Secondary Prey Research Articles

The Use of Stable Isotopes and Stomach Contents to Identify Dietary Components of the Spotted Rose Snapper, Lutjanus guttatus (Steindachner, 1869), off the Eastern Coast of the Southern Gulf of California

The food habits of the spotted rose snapper, Lutjanus guttatus (Steindachner, 1869), living off of the Eastern coast of the Southern Gulf of California (off the coast of Nayarit) are described based on their stomach contents and isotopic analysis. Fish were collected from the bycatch of shrimp trawling during the 2005-2007 shrimp fishing seasons. Twenty-six taxa were identified in the stomach contents and the geometric importance index suggested xanthid crabs and engraulidae fish are the most important species in the L. guttatus diet. Isotopic analysis and mixing models also led to the identification of crustaceans as important species in diets, but fish were considered only as secondary prey in these models. Notably, the diet and trophic level of the spotted rose snapper tend to change as it matures; young fish mainly feed on crustaceans, while larger L. guttatus can incorporate fish into their diets. Furthermore, Morisita-Horn index suggests that there are significant differences between the diets of juvenile and adult fish. The estimates of the trophic level for L. guttatus from stomach contents (TLsc = 3.7) and isotopic analysis (TLiso = 3.5) are very similar.

Diet composition of the Persian sturgeon Acipenser persicus at the coast of the Guilan Province, Caspian Sea

Sturgeons are one of the most important and valuable species in the Caspian Sea. This investigation (2000–2002) was aimed at determining the feeding habits and the main and secondary prey organisms consumed by the Persian sturgeon (Acipenser persicus Borodin, 1897) living at depths less than 10 m in the Caspian Sea (Guilan Province). The food spectrum of immature A. persicus was comprised of benthic invertebrates of seven genera and fishes of three genera. The main prey items of immature A. persicus were polychaete worms and the secondary prey were crustaceans. The study of stomach contents showed that polychaetes were the most important component of the diet of immature A. persicus of the length class up to 25 cm.

Development and application of DNA techniques for validating and improving pinniped diet estimates

Polymerase chain reaction techniques were developed and applied to identify DNA from >40 species of prey contained in fecal (scat) soft-part matrix collected at terrestrial sites used by Steller sea lions (Eumetopias jubatus) in British Columbia and the eastern Aleutian Islands, Alaska. Sixty percent more fish and cephalopod prey were identified by morphological analyses of hard parts compared with DNA analysis of soft parts (hard parts identified higher relative proportions of Ammodytes sp., Cottidae, and certain Gadidae). DNA identified 213 prey occurrences, of which 75 (35%) were undetected by hard parts (mainly Salmonidae, Pleuronectidae, Elasmobranchii, and Cephalopoda), and thereby increased species occurrences by 22% overall and species richness in 44% of cases (when comparing 110 scats that amplified prey DNA). Prey composition was identical within only 20% of scats. Overall, diet composition derived from both identification techniques combined did not differ significantly from hard-part identification alone, suggesting that past scat-based diet studies have not missed major dietary components. However, significant differences in relative diet contributions across scats (as identified using the two techniques separately) reflect passage rate differences between hard and soft digesta material and highlight certain hypothesized limitations in conventional morphological-based methods (e.g., differences in resistance to digestion, hard part regurgitation, partial and secondary prey consumption), as well as potential technical issues (e.g., resolution of primer efficiency and sensitivity and scat subsampling protocols). DNA analysis of salmon occurrence (from scat soft-part matrix and 238 archived salmon hard parts) provided species-level taxonomic resolution that could not be obtained by morphological identification and showed that Steller sea lions were primarily consuming pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon. Notably, DNA from Atlantic salmon (Salmo salar) that likely originated from a distant fish farm was also detected in two scats from one site in the eastern Aleutian Islands. Overall, molecular techniques are valuable for identifying prey in the fecal remains of marine predators. Combining DNA and hard-part identification will effectively alleviate certain predicted biases and will ultimately enhance measures of diet richness, fisheries interactions (especially salmon-related ones), and the ecological role of pinnipeds and other marine predators, to the benefit of marine wildlife conservationists and fisheries managers.

