Foraging Depth Research Articles

Mercury Stable Isotopes Reveal the Vertical Distribution and Trophic Ecology of Deep-Pelagic Organisms over the North-East Atlantic Ocean Continental Slope.

Deep-pelagic species are central to marine ecosystems and increasingly vulnerable to global change and human exploitation. To date, our understanding of these communities remains limited mainly due to the difficulty of observations, calling for complementary innovative tools to better characterize their ecology. We used mercury (Δ199Hg, δ202Hg, Δ201Hg, and Δ200Hg), carbon (δ13C), and nitrogen (δ15N) stable isotope compositions to segregate deep-pelagic species caught on the continental slope of the Bay of Biscay (NE Atlantic) according to their foraging depth and trophic ecology. Decreasing fish Δ199Hg values with corresponding depth estimates from the surface to down to 1,800 m confirmed that mercury isotopes are able to segregate deep species over a large vertical gradient according to their foraging depth. Results from isotopic compositions also identified different mercury sources, likely reflecting different trophic assemblages over the continental slope, in particular, the demersal influence for some species, compared to purely oceanic species. Overall, our results demonstrate how mercury stable isotopes can inform the vertical foraging habitat of little-known species and communities feeding in the deep.

Temporal patterns in dolphin foraging activity in the Mediterranean Sea: insights from vocalisations recorded during the ACCOBAMS Survey Initiative

Marine organisms continually adapt their physiology and behaviour to temporal variations in their environment, resulting in diurnal rhythmic behaviour, particularly when foraging. In delphinids, these rhythms can be studied by recording echolocation clicks, which can provide indicators of foraging activity. The foraging rhythms of delphinids and their relationship to temporal parameters are poorly documented and most studies so far have used moored passive acoustic systems. The present study provides, for the first time, information on the activity rhythms of delphinids investigated in relation with temporal variables at a basin scale from a moving platform, in the western and central Mediterranean Sea. We used passive acoustic recordings collected by hydrophones towed along transect lines during the ACCOBAMS Survey Initiative in the summer 2018. We extracted variables that may influence daily and monthly rhythms, including time of day, lunar cycle, lunar illumination and sea state and fitted generalised additive models. The nycthemeral and lunar cycles were the two main factors influencing dolphin activity rhythms. Echolocation activity was predominant at night, with a maximum of 0.026 acoustic events per minute at 21:00/22:00 compared to as few as 0.0007 events per minute at 11:00. These events were also more frequent during the third quarter of the moon; 0.033 acoustic events on day 22 of the lunar cycle as opposed to 0.0008 on day 8 of the lunar cycle, corresponding to the first quarter of the moon. Variations in the echolocation activity of delphinids in the Mediterranean Sea could reflect variation in their foraging effort and be related to prey density, composition, accessibility and catchability within dolphin foraging depth range. These results should also improve interpretation of passive acoustic monitoring data.

Mercury stable isotopes suggest reduced foraging depth in oxygen minimum zones for blue sharks

Oxygen minimum zones (OMZs) are currently expanding across the global ocean due to climate change, leading to a compression of usable habitat for several marine species. Mercury stable isotope compositions provide a spatially and temporally integrated view of marine predator foraging habitat and its variability with environmental conditions. Here, we analyzed mercury isotopes in blue sharks Prionace glauca from normoxic waters in the northeastern Atlantic and from the world's largest and shallowest OMZ, located in the northeastern Pacific (NEP). Blue sharks from the NEP OMZ area showed higher Δ199Hg values compared to sharks from the northeastern Atlantic, indicating a reduction in foraging depth of approximately 200 m. Our study suggests for the first time that blue shark feeding depth is altered by expanding OMZs and illustrates the use of mercury isotopes to assess the impacts of ocean deoxygenation on the vertical foraging habitat of pelagic predators.

Seasonal resource pulses and the foraging depth of a Southern Ocean top predator.

