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

Abstract

Small pelagic fish lack clear stock–recruitment relationships. This is a problem because such relationships are taken to be the primary descriptors of density dependence, responsible for regulating population density. In this paper, we show that small pelagic fish species, anchovy (Engraulis spp., Engraulidae), living in a stochastic environment, can be strongly regulated without a stock–recruitment relationship emerging. This is done through numerical analysis of a size‐spectrum model, in which fish grow by eating and die in part from being eaten, with the result that birth, growth and death are all density‐dependent. The model includes cannibalism, and growth‐dependent larval mortality, both of which have been suggested as regulatory mechanisms in anchovy, together with growth and reproduction later in life. Despite the lack of a clear stock–recruitment relationship in the presence of stochasticity, signals of density dependence in the vital rates remain clear, suggesting that they might prove to be better indicators of density dependence than stock–recruitment relationships in small pelagic fish.

Density-dependent attributes of schooling in small pelagic fishes

“…In small pelagic fishes, a strong density-dependent decline in body growth is often observed during times of increasing population density (Canales et al ., 2020; Kamimura et al ., 2021a; Wada et al ., 1995; Watanabe & Yatsu, 2004). Density-dependent growth reduction is mainly caused by intraspecific competition for food (Hansen et al ., 1999; Jenkins et al ., 1999; Kamimura et al ., 2021a; Post et al ., 1999).…”

Projected Shifts in 21st Century Sardine Distribution and Catch in the California Current

“…While density-dependent mortality or crowding effects on food limitation may help improve the accuracy of future projections, such additions to the model would significantly increase uncertainty because of alternative ways to combine larval and juvenile processes to mimic the historical spawner-recruit relationship. Furthermore, the historical spawner-recruit relationship for forage species like sardine has its own uncertainties about how it affects population dynamics (Canales et al, 2020), and the sardine relationship for the CCS exhibits high variability, reflects past management actions, and will likely change under future conditions. The possibility of density-dependence in the adult stage must also be considered (Lorenzen, 2008;Andersen et al, 2017).…”

Fishing for biodiversity by balanced harvesting

“…Strong-enough self-limitation of this kind can lead to coexistence, analogous to diagonal dominance of unstructured Lotka-Volterra food webs (Hofbauer and Sigmund, 1988, p. 193). However density-dependent feedbacks are already built into size-spectrum models, and stockrecruitment relationships can emerge naturally from them in a deterministic setting (Canales et al, 2020). So we avoided imposing arbitrary stock-recruitment relationships, and instead did three things to facilitate coexistence.…”

“…First, an important negative feedback at the larval stage was incorporated (Ricker and Foerster, 1948;MacCall, 1980;Canales et al, 2020). The feedback operates in the following way.…”

“…Conversely, when larval density is relatively high, growth is slower and the accumulated risk of death in the larval stage becomes greater. This natural feedback mechanism has a strong stabilizing effect when plankton are coupled to single-species size-spectra of fish (Canales et al, 2020). It needs only a high larval mortality rate to be included in the model; food-dependent growth, built into size-spectrum models, does the rest.…”