Using the diet of the barn owl (Tyto alba) as an indicator of small vertebrate abundance in the Channel Country, south-western Queensland

The diet of the barn owl (Tyto alba) was determined by analysing pellets and bulk pellet debris found in the ruins of Baryulah Homestead, south-western Queensland. Nine species of mammal, at least eight bird, five reptile and three frog species were identified. The majority of prey consisted of small mammals and was dominated by Mus musculus, which accounted for almost 40 Prey Unit percent (PU%) of all prey. Rattus villosissimus was an important secondary prey species, which, due to its comparatively large mass, contributed 21.79 PU%. Other native mammals were present in low frequency only. Reptiles (primarily geckos) were more abundant than expected, collectively contributing >15 PU%, suggesting that they were an important component of the barn owl’s diet.

Evidence-based culling of a facultative predator: Efficacy and efficiency components

Human activities have greatly modified predator–prey dynamics within seabird communities by favouring a rapid increase in density of large predatory gulls. To counteract such a subsidized growth, conservation agencies perform massive random culling programs, which generally fail to restore the original predator–prey relationship. We used long-term individual-based information to evaluate the effects of a selective culling of a top seabird predator, the yellow-legged gull ( Larus michahellis), on the predatory pressure, survival and reproductive success probabilities of a secondary prey, the vulnerable European storm-petrel ( Hydrobates pelagicus). The selective removal of only 16 gulls in 3 years led to a reduction of ca. 65% in the number of petrels killed, and to a relative increase in their survival and breeding success probabilities of 16% and 23%, respectively. Our results show that only a few specialised predators were responsible for the bulk of the impact on a secondary prey and that the removal of those specialised individuals was an effective and efficient way to improve prey demographic parameters.

Feeding habits of blackmouth catsharkGaleus melastomusRafinesque, 1810 and velvet belly lantern sharkEtmopterus spinax(Linnaeus, 1758) in the western Mediterranean

Summary Feeding habits of blackmouth catshark Galeus melastomus and velvet belly lantern shark Etmopterus spinax were studied throughout the Spanish Mediterranean, from the Alboran Sea to the Gulf of Lion, between 400 and 790 m depth. Diets were studied taking into account size and depth differences. Included within the trophic guild of non-migratory macroplankton feeders, both species preferably exploited mesopelagic resources (mainly natantian decapods, euphausiids, mesopelagic fish and cephalopods). G. melastomus mostly preyed on decapod crustaceans (46% in terms of IRI), with cephalopods, euphausiids and mesopelagic fish as a secondary prey item. The diet of E. spinax was composed primarily of mesopelagic fish (61.4% in terms of IRI), with decapod crustaceans and cephalopods of secondary importance. Both species showed ontogenetic changes in their diets: small blackmouth catshark specimens (between 150 and 350 mm total length) mainly consumed cephalopods, medium size individuals (351–450 mm TL) consumed decapod crustaceans, while larger specimens (larger than 451 mm TL) seemed to be more generalist-feeders. Smaller specimens of E. spinax (150– 250 mm TL) mostly fed on small crustaceans and cephalopods, whilst an increase in the consumption of mesopelagic fish (mycthophids and Stomiiformes) was detected in larger individuals (251–450 mm TL). Diet of G. melastomus also changed throughout the narrow depth range explored, mainly consumed euphausiids and mesopelagic fish between 400 and 500 m depth, whilst preferably exploiting natantian decapods and cephalopods below 500 m of depth. However, this trend may be correlated to the larger-deeper trend found for this species. Slight but not significant differences were found in E. spinax diet by depth, with euphausiids mainly consumed at lower depths (400–500 m). In a multispecies MDS analysis, diets of G. melastomus and E. spinax were separated and the ANOSIM test proved evidence for significant differences in the diets of the two species (R = 0.25; P = 0.05), mainly attributed to the stronger pelagic habits of E. spinax in comparison with G. melastomus. Low overlap (by Schoener Index) also occurred when comparing specimens of the same size range. In general, the higher occurrence of benthic prey in the diet of G. melastomus (i.e. the brachyuran crab Geryon longipes, the thalassinid shrimp Calocaris macandrae) than in E. spinax pointed to a stronger pelagic behavior for the velvet belly lantern shark. Both multivariate analysis and the Levins Index pointed to a narrow niche breadth for the two sharks. A trend of increasing fullness was found for both species in the highly productive areas of the Alboran Sea and Vera Gulf, probably related to higher resource availability, enhanced by local upwellings.

