Trophic position scales positively with body size within but not among four species of rocky reef predators

Abstract

Intra- and interspecifically, larger-bodied predators generally occupy higher trophic positions (TPs). With widespread declines in large predators, there is a need to understand their size-based trophic roles to predict ecosystem-level responses. In British Columbia, Canada, we examined size-based trophic interactions between predatory fishes—3 rockfish species (genus Sebastes) and lingcod Ophiodon elongatus—and their prey, converting predator δ15N signatures to TP and analyzing stomach contents. Intraspecifically, TP scaled positively with predator length and gape width, but the rates of change varied by species. Interspecifically, TP did not scale positively with the observed mean sizes or known maximum sizes of species. Lingcod TP was lower than that of yelloweye and quillback rockfishes, which were 51 and 37%, respectively, smaller than lingcod. Yellowtail rockfish had the smallest average size, yet their mean TP did not differ significantly from that of lingcod. Neither species differences in some morphometric traits known to influence body size-TP relationships nor phylogenetic history explained these results. Most prey consumed were <20% of the predator’s size, which might partially explain the lack of a size-based trophic hierarchy among species. Currently, large size classes of rockfishes are being lost due to fisheries and perhaps climate-driven changes. Our findings on intraspecific size-TP relationships indicate that fishery removals of large individuals may diminish trophic structures. Interspecific comparisons of TP suggest that, along with size, species remain an important factor in understanding trophic dynamics. In addition, smaller-bodied predator species may have significant ecological roles to be considered in ecosystem-based fisheries management.

Growth parameter k and location affect body size responses to spatial protection by exploited rockfishes

“…Critically, the trophic position of rockfishes increases with their individual body sizes ( Trebilco et al, 2016 ; Olson et al, 2020 ). Larger size classes of yelloweye rockfish ( S. ruberrimus )—a long-lived demersal species inherent to Indigenous diets and undergoing size declines ( Eckert et al, 2017 ; McGreer & Frid, 2017 )—occupy a particularly high trophic position in rocky reefs ( Olson et al, 2020 ). Ensuring that RCAs effectively restore large size structures for exploited species, therefore, is essential for ecosystem-based fishery management.…”

“…Understanding factors affecting body size distributions is particularly important because fisheries remove the largest individuals, often at great ecological cost (Strong & Frank, 2010;Hixon, Johnson & Sogard, 2014). Larger individuals tend to occupy higher trophic positions (e.g., Trebilco et al, 2016;Olson et al, 2020), and their removal may alter food web structure (Shackell et al, 2010;Zgliczynski & Sandin, 2017). Further, for many fish taxa the relationship between fecundity and body size is hyperallometric (a power function with exponents > 1), such that larger females contribute disproportionately more offspring (per unit of body size) than smaller females (Dick et al, 2017;Barneche et al, 2018).…”

Hotspots for rockfishes, structural corals, and large-bodied sponges along the central coast of Pacific Canada

“…They include upper-level predators (e.g. yelloweye rockfish 10 ) that may indirectly benefit smaller organisms via trophic cascades 11 , anadromous species that transport nutrients from offshore areas to estuaries and riparian ecosystems (e.g. Pacific salmon 12 , eulachon 13 ), and foundation species that create biogenic habitats (e.g.…”

“…Rockfish are well-suited for hotspot delineation at small spatial scales. They include sedentary species with long maximum lifespans (>100 years) 36 and high trophic positions 10 , and therefore are fishery vulnerable 37 and ecologically important 11 . The genus has been an important food for coastal peoples for over 2500 years 38 .…”

Peer Review #1 of "Growth parameter k and location affect body size responses to spatial protection by exploited rockfishes (v0.1)"

“…Larger individuals tend to occupy higher trophic positions (e.g. Trebilco et al, 2016;Olson et al, 2020), and their removal may alter food web structure (Shackell et al, 2010;Zgliczynski & Sandin, 2017). Further, for many fish taxa the relationship between fecundity and body size is hyperallometric (a power function with exponents >1), such that larger females contribute disproportionately more offspring (per unit of body size) than smaller females (Dick et al, 2017a; Barneche et al, 2018).…”