Mobile Predators Research Articles

Giant kelp, Macrocystis pyrifera, increases faunal diversity through physical engineering.

Foundation species define the ecosystems they live in, but ecologists have often characterized dominant plants as foundational without supporting evidence. Giant kelp has long been considered a marine foundation species due to its complex structure and high productivity; however, there is little quantitative evidence to evaluate this. Here, we apply structural equation modelling to a 15-year time series of reef community data to evaluate how giant kelp affects the reef community. Although species richness was positively associated with giant kelp biomass, most direct paths did not involve giant kelp. Instead, the foundational qualities of giant kelp were driven mostly by indirect effects attributed to its dominant physical structure and associated engineering influence on the ecosystem, rather than by its use as food by invertebrates and fishes. Giant kelp structure has indirect effects because it shades out understorey algae that compete with sessile invertebrates. When released from competition, sessile species in turn increase the diversity of mobile predators. Sea urchin grazing effects could have been misinterpreted as kelp effects, because sea urchins can overgraze giant kelp, understorey algae and sessile invertebrates alike. Our results confirm the high diversity and biomass associated with kelp forests, but highlight how species interactions and habitat attributes can be misconstrued as direct consequences of a foundation species like giant kelp.

A global perspective on the trophic geography of sharks.

Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.

Spatial conservation of large mobile elasmobranchs requires an understanding of spatio-temporal seascape utilization

The positioning of habitats interacts with variability in abiotic factors (e.g. seasonal changes in temperature and extreme weather events) to change how animals use a land or seascape. Marine reserves can regulate how human activities alter fish communities and increase the abundance of targeted species, but the combined influence of reserves and seascape context on species habitat use remains uncertain in many ecosystems. Further, marine reserve effectiveness might be low for mobile species if the size of the reserve is less than a species usual range, reducing the overall time a individual may be protected. In this study, we tracked 19 giant shovelnose rays (Glaucostegus typus), an IUCN listed vulnerable species within the Moreton Bay Marine Park in eastern Australia. We used an array of 28 acoustic receivers within a complex mosaic of seagrass patches, bare sand, mangrove forests and deep-water channels and used regression tree analyses to determine which spatial, temporal and protection factors contributed most to G. typus habitat use. Overall, 50% of the total detections in the study occurred inside marine reserves containing large seagrass beds (>7.09 m2) and in close proximity to mangroves ( 20.2 °C), the habitat use of individuals is contained in smaller centres of activity compared with winter, however, protection effects varied. Our results show that seascape context and marine reserves combine to provide the optimal areas for G. typus habitat selection. Limited food resources likely caused larger centres of activity during winter. Identifying priority habitats for vulnerable species is critical for ongoing protection and maintaining effective conservation initiatives. We have shown here that incorporating spatial features into the design of marine reserves can improve conservation outcomes for mobile benthic predators such as G. typus and other species that use such seascapes.

Gut microbiomes of mobile predators vary with landscape context and species identity.

Landscape context affects predator–prey interactions and predator diet composition, yet little is known about landscape effects on insect gut microbiomes, a determinant of physiology and condition. Here, we combine laboratory and field experiments to examine the effects of landscape context on the gut bacterial community and body condition of predatory insects. Under laboratory conditions, we found that prey diversity increased bacterial richness in insect guts. In the field, we studied the performance and gut microbiota of six predatory insect species along a landscape complexity gradient in two local habitat types (soybean fields vs. prairie). Insects from soy fields had richer gut bacteria and lower fat content than those from prairies, suggesting better feeding conditions in prairies. Species origin mediated landscape context effects, suggesting differences in foraging of exotic and native predators on a landscape scale. Overall, our study highlights complex interactions among gut microbiota, predator identity, and landscape context.

The influence of habitat accessibility on the dietary and morphological specialisation of an aquatic predator

Individual diet and habitat specialisation are widespread in animal taxa and often related to levels of predation and competition. Mobile consumers such as predatory fish can stabilise lake food webs by ranging over a larger area than their prey, thereby switching between habitats. Although, this switching assumes that the predator has equal preference for the available prey, individual diet specialisation and morphological adaptations to different habitats could potentially prevent individuals from switching between habitats. In this study, we assessed the niche width and individual specialisation in Eurasian perch Perca fluviatilis in response to a shift in habitat use by manipulating the ability for this top predator to couple habitats. We ran an eight weeks pond experiment, to test the effect of habitat switching on diet and morphological specialisations. We show that habitat coupling influenced individual diet specialisation and niche use in expected directions where specialisation increased with decreasing habitat switching. In contrast to expectations, the morphological variation decreased with increasing diet specialisation. Our results expand on previous work and suggest that individual specialisation and niche width can impact the ability of mobile predators to couple habitats. Furthermore, it shows the importance of individual specialisations in relation to habitat coupling.