Seasonal resource pulses can have enormous impacts on species interactions. In marine ecosystems, air-breathing predators often drive their prey to deeper waters. However, it is unclear how ephemeral resource pulses such as near-surface phytoplankton blooms alter the vertical trade-off between predation avoidance and resource availability in consumers, and how these changes cascade to the diving behaviour of top predators. We integrated data on Weddell seal diving behaviour, diet stable isotopes, feeding success and mass gain to examine shifts in vertical foraging throughout ice break-out and the resulting phytoplankton bloom each year. We also tested hypotheses about the likely location of phytoplankton bloom origination (advected or produced in situ where seals foraged) based on sea ice break-out phenology and advection rates from several locations within 150 km of the seal colony. In early summer, seals foraged at deeper depths resulting in lower feeding rates and mass gain. As sea ice extent decreased throughout the summer, seals foraged at shallower depths and benefited from more efficient energy intake. Changes in diving depth were not due to seasonal shifts in seal diets or horizontal space use and instead may reflect a change in the vertical distribution of prey. Correspondence between the timing of seal shallowing and the resource pulse was variable from year to year and could not be readily explained by our existing understanding of the ocean and ice dynamics. Phytoplankton advection occurred faster than ice break-out, and seal dive shallowing occurred substantially earlier than local break-out. While there remains much to be learned about the marine ecosystem, it appears that an increase in prey abundance and accessibility via shallower distributions during the resource pulse could synchronize life-history phenology across trophic levels in this high-latitude ecosystem.

Variation in dive behavior of Cuvier’s beaked whales with seafloor depth, time-of-day, and lunar illumination

Depth distributions were analyzed from a study of 19 Cuvier’s beaked whalesZiphius cavirostristhat were tagged with satellite transmitting instruments off southern California, USA. Over 113000 depth measurements were made over the equivalent of ~200 sampling days. The mean foraging depth was 1182 m (SD = 305 m), and the mean of the maximum depth of all foraging dives was 1427 m (SD = 298 m). Mean foraging depths increased with seafloor depths up to a maximum of ~1300 m at a seafloor depth of 1900 m, but decreased slightly to a mean of ~1200 m at seafloor depths of 2000-4000 m. Near-bottom habitat appears to be important for foraging; whales spent ~30% of their foraging time within 200 m of the bottom at seafloor depths of 1000-2000 m. However, little foraging time was spent near the bottom at seafloor depths greater than 2000 m. The percentage of time spent at near-surface depths (<50 m) was more than twice as high at night (25%) than during the day (12%). Lunar light also appears to affect diving, with 28% of dark nights and only 17% of brightly moonlit nights spent at these near-surface depths. The apparent avoidance of surface waters during daytime and on brightly moonlit nights is consistent with avoidance of visual predators. A considerably greater fraction of time was spent foraging at night (24.8%) than during the day (15.7%), possibly due to energetic constraints imposed by predator avoidance during the day.

Foraging ecology of masked boobies (Sula dactylatra) in the world\u2019s largest \u201coceanic desert\u201d

The South Pacific Gyre has the most hyper-oligotrophic waters in the world and is considered the largest “oceanic desert.” Rapa Nui (Easter Island), located within the South Pacific Gyre, is a breeding ground for masked boobies (Sula dactylatra), which are seabirds with a foraging range that effectively confines them within the gyre. The foraging ecology of this species in the gyre was examined by attaching GPS and time-depth devices to chick-rearing adult birds (9 and 14 birds in 2016 and 2017, respectively) and by collecting regurgitates (18 and 15 samples in 2016 and 2017, respectively). In addition, the birds’ foraging ecology between years was compared. Masked boobies traveled in various directions, dived at unspecific locations, and explored areas < 110 km from the colony. Local environmental conditions were not significantly different between years, and differences in foraging parameters (maximum foraging range, trip duration, and dive depth) were greater among individuals than between years. The foraging characteristics of masked boobies suggest that resources were ephemerally distributed around the colony, with similar abundances across years. Under these conditions, traveling to unspecific locations may increase the area covered and the probability of prey encounter. The spatial and temporal consistencies in environmental conditions explain the uniformity of foraging parameters between years. The ability of masked boobies to exploit ephemerally distributed resources in seascapes like Rapa Nui may help explain its pantropical distribution.

