 2009), and their diets exhibit substantial seasonal and interannual variability , Weitkamp & Sturdevant 2008.…”

Estimating behavior in a black box: how coastal oceanographic dynamics influence yearling Chinook salmon marine growth and migration behaviors

“…Therefore, we relied on two simplifying assumptions: 1) the energy density of prey remained constant at 4185 J/g, and 2) the total consumed biomass consisted of half invertebrates and half fish (Daly et al 2009). All growth parameters were scaled to the 15-minute time step of the simulation.…”

“…Second, it could be the result of an ontogenetic shift in primary diet from invertebrates to fish, which occurs at about the time salmon begin the juvenile marine migration (Daly et al 2009). Correspondingly, longer coastal residence times would increase the probability that the salmon will reach a size that allows piscivorous foraging, which provides higher energetic returns and further enhances growth in a self-reinforcing manner.…”

“…In reality, food quality shifts seasonally (Daly et al 2009) and annually (Brodeur et al 2007). It is possible that differences in estimated consumption were due to differences in the relative abundance of high-quality food in the diet among months and years.…”