Seabird‐induced natural mortality of forage fish varies with fish abundance: Evidence from five ecosystems

Abstract

Forage fish populations often undergo large and rapid fluctuations in abundance. However, most of their predators are buffered against such fluctuations owing to their slower pace of life, which allows them to maintain more stable populations, at least during short periods of food scarcity. In this study, we investigated top-down processes exerted by seabirds on forage fish stocks in five contrasted marine ecosystems, compiling numerous datasets on seabird counts, diets, energetic needs and prey energy content and abundance. Off Norway, South Africa, Peru, Sweden and Scotland, we found that predation pressure-estimated as the proportion of a fish stock consumed by seabirds-was generally low (median = 1%), but increased sharply at low levels of prey abundance. When prey biomass decreased below 15 to 18% of its maximum recorded value, predation by seabirds became a source of important additional pressure on prey stocks (~20% of prey biomass is consumed by seabirds). An earlier empirical study advocated for keeping forage stocks from falling below a threshold of 33% of long-term maximum prey biomass in order to safeguard seabird breeding success, but here we further suggest that a threshold of 18% should be considered as a limit not to be exceeded for the sake of the forage fish themselves, and below which extra cautious management of fisheries may be required. Nevertheless, despite exceptionally high rates of predation on some occasions, predation pressure was not correlated with prey dynamics, suggesting an absence of prey entrapment due to seabirds alone in these five ecosystems.

Integrating seabird dietary and groundfish stock assessment data: Can puffins predict pollock spawning stock biomass in the North Pacific?

“…The relative proportion of a fish species represented in the predator diet is commonly used and has been correlated with abundance of age‐0 fish in local environments (e.g., Mills et al, 2007; Piatt et al, 2018), but this relationship varies with degree of diet specialization and spatio‐temporal overlap between predator and prey. Recognizing the potential significance of sampling of age‐0 fish early on, Cairns (1992) suggested in a unique paper that several seabird‐based dietary parameters could be used as leading indicators of potential recruitment and year class strength, as well as direct inputs needed for models of variable natural mortality (e.g., Hill et al, 2020; Saraux et al, 2021). Notably, Cairns assumed that relationships between seabird diet and year class strength would be non‐linear.…”

A Cursory Look at the Fishmeal/Oil Industry From an Ecosystem Perspective

“…Homing in on seabirds, a global comparative study based on observational data showed a significant decline in the breeding success of seabirds across multiple marine ecosystems when their prey bases dropped to below one third of maximum abundance levels (Cury et al, 2011). Furthermore, a recent study drawing on data from the same ecosystems suggests that forage fish should be carefully managed so as to remain at levels above 18% of maximum abundance levels, since predation mortality inflicted on forage fish by seabird predators significantly increases below this fish abundance level (Saraux et al, 2020). The need for conservative management measures at low stock levels may be even more crucial as we suffer the effects of climate change.…”

“…Several marine species inhabiting South African waters, and relying on forage fish as prey, are either charismatic and important for ecotourism, commercially valuable and/or are species of conservation concern, including three endemic species of endangered seabirds (African penguin Spheniscus demersus, Cape gannet Morus capensis, and Cape Cormorant Phalacrocorax capensis), great white shark Carcharodon carcharias, Bryde's whale Balaenoptera edeni, common dolphin Delphinus delphis, dusky dolphin Lagenorhynchus obscurus, and Cape fur seal Arctocephalus pusillus. Sardine Sardinops sagax and anchovy Engraulis encrasicolus comprise notable portions of the diet of many top predators off South Africa: African penguin -79%, Cape gannet -58%, Cape Cormorant -98%, and snoek Thyrsites atun -46%, Cape hake Merluccius capensis and M. paradoxus -10 to 40% depending on hake size class considered, yellowtail Seriola lalandei -30%, geelbek Atractoscion aequidens -20%, Bryde's Whale -82%, cetaceans in general -37%, and Cape fur seal -30% (Saraux et al, 2020). Economically valuable predatory fish include geelbek and yellowtail, both especially caught in the line fishery, and both heavily dependent on availability of anchovy and sardine prey.…”

“…With respect to sustaining ecosystem functioning, there is high variability in both direction and magnitude of change in biomass of predatory fish in response to changes in catch of forage fish, dependent on ecosystem, local predator-prey relationships, and the spatial and temporal scales examined (e.g., Smith et al, 2011;Koehn et al, 2017). Seabirds and marine mammals often show biomass (and also foraging and breeding) responses closely reflecting forage fish availability (Koehn et al, 2017;Campbell et al, 2019;Saraux et al, 2020). Further, it is no simple matter to compare economic trade-offs between forage fish and their predators (Koehn et al, 2017;Konar et al, 2019), and a full socio-economic analysis is warranted at the local scale.…”

Diet and prey size preference in Razorbills Alca torda breeding at Stora Karlsö, Sweden

“…Seabirds are important indicator species for changes in fish stock composition, and thus the status of the marine food web and ecosystem as a whole (Furness & Camphuysen 1997, HELCOM 2018. As seabirds compete with forage fisheries for prey, estimating their prey requirements is essential for achieving good conservation status (Sydeman et al 2017, Saraux et al 2020, Hentati-Sundberg et al 2021. Diet and prey choice are key parameters for estimating prey consumption and prey requirements, and good estimations of these parameters are necessary for evaluating potential overlap between fisheries and marine top predators (Hansson et al 2018).…”