The Potential Vertical Distribution of Bigeye Tuna (Thunnus obesus) and Its Influence on the Spatial Distribution of CPUEs in the Tropical Atlantic Ocean

Understanding the potential vertical distribution of bigeye tuna (Thunnus obesus) is necessary to understand the catch rate fluctuations and the stock assessment of bigeye tuna. To characterize the potential vertical distribution of this fish while foraging and determine the influences of the distribution on longline efficiency in the tropical Atlantic Ocean, the catch per unit effort (CPUE) data were compiled from the International Commission for the Conservation of Atlantic Tunas and the Argo buoy data were downloaded from the Argo data center. The raw Argo buoy data were processed by data mining methods. The CPUE was standardized by support vector machine before analysis. We assumed the depths with the upper and lower limits of the optimum water temperatures of 15°C and 9°C as the preferred swimming depth, while the lower limit of the temperature (12°C) associated with the highest hooking rate as the preferred foraging depth (D12) of bigeye tuna during the daytime in the Atlantic Ocean. The preferred swimming depth and foraging depth range in the daytime were assessed by plotting the isobath based on Argo buoy data. The preferred swimming depth and vertical structure of the water column were identified to investigate the spatial effects on the CPUE by using a generalized additive model (GAM). The empirical cumulative distribution function was used to assess the relationship between the spatial distribution of CPUE and the depth of 12°C isolines and thermocline. The results indicate that 1) the preferred swimming depth of bigeye tuna in the tropical Atlantic is from 100 m to 400 m and displays spatial variation; 2) the preferred foraging depth of bigeye tuna is between 190 and 300 m and below the thermocline; 3) the number of CPUEs peaks at a relative depth of 30-50 m (difference between the 12°C isolines and the lower boundary of the thermocline); and 4) most CPUEs are within the lower depth boundary of the thermocline levels (LDBT) which is from 160 m to 230 m. GAM analysis indicates that the general relationship between the nominal CPUE and LDBT is characterized by a dome shape and peaks at approximately 190 m. The oceanographic features influence the habitat of tropical pelagic fish and fisheries. Argo buoy data can be an important tool to describe the habitat of oceanic fish. Our results provide new insights into how oceanographic features influence the habitat of tropical pelagic fish and fisheries and how fisheries exploit these fish using a new tool (Argo profile data).

Siscowet lake charr (Salvelinus namaycush siscowet) visual foraging habitat in relation to daily and seasonal light cycles

Using siscowet lake charr (Salvelinus namaycush siscowet) as an example organism, we modeled visual foraging habitat in relation to: (i) daily solar and lunar intensity, (ii) seasonal changes in maximum solar and lunar altitude, (iii) foraging for pelagic or benthic prey, and (iv) increased turbidity that may occur with climate change. Siscowet foraging success increased with light intensity and was higher for pelagic prey than benthic prey. Daily and seasonal siscowet foraging patterns were similar for benthic and pelagic prey types. Predicted day-time foraging depths were deepest in summer and shallowest in winter (range 172–233 m for pelagic prey and 210–283 m for benthic prey), and night-time foraging depths were deepest in winter and shallowest in summer (range 25–32 m for pelagic prey and 63–81 m for benthic prey). Within the Lake Superior basin, extreme precipitation events and associated sediment plumes can cause localized declines in light attenuation. Increases in turbidity associated with these sediment plumes can reduce predicted foraging depths by 65% and 80%, when compared to normal lake attenuation values. The model can be applied to predict how solar and lunar patterns influence foraging patterns in any aquatic organism that displays light-mediated behavior.

Increased proximity of vessels reduces feeding opportunities of blue whales in the St. Lawrence Estuary, Canada