Searching for Biotic Multipliers of Climate Change.

As climates change, biologists need to prioritize which species to understand, predict, and protect. One way is to identify key species that are both sensitive to climate change and that disproportionately affect communities and ecosystems. These "biotic multipliers" provide efficient targets for research and conservation. Here, we propose eight mechanistic hypotheses related to impact and sensitivity that suggest that top consumers might often act as biotic multipliers of climate change. For impact, top consumers often affect communities and ecosystems through strong top-down effects. For sensitivity, metabolic theory and data suggest that photosynthesis and respiration differ in temperature responses, potentially increasing the sensitivity of consumers relative to plants. Larger-bodied organisms are typically more thermally sensitive than smaller ones, suggesting how large top consumers might be more sensitive than their smaller prey. In addition, traits related to predation are more sensitive than defensive traits to temperature. Top consumers might also be more sensitive because they often lag behind prey in phenological responses. The combination of low population sizes and demographic traits of top consumers could make them more sensitive to disturbances like climate change, which could slow their recovery. As top consumers are positioned at the top of the food chain, many small effects can accumulate from other trophic levels to affect top consumers. Finally, top consumers also often disperse more frequently and farther than prey, potentially leading to greater sensitivity to climate-induced changes in ranges and subsequent impacts on invaded communities. Overall, we expect that large, ectothermic top consumers and mobile predators might frequently be biotic multipliers of climate change. However, this prediction depends on the particular features of species, habitats, and ecosystems. In specific cases, herbivores, plants, or pathogens might be more sensitive than top consumers or have greater community impacts. To predict biotic multipliers, we need to compare sensitivities and impacts across trophic groups in a broader range of ecosystems as well as perform experiments that uncouple proposed mechanisms. Overall, the biotic multiplier concept offers an alternative prioritization scheme for research and conservation that includes impacts on communities and ecosystems.

An Assessment of Mobile Predator Populations along Shallow and Mesophotic Depth Gradients in the Hawaiian Archipelago

Large-bodied coral reef roving predators (sharks, jacks, snappers) are largely considered to be depleted around human population centers. In the Hawaiian Archipelago, supporting evidence is primarily derived from underwater visual censuses in shallow waters (≤30 m). However, while many roving predators are present or potentially more abundant in deeper strata (30–100 m+), distributional information remains sparse. To partially fill that knowledge gap, we conducted surveys in the remote Northwestern Hawaiian Islands (NWHI) and populated Main Hawaiian Islands (MHI) from 2012–2014 using baited remote underwater stereo-video. Surveys between 0–100 m found considerable roving predator community dissimilarities between regions, marked conspicuous changes in species abundances with increasing depth, and largely corroborated patterns documented during shallow water underwater visual censuses, with up to an order of magnitude more jacks and five times more sharks sampled in the NWHI compared to the MHI. Additionally, several species were significantly more abundant and larger in mesophotic versus shallow depths, which remains particularly suggestive of deep-water refugia effects in the MHI. Stereo-video extends the depth range of current roving predator surveys in a more robust manner than was previously available, and appears to be well-suited for large-scale roving predator work in the Hawaiian Archipelago.

Bayesian joint models with INLA exploring marine mobile predator-prey and competitor species habitat overlap.

Understanding spatial physical habitat selection driven by competition and/or predator–prey interactions of mobile marine species is a fundamental goal of spatial ecology. However, spatial counts or density data for highly mobile animals often (1) include excess zeros, (2) have spatial correlation, and (3) have highly nonlinear relationships with physical habitat variables, which results in the need for complex joint spatial models. In this paper, we test the use of Bayesian hierarchical hurdle and zero‐inflated joint models with integrated nested Laplace approximation (INLA), to fit complex joint models to spatial patterns of eight mobile marine species (grey seal, harbor seal, harbor porpoise, common guillemot, black‐legged kittiwake, northern gannet, herring, and sandeels). For each joint model, we specified nonlinear smoothed effect of physical habitat covariates and selected either competing species or predator–prey interactions. Out of a range of six ecologically important physical and biologic variables that are predicted to change with climate change and large‐scale energy extraction, we identified the most important habitat variables for each species and present the relationships between these bio/physical variables and species distributions. In particular, we found that net primary production played a significant role in determining habitat preferences of all the selected mobile marine species. We have shown that the INLA method is well‐suited for modeling spatially correlated data with excessive zeros and is an efficient approach to fit complex joint spatial models with nonlinear effects of covariates. Our approach has demonstrated its ability to define joint habitat selection for both competing and prey–predator species that can be relevant to numerous issues in the management and conservation of mobile marine species.

