Assessing the Potential Impact of Cane Toads on Australian Snakes

Abstract:   Cane toads ( Bufo marinus ) are large, highly toxic anurans that were introduced into Australia in 1937. Anecdotal reports suggest that the invasion of toads into an area is followed by dramatic declines in the abundance of terrestrial native frog-eating predators, but quantitative studies have been restricted to nonpredator taxa or aquatic predators and have generally reported minimal impacts. Will toads substantially affect Australian snakes? Based on geographic distributions and dietary composition, we identified 49 snake taxa as potentially at risk from toads. The impact of these feral prey also depends on the snakes' ability to survive after ingesting toad toxins. Based on decrements in locomotor (swimming) performance after ingesting toxin, we estimate the LD₅₀ of toad toxins for 10 of the at-risk snake species. Most species exhibited a similar low ability to tolerate toad toxins. Based on head widths relative to sizes of toads, we calculate that 7 of the 10 taxa could easily ingest a fatal dose of toxin in a single meal. The exceptions were two colubrid taxa (keelbacks [  Tropidonophis mairii ] and slatey-grey snakes [  Stegonotus cucullatus ]) with much higher resistance to toad toxins (up to 85-fold) and one elapid (swamp snakes [  Hemiaspis signata ]) with low resistance but a small relative head size and thus low maximum prey size. Overall, our analysis suggests that cane toads threaten populations of approximately 30% of terrestrial Australian snake species.     

Food and feeding of Amphiesma stolatum (Linnaeus 1758)

The food and feeding behaviour of Amphiesma stolatum was studied. A total of 117 snakes were examined to analyse the diet composition. Ten live snakes were maintained in enclosure for evaluating the feeding habit, choice of food and rate of assimilation. The anurans constituted the major diet of the food spectrum. In captivity, fishes (fry), amphibians and reptiles were consumed by the snakes. A. stolatum exihibited Elaphe type of hunting behaviour. In most cases the snake struck the prey at its head. The assimilation rate with frog was highest.     

Influence of season on the feeding habits of the common sole Solea solea

The object of the study was to determine whether the composition of the diet of sole Solea solea Linnaeus, 1758 throughout the year is influenced only by the presence and abundance of prey or whether in addition it is influenced by a selection procedure resulting from its energetic and nutritional needs. Feeding habits were established by examining the stomach contents of males and females throughout the year. We recorded the presence or absence of each prey item and identified dominant, occasional and accidental prey. Differences in the seasonal composition of the basic diet were analysed (chisquare). Throughout the year, the diet consisted mainly of crustaceans, except in autumn when polychaetes were the most abundant prey. Significant differences (chisquare) in feeding habits were established between seasons and between sexes in each season. Some of the dominant prey (Ampeliscidae and Callianassidae) live exclusively in estuaries and bays, and occur in highest abundance during winter, the season in which sole enter estuaries to spawn. These prey items were present in higher numbers in the stomach contents of the sole during this period, reflecting the dependence of its diet on prey availability.     

Increased nitrogen availability influences predator–prey interactions by altering host-plant quality

Little is known about how plant nutritional and defensive qualities interact to influence predator–prey interactions. To address this need, we provided the neo-tropical milkweed, Asclepias curassavica, with two levels of nitrogen availability and examined how altered host-plant quality influenced the responses of a specialist aphid, Aphis nerii, and a coccinellid predator, Harmonia axyridis. Aphis nerii uses A. curassavica for multiple resources, including nutrition and sequestration of cardenolides for defense against natural enemies. Increased nitrogen availability improved A. curassavica quality by decreasing carbon-to-nitrogen (C:N) ratios and cardenolide concentrations, resulting in A. nerii that also had lower C:N ratios and cardenolide concentrations. Aphis nerii population growth was higher on plants with high nitrogen availability, compared with aphids on plants with low nitrogen availability. In no-choice feeding trials, Harmonia axyridis consumed more high C:N ratio aphids, suggesting a potential compensatory response to reduced aphid nutritional quality. Additionally, H. axyridis were able to consume more low-quality aphids at the expense of increasing exposure to increased cardenolide concentrations, suggesting that interactions between H. axyridis and A. nerii may be strongly influenced by prey nutritional quality. This work highlights the need to consider how variation in plant quality influences herbivore nutritional and defensive quality when examining mechanisms that influence predator–prey interactions.     