ESR Endangered Species Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials ESR 32:351-361 (2017) - DOI: https://doi.org/10.3354/esr00825 Increased proximity of vessels reduces feeding opportunities of blue whales in the St. Lawrence Estuary, Canada Véronique Lesage1,*, Abdelghani Omrane1,2, Thomas Doniol-Valcroze1, Arnaud Mosnier1 1Maurice Lamontagne Institute, Fisheries and Oceans Canada, Mont-Joli, Quebec G5H 3Z4, Canada 2Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski, Rimouski, Québec G5L 3A1, Canada *Corresponding author: veronique.lesage@dfo-mpo.gc.ca ABSTRACT: Blue whales Balaenoptera musculus occur seasonally in the St. Lawrence Estuary, Canada, where they spend most of their time foraging. Their recurrent presence has stimulated the development of a large whale-watching industry. Here, we examine the effect of vessel distance on blue whale foraging behaviour by measuring changes in surface and diving patterns. Vessels were within 2000 m of blue whales during 70% of 33 follows, and 59% of total observation time. At vessel distances ≤400 m, surface and dive times were on average 49 and 36% shorter, respectively, and the number of breaths taken by the whales was reduced by 51% compared to control observations without vessel presence within 2000 m of whales. The consequent reduction in foraging time was likely greater than 36%, given that transit time is incompressible and foraging depth is dictated by where krill densities are located. We showed that the relative proportion of lost foraging time from vessel exposure increased exponentially with prey depth. Whales were unable to compensate for lost feeding opportunities by increasing diving rate or swim speed, except when feeding within 10 to 15 m of the surface. Our results indicate that preventing vessels from entering within a 400 m radius around blue whales can help reduce the negative effects of marine recreational activities on blue whale foraging. KEY WORDS: Disturbance · Behaviour · Whale-watching · Foraging · GLMM · GEE Full text in pdf format PreviousNextCite this article as: Lesage V, Omrane A, Doniol-Valcroze T, Mosnier A (2017) Increased proximity of vessels reduces feeding opportunities of blue whales in the St. Lawrence Estuary, Canada. Endang Species Res 32:351-361. https://doi.org/10.3354/esr00825 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in ESR Vol. 32. Online publication date: May 04, 2017 Print ISSN: 1863-5407; Online ISSN: 1613-4796 Copyright © 2017 Inter-Research.

Acoustic multi-tasking in beaked whales: The use of alternating echolocation regimes during the descent phase of deep foraging dives

Beaked whales use echolocation to navigate and hunt at extreme depths, and the timing of their echolocation clicks while actively searching for prey is generally characterized by a stable inter-click interval (ICI). However, analysis of autonomous, long-term passive acoustic data revealed a markedly different pattern during the vocal portion of their descent towards the seafloor. Diving beaked whales were found to alternate between two ICI regimes, while also gradually increasing their overall click rate. This strategy is presumably used to simultaneously monitor two different target ranges as they approach their preferred foraging depth. One ICI regime likely corresponds to the two-way-travel time of sound to the seafloor, and by examining the rate at which the time interval between clicks decreased in this seafloor-tracking ICI regime, we calculated estimates of dive descent rate. The second ICI regime was consistently more rapid, and represents a shorter search range that is likely used by the whale to inspect the nearby water column for the presence of prey and other features. This unique echolocation behavior was identified in acoustic encounters of four different species of beaked whales recorded at a variety of sites in the North Pacific Ocean and the Gulf of Mexico.

Life history consequences of larval foraging depth differ between two competingAnopheles mosquitoes

1. Anopheline larvae are surface feeders and allocate most of their time to search for food at the water surface. However, species of theAnopheles gambiaeGiles complex may also show bottom feeding. The consequences of this foraging tactic for life history are unknown, yet may be relevant to understand inter‐specific competition patterns.2. The diving ability and activity of larvae of the mainAfrican malaria vectors,An. coluzziiandAn. gambiae, at two different water depths (14 and 30 cm) were assessed. We further explored the biological relevance of diving for food harvesting by monitoring key life history traits in two species treatments (single or mixed species) and two food treatments (surface or bottom feeding).3. Overall,An. coluzziilarvae showed more diving activity thanAn. gambiae. When feeding at the bottom both species, and especiallyAn. gambiae, showed a delayed emergence and a reduced emergence rate. Moreover,An. gambiaealso suffered a reduced wing length.4. Mixed‐species rearing had a detrimental effect on the life history traits ofAn. gambiaebut not onAn. coluzzii, suggesting a competitive advantage for the latter in our experimental conditions.5. The present results confirm that anopheline larvae are able to forage for food at the bottom of their breeding site and thatAn. coluzziishows a superior diving activity thanAn. gambiaeand this at a lower cost. These behavioural differences probably reflect specific adaptations to different aquatic habitats, and may be important in shaping species distributions and the population biology of these important vector mosquitoes.