Hydrodynamic Hunters

The Gram-negative Bdellovibrio bacteriovorus (BV) is a model bacterial predator that hunts other bacteria and may serve as a living antibiotic. Despite over 50 years since its discovery, it is suggested that BV probably collides into its prey at random. It remains unclear to what degree, if any, BV uses chemical cues to target its prey. The targeted search problem by the predator for its prey in three dimensions is a difficult problem: it requires the predator to sensitively detect prey and forecast its mobile prey’s future position on the basis of previously detected signal. Here instead we find that rather than chemically detecting prey, hydrodynamics forces BV into regions high in prey density, thereby improving its odds of a chance collision with prey and ultimately reducing BV’s search space for prey. We do so by showing that BV’s dynamics are strongly influenced by self-generated hydrodynamic flow fields forcing BV onto surfaces and, for large enough defects on surfaces, forcing BV in orbital motion around these defects. Key experimental controls and calculations recapitulate the hydrodynamic origin of these behaviors. While BV’s prey (Escherichia coli) are too small to trap BV in hydrodynamic orbit, the prey are also susceptible to their own hydrodynamic fields, substantially confining them to surfaces and defects where mobile predator and prey density is now dramatically enhanced. Colocalization, driven by hydrodynamics, ultimately reduces BV’s search space for prey from three to two dimensions (on surfaces) even down to a single dimension (around defects). We conclude that BV’s search for individual prey remains random, as suggested in the literature, but confined, however—by generic hydrodynamic forces—to reduced dimensionality.

Exposure of rangia clams to hypoxia enhances blue crab predation

This experimental study questions whether exposure of non-mobile prey to episodic hypoxia might enhance predation by a mobile predator, which moves into the former hypoxic area immediately after a shift back to normoxic conditions. We used Rangia cuneata (common rangia clam) and Callinectes sapidus (blue crab) from an oligohaline estuary where mature clams are rare in areas subject to episodic anoxia and hypoxia. Clams were exposed to severe laboratory hypoxia for 72h. One clam stressed by hypoxia and another clam maintained under aeration (normoxia) were placed in aerated aquaria containing a crab. Feeding choice of hypoxic vs. normoxic clams was then monitored for 12h. We used 20 different crabs for two experimental replicates each for a total of 40 replicates. To test for homogeneity of the feeding response, we used a 1-tail binomial test with 0.5 expected probabilities. Eleven of the 20 crabs fed (55%), and 16 out of 18 hypoxia-stressed clams were eaten first compared to two out of 18 clams kept under normoxic conditions (p=0.001). The significant frequency choice of stressed clams indicates that in this experimental study, exposure of clams to hypoxia enhanced crab predation.

Long-term residency and movements of yellowmouth barracuda (Sphyraena viridensis) at a shallow seamount

The yellowmouth barracuda Sphyraena viridensis, a highly mobile predator, is considered a migratory seasonal visitor to the Azores Islands by local fishers. This perception is challenged here through a combined analysis of multi-annual commercial landings, underwater visual census and telemetry data from acoustically tagged fish to investigate the long-term movement patterns and residency (up to 4 years) in a seamount marine reserve. Although the landing patterns suggested a strong seasonal variation in abundance, the visual census and the telemetry experiment results were consistent with year-round residency in the Azores. Barracudas occurred consistently in underwater census throughout the year, and most tagged fish were almost continuously detected at the seamounts for up to four consecutive years. Detection probability did, however, decrease over time and showed seasonal variability as tagged fish were less mobile with longer absences during summer and fall. Given the intrinsic vulnerability of fish aggregations to exploitation, our results highlight the need to protect shallow seamount habitats and preserve sexually mature fish aggregations and their reproductive potential, contributing to the sustainable management and conservation of these vulnerable ecosystems.