Do predators control prey species abundance? An experimental test with brown treesnakes on Guam

The effect of predators on the abundance of prey species is a topic of ongoing debate in ecology; the effect of snake predators on their prey has been less debated, as there exists a general consensus that snakes do not negatively influence the abundance of their prey. However, this viewpoint has not been adequately tested. We quantified the effect of brown treesnake (Boiga irregularis) predation on the abundance and size of lizards on Guam by contrasting lizards in two 1-ha treatment plots of secondary forest from which snakes had been removed and excluded vs. two 1-ha control plots in which snakes were monitored but not removed or excluded. We removed resident snakes from the treatment plots with snake traps and hand capture, and snake immigration into these plots was precluded by electrified snake barriers. Lizards were sampled in all plots quarterly for a year following snake elimination in the treatment plots. Following the completion of this experiment, we used total removal sampling to census lizards on a 100-m2 subsample of each plot. Results of systematic lizard population monitoring before and after snake removal suggest that the abundance of the skink, Carlia ailanpalai, increased substantially and the abundance of two species of gekkonids, Lepidodactylus lugubris and Hemidactylus frenatus, also increased on snake-free plots. No treatment effect was observed for the skink Emoia caeruleocauda. Mean snout-vent length of all lizard species only increased following snake removal in the treatment plots. The general increase in prey density and mean size was unexpected in light of the literature consensus that snakes do not control the abundance of their prey species. Our findings show that, at least where alternate predators are lacking, snakes may indeed affect prey populations.     

Prey selection by the hogchoker, Trinectes maculatus (Pisces: Soleidae), along summer salinity gradients in Chesapeake Bay, USA

We investigated feeding by the hogchoker, Trinectes maculatus (Bloch and Schneider), in freshwater, oligohaline, mesohaline, and polyhaline regions of Chesapeake Bay, USA, and examined prey selection in relation to food availability. Otter trawling for fish and Van Veen grab sampling for benthic macrofauna occurred in July and August 1992 and August and September 1993. Hogchokers exhibited both opportunistic and selective feeding patterns along the estuarine salinity gradient in four tributaries (Potomac, Rappahannock, York, and James Rivers) and in the mainstem Chesapeake Bay. Major prey taxa included annelids, arthropods, and tellinid siphons. In polyhaline habitat, polychaetes dominated both the benthos and gut contents numerically and gravimetrically. On the other hand, oligochaetes were numerically dominant in freshwater/oligohaline areas but were rarely eaten, perhaps because of their burial depth. Arthropods (mostly amphipods) occurred at most salinities, were common in gut contents in low-salinity areas, and were replaced as prey by larger proportions of polychaetes in polyhaline regimes. Although hogchokers ate tellinid siphons, they rarely consumed whole bivalves or gastropods. These diet patterns (and especially the importance of siphon nipping) are similar to those of juvenile or small flatfish elsewhere in Europe, Africa, and North America. A size–salinity relationship for hogchokers occurred along the summer salinity gradient, with smaller fish predominating upstream and larger fish downstream. It was not clear from our data if variation in diet composition reflected changes in prey composition along the salinity gradient rather than changes in fish size. Received: 14 June 1997 / Accepted: 27 June 1997     

Feeding habits of the Madeira rockfish <Emphasis Type="Italic">Scorpaena maderensis</Emphasis> from central Mediterranean Sea

In order to assess diet composition and niche breadth of this species, we analysed the stomach content of 182 specimens collected monthly along the eastern coast of Sicily (Central Mediterranean Sea). Overall, 50 prey taxa belonging to five major groups (algae, gastropods, crustaceans, polychaetes, fishes) were identified in 102 full stomachs. Benthic or epibenthic crustaceans, such as decapods, amphipods and isopods were the most important prey, whereas algae, gastropods, polychaetes and fishes were only occasionally ingested. In terms of composition by species, the diet of Scorpaena maderensis was characterized by a variety of rare or unimportant prey, which was consumed by few individuals only, although sometimes in large amount. As a result, S. maderensis can be considered a generalized and opportunistic feeder. The feeding intensity followed roughly a seasonal trend, with a minimum food intake in summer. The individual fish size was the most important factor affecting diet. According to the observed ontogenetic shift, small-sized individuals fed primarily on small crustaceans (i.e. amphipods and isopods), whereas large-sized specimens consumed preferably bigger and more vagile prey, such as walking and swimming decapods. No significant differences in diet were observed in relation to sex of predator and sampling season.     

