Matthew G. Burgess

Predicting overfishing and extinction threats in multispecies fisheries

Significance Threats to species from commercial fishing are rarely identified until species have suffered large population declines, by which time recovery can require costly remedial actions, such as fishery closures. We present a mechanistic approach to predicting the threats of future extinction or severe depletion posed by current multispecies fishing practices to a given population. We show that severe depletions recently experienced by four Pacific tuna and billfish populations could have been predicted in the 1950s, using our approach. Our results demonstrate that threatened species can be identified long before they experience severe population declines, providing time for preventative adjustments in fishing practices before consequences become severe and fishery closures or other socioeconomically disruptive interventions are required to protect species. Threats to species from commercial fishing are rarely identified until species have suffered large population declines, by which time remedial actions can have severe economic consequences, such as closure of fisheries. Many of the species most threatened by fishing are caught in multispecies fisheries, which can remain profitable even as populations of some species collapse. Here we show for multispecies fisheries that the biological and socioeconomic conditions that would eventually cause species to be severely depleted or even driven extinct can be identified decades before those species experience high harvest rates or marked population declines. Because fishing effort imposes a common source of mortality on all species in a fishery, the long-term impact of a fishery on a species is predicted by measuring its loss rate relative to that of species that influence the fishery’s maximal effort. We tested our approach on eight Pacific tuna and billfish populations, four of which have been identified recently as in decline and threatened with overfishing. The severe depletion of all four populations could have been predicted in the 1950s, using our approach. Our results demonstrate that species threatened by human harvesting can be identified much earlier, providing time for adjustments in harvesting practices before consequences become severe and fishery closures or other socioeconomically disruptive interventions are required to protect species.

High fishery catches through trophic cascades in China

Significance Fishing marine ecosystems indiscriminately and intensely can have negative impacts on biodiversity, but it may increase the biomass of fish available for capture in the system. We explore the possibility that China’s high fishery catches are a result of predator removal using an ecosystem model of the East China Sea (ECS). We show that China’s high fishery catches can be explained by the removal of larger predatory fish and consequent increases in the production of smaller fish. We project that single-species management would decrease catches in the ECS by reversing these ecosystem effects. Fisheries similar to those in China produce a large fraction of global catch; management reform in these areas must consider the entire ecosystem, rather than individual species. Indiscriminate and intense fishing has occurred in many marine ecosystems around the world. Although this practice may have negative effects on biodiversity and populations of individual species, it may also increase total fishery productivity by removing predatory fish. We examine the potential for this phenomenon to explain the high reported wild catches in the East China Sea—one of the most productive ecosystems in the world that has also had its catch reporting accuracy and fishery management questioned. We show that reported catches can be approximated using an ecosystem model that allows for trophic cascades (i.e., the depletion of predators and consequent increases in production of their prey). This would be the world’s largest known example of marine ecosystem “engineering” and suggests that trade-offs between conservation and food production exist. We project that fishing practices could be modified to increase total catches, revenue, and biomass in the East China Sea, but single-species management would decrease both catches and revenue by reversing the trophic cascades. Our results suggest that implementing single-species management in currently lightly managed and highly exploited multispecies fisheries (which account for a large fraction of global fish catch) may result in decreases in global catch. Efforts to reform management in these fisheries will need to consider system wide impacts of changes in management, rather than focusing only on individual species.