Movement tactics of a mobile predator in a meta‐ecosystem with fluctuating resources: the arctic fox in the High Arctic

Animal movement is a fundamental process shaping ecosystems at multiple levels, from the fate of individuals to global patterns of biodiversity. The spatio‐temporal dynamic of food resources is a major driver of animal movement and generates patterns ranging from range residency to migration and nomadism. Arctic tundra predators face a strongly fluctuating environment marked by cyclic microtine populations, high seasonality, and the potential availability of sea ice, which gives access to marine resources in winter. This type of relatively poor and highly variable environment can promote long‐distance movements and resource tracking in mobile species. Here, we investigated the winter movements of the arctic fox, a major tundra predator often described as a seasonal migrant or nomad. We used six years of Argos satellite telemetry data collected on 66 adults from Bylot Island (Nunavut, Canada) tracked during the sea ice period. We hypothesized that long‐distance movements would be influenced by spatio‐temporal changes in resource availability and individual characteristics. Despite strong annual and seasonal changes in resource abundance and distribution, we found that a majority of individuals remained resident, especially those located in an area characterized by highly predictable pulse resources (goose nesting colony) and abundant cached food items (eggs). Foxes compensated terrestrial food shortage by commuting to the sea ice rather than using long‐distance tracking or moving completely onto the sea ice for winter. Individual characteristics also influenced movement patterns: age positively influenced the propensity to engage in nomadism, suggesting older foxes may be driven out of their territories. Our results show how these mammalian predators can adjust their movement patterns to favor range residency despite strong spatio‐temporal fluctuations in food resources. Understanding the movement responses of predators to prey dynamics helps identifying the scales at which they work, which is a critical aspect of the functioning and connectivity among meta‐ecosystems.

Movement patterns of reef predators in a small isolated marine protected area with implications for resource management

Reef predators play a critical role in maintaining the balance of marine ecosystems and are an important component of Hawaii’s recreational and commercial fisheries. In response to the increasing anthropogenic demands on these populations across the main Hawaiian Islands, the study of predator movements in marine protected areas has become a research priority. To this aim, we used passive acoustic telemetry to investigate the spatial and temporal movement patterns of five reef predator species: bluefin trevally (Caranx melampygus), giant trevally (Caranx ignobilis), green jobfish (Aprion virescens), whitetip reef sharks (Triaenodon obesus) and gray reef sharks (Carcharhinus amblyrhynchos) at the 31 ha Molokini Marine Life Conservation District (MLCD) off Maui, Hawaii (Lat: 20°37′56.70″N, Lon: 156°29′44.52″W) from November 13, 2013 to August 28, 2015. Our results indicate that the predator assemblage in the MLCD was dominated by teleost fishes during the day and sharks at night. Residency was variable across species, with bluefin trevally exhibiting the highest residency in the MLCD, green jobfish the lowest, and long-distance movements between the Molokini MLCD and the other islands of the Maui Nui Complex were common for gray reef sharks and giant trevally. These results indicate that despite its small size, the Molokini MLCD provides a high level of protection to resident species such as bluefin trevally. However, this MLCD is less effective at protecting more mobile predators such as green jobfish, gray reef sharks and giant trevally.

Cover crops have neutral effects on predator communities and biological control services in annual cellulosic bioenergy cropping systems

Maize stover is beginning to be used as a cellulosic biofuel feedstock in the Midwestern United States; however, there are concerns that stover removal could result in increased soil erosion and loss of soil organic matter. Use of a winter cover crop following maize harvest has the potential to mitigate these impacts and may have additional benefits by providing continuous living cover in annual crop habitats leading to changes in insect predator communities and increased biocontrol services. However, cover crops may also be harvested in cellulosic biofuel systems, adding a disturbance event that may negatively affect biocontrol. We contrasted insect predator communities and their impacts in four potential bioenergy cropping systems in Michigan and Wisconsin (USA) during the 2013 and 2014 growing seasons. Two annual maize systems were harvested for grain and stover; one maize system included a cereal rye/Austrian winter pea cover crop. Two perennial systems, switchgrass and a mixed prairie grasses and forbs, were harvested in the fall for biomass. Predatory insect abundance and diversity were lower in both annual cropping systems than in the perennial cropping systems and the inclusion of the cover crop did not significantly alter these responses. Similarly, removal of sentinel insect egg prey was also lower in the annual versus perennial cropping systems, with no significant influence of cover crop. We also explored the potential for cover crops to harbor prey populations in the spring that might encourage oviposition by mobile predators and potentially lead to local population sources or sinks of predators depending on the timing and effect of cover crop harvest. We found that existing predator communities in the cover crop treatments effectively suppressed aphid population growth, limiting their attractiveness to mobile predators. While we found no significant positive or negative impacts of this cover crop system on biocontrol services, bioenergy cover cropping systems could be managed to increase multiple ecosystem services by altering cover crop identity, or timing of planting and harvest.