Size matters: variation in the diet of chick and adult crested terns

We investigated ontogenetic, temporal and spatial patterns in the composition and size of prey in the diet of crested terns, Sterna bergii. Diet analyses indicated that crested terns are a generalist predator on surface-schooling clupeids (Australian anchovy Engraulis australis, sardine Sardinops sagax and blue sprat Spratelloides robustus), Degens leatherjacket Thamnaconus degeni, southern sea garfish Hyporhamphus melanochir, Australian herring Arripis georgianus, slender bullseye Parapriacanthus elongatus and barracouta Thyrsites atun. Ontogenetic differences in prey size indicated that adults are constrained in their foraging behaviour during the early chick-provisioning period by the need to self feed and select smaller prey that can be ingested by their chicks. Chicks consumed significantly higher proportions of clupeids than adults, which consumed mainly Degens leatherjackets and barracouta, suggesting that adults may select higher quality prey for their chicks compared to what they consume themselves. Spatial differences in prey composition were driven by differing proportions of sardine, Australian anchovy and Degens leatherjacket and could reflect local differences in the abundances of these prey. The size of prey taxa consumed by adults also reflected a North–South gradient in prey size. The large component of juvenile sardine in the diet of crested terns suggests future dietary measures may inform fisheries managers about changes in local juvenile sardine abundance. These data could assist in highlighting any fishery-related decreases in sardine recruitment and help ensure commercial fishing practices address principals of Ecologically Sustainable Development developed for Australian fisheries.     

Effects of limiting access to prey on development of first zoeal stage of the brachyuran crabs Cancer magister and Hemigrapsus oregonensis

We tested the influence of limiting access to prey on larval development of the crabs Cancer magister and Hemigrapsus oregonensis by raising their Stage 1 larvae in the laboratory on different prey densities and with various periods of access to prey. Experiments were conducted in 1995 and 1996 at the Shannon Point Marine Center in Anacortes, Washington, USA. Our results show that crab larvae do not require continuous access to prey for optimal development nor do they appear to require light for prey capture. Survival and duration of Stage 1 C. magister fed continuously on only one-fourth the amount of the control density of prey and those fed at the control density for only 6 h per day were the same as for larvae fed continuously at the control density (20 ml⁻¹). Larvae with cyclic access to prey at the control density for 24 h and then starved for 72 h showed significantly lower survival and longer instar duration to Stage 2. Experiments on Stage 1 H. oregonensis which investigated a combination of prey density, period of access to prey and light/dark conditions during feeding revealed that survival decreased with decreasing prey density or with decreasing feeding period, but no differences were observed during periods of limited prey availability as a function of light or dark conditions. Stage duration was not affected by reduced prey density nor by the light/dark condition at the time of feeding, but it was prolonged when the period of access to prey was limited. The period of access to prey did not affect the weight of Day 1 Stage 2 larvae. Larvae fed high densities of prey for 4 h followed by 20 h of reduced-density diet exhibited the same survival and stage duration as controls that were continuously fed high-density prey. Our results define sub-optimal diets that can be used experimentally to determine the nutritional contributions made by naturally-occurring prey organisms during larval development in the two species. In nature, larvae may satisfy nutritional requirements through periodic encounters with dense prey patches during vertical migrations by day or night. Received: 12 August 1997 / Accepted: 5 February 1998     

Chemical investigations of defensive steroid sequestration by the Asian snake Rhabdophis tigrinus

