Interspecific comparisons of growth and diet among late larvae of three co-occurring clupeoid species in the Kii Channel,Japan

Growth and diet were compared among larvae of Japanese anchovy Engraulis japonicus, Japanese sardine Sardinops melanostictus and Pacific round herring Etrumeus teres. Compositions of prey items of the three species in the same month showed greater similarity than for the same species in different months. Prey size as well as prey taxa of the three species overlapped considerably with one another. Therefore, interspecific prey competition is likely in the case of limited food availability. The most abundant species tended to change from anchovy to round herring in early winter, from round herring to sardine in late winter and from sardine to anchovy in early spring, indicating a temporal segregation in use of the nursery grounds. Similar seasonal changes in growth rates were observed for the three species. Although interspecific prey competition is likely, the temporal segregation and similar temporal changes in growth rates could favor their coexistence.     

Spatial and temporal variation in the diet of a high trophic level predator,the Australian fur seal (<Emphasis Type="Italic">Arctocephalus pusillus doriferus</Emphasis>)

This study quantifies the manner in which Australian fur seals, Arctocephalus pusillus doriferus, use their prey in a spatial and temporal context. We analysed 977 scat and 66 regurgitate samples collected from Tasmanian breeding colonies and haul-outs between 1994 and 2000. Diagnostic prey remains identified in the scats represented 35 fish taxa and 8 cephalopod taxa. The main taxa identified in scats, where frequency of occurrence was 10%, were leatherjacket species (family Monocanthidae), redbait (Emmelichthys nitidus), barracouta (Thyrsites atun), jack mackerel (Trachurus declivis) and red cod (Pseudophysis bachus). Regurgitates were dominated by cephalopods, primarily Goulds squid (Nototodarus gouldi), Octopus maorum, O. berrima/pallidus and Sepia apama. Discriminant function analyses indicated that there were generally no significant differences in the composition of the diet between colonies within a year, suggesting that prey distribution is fairly uniform throughout Bass Strait at those time scales. The diet at breeding colonies, however, exhibited significant inter- and intra-annual variation, determined by the presence of several key taxa, such as barracouta and a species of scorpionfish (family Scorpaenidae). The diet composition also varied regionally, between Bass Strait and southern Tasmania in spring 1999 and autumn 2000, with redbait, barracouta and a species of scorpionfish identified as the main taxa contributing to this difference. Redbait occurred in the diet only in southern Tasmania, whereas barracouta and scorpionfish occurred only in Bass Strait.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00227-003-1219-0.Communicated by G.F. Humphrey, Sydney     

Diet of the Humboldt penguin (<Emphasis Type="Italic">Spheniscus humboldti</Emphasis>) in northern and southern Chile

The diet of the Humboldt penguin (Spheniscus humboldti) was examined and compared in two colonies in Chile. Field work was conducted on Pan de Azúcar Island in northern Chile in the breeding season 1998/1999 and on the Puñihuil Islands in southern Chile over two successive breeding seasons during 1997/1998 and 1998/1999. Penguin diet was studied by stomach-pumping birds and analysed by species composition, size and mass of prey. Fish were the dominant prey item at both sites, the contribution of cephalopods and crustaceans varying between sites. The fish prey consisted predominantly of school fish, but there were clear latitudinal differences in fish prey taken. Penguins in the northern colony consumed primarily garfish (Scomberesox saurus), while birds at the southern colony of Puñihuil fed primarily on anchovy (Engraulis ringens), Araucanian herring (Strangomera bentincki) and silverside (Odontesthes regia). The results showed significant differences in terms of numbers of fish taken between the two breeding seasons at Puñihuil. In 1997/1998 penguins consumed almost exclusively anchovy, while they fed primarily on silversides in the successive year. Almost all prey, except stomatopods, were characterised as being pelagic species that occur in relatively inshore water, consistent with the foraging behaviour of Humboldt penguins. The dependence of Humboldt penguins on commercially exploited, schooling prey species makes the species particularly susceptible to changes in prey stocks, due to non-sustainable fisheries management.Communicated by O. Kinne, Oldendorf/Luhe     

Comparison of feeding habits of myctophid fishes and juvenile small epipelagic fishes in the western North Pacific

