Mathematical and statistical models underlie many of the world's most important fisheries management decisions. Since the 19th century, difficulty calibrating and fitting such models has been used to justify the selection of simple, stationary, single-species models to aid tactical fisheries management decisions. Whereas these justifications are reasonable, it is imperative that we quantify the value of different levels of model complexity for supporting fisheries management, especially given a changing climate, where old methodologies may no longer perform as well as in the past. Here we argue that cost-benefit analysis is an ideal lens to assess the value of model complexity in fisheries management. While some studies have reported the benefits of model complexity in fisheries, modeling costs are rarely considered. In the absence of cost data in the literature, we report, as a starting point, relative costs of single-species stock assessment and marine ecosystem models from two Australian organizations. We found that costs varied by two orders of magnitude, and that ecosystem model costs increased with model complexity. Using these costs, we walk through a hypothetical example of cost-benefit analysis. The demonstration is intended to catalyze the reporting of modeling costs and benefits.

Population irruptions of the Pacific crown-of-thorns starfish (CoTS, Acanthaster cf. solaris) are a key source of coral loss on Australia’s Great Barrier Reef (GBR). CoTS management currently involves their manual control (culling) to threshold densities below which net coral decline theoretically ceases based on analysis of a validated population dynamics model. Spatial variability in coral growth and community composition, and their predicted changes under continuing global warming, necessitate further consideration of current coral representation in CoTS models. Here, we consider the sensitivity of equilibrium coral-CoTS thresholds to coral growth rates and consider how the demographic composition of CoTS at a site may relate to culling thresholds. We found thresholds should be location-specific if the objective of CoTS control is coral recovery, but location-specific thresholds may not be needed if the objective is to limit coral cover loss based on coral growth and CoTS demography. The consequence of using a higher CPUE threshold than the analytical equilibrium coral-CoTS thresholds in terms of coral cover loss is suggested to be limited at coral cover < 40

Integrated pest management (IPM) leverages our understanding of ecological interactions to mitigate the impact of pest species on economically and/or ecologically important assets. It has primarily been applied in terrestrial settings (e.g., agriculture), but has rarely been attempted for marine ecosystems. The crown-of-thorns starfish (CoTS), Acanthaster spp., is a voracious coral predator throughout the Indo-Pacific where it undergoes large population increases (irruptions), termed outbreaks. During outbreaks CoTS act as a pest species and can result in substantial coral loss. Contemporary management of CoTS on the Great Barrier Reef (GBR) adopts facets of the IPM paradigm to manage these outbreaks through strategic use of direct manual control (culling) of individuals in response to ecologically based target thresholds. There has, however, been limited quantitative analysis of how to optimize the implementation of such thresholds. Here we use a multispecies modeling approach to assess the performance of alternative CoTS management scenarios for improving coral cover trajectories. The scenarios examined varied in terms of their ecological threshold target, the sensitivity of the threshold, and the level of management resourcing. Our approach illustrates how to quantify multidimensional trade-offs in resourcing constraints, concurrent CoTS and coral population dynamics, the stringency of target thresholds, and the geographical scale of management outcomes (number of sites). We found strategies with low target density thresholds for CoTS (≤0.03 CoTS min-1 ) could act as "Effort Sinks" and limit the number of sites that could be effectively controlled, particularly under CoTS population outbreaks. This was because a handful of sites took longer to control, which meant other sites were not controlled. Higher density thresholds (e.g., 0.04-0.08 CoTS min-1 ), tuned to levels of coral cover, diluted resources among sites but were more robust to resourcing constraints and pest population dynamics. Our study highlights trade-off decisions when using an IPM framework and informs the implementation of threshold-based strategies on the GBR.

Stock assessment models often assume natural mortality rates (M) are constant and attribute the variability in a stock’s overall mortality rate to variability in fishing mortality and recruitment. However, this assumption may not be valid if M varies due to both direct and indirect trophic interactions as well as environmental variability. This is particularly the case when there are substantial changes in the overlapping abundances of a key predator or prey species, or extreme environmental changes impacting an ecosystem. Hence multispecies models (MSMs) are used to explicitly capture variations in natural mortality rates and improve the ability to discriminate between mortality due to fishing versus natural mortality, plus quantify the influences of fishing on the broader ecosystem, consistent with an ecosystem approach to fishing (EAF). There are a growing number of approaches to modelling natural mortality rates in MSMs, with models developed for tactical applications requiring a higher level of rigour and consideration of uncertainty than broad strategic ecosystem models. We overview approaches to modelling natural mortality in Models of Intermediate Complexity for Ecosystem assessments (MICE) and use four case studies to highlight lessons learnt, applications and provide some guidelines going forward. We identify ten broad application categories for MSMs (incorporating explicit representation of components of M), to advance EAF. These include: (1) quantify variability in natural mortality by age and over time as inputs to single species stock assessment models, because of the recognition that predation mortality (M2) is a large and variable portion of total M; (2) inform ecosystem reference points; (3) use as operating models in Management Strategy Evaluation frameworks; (4) simulation test broader management levers; (5) optimise multispecies harvest strategies; (6) inform on management of bycatch and ‘choke’ species; (7) represent and forward project trajectories and model recovery scenarios for threatened and protected species; (8) correctly attribute sources of mortality to support management efforts; (9) pest management and (10) broader applications (e.g. habitat alterations). Using well-constructed MSMs to correctly specify M reduces bias in model parameters, reference points and projections and is increasingly important as ecosystems respond to a more variable and changing climate.