Dietary Evidence of Mesopelagic and Pelagic Foraging by Atlantic Bluefin Tuna (Thunnus thynnus L.) during Autumn Migrations to the Iceland Basin

Atlantic bluefin tuna (ABFT; Thunnus thynnus) is a large highly mobile predator fish species in the North Atlantic Ocean and Mediterranean Sea. Knowledge of its trophic role in marine food webs in summer feeding areas is presently based on recent (1980s-2010s) sampling in the Bay of Biscay, Gulfs of Maine and St. Lawrence, and from historical (1950s-1960s) sampling in the Norwegian-North Sea-Kattegat. No study has yet investigated the diets of ABFT in Icelandic waters, where it supported an experimental fishery during 1996-2005, nor in any region north of the Bay of Biscay since the 1960s. However north Atlantic temperatures and fish species distributions, including some ABFT prey species (e. g., mackerel) have been changing in the 2000s. New knowledge of ABFT diets in previously understudied parts of the species range will be useful for understanding factors affecting the trophic role, migration behavior and bioenergetics of ABFT. Here we report the dietary composition of ABFT during autumn migrations to the Iceland Basin south of the continental shelf of Iceland. A total of 36 prey species or higher taxa were observed in 421 stomach samples: 17 teleost fishes, 4 squid, 1 octopus, 11 crustaceans and 2 other invertebrate species. The most important prey species European flying squid (Todarodes sagittatus) (%N=16.70, %W=48.89; %FO=87.65), barracudinas (Paralepididae) (%N=14.05, %W=28.59, %FO=76.48) and gonate squid (Gonatus sp.) (%N=9.17, %W=7.85, %FO=75.06). Prey sizes were highly variable relative to ABFT sizes indicating highly opportunistic feeding on diverse sizes. The presence of a large proportion of mesopelagic species in the diet indicates feeding in the mesopelagic layer and extensive dive behavior. These results give new baseline knowledge for future comparison with anticipated oceanographic-biological changes in the region in the coming decades and can be used to help parameterize new models of ABFT migration behavior and trophic role.

School level structural and dynamic adjustments to risk promote information transfer and collective evasion in herring

Many large-scale animal groups have the ability to react in a rapid and coordinated manner to environmental perturbations or predators. Information transfer among organisms during such events is thought to confer important antipredator advantages. However, it remains unknown whether individuals in large aggregations can change the structural properties of their collective in response to higher predation risk, and if so whether such adjustments promote responsiveness and information transfer. We examined the role of risk perception on the schooling dynamics and collective evasions of a large herring, Clupea harengus, school (ca. 60 000 fish) during simulated-predator encounters in a sea cage. Using an echosounder, high-resolution imaging sonar and acoustic video analysis, we quantified swimming dynamics, collective reactions and the speed of the propagating waves of evasion induced by a mobile predator model. In the higher risk condition, fish swam faster, exhibited a stronger circular swimming pattern, and we found an increased correlation strength indicating that the school had a greater ability to collectively respond to a perturbation. When exposed to a simulated threat, collective evasions were stronger and behavioural change (evasion manoeuvres) propagated more quickly within the school under environmental conditions perceived as being more risky. Our results demonstrate that large schools make structural and behavioural adjustments in response to perceived risk in a way that improves collective information transfer, and thus responsiveness, during predator attacks.

Shallow water tidal flat use and associated specialized foraging behavior of the great hammerhead shark (Sphyrna mokarran)

Evidence suggests the great hammerhead shark, Sphyrna mokarran, is vulnerable to a variety of anthropogenic stressors, and is an understudied species of shark due to its cryptic nature and wide-ranging movements. While recognized as both a pelagic-coastal and a highly mobile predator, minimal anecdotal evidence exist describing shallow water habitat use by this species. This report describes six cases in which a great hammerhead shark utilizes an inshore shallow water flats environment (<1.5 m in depth), five of which involve prey capture. These observations permitted identification of two novel behaviors that may allow great hammerheads to inhabit these shallow habitats: a (1) prey-capture technique termed ‘grasp-turning’ that involves burst swimming at tight turning angles while grasping prey and (2) a post-predation recovery period whereby the shark maintains head-first orientation into the current that may facilitate respiration and prey consumption. These behavioral observations provide insights into the natural history of this species.