Rhabdophis tigrinus is an Asian natricine snake that possesses unusual defensive glands on the dorsal surface of its neck. These nuchal glands typically contain cardiotonic steroidal toxins known as bufadienolides, which are also abundant in the skin of toads. Feeding experiments demonstrated that toads consumed as prey are the ultimate sources of the bufadienolides in nuchal glands of R. tigrinus. Indeed, snakes on a toad-free Japanese island (Kinkasan, Miyagi Prefecture) lack these compounds in their nuchal glands, confirming that these snakes are unable to synthesize defensive bufadienolides. However, when snakes from Kinkasan are fed toads in the laboratory, they accumulate bufadienolides in their nuchal glands, indicating that they have not lost the ability to sequester defensive compounds from prey. In contrast, R. tigrinus from a toad-rich island (Ishima, Tokushima Prefecture) possess large quantities of bufadienolides, reflecting the abundance of toads from which these compounds can be sequestered. Feeding experiments involving gravid R. tigrinus demonstrated that bufadienolides can be provisioned to offspring so that hatchlings are chemically defended before their first toad meal. Maternal provisioning of bufadienolides can take place through two routes: by deposition in yolk and by diffusion in utero, even late in gestation. We applied bufadienolides to the surface of eggs from Kinkasan and found that the embryos are able to take up these compounds into their nuchal glands, demonstrating the feasibility of uptake across the eggshell. Female R. tigrinus provision bufadienolides to their offspring in direct proportion to their own level of chemical defense. By feeding toad-derived bufotoxins to R. tigrinus hatchlings, we determined that the sequestration of these compounds involves at least three types of modification: hydrolytic cleavage of suberylarginine side chains, hydroxylation, and epimerization.     

Food regulates reproduction differently in different habitats: experimental evidence in the Goshawk

Food supplementation experiments have been widely used to get detailed insight into how food supply contributes to the reproductive performance of wild animals. Surprisingly, even though food seldom is distributed evenly in space, variation in local habitat quality has usually not been controlled for in food supplementation studies. With results from a two-year feeding experiment involving a habitat-sensitive avian top predator, the Northern Goshawk Accipiter gentilis, we show that treatment effects on goshawk reproductive performance are habitat dependent. Extra food reduced nestling mortality in low-quality territories where prime habitat (forest) is scarce, but not in high-quality territories where prime habitat is abundant. Consequently, brood size did not differ between treatment categories in heavily forested territories, but fledgling numbers differed between unfed and fed goshawk pairs breeding in territories where forest is scarce. However, because extra food was not superabundant, this artificial increase in offspring number induced a dramatic decrease in nestling condition in low-quality territories. Treatment effects were detected even after controlling statistically for other potentially confounding effects (year, territory identity) and strongly covaried with territory-specific abundances of the most important summer prey species. These results highlight the importance of acknowledging the effect that small-scale variation in habitat quality and availability of natural food may have on the results of food supplementation experiments. In order to assess the generality of food supplementation effects, the integration of habitat heterogeneity and variation in food abundance is thus needed, especially among species in which small-scale variation in habitat quality influences demographic patterns.     

Feeding opportunities of larval and juvenile cod (Gadus morhua) in a Greenlandic fjord: temporal and spatial linkages between cod and their preferred prey

Feeding of fish depends on a spatial and temporal match with prey, and since larval and juvenile feeding can be highly selective, their preferences for given prey sizes and taxa should be considered when quantifying the actual availability of potential prey. We investigated the diet and prey preferences of the early-life stages of Atlantic cod (Gadus morhua) to quantify the availability of prey during a spring-summer season in a West Greenlandic fjord. We hypothesized that abundances of larval and juvenile cod at size were synchronized to optimal availability of preferred prey in space and time. The present analysis is based on nine cruises each covering 5 stations visited between 24 May and 5 August 2010 comparing zooplankton abundance, cod gut content and distribution patterns. Cod 4–25 mm in length preferred prey of about 5 % of their own length. During ontogeny, their preferences changed from calanoid nauplii towards Pseudocalanus spp. and Calanus spp. copepodites. The larvae/juvenile had an exceptionally high dietary contribution from cladocerans, which were highly preferred by cod larger than 9 mm, while the abundant Metridia longa and the non-calanoid copepods contributed less. These findings stress the importance of focusing on abundance of preferred prey when assessing the actual prey availability to young fish. We found a spatio-temporal overlap between cod and their preferred prey, and observations suggest that advection of both zooplankton and cod contributed to this overlap. Hence, the larval feeding opportunities might be sensitive to climate-related changes affecting the circulation patterns in this fjord.     