Feeding habits of the exotic black bullheadAmeiurus melas(Rafinesque) in the Iberian Peninsula: first evidence of direct predation on native fish species

The feeding ecology of the exotic invasive black bullheadAmeiurus melaswas conducted in the Iberian Peninsula for the first time. Dietary analysis based on the stomach contents of individuals caught in several Iberian basins was carried out as a first step to evaluate its potential threat for the native Iberian ichthyofauna. Aquatic macroinvertebrates (mainly Chironomidae) dominated the black bullhead’s diet in all size‐classes and sites, irrespective of natural riverine or artificial lentic habitats. Secondary prey items were responsible for the observed between‐sites (microcrustaceans in artificial lentic habitat; oligochaeta and caddisfly larvae in natural riverine habitats) and ontogenetic diet differences (from microcrustaceans to larger prey). These diet variations were also detected in trophic diversity values and feeding strategy plots. Black bullheads consumed plant material, terrestrial prey and co‐occurring fish species (native or exotic) and thus they could be considered as generalist or opportunistic, foraging on the most abundant and available prey. There was no positive relationship between black bullhead size (total length) and fish prey size, probably indicating piscivory on dead or dying vulnerable fishes as well as predation on smaller‐sized active fishes. The results showed that the black bullhead could negatively affect native Iberian ichthyofauna throughout direct predation and competition. Aspects of potential conservation and management implications of fishes resulting from the undesirable presence of the black bullhead in Iberian water bodies are discussed.

Is the red spotted green frog Hypsiboas punctatus (Anura: Hylidae) selecting its preys? The importance of prey availability

The study of the feeding ecology of amphibians is an old issue in herpetology. Notwithstanding, the lack of food resources data in many studies of amphibians feeding has lead to partial understanding of frog feeding strategies. In this study we evaluate the trophic selectivity of a red spotted green frog (Hypsiboas punctatus) population from a Middle Paraná River floodplain pond in Argentina, and discuss the importance of prey availability data when interpreting results from diet analysis. We analyzed the gut contents of 47 H. punctatus adults and compared frog's diet with the environmental food resources. Prey availability was estimated by systematically seep-netting the microhabitat where anurans were localized foraging. We identified 33 taxonomic categories from gastrointestinal contents. Numerically, the most important prey categories were dipterans, followed by hemipterans, homopterans and coleopterans. The diet similarity between males and females was high and no statistical differences in diet composition were found. The most abundant food resources in the environment were dipterans, coleopterans, homopterans and collembolans. In order to assess whether frogs were selecting their preys, we calculated Pianka's niche overlap index and Jacobs' electivity index comparing gut contents to prey availability data. Trophic niche overlap was medium but significantly higher than expected by chance. The electivity index indicated that H. punctatus foraged dipterans slightly above their environmental abundance. Among the secondary preys, hemipterans were foraged selectively, homopterans were consumed in the same proportion to their occurrence in the environment, coleopterans were foraged quite under their availability and collembolans were practically ignored by frogs. Without food resources data, H. punctatus could be classified as a specialist feeder, but dipterans also were quite abundant in the environment. Our results show that H. punctatus fit better as a generalist feeder, foraging on their main food item and some secondary preys in similar proportion to their environmental availability; even though other secondary preys are being selectively preferred or ignored by frogs. Our data illustrate the importance of including the resource availability data on diet studies to improve the understanding of amphibian feeding ecology.

An Emerging Infectious Disease Triggering Large-Scale Hyperpredation

Hyperpredation refers to an enhanced predation pressure on a secondary prey due to either an increase in the abundance of a predator population or a sudden drop in the abundance of the main prey. This scarcely documented mechanism has been previously studied in scenarios in which the introduction of a feral prey caused overexploitation of native prey. Here we provide evidence of a previously unreported link between Emergent Infectious Diseases (EIDs) and hyperpredation on a predator-prey community. We show how a viral outbreak caused the population collapse of a host prey at a large spatial scale, which subsequently promoted higher-than-normal predation intensity on a second prey from shared predators. Thus, the disease left a population dynamic fingerprint both in the primary host prey, through direct mortality from the disease, and indirectly in the secondary prey, through hyperpredation. This resulted in synchronized prey population dynamics at a large spatio-temporal scale. We therefore provide evidence for a novel mechanism by which EIDs can disrupt a predator-prey interaction from the individual behavior to the population dynamics. This mechanism can pose a further threat to biodiversity through the human-aided disruption of ecological interactions at large spatial and temporal scales.

Niche overlap between marsupial and eutherian carnivores: does competition threaten the endangered spotted‐tailed quoll?

