Stock–recruitment relations controlled by feeding interactions alone

Abstract

IntroductionEmpirical stock recruitment (S-R) relationships exhibit clear evidence of density dependence. Since the early works of Ricker (1954), Schaefer (1954), Beverton and Holt (1957) and others, characterizing and interpreting these relationships, much progress has been made in linking this density-dependence to ecological mechanisms. Reviewing our current understanding, Houde ( , 2009) stresses the distinction between a variety of oceanographic, climatic, and ecological mechanisms controlling recruitment and manifestly density-dependent mechanisms contributing to population regulation. While all controlling mechanisms combine to generate the observed large variability in recruitment even at constant spawning-stock biomass (SSB), only regulating mechanisms can explain the observed non-linear dependence of recruitment R on SSB. Among possible regulating mechanisms that have been considered are competition for refuge or territory, diseases or parasitism at various life stages, larval competition for oxygen, and densitydependent trophic (feeding) interactions, i.e., food intake and predation mortality, with cannibalism as a special case (Houde 2009).The goal of the present study is to test the plausibility of the hypothesis that S-R relationships are primarily shaped by the density dependencies of trophic interactions. In addition to its relevance for the interpretation of S-R data, this hypothesis also has a bearing on questions such as to what extent trophic interactions are relevant for management models or what the mechanisms are that determine the carrying capacity of individual stocks and entire communities (Batchelder and Kim 2008). Management aiming at maximum sustainable yield (MSY), in turn, hinges on a good understanding of density dependencies and carrying capacity. This is clear already from the textbook analysis of MSY for the logistic model (Conroy and Carroll 2009), where the optimal population biomass comes out as exactly half the carrying capacity.Our approach to this set of interrelated problems is to construct a species-rich fish-community model in which trophic interactions are the only source of density dependence, and to study in detail the resulting S-R relationships, relating them to observations. The model we study is a variant of the Fish Community Size Resolved Model (FCSRM) (Hartvig et al. 2011;Andersen and Pedersen 2010;Houle et al. 2012), developed based on theory by . In this model, individual fish interact exclusively through predator-prey relationships. Just as in the original model formulation by Hartvig et al. (2011), individual reproductive output is a linear function of food intake (with negative intercept), that is, S-R relationships are not a priori built into the model. Density dependencies in the model arise only from the dependence of growth and fecundity on food availability and the dependence of mortality on the abundance of predators. Individuals are characterized by their momentary body mass and their species identity, with the latter determining prey, predators a...

Dome patterns in pelagic size spectra reveal strong trophic cascades

“…1). The SSSM belongs to a family of similar models 24,46,47 going back to the Fish Community Size-Resolved Model 48–50 . Structure and motivation of the SSSM are best understood in the wider context of size-spectrum theory 16 .…”

“…A natural way to overcome this limitation is to acknowledge that, while body mass is an important trait affecting feeding interactions, the combined influence of all other traits can be even stronger 53,54 . In models this is implemented by either equipping species with abstract secondary traits 54,55 (usually assigned at random) that affect the strength of feeding interactions 56,57 , or by directly multiplying the size-dependent interaction strength with a non-negative factor sampled at random for each species pair 48,50 . Both approaches lead to food-web models in which species of similar size can have very different prey and predators, implying reduced competition and a lower likelihood of competitive exclusion.…”

The consequences of balanced harvesting of fish communities

“…Ecosystem models need one or several controlling processes to generate coexistence of a larger number of species. Specific examples are ratio-dependent functional response [35], stock-recruitment relationships [36], prey switching [37] or food webs with more weakly linked components [21,38]. The implemented stock-recruitment relationship is characterized by two things: the slope at the origin that controls the degree of density dependence, and maximum recruitment.…”

Regulation of fish stocks without stock–recruitment relationships: The case of small pelagic fish

“…This is the approach we take here. Models of multispecies, size‐spectra dynamics, have often imposed an SRR as an input (Scott, Blanchard, & Andersen, 2014), but it is not necessary to do so (see also: Andersen et al., 2017; Rossberg et al., 2013).…”

“…Density dependence in somatic growth and reproduction is built into this modelling framework, so it can be used to examine the potential role (a) of growth through the larval stage, and (b) of competition for food for reproduction (MacCall, 1980; Takasuka et al., 2019a). Previous studies using size‐spectrum models in this research area include analysis of the role of trophic interactions in generating SRRs in multispecies assemblages (Rossberg, Houle, & Hyder, 2013), effects of spatial dependence on the stage in life at which regulation of fish stocks could occur (Andersen et al., 2017), and exploration of whether strength of density‐dependent growth in fish stocks is sufficiently strong to reduce optimal fishery size‐at‐entry to below size‐at‐maturity (van Gemert & Andersen, 2018).…”