Vulnerability of the copepod Acartia tonsa to predation by the scyphomedusa Chrysaora quinquecirrha : effect of prey size and behavior

Although scyphomedusae have received increased attention in recent years as important predators in coastal and estuarine environments, the factors affecting zooplankton prey vulnerability to these jellyfish remain poorly understood. Current models predicting feeding patterns of cruising entangling predators, such as Chrysaora quinquecirrha (Desor, 1948), fail to account for the selection of fast-escaping prey such as copepods. Nevertheless, our analysis of gastric contents of field-collected medusae showed that this scyphomedusa fed selectively on the calanoid copepod Acartia tonsa (Dana, 1846) and preferentially ingested adult over copepodite stages. We measured feeding rates in a planktonkreisel while simultaneously videotaping predator–prey interactions. C. quinquecirrha consumed adult A. tonsa ten times faster than copepodites. Differences in prey behavior, in the form of predator–prey encounter rates or post-encounter escape responses, could not account for the elevated feeding rates on adults. Prey size, however, had a dramatic impact on the vulnerability of copepods. In experiments using heat-killed prey, feeding rates on adults (1.5 times longer than copepodites) were 11 times higher than on copepodites. In comparison, medusae ingested heat-killed prey at only two to three times the rate of live prey. These results suggest that during scyphomedusan–copepod interactions, prey escape ability is important, but ultimately small size is a more effective refuge from predation. Received: 26 September 1997 / Accepted: 20 May 1998     

Diet of Neotropic cormorant (Phalacrocorax brasilianus) in an estuarine environment

The diet of the Neotropic cormorant (Phalacrocorax brasilianus) was studied by analysing 289 regurgitated pellets collected from a roosting site at Lagoa dos Patos estuary, southern Brazil, between November 2001 and October 2002 (except April to June). In total, 5,584 remains of prey items from 20 food types were found. Fish composed the bulk of the diet representing 99.9% by mass and 99.7% by number. The main food items were White croaker (Micropogonias furnieri) (73.7% by frequency of occurrence, 48.9% by mass and 41.2% by number), followed by Catfish (Ariidae) and anchovies (Engraulididae). In Lagoa dos Patos estuary the generalist Neotropic cormorant fed mainly on the two most abundant demersal fishes (White croaker and Catfish), which accounted for the low niche breadth calculated. The total length of all fish preyed varied from 27.2 to 318.3 mm (113.5 ± 48.0 mm), and preyed White croakers’ size differed between months. Neotropic cormorants seem to prey on most abundant class sizes of White croaker instead of selecting similar prey size throughout the time. However, temporary changes in diet in terms of food items, abundance and prey size were detected, revealing a high ecological plasticity of the species. Individual daily food intake of Neotropic cormorants estimated by pellets and metabolic equations corresponded to 23.7 and 27.1% of their body mass, falling in the range of other cormorant species. Annual food consumption of the population estimated by both methods was 73.4 and 81.9 tonnes, comprising mainly immature and subadult White croaker and Catfish which are commercially important. Temporal variations in diet composition and fish size preyed by Neotropics cormorants, a widespread and generalist species, suggest shifts according to fluctuations in the abundance of prey. The plasticity of this cormorant is also revealed by their ability to adjust feeding behaviour in response to temporal or local changes in the environment, from a generalist at the species level to a specialist at the individual or local population level.     

