Musical chairs mortality functions: density-dependent deaths caused by competition for unguarded refuges

Structural refuges within which prey can escape from predators can be an important limiting resource for the prey. In a manner that resembles the childhood game of musical chairs, many prey species rapidly retreat to shared, unguarded refuges whenever a predator threatens, and only when refuges are relatively abundant do all prey individuals actually escape. The key feature of this process is that the per capita prey mortality rate depends on the ratio of prey individuals to refuges. We introduce a new class of mortality functions with this feature and then demonstrate statistically that they describe field mortality data from a well-studied coral reef fish species, the Caribbean bridled goby Coryphopterus glaucofraenum, substantially better than do several mortality functions of more conventional form

Identifying Thresholds for Ecosystem-Based Management

Background One of the greatest obstacles to moving ecosystem-based management (EBM) from concept to practice is the lack of a systematic approach to defining ecosystem-level decision criteria, or reference points that trigger management action. Methodology/Principal Findings To assist resource managers and policymakers in developing EBM decision criteria, we introduce a quantitative, transferable method for identifying utility thresholds. A utility threshold is the level of human-induced pressure (e.g., pollution) at which small changes produce substantial improvements toward the EBM goal of protecting an ecosystem\u27s structural (e.g., diversity) and functional (e.g., resilience) attributes. The analytical approach is based on the detection of nonlinearities in relationships between ecosystem attributes and pressures. We illustrate the method with a hypothetical case study of (1) fishing and (2) nearshore habitat pressure using an empirically-validated marine ecosystem model for British Columbia, Canada, and derive numerical threshold values in terms of the density of two empirically-tractable indicator groups, sablefish and jellyfish. We also describe how to incorporate uncertainty into the estimation of utility thresholds and highlight their value in the context of understanding EBM trade-offs. Conclusions/Significance For any policy scenario, an understanding of utility thresholds provides insight into the amount and type of management intervention required to make significant progress toward improved ecosystem structure and function. The approach outlined in this paper can be applied in the context of single or multiple human-induced pressures, to any marine, freshwater, or terrestrial ecosystem, and should facilitate more effective management

Footprints of fixed-gear fisheries in relation to rising whale entanglements on the U.S. West Coast

On the U.S. West Coast, reports of whales entangled in fishing gear increased dramatically in 2014. In this study, a time series of fishing activity maps was developed from 2009 to 2016 for the four fixed-gear fisheries most commonly implicated in entanglements. Maps were generated using vessel monitoring system (VMS) data linked to port-level landings databases, which were related to entangled whale reports over the same time period and with modelled distributions of humpback whales Megaptera novaeangliae Borowski. Over the full study period, neither marked increases in fishing activity nor changes in fisheries footprints within regions with high whale densities were detected. By contrast, a delayed fishery opening in California due to a harmful algal bloom in spring of 2016 led to ~5–7 times average levels of Dungeness crab Metacarcinus magister (Dana) fishing activity, which was consistent with a high rate of entanglement in that year. These results are consistent with current hypotheses that habitat compression caused by a marine heatwave increased the overlap of whales with fishing activity, despite minimal changes in the fisheries themselves. This study adds to literature on bycatch of protected species in otherwise sustainable fisheries, highlighting the value of using VMS data for reducing human–wildlife conflict in the ocean

Spatial Density Dependence Scales up but Does Not Produce Temporal Density Dependence in a Reef Fish

Field experiments provide rigorous tests of ecological hypotheses but are typically of short duration and use small spatial replicates. We assessed empirically whether the results of experiments testing for density dependence applied at larger spatial domains and explained temporal population dynamics. We studied a small coral reef fish, the goldspot goby {Gnatholepis thompsoni), in the Bahamas. We assessed the effects of interactions with conspecifics and with an ecologically similar species, the bridled goby {Coryphopterus glaucofraenum). Two density manipulations on small reef patches revealed that goldspot goby mortality over one month increased as conspecifics became crowded. On five large natural reefs, we correlated the initial year-class density of both species (annual larval settlement) with the subsequent decline of goldspot goby year-classes for five years. Mortality was correlated with conspecific density among reefs for all years, but not among years for all reefs. Thus, spatial density dependence in mortality scaled up qualitatively from small patches to entire reefs but was not associated with temporal density dependence. Our results support the conclusion that field experiments may be extrapolated to larger spatial domains with care, but that using small spatial comparisons to predict temporal responses is difficult without knowing the underlying biological mechanisms

Retrospective analysis of measures to reduce large whale entanglements in a lucrative commercial fishery

Recovering marine animal populations and climate-driven shifts in their distributions are colliding with growing ocean use by humans. One such example is the bycatch of whales in commercial fishing, which poses a significant threat to the conservation and continued recovery of these protected animals and is a major barrier to sustainable fisheries. Long-lasting solutions to this problem need to be robust to variability in ecological dynamics while also addressing socio-cultural and economic concerns. We assessed the efficacy of gear reductions as an entanglement mitigation strategy during 2019 and 2020 in the highly valuable Dungeness crab fishery (Washington State, USA) in terms of changes in the entanglement risk to protected blue and humpback whales, and in terms of economic consequences for the fishery. Using a combination of fishery logbooks, landings data, and whale habitat models, we found that in the two seasons with mandatory crab pot reductions, entanglement risk was reduced by up to 20 % for blue whales, and 78 % for humpback whales, compared to seasons with no regulations. Spatio-temporal variability in the distribution of each whale species was a key factor in determining risk. Importantly, the conservation measure did not have a substantial negative effect on fleet-level fishery performance metrics, despite a reduction in fishing effort. Results indicated that a simple, fixed management strategy achieved the desired conservation goals in an economically sustainable way. Our findings underscore the value of carefully considering the dynamic nature of species\u27 spatial distributions and key social and economic impacts that together determine conservation efficacy

