Monitoring of the trophic ecology of pipefish species (Syngnathus abaster, Syngnathus acus) in an alluvial lake habitat (Lake Bafa, Turkey)

The present study investigated the trophic ecology of two species living in an alluvial lake ecosystem in terms of season, length groups, sex, and mouth morphology. The fish were caught in a coastal lake area between December 2014 and March 2016 using a seine. In general, C. aquaedulcis was the most important prey for S. abaster, accounting for 10.99% of all prey, while amphipods accounted for 54.04% of the S. acus diet. Amphipods were the most frequently consumed prey in autumn, while C. aquaedulcis was the most important prey in spring. This result indicates that the species preferred similar prey groups and generated food competition depending on the season. The short-length group contained samples of S. abaster with empty guts, whereas S. acus in the same group consumed C. aquaedulcis. Length group II and III of individuals indicated that both species consumed amphipods and juvenile Syngnathus sp. Thus, the two species had statistically different preferences in terms of prey length. The mouth width of S. abaster was larger than that of S. acus. In conclusion, zooplankton constituted the main prey for both species in the lake and the morphological differences between the mouth apparatus of the species were due to the differences in feeding habits.

No edge effect on quantity of prey captured in the forest-dwelling tetragnathid orb spider Metellina mengei

Among the most damaging anthropogenic effects for ecosystems is habitat fragmentation. One of its consequences is the creation of edges, which results in more exposed habitats that have different ecological and behavioural effects on the different species that live there. However, the nature and magnitude of these effects remain unknown for most of the animals and plants inhabiting these edge habitats. This study intends to determine if quantity of prey capture by a woodland population of the orb spider Metellina mengei is subjected to edge effects. By observing the prey capture of this species at edge and interior locations of a woodland, we found no significant effects of edge on the number of prey captured or the average prey length. Instead, we found that inclination of the web, but not web area or other measured web parameters, had a significant effect on prey capture. Therefore, this species of spider may be minimally affected by its location within the woodland and more affected by its surrounding microhabitat, which raises the possibility that non-specialised invertebrate predators could be less impacted by fragmentation than generally recognised.

Swordfish Xiphias gladius diet in the Florida Straits

Swordfish Xiphias gladius inhabit the Florida Straits year-round and provide a significant role in the food web as top-level predators. However, little is known about the diet composition and thus ecological role of swordfish in Florida. This study investigated swordfish diet by analyzing stomach contents of 131 swordfish in the Florida Straits from April 2007 to December 2008. Identifiable species included 13 teleost species, 3 cephalopod species, and 1 crustacean species. Cephalopods dominated the swordfish diet by weight (72.4%) and number (69.9%), and ranked highest in importance in the diet by the index of relative importance (IRI; 81.5%). Teleosts occurred the most (99.1%) but represented the second highest importance in diet by weight (25.2%), number (26.3%), and IRI (17.6%). Illex sp. was the prey with the greatest dietary importance, followed by unidentifiable ommastrephid squids. Stomach fullness index values ranged from 0 (empty) to 8.98 (mean = 0.37). A positive significant correlation between swordfish length and prey length was found (r2 = 0.104, P = 0.037), and no significant correlations were found between swordfish weight and prey weight (r2 = –0.075, P = 0.065). This is the first study to report a comprehensive diet of swordfish inhabiting waters in the Florida Straits region and suggests swordfish may be opportunistic feeders, altering their diet when abundance or presence of prey changes.