Trade-offs associated with dietary specialization in corallivorous butterflyfishes (Chaetodontidae: <Emphasis Type="Italic">Chaetodon</Emphasis>)

Increasing dietary specialization is an inherently risky strategy because it increases a species’ vulnerability to resource depletion. However, risks associated with dietary specialization may be offset by increased performance when feeding on preferred prey. Although rarely demonstrated, highly specialized species are expected to outperform generalists when feeding on their preferred prey, whereas generalists are predicted to have more similar performance across a range of different prey. To test this theory, we compared the growth rates of two obligate coral-feeding butterflyfishes (Chaetodon trifascialis and Chaetodon plebeius) maintained on exclusive diets of preferred vs nonpreferred prey. In the field, C. trifascialis was the most specialized species, feeding almost exclusively on just one coral species, Acropora hyacinthus. C. plebeius meanwhile, was much less specialized, but fed predominantly on Pocillopora damicornis. During growth experiments, C. trifascialis grew fastest when feeding on A. hyacinthus and did not grow at all when feeding on less preferred prey (P. damicornis and Porites cylindrica). C. plebeius performed equally well on both A. hyacinthus and P. damicornis (its preferred prey), but performed poorly when feeding on P. cylindrica. Both butterflyfishes select coral species that maximize juvenile growth, but contrary to expectations, the more specialized species (C. trifascialis) did not outperform the generalist species (C. plebeius) when both consumed their preferred prey. Increased dietary specialization, therefore, appears to be a questionable strategy, as there was no evidence of any increased benefits to offset increases in susceptibility to disturbance.     

Diet of young Atlantic bluefin tuna (<Emphasis Type="Italic">Thunnus thynnus</Emphasis>) in eastern and western Atlantic foraging grounds

Atlantic bluefin tuna (Thunnus thynnus) are highly migratory predators whose abundance, distribution, and somatic condition have changed over the past decades. Prey community composition and abundance have also varied in several foraging grounds. To better understand underlying food webs and regional energy sources, we performed stomach content and stable isotope analyses on mainly juvenile (60–150 cm curved fork length) bluefin tuna captured in foraging grounds in the western (Mid-Atlantic Bight) and eastern (Bay of Biscay) Atlantic Ocean. In the Mid-Atlantic Bight, bluefin tuna diet was mainly sand lance (Ammodytes spp., 29% prey weight), consistent with historic findings. In the Bay of Biscay, krill (Meganyctiphanes norvegica) and anchovy (Engraulis encrasicolus) made up 39% prey weight, with relative consumption of each reflecting annual changes in prey abundance. Consumption of anchovies apparently declined after the local collapse of this prey resource. In both regions, stable isotope analysis results showed that juvenile bluefin tuna fed at a lower trophic position than indicated by stomach content analysis. In the Mid-Atlantic Bight, stable isotope analyses suggested that >30% of the diet was prey from lower trophic levels that composed <10% of the prey weights based upon traditional stomach content analyses. Trophic position was similar to juvenile fish sampled in the NW Atlantic but lower than juveniles sampled in the Mediterranean Sea in previous studies. Our findings indicate that juvenile bluefin tuna targeted a relatively small range of prey species and regional foraging patterns remained consistent over time in the Mid-Atlantic Bight but changed in relation to local prey availability in the Bay of Biscay.     

Higher trophic level prey does not represent a higher quality diet in a threatened seabird: implications for relating population dynamics to diet shifts inferred from stable isotopes

Diet quality is a key determinant of population dynamics. If a higher trophic level, more fish-based diet is of higher quality for marine predators, then individuals with a higher trophic level diet should have a greater body mass than those feeding at a lower trophic level. We examined this hypothesis using stable isotope analysis to infer dietary trophic level and foraging habitat over three years in eastern rockhopper penguins Eudyptes chrysocome filholi on sub-Antarctic Campbell Island, New Zealand. Rockhopper penguins are ‘Vulnerable’ to extinction because of widespread and dramatic population declines, perhaps related to nutritional stress caused by a climate-induced shift to a lower trophic level, lower quality diet. We related the stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope values of blood from 70 chicks, 55 adult females, and 55 adult males to their body masses in the 2010, 2011, and 2012 breeding seasons and examined year, stage, age, and sex differences. Opposite to predictions, heavier males consumed a lower trophic level diet during incubation in 2011, and average chick mass was heavier in 2011 when chicks were fed a more zooplankton-based, pelagic/offshore diet than in 2012. Contrary to the suggested importance of a fish-based diet, our results support the alternative hypothesis that rockhopper penguin populations are likely to be most successful when abundant zooplankton prey are available. We caution that historic shifts to lower trophic level prey should not be assumed to reflect nutritional stress and a cause of population declines.     