Range contraction enables harvesting to extinction

Significance Many threatened species including elephants, sturgeons, and bluefin tunas are harvested for high-value products. Species can be driven extinct if incentives to harvest do not diminish as populations decline; this occurs if harvest prices rise faster than costs with declining stock. Whereas recent conservation attention for these species has largely focused on market demand, we show—using a theoretical model and an empirical review—that contractions in species’ geographic ranges, which stabilize costs and may be especially common among terrestrial species, might often play a larger role in maintaining harvest incentives. Forces impacting ranges—such as patchy and declining habitats, schooling/herding behavior, and climate change—therefore merit greater attention in assessing overharvesting threats. Economic incentives to harvest a species usually diminish as its abundance declines, because harvest costs increase. This prevents harvesting to extinction. A known exception can occur if consumer demand causes a declining species’ harvest price to rise faster than costs. This threat may affect rare and valuable species, such as large land mammals, sturgeons, and bluefin tunas. We analyze a similar but underappreciated threat, which arises when the geographic area (range) occupied by a species contracts as its abundance declines. Range contractions maintain the local densities of declining populations, which facilitates harvesting to extinction by preventing abundance declines from causing harvest costs to rise. Factors causing such range contractions include schooling, herding, or flocking behaviors—which, ironically, can be predator-avoidance adaptations; patchy environments; habitat loss; and climate change. We use a simple model to identify combinations of range contractions and price increases capable of causing extinction from profitable overharvesting, and we compare these to an empirical review. We find that some aquatic species that school or forage in patchy environments experience sufficiently severe range contractions as they decline to allow profitable harvesting to extinction even with little or no price increase; and some high-value declining aquatic species experience severe price increases. For terrestrial species, the data needed to evaluate our theory are scarce, but available evidence suggests that extinction-enabling range contractions may be common among declining mammals and birds. Thus, factors causing range contraction as abundance declines may pose unexpectedly large extinction risks to harvested species.

U.S. seafood import restriction presents opportunity and risk

Marine mammal protections require increased global capacity On 1 January 2017, the U.S. National Oceanic and Atmospheric Administration (NOAA) will enact a new rule (1) requiring countries exporting seafood to the United States to demonstrate that their fisheries comply with the U.S. Marine Mammal Protection Act (MMPA). The United States is the worlds largest seafood importer (2); the MMPA is among the worlds strongest marine mammal protection laws; and most of the worlds ∼125 marine mammal species are affected by fisheries bycatch (accidental entanglement in fishing gear) (3). This regulation could thus have significant conservation benefits, potentially spilling over to other areas of marine governance, if it is accompanied by substantial investments to boost scientific and compliance capacity in developing countries. Otherwise, it risks having little effect besides inflicting economic hardship on already poor communities.

Sub-optimal pit construction in predatory ant lion larvae (Myrmeleon sp.)

The impacts on energy gains of two aspects of ant lion pit architecture were investigated in a natural population of pit-building ant lion larvae (Myrmeleon sp.) in Costa Rica. Field and laboratory settings were used to examine the impacts of circumference and depth of the pit on net energy gain rate. An optimization model predicted a point optimum circumference and angle of depression in an unconstrained system, and positive correlations between body mass, pit circumference, and pit angle of depression in the presence of physiological constraints on both measures. Such a physiological constraint is possible in this system due to a large one-time construction cost. All of these correlations were observed in a lab setting with filtered substrate and no competition; though none were significant in the field. Individuals additionally constructed wider, shallower pits in the field. These results are consistent with an angle of depression that is limited by the angle of repose of the substrate in the field, rather than physiology. These results provided suggestive evidence for sub-optimal pit dimensions in Myrmeleon sp., and for the importance of substrate type in understanding the architecture of natural ant lion pits. The model predicted that the frequency of relocation would not affect the optimal angle of depression, but it would affect the optimal pit circumference to a degree proportional to the square root of the change in the average time an ant lion occupies a single pit. These findings challenge the widely held assumption of adaptive optimality in animal foraging.

A computational approach to managing coupled human–environmental systems: the POSEIDON model of ocean fisheries

Sustainable management of complex human–environment systems, and the essential services they provide, remains a major challenge, felt from local to global scales. These systems are typically highly dynamic and hard to predict, particularly in the context of rapid environmental change, where novel sets of conditions drive coupled socio-economic-environmental responses. Faced with these challenges, our tools for policy development, while informed by the past experience, must not be unduly constrained; they must allow equally for both the fine-tuning of successful existing approaches and the generation of novel ones in unbiased ways. We study ocean fisheries as an example class of complex human–environmental systems, and present a new model (POSEIDON) and computational approach to policy design. The model includes an adaptive agent-based representation of a fishing fleet, coupled to a simplified ocean ecology model. The agents (fishing boats) do not have programmed responses based on empirical data, but respond adaptively, as a group, to their environment (including policy constraints). This conceptual model captures qualitatively a wide range of empirically observed fleet behaviour, in response to a broad set of policies. Within this framework, we define policy objectives (of arbitrary complexity) and use Bayesian optimization over multiple model runs to find policy parameters that best meet the goals. The trade-offs inherent in this approach are explored explicitly. Taking this further, optimization is used to generate novel hybrid policies. We illustrate this approach using simulated examples, in which policy prescriptions generated by our computational methods are counterintuitive and thus unlikely to be identified by conventional frameworks.