Diet of the Bronze Skink Eutropis macularius (Reptilia: Squamata: Scincidae) from Thua Thien Hue Province, Central Vietnam

In this study, we examined the diet of 149 males and 147 females of Eutropis macularius from Thua Thien Hue Province, central Vietnam using a nonlethal stomach-flushing technique. The prey items of E. macularius composed of Araneae, Insecta (Blattodea, Coleoptera, Hymenoptera, Isoptera, Odonata, and Orthoptera), Mollusca (Philomycidae), and plant materials. The most important prey items were insect larvae, hymenopterans (including ants), grasshoppers, and termites, for both sexes in three populations. Plant materials were also found in the stomach of E. macularius with an index of relative importance of 7.19%, suggesting that E. macularius is an omnivorous species. However, the dominant prey categories of E. macularius were insects, including insect larvae, hymenopterans, grasshoppers, and termites, with many small, sedentary, clumped prey items. Simpson’s heterogeneity index of skinks from three populations from Bach Ma National Park as well as from Aluoi and Huong Tra districts were 10.07, 7.85, 3.94, respectively. Eutropis macularius showed significant positive correlations between mouth width (MW) and prey width (P = 0.001) and between MW and prey volume (P 0.0001). There are significant positive correlations between snout-vent length (SVL) and prey sizes consumed: between SVL and prey length, P 0.0001; SVL and prey width, P 0.0001; and between SVL and prey volume, P 0.0001. These results indicated that SVL and MW are the limiting factors on the size of prey consumed in this skink.

The biggest or the most abundant? Predation of the Black Drum Pogonias cromis (Perciformes, Sciaenidae) on benthic organisms in southern Brazil

Predators that consume larger prey acquire a greater net return of energy per individual, even though they are less abundant. The objective of this work is to analyze the feeding biology of Pogonias cromis in southern Brazil, in order to test for the occurrence of ontogenetic changes in diet as fish reach larger sizes, by consuming larger prey as they grow. Between August 2014 and May 2016, 347 specimens were collected from catches of the fishing fleet that operate in the Patos lagoon estuary and in the adjacent marine area of Cassino beach, that use artisanal fishing gillnets. The prey-specific relative importance index, food overlap, niche breadth and prey length preference were calculated for three length classes, class 1 (27.8–48.73 cm), class 2 (48.73–69.66 cm) and class 3 (69.66–90.60 cm). A total of 13 food items (6 species of crustaceans, 5 species of mollusks, fish fragments and non-animal fragments) were identified, where two species of mollusks (Heleobia australis and Erodona mactroides) represented 90.49% of the diet. The overlap index was moderate between classes 1 and 2, high between 2 and 3 and moderate between 1 and 3. There was a low niche breadth at the population level and for each length class. There were no significant differences in the length of prey consumed among classes. The data obtained here indicates that P. cromis can be classified as a predator specialized in mollusks, with low tendency to ontogenetic changes in southern Brazil. Considering information from the population of P. cromis from Argentina, it can be inferred that the species in the waters of southern South America is a benthic predator adapted to local conditions.

Importance of the Landlocked Pond Smelt Hypomesus nipponensis as a food resource of the Little Tern Sternula albifrons on inland Andong Lake of Korea: A video image analysis

We carried out the diet study of the little tern on the sandy islet in inland Andong Lake, Korea, during the beeding season (April to July 2018). To identify its diet and examine the importance of the main prey species as a food resource, we set two remote-control video cameras with 4K-resolution on the islet. One thousand two hundred seventy-five still images that the tern had prey in its bill were identified at the species level and measured on a monitor. Then, they were classified to five length-categories and compared among months and breeding stages. Freshwater fishes dominated the diet (100%; eleven species overall), where the landlocked pond smelt Hypomesus nipponensis (80.8%) and largemouth bass Micropterus salmoides (13.7%) were the primary and next essential prey species, respectively. The average prey item length was 51.04 ± 20.89 mm and significantly differed among months and breeding stages (P < 0.001, respectively). 50–75 mm prey length category was the most frequent in the diet (42.2%). In April and May, larger fish >50 mm constituted the greater part of their diet (93.1%, 66.3%, respectively), whereas the diet in June and July consisted of smaller fishes <50 mm (56.2%, 68.8% respectively). The occurrence frequency of prey length categories also varied significantly among the breeding stages (P < 0.001): 1–25 mm and 50–75 mm were overrepresented and underrepresented, respectively, at the chicks in the nest stage. On the other hand, 50–75 mm was preferred for the pre-laying and incubation stages. In terms of the survival condition of pond smelts, the before- and after water surface temperatures of the day when terns flew away showed a significant difference (P = 0.004), where a threshold looks like between 29.11°C and 30.04°C. These results support the prey abundance hypothesis that, when cold-water pond smelts might wholly swim down into the deeper lake in the hot summer, the terns might also leave their colony for another foraging place with higher prey availability.

