Bottom-up and top-down control of small pelagic forage fish: factors affecting age-0 herring in the Strait of Georgia, British Columbia

Abstract

Small pelagic fish are key planktivores and prey in marine ecosystems, and their population abundances undergo strong temporal and spatial variability. Top-down (predator controlled) and bottom-up (prey-driven) processes during early life history are important for determining forage fish survival and recruitment. We examined biological and environmental factors hypothesized to influence age-0 Pacific herring Clupea pallasi in the Strait of Georgia (SOG), British Columbia, Canada. Primarily bottom-up processes affected interannual variability in age-0 herring abundance and condition, with some evidence of top-down effects on condition. Age-0 herring abundance increased with increasing adult spawning biomass and peaked when most adults spawned about 20 d prior to the peak spring primary production bloom. This timeline would temporally align first-feeding herring larvae with their prey, such as small copepods. Age-0 herring abundance also increased with increasing juvenile salmon abundance, indicating that conditions favourable for herring were also favourable for their predators and competitors. Age-0 herring condition decreased with increasing spawning biomass, increased when most adults spawned closer to the peak spring bloom, increased with increasing temperatures above 8.2°C, and increased then stabilized with increasing prey zooplankton density. Age-0 herring condition had a dome-shaped relationship with predator abundance, indicating that high predator abundances negatively affected fish condition. Study results suggest that density-dependent processes, such as intraspecific competition, may be important in the SOG. A positive correlation between age-0 herring abundance and subsequent age-3 recruit abundance may provide a leading indicator of low recruitment years.

Density‐dependent marine survival of hatchery‐origin Chinook salmon may be associated with pink salmon

“…Pink salmon are known to feed more heavily on zooplankton (especially epibenthic harpacticoid copepods and calanoid copepods) than Chinook salmon, who consume mostly insects and gammarid amphipods in the nearshore environments, then focus initially on decapods (crab larvae in particular) as they move offshore, and then become progressively more piscivorous by late summer or early fall (Kaczynski et al 1973, Duffy et al 2010, Osgood et al 2016. A trophic cascade may occur between the copepods preyed upon in some years by pink salmon that would otherwise be consumed by young-of-year (age-0) Pacific herring, insects, amphipods, and decapods, which support Chinook salmon (Boldt et al 2019). Chinook salmon marine survival is especially related to their feeding in offshore habitats of the Salish Sea in June-July , and changes in the prey base at lower trophic levels have been directly linked to Chinook salmon survival in the coastal ocean (Losee et al 2014).…”

“…Pink salmon diets are similar to those of forage fishes like young-of-year Pacific herring (Osgood et al 2016, Boldt et al 2019, which are very abundant in the offshore environment of the Salish Sea (Therriault et al 2009, Siple andFrancis 2016). Young-of-year herring are key prey for juvenile Chinook salmon in the Salish Sea in the summer and fall (Duffy et al 2010).…”

“…In the North Pacific Ocean, high pink salmon abundance has been thought to decrease zooplankton biomass, inducing trophic cascades down to the phytoplankton level (Shiomoto et al 1997, Batten et al 2018) that can depress the availability of prey resources for numerous species including salmon (Ruggerone et al 2003, Ruggerone and Nielsen 2004, Kaga et al 2013) and seabirds (Toge et al 2011, Springer et al 2018. High pink salmon abundance can also depress Pacific herring (Clupea pallasii) stocks through competition or predation (Deriso et al 2008, Pearson et al 2012, though this is not always the case (Boldt et al 2019). Density-dependent interactions between pink and Chinook salmon (O. tshawytscha) have also been previously hypothesized to occur during the first ocean year of the salmon in the Salish Sea Goetz 2004, Ruggerone et al 2019;Claiborne et al, in press), a rich and diverse but highly impacted inland sea in Washington State and British Columbia.…”

Kelp Forest Zooplankton, Forage Fishes, and Juvenile Salmonids of the Northeast Pacific Nearshore

“…Boldt et al. () concluded that prey resources may dictate forage fish abundance. Kelps and seagrasses provide a source of zooplankton prey, such as gammarid amphipods and harpacticoid copepods, for resident fish like young‐of‐the‐year rockfishes (Olson et al.…”

“…Juvenile salmonids and forage fishes are present seasonally in kelp habitats (Shaffer 2004;Shaffer and Ritchie 2007). Boldt et al (2019) concluded that prey resources may dictate forage fish abundance. Kelps and seagrasses provide a source of zooplankton prey, such as gammarid amphipods and harpacticoid copepods, for resident fish like young-of-theyear rockfishes (Olson et al 2019).…”