Columbia River plume fronts. II. Distribution, abundance, and feeding ecology of juvenile salmon

Abstract

Well-defined fronts develop at the seaward edge of riverine plumes where suspended materials and planktonic organisms are concentrated by convergent water flows. Riverine plume fronts have been hypothesized to be favorable fish habitats because they can lead to localized prey aggregations. We examined the spatial distribution of juvenile Pacific salmonids Oncorhynchus spp. in and around plankton-rich frontal regions of the Columbia River plume to test the hypothesis that juvenile salmonids aggregate at riverine plume fronts to feed. Juvenile salmonids tended to be abundant in the frontal and plume regions compared to the more marine shelf waters, but this pattern differed among species and was not consistent across the 2 study years. Stomach fullness tended to be higher in the more marine shelf waters than either the front or plume areas, which does not support the hypothesis that salmonids consistently ingest more prey at frontal regions. Many prey organisms were disproportionately abundant at these fronts, but salmon stomach-content analysis did not reveal higher stomach contents at fronts or identify prey groups indicative of feeding in the frontal areas. Although our results indicate that the Columbia River plume influences the distributions of juvenile salmon, our observations do not support the hypothesis that juvenile salmonids congregate to feed at fronts at the leading edge of the Columbia River plume. The short persistence time of these fronts may prevent juvenile salmon from exploiting these food-rich, but ephemeral, features.

Have droughts and increased water extraction from the Murray River (Australia) reduced coastal ocean productivity?

“…Although a limited number of studies reflect on potential implications of a complete lack of river outflows on marine environments, it is generally understood that changing the scale, frequency and seasonality of river outflows can have severe impacts on species that require food pulses for successful reproduction or larval survival (Drinkwater and Frank 1994;Whitfield and Marais 1999). Several studies have identified important relationships between outflow-stimulated productivity and populations of marine species and habitats as a result of these controls (Burrage et al 2002;De Robertis et al 2005;Burla et al 2010;Brookes et al 2015;Black et al 2016). There is also a critical period at the point of first feeding by larvae where the presence of suitable amounts and types of food determine year-class strength (Hjort 1914).…”

Seasonal Juvenile Salmonid Presence and Migratory Behavior in the Lower Columbia River

“…During 12 days of surface trawling in May of 2001 and 2002, densities of juvenile yearling coho salmon, yearling Chinook salmon, and steelhead were generally higher in the plume compared to more marine waters, reflecting the outmigration of large numbers of juvenile salmon from the Columbia River estuary into coastal waters during this time (De Robertis et al 2005). Chum salmon were rare in the plume, possibly because Columbia River chum salmon populations were at low levels during the time of this study (De Robertis et al 2005).…”

“…Many Columbia River salmonids, such as yearling coho salmon, yearling Chinook salmon, chum salmon, and steelhead, enter the ocean during late spring and early summer when river flows are high and the low-salinity plume is well developed (De Robertis et al 2005). During 12 days of surface trawling in May of 2001 and 2002, densities of juvenile yearling coho salmon, yearling Chinook salmon, and steelhead were generally higher in the plume compared to more marine waters, reflecting the outmigration of large numbers of juvenile salmon from the Columbia River estuary into coastal waters during this time (De Robertis et al 2005).…”

“…While in the plume and adjacent waters, yearling coho and yearling Chinook salmon feed on a variety of forage fish, crab megalopae, adult euphausiids, pteropods, and hyperiid amphipods, whereas juvenile chum salmon seem to feed exclusively on planktonic prey (De Robertis et al 2005). Juvenile salmonids captured outside the plume are often larger in size and have fuller stomachs than those captured in the plume (Fisher and Pearcy 1995;De Robertis et al 2005).…”

“…Juvenile salmonids captured outside the plume are often larger in size and have fuller stomachs than those captured in the plume (Fisher and Pearcy 1995;De Robertis et al 2005). These results suggest that once fish made the…”