A length-based approach to predator–prey relationships in marine predators

Body size is a critical feature of the ecology of most organisms and has been used to describe and understand predator–prey interactions in both terrestrial and aquatic environments. Most previous studies have used prey mass to examine the relationships between predator size and prey size; however, using prey lengths may provide a different perspective, particularly for gape-limited fishes. Using a large database of predator and prey lengths for marine aquatic predators, I found the expected positive wedge-shaped relationship between predator length and prey length and a negative converging relationship between relative prey length (prey–predator length ratio = a measure of trophic niche breadth) and predator length. Distinct patterns in the size scaling of this measure of trophic niche breadth were identified using quantile regression: converging relationships were common among adults but absent among larvae. This difference suggests contrasting ontogenetic foraging opportunities between adults and larvae: a lack of large relative prey sizes for the largest adult predators, and a greater ability of larvae to include larger prey items in their diet as they grow.

Prey capture in the Venus flytrap: collection or selection?

Charles Darwin first proposed that the Venus flytrap ( Dionaea muscipula Ellis) functions optimally by capturing and digesting large prey, the small prey escaping through openings at the trap margins. This hypothesis, although intuitively sound in the context of trap mechanics or plant allocation theory, has not been tested adequately with populations of plants growing in the field. Here, with traps collected in the endemic habitat over 9 months, we show that prey capture in the Venus flytrap is opportunistic rather than selective. While there was no effect of trap size on prey capture success, there was a significant but weak positive relationship between trap length and prey length. Prey sizes were well below the theoretical maximum holding capacities of traps and relatively small insects were represented across the range of trap sizes. Our results show that prey capture was not biased toward large invertebrates. Instead, we suggest that nonselective prey capture across the observed range of trap sizes is the best-fit explanation of trap function in the context of relatively limited ability to change allocation in response to sudden increases in resource availability.

Modelling the growth, mortality, and predation interactions of cod juveniles and capelin larvae in the Barents Sea using a novel proto-moment population dynamics model

Hallfredsson, E. H., and Pope, J. G. 2007. Modelling the growth, mortality, and predation interactions of cod juveniles and capelin larvae in the Barents Sea using a novel proto-moment population dynamics model. – ICES Journal of Marine Science, 64: 1313–1323. Proto-moments of a fish population, the sums of products of powers of length with numbers at length, relate both to the traditional statistical measures, the mean, the variance, the skewness, and the kurtosis of their size distribution, and to the biologically important measures of the abundance and the biomass of the population. Population models based on this approach are constructed as matrix delay-difference equations. They model moments of the length distributions rather than the age distributions, and express population dynamic problems in an analytically tractable form. Here, a modification of this approach is explored for a case involving a predator–prey relationship among young-of-the-year fish. The modelled species are juvenile cod (the predator) and capelin larvae (the prey) in the Barents Sea. Their population dynamics are modelled by the proto-moment method, but using two different approaches for the derivation of the predation mortality term. The first approach, a published matrix-based formulation, is formed purely in terms of the proto-moments. The second approach, developed here, converts the proto-moments back to consistent size distributions to calculate the rates of predation mortality on each proto-moment. The latter model produces a realistic development in time for the predator and prey length distributions, density in numbers, and biomass for their first summer. It also provides estimates of growth for both species along with estimates of the predation mortality that cod generate on capelin in this time period. The estimates permitted the accuracy of the matrix-based approach to be investigated, better understood, and improved.