Exploring trade‐offs in mixed fisheries by integrating fleet dynamics into multispecies size‐spectrum models

Abstract

1. Ecosystem-based fisheries management aims to ensure ecologically sustainable fishing while maximising socio-economic benefits. Achieving this goal for mixed fisheries requires better understanding of the effects of competing fishing fleets on shared resources and economic performance. Proposed management strategies that promote either specialisation or diversification of catches may result in unintended consequences for ecosystem-based management. Here, we ask the following questions: does increased or decreased competition among fleets lead to better ecological and socio-economic fishery outcomes? How effective are currently proposed management strategies for achieving these outcomes? 2. We integrated fleet dynamics into a multispecies size-spectrum model and parameterised this model to represent Australian Southern and Eastern Scalefish and Shark Mixed Fishery. We compared the fishery status quo to two extreme scenarios: no competition, where each species is fished only by one fleet (specialisation); and maximal competition, where all fleets catch all species (diversification). To answer our second question, we considered three more plausible scenarios resulting from proposed management strategies: decreased competition due to reduced bycatch, and increased competition due to increased catches of under-utilised or valuable species. We used indicators to explore scenarios' outcomes. 3. Our model reproduced observed trends in fishing effort and yield. Extreme scenarios showed that a fishery dependent on single species management structures is more likely to achieve ecosystem-based management objectives if fleets do not compete, while maximal competition can lead to socio-economic loss as management buffers the ecological impact of diversifying. The more plausible scenarios showed little improvement over the status quo, with mixed ecological and negative economic effects.4. Synthesis and applications. Our model can be applied to assess mixed fisheries ecosystem-based management strategies. Our results show that, under single species management approaches, greatest outcomes can be achieved when fleets are specialised, whereas managing fleets that catch similar species is unlikely to be successful. They question the effectiveness of these management

Temperature‐Dependence Assumptions Drive Projected Responses of Diverse Size‐Based Food Webs to Warming

“…Next, we explored temperature effects on community dynamics using six previously developed regional multi‐species size spectrum models (MSSMs; Table 1). These models were parameterized to represent ecologically and economically important species composing food webs from the eastern Bering Sea shelf (Reum, Blanchard, et al., 2019), North Sea (Blanchard et al., 2014), Baltic Sea (Lindmark, Audzijonyte, et al., 2022), central North Pacific Ocean (Woodworth‐Jefcoats et al., 2019), Southeast Australian shelf and slope (Novaglio et al., 2022), and coastal Tasmania, Australia (Audzijonyte et al., 2023; Table 1). In each MSSM, an effort was made to include species‐specific parameters controlling life history, growth rates, fishing mortality, and feeding ecology in terms of the prey and species preference of predators.…”

“…In TBMs, asymptotic size is considered a master trait that embodies a trade‐off between reproductive output and asymptotic size, and theoretical arguments linking the two set predator abundances (Andersen et al., 2016). The multispecies size spectrum models (MSSMs) in contrast were previously developed and calibrated to different regional ecosystems spanning subpolar to tropical latitudes, and included oceanic, continental shelf, and nearshore demersal food webs (Audzijonyte et al., 2023; Blanchard et al., 2014; Lindmark, Audzijonyte, et al., 2022; Novaglio et al., 2022; Reum, Blanchard, et al., 2019; Woodworth‐Jefcoats et al., 2019). In MSSMs, species‐level detail is richer—parameters controlling growth rates, feeding interactions, and maturation and asymptotic sizes are tuned to improve ecological realism and abundances are adjusted to better match observed levels.…”

Changes in sea floor productivity are crucial to understanding the impact of climate change in temperate coastal ecosystems according to a new size-based model

“…To explore how temperature effects on fish physiology compare and interact with changes in the pelagic and benthic resource abundance and size composition, we use an MSSS modelling framework [ 13 ] and its implementation in the R package mizer [ 41 ] ( sizespectrum.org/mizer ). The framework has been used in several recent studies [ 34 , 42 – 44 ], and its theoretical basis is described in detail on sizespectrum.org , mizer vignette, and [ 13 ]. Here, we only briefly mention the key assumptions, with more details provided in S1 Text section 1.1 and Tables A and B in S1 Text .…”

“…The framework has been used in several recent studies [34,[42][43][44], and its theoretical basis is described in detail on sizespectrum.org, mizer vignette, and [13]. Here, we only briefly mention the key assumptions, with more details provided in S1 Text section 1.1 and Tables A and B in S1 Text.…”

Changes in benthic and pelagic production interact with warming to drive responses to climate change in a temperate coastal ecosystem

“…To explore how temperature effects on fish physiology compare and interact with changes in the pelagic and benthic resource abundance and size composition, we use a multi-species size spectrum (MSSS) modelling framework (Andersen 2019) and its implementation in the R package mizer (Scott et al 2014) (sizespectrum.org/mizer). The framework has been used in several recent studies (Blanchard et al 2014, Spence et al 2016, Reum et al 2019a, Novaglio et al 2021) and its theoretical basis is described in detail on sizespectrum.org, ‘mizer’ vignette and Andersen (2019). Here we only briefly mention the key assumptions, with more details provided in the Supplement.…”