Does diet in Celtic Sea fishes reflect prey availability?

Feeding preferences of Celtic Sea fishes were investigated using a database of stomach content records, collected between 1977 and 1994. The diet of cod Gadus morhua , hake Merluccius merluccius , megrim Lepidorhombus whiffiagonis , whiting Merlangius merlangus and saithe Pollachius virens changed markedly as the animals grew larger, and although large predators generally chose larger bodied prey, the variability of prey sizes consumed also increased. Large predators continued to select small, low value, benthic prey ( e.g . Callionymus spp. and Trisopterus spp.) which were easier to catch, rather than larger, more energy lucrative pelagic prey ( e.g . mackerel Scomber scombrus ), even though these pelagic prey‐fishes were nearly always available and were often very abundant. Stock estimates of the International Council for the Exploration of the Sea and U.K. groundfish survey catches were used as indices of prey abundance. Blue‐whiting Micromesistius poutassou and other small pelagic fishes ( Argentina spp. and clupeoids) were identified as being particularly important, and were consumed by some predators more often than would be expected given the abundance of these prey in the environment. There was no evidence for density‐dependent feeding by predators on mackerel and only hake exhibited density‐dependent feeding on horse‐mackerel. Hake, cod and megrim consumed more blue‐whiting when this prey was at higher abundance in the environment. In choosing what prey to consume, predators must balance costs and benefits, considering the quality of prey and the energy expended during search, capture and handling.     

A multivariate approach to the feeding habits of small fishes in the Guadalquivir Estuary

In the analysis of the feeding habits of the 11 most abundant fish species in the Guadalquivir Estuary, collected monthly (February 1998 to January 1999) at two different sampling sites, a total of 46 prey taxa were identified. Classifications (based on Bray–Curtis similarities derived from occurrence, number and mass data) of the different fish categories (postlarvae and juvenile–adults of each species) revealed two main trophic guilds, whose preferential prey (SIMPER analysis) were mysids and copepods, respectively. The similarity matrices derived from occurrence, number and mass data were always significantly correlated (RELATE: r >0.636; P <0.01), indicating that a good agreement in feeding patterns emerged from these variables. The seasonal coincidence of maximal fish and key-prey species densities suggests that food availability may be a principal factor influencing the nursery function of the Guadalquivir Estuary.     

Quantitative evaluation of the point method for fish stomach contents analysis

To test the accuracy of the point methods a simulation was conducted using simulated stomachs. Results from seven participants told to analyse the stomachs by the point and percentage methods differed considerably among subjects and from the control.     

Molecular identification of prey in the stomach contents of Harp Seals (Pagophilus groenlandicus) using species-specific oligonucleotides

All methods of diet analysis in marine mammals, including hard part analysis (HPA), have biases affecting the accuracy of prey-species identification and frequency in the estimated diet due to differential consumption, digestion and retention. Using PCR amplification of specific prey DNA with species-specific primers, we developed a DNA-based method that complements HPA and provides an alternative means to detect prey from stomach contents of Harp Seals (Pagophilus groenlandicus). The target size that could be reliably amplified was determined using a digestion time-series of Atlantic Cod (Gadus morhua) tissue in simulated seal stomachs. Various target lengths were trialed using general teleost primers; amplicons of approximately 800 bp or less were consistently obtained. Prey species-specific PCR primers for Atlantic Cod, Arctic Cod (Boreogadus saida) and Capelin (Mallotus villosus) were designed and tested with DNA from the stomach contents of 31 Harp Seals. Amplicons were obtained for all three species-specific primer sets. Amplification results compared with HPA revealed: (i) Atlantic Cod hard parts were found in five stomachs where no Atlantic Cod DNA amplified, suggesting that Atlantic Cod may be over-represented in the estimated diet, (ii) amplification of Arctic Cod DNA occurred for 17 stomachs, including all 12 stomachs with, and five stomachs without, Arctic Cod hard parts, and (iii) Capelin DNA amplified for four of five stomachs with Capelin hard parts and for one stomach without Capelin hard parts. We conclude that PCR amplification of specific prey DNA provides a viable means to complement Harp Seal diet analysis by HPA, but suggest that valuable information for quantitative diet analysis rests in a quantitative PCR approach.     

What do tadpoles really eat? Assessing the trophic status of an understudied and imperiled group of consumers in freshwater habitats

1. Understanding the trophic status of consumers in freshwater habitats is central to understanding their ecological roles and significance. Tadpoles are a diverse and abundant component of many freshwater habitats, yet we know relatively little about their feeding ecology and true trophic status compared with many other consumer groups. While many tadpole species are labelled herbivores or detritivores, there is surprisingly little evidence to support these trophic assignments. 2. Here we discuss shortcomings in our knowledge of the feeding ecology and trophic status of tadpoles and provide suggestions and examples of how we can more accurately quantify their trophic status and ecological significance. 3. Given the catastrophic amphibian declines that are ongoing in many regions of the planet, there is a sense of urgency regarding this information. Understanding the varied ecological roles of tadpoles will allow for more effective conservation of remaining populations, benefit captive breeding programmes, and allow for more accurate predictions of the ecological consequences of their losses.