Summary The significance of top‐down regulation by carnivores is receiving increasing global recognition. As a consequence, key objectives in many programmes that seek to maintain ecosystem function now include conserving carnivores and understanding their interactions. This study examined overlap in resource use (space and diet) of introduced eutherian carnivores and an endangered marsupial carnivore, the spotted‐tailed quoll Dasyurus maculatus, in eastern Australia. We also investigated mechanisms of niche partitioning and evidence for interspecific aggression. Dietary overlap between quolls, red foxes Vulpes vulpes and wild dogs Canis lupus ssp. was assessed by analysis of scats. Trapping, radio‐tracking and direct observations were used to quantify spatial overlap between quolls, foxes, wild dogs and feral cats Felis catus. Dietary overlap among the carnivores was extensive. Medium‐sized mammals were the most important prey for all three predators, indicating potential for exploitative interactions. However, hunting of different size classes of secondary prey and consumption by quolls of more arboreal prey than their counterparts may assist coexistence. Remains of quoll were found in two dog scats, and cat hair in another, possibly indicating intraguild predation. We observed extensive spatial overlap between quolls and eutherian carnivores. However, we inferred from dietary data that quolls foraged primarily in forested habitat, while canids foraged mainly in cleared habitat. Synthesis and applications. Our results indicate strong potential for competition between spotted‐tailed quolls and eutherian carnivores, and thus a situation where control of introduced predators may be desirable, not only for the conservation of prey species but also for the protection of native carnivores. Concern over potential non‐target mortality of quolls has hindered efforts to control foxes in eastern Australia using poison baits. We contend that, rather than harming quoll populations, baiting for foxes should aid the conservation of quolls and should be implemented in areas of sympatry where fox numbers are high.

Nesting with an enemy: The abundance and distribution of preferred and secondary prey near nesting Cooper's hawks, Accipiter cooperii

By nesting near their predators, birds may gain safe nest sites because predators defend a region around their own nests. For some species, this benefit comes at the potential cost of increased predation risk due to the predators. We hypothesized that birds less vulnerable to predation were more likely to associate with predatory birds. We surveyed different bird species near active Cooper's hawk, Accipiter cooperii, nests, 200 m from each hawk nest, and at control sites that did not contain Cooper's hawk nests. The birds surveyed were categorized by weight, foraging height, and nest type. Canopy foraging birds were found in relatively higher densities near the Cooper's hawk nests than at other sites, whereas ground and mid-level foragers were more common away from Cooper's hawk nests. The association with Cooper's hawks was most pronounced for those canopy foraging birds that were small and built cup nests. Birds less vulnerable to predation by Cooper's hawks are more likely to nest in association with these hawks than are more vulnerable birds.

Food resource variability in an Australian dryland river: evidence from the diet of two generalist native fish species

To examine how food resource availability links with natural variation in primary productivity in the Moonie River, south-west Queensland, the diets of two native Australian fish species (Nematalosa erebi and Macquaria ambigua) were examined from fifteen waterholes in February, May and September 2006. N. erebi diets reflected strong ‘boom and bust’ patterns of food consumption, with high concentrations of benthic (non-filamentous) algae during boom (flow) times, moving to higher concentrations of filamentous algae and detritus during bust (no flow) periods. M. ambigua diets were primarily dominated by aquatic insects in all sampling periods. Although there was no clear ‘boom to bust’ pattern in relation to flow, M. ambigua secondary prey consumption revealed a compensatory switch between high energy prey (crustaceans) during more productive periods with terrestrial insects during less productive periods. The ability of both species of fish to switch from high to low concentrations of food quality under a variable environmental background allows them to persist through both high productive and low productive periods. This interaction between native biota and variable ‘boom’ and ‘bust’ conditions, and how changes to the natural hydrology will affect it is an important consideration of any future water resource development plans.

Predictors of clutch predation of a globally significant avifauna in New Zealand's braided river ecosystems

AbstractPredation by introduced mammals is decimating New Zealand's indigenous fauna. Understanding factors that influence this process allows resources for predator control to be applied with maximum effect. This study examines how predation of a secondary prey species (a relatively common but declining native plover, the banded dotterel Charadrius bicinctus) varied with reductions in abundance of a major prey source (rabbits), kill‐trapping of predators, nest density and habitat complexity. Banded dotterels mostly nest in open braided riverbeds alongside a number of endemic threatened species. We measured the fate of 753 dotterel clutches exposed to predation by cats, ferrets and hedgehogs. We found key times and places of high predation risk. Immediately after widespread reduction in rabbit populations by rabbit haemorrhagic disease (RHD), clutch predation rates were almost as high (mean, 50%) as those recorded during past rabbit poisoning programmes (mean, 57%). Both rates were significantly higher than the mean predation rate of 22% without rabbit control, suggesting a shift in predator diet immediately after rabbit population declines. Unlike after rabbit poisoning, clutch predation rate remained high in the years after RHD. Other patterns observed included higher clutch predation rate where nest density was lower, suggesting that predation can potentially cause local extinction. Clutch predation was also higher along riverbed margins where vegetation was dense. There was equivocal evidence for an effect of predator kill‐trapping on clutch predation rate. Management strategies that could potentially reduce clutch predation risks include focusing predator mitigation measures during periods of rabbit decline, maintaining them for more than one breeding season if the rabbit declines are widespread (e.g. RHD epidemics), and applying greater effort at sites with relatively low nest density and along riverbed margins where predator use is more frequent.