Management of coral predators, corallivores, is recommended to improve coral cover on tropical coral reefs under projected increasing levels of accumulated thermal stress, but whether corallivore management can improve coral cover, which is necessary for large-scale operationalisation, remains equivocal. Here, using a multispecies ecosystem model, we investigate intensive management of an invertebrate corallivore, the Crown-of-Thorns Starfish ( Acanthaster cf. solaris ), and show that culling could improve coral cover at sub-reef spatial scales, but efficacy varied substantially within and among reefs. Simulated thermal stress events attenuated management-derived coral cover improvements and was dependent on the level of accumulated thermal stress, the thermal sensitivity of coral communities and the rate of corallivore recruitment at fine spatial scales. Corallivore management was most effective when accumulated thermal stress was low, coral communities were less sensitive to heat stress and in areas of high corallivore recruitment success. Our analysis informs how to manage a pest species to promote coral cover under future thermal stress events.

The crown-of-thorns starfish (COTS), Acanthaster cf. solaris, is one of the main contributors to declines in coral cover on the Great Barrier Reef (GBR) and remains one of the major acute disturbances on coral reefs throughout much of the Indo-Pacific. Extensive control programs on the GBR involve manual culling of COTS in the field, and research is needed to inform these management efforts. Data from the Great Barrier Reef Marine Park Authority’s (GBRMPA) COTS control program provide near-real-time CPUE (Catch-Per-Unit-Effort, COTS culled per minute) data ideal for operational decision-making but these must be converted to density estimates before they can be related to ecological status of reefs or incorporated into ecological models. We developed conversions between common COTS field survey methods (i.e. manta tow, SCUBA transect searches) and COTS control program CPUE data using estimates of sightability and detectability. We used a population model and COTS size-structure data from COTS control program culling efforts to estimate that, on average, only 19% of 1-yr-old COTS (1–15 cm) are available to be culled. Finally, we developed a CPUE-COTS density relationship to estimate the threshold levels of COTS that prevent net growth of hard corals. Culling programs should therefore aim to achieve CPUEs below these ecological thresholds in order to effectively promote coral growth and recovery. These ecologically sustainable thresholds of COTS density varied depending on hard coral cover. For example, for 35% fast-growing coral cover, COTS culling needs to continue until CPUE decreases to below 0.05 COTS/min (1 COTS per 20 min) in order to prevent coral decline, whereas if coral cover is higher (80%), then a higher target threshold CPUE of ca. 0.08 COTS/min (ca. 3 COTS per 40 min) may be ecologically sustainable. These estimates underpin the current pest management rules being implemented by the GBRMPA in its COTS control program.

We investigated how density and aggregation influence crown-of-thorns starfish Acanthaster planci reproductive success, using an empirically-tuned, individual-based simulation model that incorporates spatial and temporal biological stochasticity associated with spawning, and a kinetics model of fertilisation that explicitly incorporates the probability of polyspermy. Greater aggregation of individuals relieved Allee fertilisation dynamics, particularly at low densities, leading to higher rates of successful monospermic fertilisation and allowing populations to produce many more zygotes. This is likely more important to smaller, rather than larger, populations, due to limited and more variable reproductive success. In higher density populations a fertilisation optimum was observed at moderate levels of aggregation, above which monospermic fertilisation plateaued or even declined. This was likely due to 2 factors: the spatial dynamics of gamete plume dispersal and polyspermic fertilisation. Comparison of in situ natural spawning aggregation with model results indicates a cost-benefit equilibrium may exist between aggregation and reproductive success, and that relief from mechanisms limiting aggregation (for example, decreased relative predator abundance) may permit increased aggregation resulting in greater fertilisation and zygote production. We propose an Allee threshold of 3 starfish ha(-1) (for starfish of a mean diameter 345 mm), below which reproductive capacity is greatly reduced regardless of aggregation level. These preliminary findings posit aggregation as a key factor in outbreak formation that may feasibly be incorporated into preventative management strategies to detect and define incipient outbreak conditions and to mitigate subsequent risk.