Movement patterns of the seastar Heliaster helianthus in central Chile: relationship with environmental conditions and prey availability

The rates and patterns of feeding and displacement of predators constitute two of the most important plastic behavioral responses that allow individuals to respond quickly to changes in abundance of their prey, predation risks and to rapid alterations in environmental conditions. In this study, we quantified seasonal and spatial variation in displacement (net changes in location in 12 or 24 h periods) and prey consumed of marked individuals of the keystone seastar Heliaster helianthus at six sites spanning 600 km along the coast of north-central Chile. We evaluated the hypotheses that: (1) at sites with low availability (cover) of the main prey, the mussel Perumytilus purpuratus, Heliaster displays larger displacements and consumes a greater proportion of other prey (e.g. mobile species) than at sites with high mussel cover, (2) daily displacements will be correlated with sea surface temperature (SST) and (3) increased wave action will reduce seastar daily displacement. Our results show that Heliaster displacement is higher at sites with lower availability of P. purpuratus; and at these sites, a larger proportion of Heliaster individuals are observed feeding, mostly on other prey (e.g. limpets), which could offset the higher costs associated with increased movement. In addition, wave forces affected the activity of Heliaster negatively. Contrary to our expectations, the daily displacements did not show any relationship with SST measured on the day or the previous days of the surveys, despite the fact that average displacement was generally higher in summer than in winter months. Future studies should examine Heliaster movement during single foraging excursions and determine whether these responses affect the growth and reproductive output of individuals. Such information is vital to understand how changes in prey abundance and environmental conditions alter the behavior and energy budget of this predator and its ability to control prey populations.     

Essential fatty acid (docosahexaenoic acid, DHA) availability affects growth of larval herring in the field

Larval fish growth and survival depends not only on prey quantity, but also on prey quality. To investigate effects of prey fatty acid concentration on larval herring growth, we collected different prey organisms and larval herring (Clupea harengus L.) in the Kiel Canal during the spring season of 2009. Along with biotic background data, we analysed fatty acids both in prey organisms and in the larvae and used biochemically derived growth rates of the larvae as the response variable. Larval herring reached their highest RNA/DNA derived growth rates only at high docosahexaenoic acid (DHA) concentration. When the ratio of copepodids to lesser quality cirriped nauplii was low, larval growth and larval DHA concentration were both significantly negatively affected. This was true even as prey abundance was increasing. This finding indicates that even in mixed, natural feeding conditions, growth variations are associated with DHA availability in larval fish.     

Seasonal changes in the diet of a critically endangered seabird and the importance of trawling discards

Pelagic seabirds obtain food from oceans where the availability of their prey changes rapidly both seasonally and spatially. Here, we investigated changes in the trophic habits of the critically endangered Balearic shearwater (Puffinus mauretanicus) through the breeding season and tested for dietary differences between sexes and age classes. We analysed δ¹⁵N and δ¹³C values in blood of adults during the pre-incubation, incubation and chick-rearing periods and of their chicks. Using a two-isotope mixing model, we estimated dietary contributions based on isotope values from potential prey species which included small pelagic species available naturally and demersal fish species available only from trawling discards. Balearic shearwaters showed clear isotopic and dietary variation through the breeding season. During pre-incubation, breeding adults appeared to exploit demersal fish, whereas during the incubation and chick-rearing period, they fed mainly on pelagic anchovies (Engraulis encrasicolus) and pilchards (Sardina pilchardus). Similarly, chicks were fed mainly with anchovies, a resource with a high energetic value. This variation in the dietary habits of adult shearwaters during the breeding season was probably related to both natural and fishery-induced seasonal changes in the availability of potential prey species within their main feeding grounds. However, changes in the nutritional requirements of the shearwaters could also play an important role. Indeed, diet differed between sexes during pre-incubation: females fed less on trawling discards and more on small pelagic fish than males. This sexual segregation in diet could be the consequence of higher nutritional requirements of females during this period. Our study reveals the differential importance of both trawling discards and small pelagic fish species for a pelagic seabird depending on the breeding period and illustrates the importance of considering the entire breeding season when making inferences about the importance of specific prey in seabird dietary studies.