Food habits of the sandpaper skate, Bathyraja kincaidii (Garman, 1908) off central California: seasonal variation in diet linked to oceanographic conditions

The stomachs of 130 sandpaper skates, Bathyraja kincaidii (Garman, 1908), were sampled from off central California to determine their diet composition. The overall diet was dominated by euphausiids, but shrimps, polychaetes and squids were also important secondary prey. A three-factor MANOVA demonstrated significant differences in the diet by sex, maturity status and oceanographic season using numeric and gravimetric measures of importance for the major prey categories. These three main factors explained more variation in diet than interactions between the factors, and season explained the most variance overall. A detailed analysis of the seasonal variation among the prey categories indicated that abundance changes in the most important prey, euphausiids, were coupled with seasonal changes in the importance of other prey. When upwelling occurred and productivity was great (Upwelling and Oceanic seasons), euphausiids were likely highly abundant in the study area and were the most important prey for B. kincaidii. As productivity declined (Davidson Current season), euphausiids appeared to decrease in abundance and B. kincaidii switched to secondary prey. At that time, gammarid amphipods and shrimps became the most important prey items and polychaetes, mysids and euphausiids were secondary.

Feeding ecology of the Iberian lynxLynx pardinus in eastern Sierra Morena (Southern Spain)

The feeding ecol ogy of the critically endangered Iberian lynxLynx pardinus Temminck, 1827, was studied in the most important remnant population, located in eastern Sierra Morena (Sierra de Andujar Natural Park, Spain). Three hundred sixty scats were collected from October 2001 through Sep tember 2002. RabbitsOryctolagus cuniculus dominated the lynx diet, as showed by frequency of occurrence (94%) and percent age of volume (91%). Red-legged partridgesAlectoris rufa were the most important secondary prey. No significant seasonal variation was observed regarding the diet preferences of the lynx. No difference was found in lynx diet by comparing two areas with high and low rabbit availability. Juvenile rabbits were positively selected when they were available, during late winter, spring and early summer.

Food and feeding ecology of Northeast Atlantic swordfish (Xiphias gladius) off the Bay of Biscay

Abstract As part of a larger project on the feeding ecology of large pelagic predators off the Bay of Biscay, this study analyses the diet of the swordfish, Xiphias gladius. Stomachs were collected from 86 swordfish. The diet was analysed in terms of prey occurrence, relative abundance, reconstituted mass, and size distribution. It consisted mainly of fish, 40.5% by mass (%M) and cephalopods, 59.3%M; crustaceans, 0.2%M, were considered secondary prey. When considering only the fresh fraction to allow for differential digestion rates, these figures were 77.3%M, 22.7%M, and trace amounts, respectively. Lanternfish, including Notoscopelus kroeyeri and Symbolophorus veranyi, were abundant, but paralepidids, Atlantic pomfret (Brama brama), and the squid Todarodes sagittatus, Ommastrephes bartramii, and Gonatus steenstrupi, dominated the diet by mass. The overall prey size range was 11–1420 mm, but sizes of 60–360 mm accounted for 80% of the distribution by number and of 140–760 mm for 80% of the distribution by mass. Intraspecifically, larger swordfish ate larger prey as a result of a change in species composition of the diet. The swordfish appears to show feeding plasticity both between different areas and between animals in the same area.

Macroscopic prey remains in the winter diet of foxes Vulpes vulpes in rural Britain

ABSTRACT The winter diet of foxes Vulpes vulpes was quantified in seven landscape types in Britain, using faecal samples from 87 sites. Medium‐sized mammals (0.1–5.0 kg) were consistently the most important prey group in arable and pastural landscapes, occurring in 44–72% of scats and comprising 50–75% of the mass of prey ingested. Birds and small mammals (< 0.1 kg) were important secondary prey groups. Small mammals were the most frequently recorded prey group in marginal upland (42% of scats) and upland landscapes (75%), followed by large mammals (33% and 23%, respectively). In terms of mass ingested, small mammals (38%) and large mammals (52%) were the most important prey groups in these landscapes. In all landscapes, field voles Microtus agrestis, lagomorphs, sheep/deer and passerines/galliforms dominated their respective prey groupings.