Ontogeny of early diving and foraging behavior of northern fur seal (Callorhinus ursinus) pups from Bering Island, Russia

The ontogeny of diving and foraging behavior of northern fur seal pups from a stable population on Bering Island, Russia, was recorded with animal-borne instruments during their first few months at sea, a critical period during their first year at sea. Thirty-five pups were instrumented with satellite-linked time-depth recorders and stomach temperature pills. Diving occurred predominantly at night with deeper and longer dives as the pups matured. Mean dive depths were correlated with lunar illumination, whereas mean dive durations were also correlated with time of day and sex. Foraging success did not differ between sexes, and there was no relationship between meal size (as indicated by feeding event duration and minimum stomach temperature) and lunar illumination fraction or maximum foraging depth. Although most pups were able to successfully forage within 3 days of starting their migration, the number of feeding events recorded each day remained low (mean 1.6 events day−1). There was no indication of an appreciable increase in meal size after the first 2 weeks of the migration despite an increase in dive frequency and depth. The results are consistent with observations that pups do not gain mass during their first year and emphasize the risk of starvation from infrequent foraging in cold water.

Automated detection of sperm whales (Physeter macrocephalus) creaks and association with depredation events.

Sperm whales have been depredating black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska for decades, but the behavior has now become pervasive enough that it is starting to affect government estimates of the sustainable catch, motivating further studies of this behavior. In July 2007 and June 2009, bioacoustic tags were attached to adult sperm whales off Southeast Alaska under both natural foraging conditions and situations wherein the animals were depredating sablefish from commercial longlining vessels. Tags remained attached for a cumulative hours, yielding more than 100 h of depth, orientation, and acoustic data. Results show that when depredating, sperm whales often dive at depths shallower than 50 m, compared to natural foraging depths of 300–400 m in the area. Dive durations, pitch, and roll rates are also distinctively different. Techniques such as spectrogram cross-correlation and cepstral analysis are used to automatically detect creak events, which have lower intensities and narrower frequency ranges than conventional clicks. Acoustic measurements such as creak production rate, duration, and intensity are then extracted. Correlations are derived between creak events, sudden changes in body orientation and depredation rate, with a goal of determining an acoustic measure of depredation activity and efficacy. [Work conducted under the SEA SWAP Program, supported by the North Pacific Research Board and the National Geographic Society.]

WHAT GROUNDS SOME BIRDS FOR LIFE? MOVEMENT AND DIVING IN THE SEXUALLY DIMORPHIC GALÁPAGOS CORMORANT

Flightlessness in previously volant birds is taxonomically widespread and thought to occur when the costs of having a functional flight apparatus outweigh the benefits. Loss of the ability to fly relaxes body mass constraints which can be particularly advantageous in divers, because underwater performance correlates with mass. The Galápagos CormorantPhalacrocorax harrisiis flightless and the largest of its 27‐member genus. Here, the loss of flight, and consequent reduced foraging range, could be compensated by enhanced dive performance. Over three years, 46 Galápagos Cormorants were successfully equipped with time–depth–temperature recorders, and 30 birds with GPS recorders during the breeding season. Birds foraged at a mean of 690 m from the nest and just 230 m from the nearest coast, confirming an extremely limited foraging range during the breeding season and corresponding increased potential for intraspecific competition. Although the maximum recorded dive depth of 73 m tallied with the species body mass, &gt;90% of dives were conducted in water &lt;15 m deep. The heavier males foraged in different areas and dived longer and deeper than females, which exposed males to colder water. Consideration of how plumage insulation decreases with depth indicates that diving males should lose 30% more heat than females, although this may be partially compensated by their lower surface area : volume ratio. A simple model highlights how energy expenditure from swimming underwater due to buoyancy and energy lost as heat have opposing trends with increasing depth, leading to the prediction of an optimum foraging depth defined by the volume of plumage air and water temperature. This has ramifications for all diving seabirds. It is proposed that the reduction in wing size, together with energy‐expensive flight musculature, allows the Galápagos Cormorant to be more efficient at shallow depths than other seabirds, but only in warm equatorial waters. The high prey density and predictability of benthic prey in defined areas of the Galápagos can be particularly well exploited by this flightless species, with its limited foraging range, but the Galápagos Cormorant is unlikely to be able to accommodate much change in environmental conditions.