Protecting marine mammals, turtles, and birds by rebuilding global fisheries

Healthy fisheries can reduce bycatch Bycatch of marine mammals, turtles, and birds during commercial fishing is a considerable threat. Activities intended to reduce bycatch are often thought to conflict with commercial fishing. However, Burgess et al. show that in the majority of cases, managing fishery stocks to best promote long-term sustainability would also reduce bycatch. Rebuilding fish stocks will naturally promote lower bycatch, and these factors together will facilitate sustainable profit generation from fish harvest. Science, this issue p. 1255 Rebuilding fishery stocks will also promote recovery of threatened marine mammals, turtles, and birds. Reductions in global fishing pressure are needed to end overfishing of target species and maximize the value of fisheries. We ask whether such reductions would also be sufficient to protect non–target species threatened as bycatch. We compare changes in fishing pressure needed to maximize profits from 4713 target fish stocks—accounting for >75% of global catch—to changes in fishing pressure needed to reverse ongoing declines of 20 marine mammal, sea turtle, and seabird populations threatened as bycatch. We project that maximizing fishery profits would halt or reverse declines of approximately half of these threatened populations. Recovering the other populations would require substantially greater effort reductions or targeting improvements. Improving commercial fishery management could thus yield important collateral benefits for threatened bycatch species globally.

The scale of life and its lessons for humanity

The scale of life on Earth is shaped by a confluence of biophysical, evolutionary, ecological, and, recently, human forces. Measuring the scale of life offers insights about these forces and raises many more questions. In PNAS, Bar-On et al. (1) offer the most comprehensive quantification to date of the biomass of life on Earth, broken down by major taxonomic groups, ecological strategies, and environments. Despite high uncertainty in some estimates, their findings shed fascinating light on how biomass is distributed. Although many of the detailed findings will likely surprise most readers, the study also builds a foundation for exploring major ecological, evolutionary, and environmental questions. We highlight two such questions as examples of the future impact of this work. One of the findings is the striking contrast between marine and terrestrial biomes. Can we account for the differences on the basis of what we know about how these disparate ecosystems function? The findings also raise important questions about the future. What scale of human activities can be supported by marine and terrestrial environments, looking forward? How will climate change alter the answers? Total primary productivity in the oceans [48.5 Gt C/y net primary production (NPP)] is similar to that on land (56.4 Gt C/y NPP), even though the oceans have more than twice as much surface area (2). However, despite similar total primary productivity, Bar-On et al. (1) estimate that there is roughly 80 times more biomass on land than in the oceans. Terrestrial plants—which comprise ∼80% of the total biomass on Earth—make up most of this difference. In striking contrast to the land’s dominance of producer biomass, Bar-On et al. (1) estimate that more than 70% of global animal biomass is found in the ocean. Earth has a plant-dominated landscape and an animal-dominated seascape. What could explain these fundamental … [↵][1]1To whom correspondence may be addressed. Email: mburgess{at}ucsb.edu or gaines{at}ucsb.edu. [1]: #xref-corresp-1-1

Reply to Le Pape et al.: Management is key to preventing marine extinctions

Our report (1) examines factors that maintain the profitability of harvesting a population as it declines. Without management, this can incentivize harvesting to extinction (2). Le Pape et al. (3) note that humans have not yet caused many marine extinctions, and argue that harvesting fish populations to complete extinction should be difficult because of the high fecundity of these populations. This argument is intuitive, but the extinction mechanism we examine (1, 2) does not depend directly on the harvested species’ fecundity. High fecundity might indirectly lessen this extinction threat in some cases, but the importance of management in preventing both past and future … [↵][1]1To whom correspondence should be addressed. Email: mburgess{at}ucsb.edu. [1]: #xref-corresp-1-1