The role of predation in the decline and extirpation of woodland caribou

To select appropriate recovery strategies for endangered populations, we must understand the dynamics of small populations and distinguish between the possible causes that drive such populations to low numbers. It has been suggested that the pattern of population decline may be inversely density-dependent with population growth rates decreasing as populations become very small; however, empirical evidence of such accelerated declines at low densities is rare. Here we analyzed the pattern of decline of a threatened population of woodland caribou (Rangifer tarandus caribou) in British Columbia, Canada. Using information on the instantaneous rate of increase relative to caribou density in suitable winter foraging habitat, as well as on pregnancy rates and on causes and temporal distribution of mortalities from a sample of 349 radiocollared animals from 15 subpopulations, we tested 3 hypothesized causes of decline: (a) food regulation caused by loss of suitable winter foraging habitat, (b) predation-sensitive foraging caused by loss of suitable winter foraging habitat and (c) predation with caribou being secondary prey. Population sizes of caribou subpopulations ranged from < 5 to > 500 individuals. Our results showed that the rates of increase of these subpopulations varied from -0.1871 to 0.0496 with smaller subpopulations declining faster than larger subpopulations. Rates of increase were positively related to the density of caribou in suitable winter foraging habitat. Pregnancy rates averaged 92.4% +/-2.24 and did not differ among subpopulations. In addition, we found predation to be the primary cause of mortality in 11 of 13 subpopulations with known causes of mortality and predation predominantly occurred during summer. These results are consistent with predictions that caribou subpopulations are declining as a consequence of increased predation. Recovery of these woodland caribou will thus require a multispecies perspective and an appreciation for the influence of inverse density dependence on population trajectories.

Food and feeding ecology of juvenile albacore, Thunnus alalunga, off the Bay of Biscay: a case study

AbstractAlthough immature albacore tuna, Thunnus alalunga, are of economic and social importance in the Bay of Biscay, little is known about their diet and feeding ecology there. For this study, the diet of 78 albacore caught in the French driftnet fishery during summer 1993 is analysed. Fish dominated the diet in terms of relative abundance (86%N), and reconstituted mass (60%M), the most important being Maurolicus muelleri (79%N, 23%M), Scomberesox saurus (2%N, 30%M), and Arctozenus risso (4%N, 4%M). Crustaceans were also important in the diet (12%N, 2%M), but given their small size, it is questionable whether they were primary or secondary prey. Foraging on cephalopods seemingly took place only occasionally: they represented 2%N and 39%M of the total diet, but were absent from the fresh fraction of stomach contents. Prey sizes ranged from 6 to 228 mm. Juvenile albacore consume either epipelagic prey by day, or vertically migrating mesopelagic species that reach the surface layer by night.

Feeding ecology of the gobies &lt;i&gt;Pomatoschistus minutes&lt;/i&gt; (Pallas, 1770) and &lt;i&gt;Pomatoschistus microps&lt;/i&gt; (Krøyer, 1838) in the upper Tagus estuary, Portugal

The sand goby Pomatoschistus minutus (Pallas, 1770) and the common goby Pomatoschistusmicrops (Kroyer, 1838) are among the most abundant fish species in estuaries, lagoons and along the Atlantic shores of Europe. In the Tagus estuary these species are particularly important in the food web. Fish samples were taken monthly in the upper estuary, and the gut contents of a total of 431 P.minutus and 382 P. microps were analysed in order to study preferences and seasonal changes in their diets and to understand intra- and interspecific relations between these two congeneric gobies. The sand goby had no dominant prey, although mysids were particularly important in the diet of this species. Shrimps and polychaetes were also common prey. P.microps preferentially ingested polychaetes, with isopods, amphipods, bivalves and copepods as secondary prey items. Significant seasonal variations in the ingestion of the main prey groups of both species were obtained, which reflect variations in prey availability. During the reproductive season there were differences in the food consumed by males and females, mainly for P.minutus, reflecting the different activity pattern exhibited by males and females during the breeding season, since parental care is supported by the former. The smallest classes from P.minutus increased their niche widths as they grew. Due to the different length reached by each species, the degree of interspecific overlap decreased with increasing body size.