Depth, orientation, and acoustics of sperm whales (Physeter macrocephalus) under natural and depredation foraging conditions in the Gulf of Alaska

In July 2007 bioacoustic tags were attached to adult sperm whales in the Gulf of Alaska under both natural foraging conditions, and situations wherein the animals were depredating sablefish from commercial longlining vessels. A small Rigid‐Hull Inflatable was used to approach and attach a suction‐cup acoustic recording tag on 13 occasions and stayed on animals for a total of 168 hours, yielding 80 h of depth, orientation, and acoustic data. These results, combined with passive acoustic tracking and underwater video‐camera data, indicate that sperm whales depredate at depths shallower than 50 m, compared to natural foraging depths of 300‐400 m in the area. During depredation the animals demonstrate changes in pitch and roll that are greater when compared with normal foraging behavior. Observations on the acoustic behavior of the animals are noted as well. The fact that these normally deep‐diving animals depredate so close to the surface has interesting implications for both depredation‐reduction strategies and biosonar research. [Work conducted under the SEASWAP program, supported by the North Pacific Research Board and the National Geographic Society.]

Predation risk constrains the plasticity of foraging behaviour in teals, Anas crecca: a flyway-level circumannual approach

The trade-off foragers make between predation risk and feeding efficiency is readily studied in dabbling ducks, which have stereotyped feeding methods, some of which prevent predator detection while others do not. Teals forage mostly with only the bill submerged (eyes above the water surface) in winter, but use a broader foraging repertoire in summer. Given the different environments used by teals over the year, it is likely that such a shift is due to changes in diet, but it may also be caused by differences in predation risk between habitats. However, neither predation risk nor teal behaviour has been studied with consistent methods around the year or throughout any of its flyways. Covering wintering, spring-staging, breeding and moulting sites, we combined focal observations of teals and predator flyover data from seven regions ranging from southern France to northern Sweden. Although not apparent at the scale of days within sites, teals indeed relied more on shallow foraging where predation risk was higher, i.e. at wintering sites. Average foraging depth increased gradually from September to August, i.e. from wintering to breeding sites. Foraging bout length of deeply foraging teals did not decrease over the year, suggesting that it is through selection of foraging technique, rather than by the balance between foraging and interruptions, that birds adjust to predation risk. This study highlights behavioural plasticity in response to contrasting selection regimes within a flyway, in dabbling ducks as well as long-distance migrants in general.

Foraging Depth of Flamingos in Single-species and Mixed-species Flocks at Laguna de Pozuelos, Argentina

Habitat use by Chilean Flamingo (Phoenicopterus chilensis), Andean Flamingo (Phoenicoparrus andinus) and James Flamingo (Phoenicoparrus jamesi) in single-species and mixed-species flocks were studied at Laguna de Pozuelos, Argentina, by comparing the foraging pattern of each species in the presence and absence of the others. Spatial segregation among species within a flock was found. James Flamingo used the edge and the shallow foraging depth both in single-species and mixed-species flocks. Chilean Flamingo did not use the edge or the shallow depth either in single-species or mixed-species flocks, and used moderate and deep foraging depths. Andean Flamingo exhibited the most flexible foraging pattern, grouping either with Chilean Flamingo or with James Flamingo depending on which of these species predominated in the flock. Spatial difference in the feeding behavior among the three flamingo species may be principally attributed to the habitat of prey organisms. Chilean Flamingo feeds on planktonic organisms present mainly in the upper portion of the water column, and therefore this species can access a wider range of food items if foraging at deeper depths both in single-species or mixed-species flocks. The foraging pattern of James Flamingo was indicated by its main prey, benthic and aerophilic diatoms, largely found in shallow waters near the edge of the lake. Andean Flamingo feed on benthic diatoms common in the water-sediment interface of shallow and deep waters, thus favoring its grouping behavior with James or Chilean Flamingos in mixed-species flocks.

