A structured seabird population model reveals how alternative forage fish control rules benefit seabirds and fisheries

Abstract

Fisheries for forage fish may affect the survival and reproduction of piscivorous predators, especially seabirds. However, seabirds have evolved life history strategies to cope with natural fluctuations in prey and it is difficult to separate effects of fishing on seabirds from impacts of natural variability. To date, potential impacts of forage fisheries on seabirds have mainly been explored using ecosystem models that simplify seabird–forage‐fish dynamics. We sought to explore how different forage fish harvest policies affect seabirds, accounting for structured population dynamics, life history specifics, and variation in forage fish dependencies across life stages; and how impacts vary across seabird and forage fish life histories. To explore these impacts, we developed an age‐stage structured seabird model that incorporates seabird diet specialization, foraging behavior, and reproductive strategy, as well as different functional responses between prey availability and adult survival, juvenile survival, reproductive success, and breeder propensity. We parameterized this model for two contrasting seabird life histories: (1) a low fecundity, limited foraging range, diet specialist (“restricted”); and (2) a high fecundity, wide ranging, diet generalist (“flexible”). Each was paired with two different forage fish prey archetypes that were fished under various control rules. The restricted seabird population was expectedly less robust to constant fishing pressure than the flexible seabird, and this sensitivity was mainly due to functional response parameterization, rather than other life history parameters. Particularly, the restricted seabird was highly sensitive to the relationship between prey availability and adult survival but was not sensitive to the relationship between prey and reproductive success. An adaptive biomass‐limit harvest rule for forage fish resulted in substantially higher seabird abundance compared to constant fishing across all scenarios, with minimal trade‐offs to the fishery (depending on fishery management objectives). However, mechanisms governing the impact of the forage fish fishery on the seabird varied by forage fish type. Therefore, tailoring forage fish management strategies to forage fish life history can lead to mutually acceptable outcomes for fisheries and seabirds. If data or time are limited, an adaptive control rule is likely a safe bet for meeting seabird conservation objectives with limited impacts to fisheries.

Assessing critical population thresholds under periodic disturbances

“…Stage‐structured models have been effectively used in the management of myriad taxa, especially in agricultural systems where predictions of pest and natural enemy population dynamics outcomes can critically inform management strategy decisions (Banks, Bommarco, et al, 2008; Bommarco, 2001; Westerberg & Wennergren, 2003). Applications to conservation also abound, ranging from plants (Menges, 2000) to bighorn sheep (Conner et al, 2018) to seabirds (Koehn et al, 2021) to Monarch butterflies (Grant et al, 2020) to sea turtles (Crouse et al, 1987). The latter example famously resulted in legislation being enacted aimed at bolstering conservation of an endangered species as a result of model predictions (Crowder et al, 1994).…”

Optimizing fisheries for blue carbon management: Why size matters

“…As with any approach to fisheries, focusing on a single metric can produce unintended trade-offs. In our case, managing solely for carbon storage in living tissues results in the selection of fishing strategies that could severely reduce the biomass of small species, which could jeopardize organisms like endangered seabirds that are not explicitly included in our modeled community (Koehn et al 2021). Taking an ecosystem approach to fisheries requires balancing the risk associated with management decisions.…”

Considerations for management strategy evaluation for small pelagic fishes

“…Predator-prey relationships-particularly functional responses and predation rates-may be one of the most important ecosystem processes driving the dynamics of SPFs. If possible, empirical data on functional response relationships (the shape of the relationships between predator demographic rate and prey abundance) should be used to parameterize predator responses (Koehn et al, 2021).…”

“…Predator–prey relationships—particularly functional responses and predation rates—may be one of the most important ecosystem processes driving the dynamics of SPFs. If possible, empirical data on functional response relationships (the shape of the relationships between predator demographic rate and prey abundance) should be used to parameterize predator responses (Koehn et al., 2021). These data may be available from meta‐analyses, for example, seabird functional responses to changes in forage fish abundance (Cury et al., 2011) or from case studies of individual populations (e.g.…”