Does ecological redundancy maintain functioning of marine benthos on centennial to millennial time scales?

AbstractPredicting the ability of the biosphere to continue to deliver ecosystem services in the face of biodiversity loss and environmental change is a major challenge. The results of short‐term and small‐scale experimental studies are both equivocal and difficult to extrapolate from. In this study we use data on benthic palaeocommunities covering 4,000,000 years (in the Late Jurassic when temperate coastal seas in NW Europe experienced fluctuations in oxygenation). The biological traits associated with each species in the palaeocommunities were combined to index the delivery of ecological functions. Five ecosystem functions were examined: food for large mobile predators, biogenic habitat provision, nutrient recycling/regeneration, inorganic carbon sequestration and food‐web dynamics. In modern systems these ecological functions underpin ecosystem services that are important for human well‐being. Our results show that the supply of food for higher predators was remarkably constant during the 4,000,000 years, suggesting that redundancy amongst species in the assemblage drives the biodiversity–ecosystem function (BEF) relationship. By contrast, the provision of biogenic habitat varied with the occurrence of a relatively few taxa, a pattern consistent with a rivet type model of BEF. For nutrient regeneration, carbon sequestration and food‐web dynamics the patterns were complex and suggestive of an idiosyncratic model of BEF. To our knowledge this is the first study to quantify ecological functioning through deep time and demonstrates the utility of this approach to understanding long‐term patterns of BEF in both ancient and contemporary marine ecosystems. The delivery of all five ecological functions studied became increasingly variable as the regional climate became drier, thus modifying the supply of terrigenous nutrient inputs.

Discontinuities concentrate mobile predators: quantifying organism–environment interactions at a seascape scale

AbstractUnderstanding environmental drivers of spatial patterns is an enduring ecological problem that is critical for effective biological conservation. Discontinuities (ecologically meaningful habitat breaks), both naturally occurring (e.g., river confluence, forest edge, drop‐off) and anthropogenic (e.g., dams, roads), can influence the distribution of highly mobile organisms that have land‐ or seascape scale ranges. A geomorphic discontinuity framework, expanded to include ecological patterns, provides a way to incorporate important but irregularly distributed physical features into organism–environment relationships. Here, we test if migratory striped bass (Morone saxatilis) are consistently concentrated by spatial discontinuities and why. We quantified the distribution of 50 acoustically tagged striped bass at 40 sites within Plum Island Estuary, Massachusetts during four‐monthly surveys relative to four physical discontinuities (sandbar, confluence, channel network, drop‐off), one continuous physical feature (depth variation), and a geographic location variable (region). Despite moving throughout the estuary, striped bass were consistently clustered in the middle geographic region at sites with high sandbar area, close to channel networks, adjacent to complex confluences, with intermediate levels of bottom unevenness, and medium sized drop‐offs. In addition, the highest striped bass concentrations occurred at sites with the greatest additive physical heterogeneity (i.e., where multiple discontinuities co‐occurred). The need to incorporate irregularly distributed features in organism–environment relationships will increase as high‐quality telemetry andGISdata accumulate for mobile organisms. The spatially explicit approach we used to address this challenge can aid both researchers who seek to understand the impact of predators on ecosystems and resource managers who require new approaches for biological conservation.

Using passive acoustic telemetry to infer mortality events in adult herbivorous coral reef fishes

Mortality is considered to be an important factor shaping the structure of coral reef fish communities, but data on the rate and nature of mortality of adult coral reef fishes are sparse. Mortality on coral reefs is intrinsically linked with predation, with most evidence suggesting that predation is highest during crepuscular periods. We tested this hypothesis using passive acoustic telemetry data to determine the time of day of potential mortality events (PMEs) of adult herbivorous reef fishes. A total of 94 fishes were tagged with acoustic transmitters, of which 43 exhibited a PME. Furthermore, we identified five categories of PMEs based on the nature of change in acoustic signal detections from tagged fishes. The majority of PMEs were characterised by an abrupt stop in detections, possibly as a result of a large, mobile predator. Overall, mortality rates were estimated to be approximately 59 % per year using passive acoustic telemetry. The time of day of PMEs suggests that predation was highest during the day and crepuscular periods and lowest at night, offering only partial support for the crepuscular predation hypothesis. Visually oriented, diurnal and crepuscular predators appear to be more important than their nocturnal counterparts in terms of predation on adult reef fishes. By timing PMEs, passive acoustic telemetry may offer an important new tool for investigating the nature of predation on coral reefs.