Changes in Benthic Invertebrate Communities of South Bay, Lake Huron Following Invasion by Zebra Mussels ( Dreissena polymorpha), and Potential Effects on Lake Whitefish ( Coregonus clupeaformis) Diet and Growth

In this study we evaluated changes in benthic invertebrate communities of South Bay, Lake Huron following the invasion of zebra mussels ( Dreissena polymorpha) and considered the implications for diets and growth of whitefish ( Coregonus clupeaformis), a commercially important fish in the Great Lakes. Of the ten benthic invertebrate groups identified prior to invasion (1980–81), only densities of Diporeia and Oligochaeta have changed since the appearance of the zebra mussel, and only Diporeia and Chironomidae changed in relative abundance. These changes are similar to those observed in other areas of the Great Lakes, with the exception of an increase in Oligochaeta density. Post-invasion (2002–03) shallow-water communities appear to be more homogeneous, dominated by zebra mussels and Isopoda, whereas deep-water sites are more heterogeneous due to the loss of Diporeia. Additional data on Diporeia density for several years between 1959 and 2004 indicated that current low densities are not typical of South Bay. Based on changes in the benthic communities and published literature on whitefish diets, we predict that unless whitefish are able to switch to Mysis as an alternative to Diporeia, post-invasion whitefish diets will only contain a maximum of 57 to 84% of their former energy content. These predictions are likely underestimates, as they do not take into account increased energy costs associated with reductions in total invertebrate density at historical foraging depths.

Foraging costs and accessibility as determinants of giving‐up densities in a swan‐pondweed system

We measured the patch use behaviour of Bewick's swans (Cygnus columbianus bewickii) feeding on below ground tubers of fennel pondweed (Potamogeton pectinatus). We compared the swans’ attack rates, foraging costs and giving‐up densities (GUDs) in natural and experimental food patches that differed in water depth. Unlike most studies that attribute habitat‐specific differences in GUDs to predation risk, food quality or foraging substrate, we quantified the relative importance of energetic costs and accessibility. Accessibility is defined as the extent to which the animal's morphology restricts its harvest of all food items within a food patch. Patch use behaviours were measured at shallow (ca 0.4 m) and deep (ca 0.6 m) water depths on sandy sediments. In a laboratory foraging experiment, when harvesting food patches, the swan's attack rate (m3 s−1) did not differ between depths. In deep water the energetic costs of surfacing, feeding and trampling were 1.13 to 1.21 times higher than in shallow water with a tendency to spend relatively more time trampling, the most expensive activity. Taking time allocation as measured in the field into account, foraging in deep water was 1.26 times as expensive as in shallow water. In the lake the GUD in shallow water was on average 12.9 g m−2. If differences in energetic costs were the only factor determining differences in GUDs, then the deep water GUD should be 14.2 g m−2. Instead, the mean GUD in deep water was 20.2 g m−2, and therefore energetic costs explain just 18% of the difference in GUDs. At deep sites, 24% of tuber biomass was estimated to be out of reach, and we calculated a maximum accessible foraging depth of 0.86 m. This is close to the published 0.84 m based on body measurements. A laboratory experiment with food offered at a depth of 0.89 m confirmed that it was just out of reach. The agreement between calculated and observed maximum accessible foraging depths suggests that accessibility largely explains the remaining difference in GUDs with depth, and it confirms the existence of partial prey refuges in this system.

Structure and biomechanical properties of the trachea of the striped dolphin Stenella coeruleoalba: Evidence for evolutionary adaptations to diving

This study analyzes the structure and mechanical properties of the trachea of the striped dolphin Stenella coeruleoalba, one of the most common cetacean species. The cetacean trachea is made up of closed or semiclosed cartilaginous rings without a paries membranaceus. Our results indicate that the inner lining of the trachea contains erectile tissue in which several venous lacunae permeate the mucosa. We also observed and described the presence of peripheral neurons containing nitric oxide along the rim of the venous lacunae. Data obtained from compression and tensile tests and comparison with the pig and goat tracheas indicate a higher stiffness and a different, higher breaking point for the dolphin trachea. On the whole, our data suggest that the trachea of the striped dolphin possesses structural properties that allow rapid filling with blood, possibly in relation to dive activities, and also allow modifications due to increased pressure and immediate return to the original shape without risks of permanent bending or rupture, as would happen in a terrestrial mammal. As the organ undergoes intense pressure difference during descent to optimal foraging depth and subsequent rapid ascent to surface, especially in deep dives of hundreds of meters, the specific structural and biomechanical peculiarities of the trachea of the striped dolphin may represent an evolutionary adaptation to life in the water and to diving.