The weaker sex: Male lingcod (Ophiodon elongatus) with blue color polymorphism are more burdened by parasites than are other sex–color combinations

The unusual blue color polymorphism of lingcod (Ophiodon elongatus) is the subject of much speculation but little empirical research; ~20% of lingcod individuals exhibit this striking blue color morph, which is discrete from and found within the same populations as the more common brown morph. In other species, color polymorphisms are intimately linked with host–parasite interactions, which led us to ask whether blue coloration in lingcod might be associated with parasitism, either as cause or effect. To test how color and parasitism are related in this host species, we performed parasitological dissection of 89 lingcod individuals collected across more than 26 degrees of latitude from Alaska, Washington, and California, USA. We found that male lingcod carried 1.89 times more parasites if they were blue than if they were brown, whereas there was no difference in parasite burden between blue and brown female lingcod. Blue individuals of both sexes had lower hepatosomatic index (i.e., relative liver weight) values than did brown individuals, indicating that blueness is associated with poor body condition. The immune systems of male vertebrates are typically less effective than those of females, due to the immunocompromising properties of male sex hormones; this might explain why blueness is associated with elevated parasite burdens in males but not in females. What remains to be determined is whether parasites induce physiological damage that produces blueness or if both blue coloration and parasite burden are driven by some unmeasured variable, such as starvation. Although our study cannot discriminate between these possibilities, our data suggest that the immune system could be involved in the blue color polymorphism–an exciting jumping-off point for future research to definitively identify the cause of lingcod blueness and a hint that immunocompetence and parasitism may play a role in lingcod population dynamics

Inter-Cohort Competition Drives Density Dependence and Selective Mortality in a Marine Fish

For organisms with complex life cycles, the transition between life stages and between habitats can act as a significant demographic and selective bottleneck. In particular, competition with older and larger conspecifics and heterospecifics may influence the number and characteristics of individuals successfully making the transition. We investigated whether the availability of enemy-free space mediated the interaction between adult goldspot gobies (Gnatholepis thompsoni), a. common tropical reef fish, and juvenile conspecifics that had recently settled from the plankton. We added rocks, which provide refuge from predators, to one-half of each of five entire coral reefs in the Bahamas and measured the survival and growth of recent settlers in relation to adult goby densities. We also evaluated whether mortality was selective with respect to three larval traits (age at settlement, size at settlement, and presettlement growth rate) and measured the influence of refuge availability and adult goby density on selection intensity. Selective mortality was measured by comparing larval traits of newly settled gobies (≤ 5 postsettlement) with those of survivors (2-3 week postsettlement juveniles). We detected a negative relationship between juvenile survival and adult goby density in both low- and high-refuge habitats, though experimental refuge addition reduced the intensity of this density dependence. Juvenile growth also declined with increasing adult goby density, but this effect was similar in both low- and high-refuge habitats. Refuge availability had no consistent effect on selective mortality, but adult goby density was significantly related to the intensity of size-selective mortality: bigger juveniles were favored where adults were abundant, and smaller juveniles were favored where adults were rare. Given the typically large difference in sizes of juveniles and adults, similar stage-structured interactions may be common but underappreciated in many marine species

Elicited preferences for components of ocean health in the California Current

As resource management efforts move towards more comprehensive approaches that spin multiple sectors and stakeholder groups, decision makers are faced with the challenge of deciding how important each group is, and how much weight their concerns should have, when making decisions. These decisions must be made transparently if they are to have credibility. This paper describes a systematic approach to eliciting such preferences, illustrated through a regional application of the Ocean Health Index in the California Current. The Index provides an ideal case study as it includes a comprehensive set of goals designed to-assess the benefits people derive from coasts and oceans. The approach leverages the strengths of two different methods for eliciting preferences, one based on random utility theory and the other on analytical deliberative methodologies. Results showed that the methods were accessible to individuals with diverse backgrounds and, in this case, revealed surprising consensus about fundamental values that may have been missed in deliberations around a specific action, rather than evaluating a spectrum of management priorities. Specifically, individuals, even extractive users, assigned higher weights to cultural and conservation goals compared to extractive ones. The approach offers a general procedure for eliciting explicit preferences through constructive deliberations among diverse stakeholders. (C) 2013 Elsevier-Ltd. All rights reserved.Keywords: Probabilistic inversion, Ecosystem-based management, Deliberative analytical, Marine spatial planning, Expert judgmen

Marine heatwave challenges solutions to human-wildlife conflict

Despite the increasing frequency and magnitude of extreme climate events, little is known about how their impacts flow through social and ecological systems or whether management actions can dampen deleterious effects. We examined how the record 2014-2016 Northeast Pacific marine heatwave influenced trade-offs in managing conflict between conservation goals and human activities using a case study on large whale entanglements in the U.S. west coast\u27s most lucrative fishery (the Dungeness crab fishery). We showed that this extreme climate event diminished the power of multiple management strategies to resolve trade-offs between entanglement risk and fishery revenue, transforming near win-win to clear win-lose outcomes (for whales and fishers, respectively). While some actions were more cost-effective than others, there was no silver-bullet strategy to reduce the severity of these trade-offs. Our study highlights how extreme climate events can exacerbate human-wildlife conflict, and emphasizes the need for innovative management and policy interventions that provide ecologically and socially sustainable solutions in an era of rapid environmental change