To examine the potential trophic competition between myctophids and small epipelagic fishes in the nursery grounds in spring, we compared the stomach contents of dominant myctophids (Symbolophorus californiensis, Ceratoscopelus warmingii and Myctophum asperum; n = 179) and juvenile epipelagic fishes (Japanese sardine, Sardinops melanostictus, Japanese anchovy, Engraulis japonicus, chub mackerel, Scomber japonicus, and spotted mackerel, S. australasicus; n = 78) that were simultaneously collected at nighttime with a midwater trawl net around the Kuroshio-Oyashio transition zone in the western North Pacific. It was clear that the neritic copepod Paracalanus parvus s.l. was the most abundant species in NORPAC samples (0.335 mm mesh size) taken at the same stations. Diets of dominant myctophid fishes differed from those of the juvenile epipelagic fishes; Japanese sardine and anchovy mostly preyed upon P. parvus s.l. (23.6% of stomach contents in volume) and Corycaeus affinis (16.1%), respectively. Both chub and spotted mackerels mainly preyed upon the seasonal vertical migrant copepod, Neocalanus cristatus (15.9 and 14.7%, respectively). On the contrary, myctophid fishes probably do not specifically select the abundant neritic copepods. Namely, S. californiensis mostly preyed upon a diel vertical migrating copepod, Pleuromamma piseki (22.7 and 30.6% in stomach of juvenile and adult, respectively), while C. warmingii and M. asperum preyed on Doliolida (43.0% in stomach of juvenile C. warmingii), appendicularians (11.0% in stomach of juvenile M. asperum), and Ostracoda (6.3% in stomach of adult C. warmingii). Feeding habits of myctophid fishes seem adapted to their prey animals; low rate of digested material (less than 30% in volume) in stomachs of S. californiensis may be linked to the movement of P. piseki, hence S. californiensis can easily consume this copepod at night since they are more concentrated at night than daytime. High rate of digested material (over 40%) of M. asperum and adult C. warmingii suggest that they feed not only at night but also during the daytime in the midwater layer. Thus, myctophid fishes actually fed in the surface layer but less actively than the small epipelagic fishes. These results suggest that the potential for direct food competition between myctophids and small epipelagic fishes is low in the nursery ground, but there remains a possibility of indirect effects through their prey items, since the above gelatinous animals feed on common prey items as juveniles of Japanese sardine and anchovy.     

Interannual fluctuations in acoustic biomass estimates and in landings of small pelagic fish populations in relation to hydrology in the Strait of Sicily

The main results of research work carried out since 1998 with regard to the application of hydro-acoustic technologies for the evaluation of biomass and distribution of small pelagic fish species off the southern coast of Sicily are presented, taking into account information from hydrology and from ecology of the fish populations targeted. The biomass estimates and the population‐density charts presented concern the two main species, i.e. sardine Sardina pilchardus (Walbaum, 1792) and anchovy Engraulis encrasicolus (Linnaeus, 1758). Both the sardine and anchovy populations experienced large inter-annual fluctuations, with biomass estimates ranging from 6000 to over 36,000 tonnes (t) (sardine) and from about 7000 to 23,000 t (anchovy). Acoustic estimates are largely consistent with landings recorded in Sciacca (the main fishing port for small pelagic species in the study area) during the year following the evaluation surveys. In addition, trends in sardine and anchovy biomass estimates appears to be negatively correlated with the mean sea surface temperature calculated over the time intervals January–September (sardine) and June–November (anchovy) of the preceding year, which correspond to larval and juvenile growth periods of target species. Observed patterns would suggest the importance of enrichment processes relevant to the survival of early stages, so determining recruitment success and finally higher population sizes.     

Comparative foraging ecology and ecological niche of a superabundant tropical seabird: the sooty tern Sterna fuscata in the southwest Indian Ocean

Over 6-million pairs of sooty terns Sterna fuscata breed once a year in the southwest Indian Ocean, mostly on three islands of the Mozambique Channel (Europa, Juan de Nova and Glorieuses) and in the Seychelles region. Seasonal reproduction in either winter or summer is the dominant strategy in the area, but non-seasonal reproduction also occurred in some places like at Glorieuses Archipelago. The feeding ecology of the sooty tern was investigated during the breeding seasons to determine whether terns showed significant differences in their trophic ecology between locations. Regurgitations were analyzed to describe the diet of individuals when breeding, and stable isotopes and mercury concentrations were used to temporally integrate over the medium-term of the trophic ecology of both adults and chicks. Overall, the diet was composed of fish, flying squid and fish larvae in different proportions. At Europa and Aride in the Seychelles, where winter reproduction occurs, large epipelagic prey like flying fish or squid dominated the diet. At Juan de Nova, sooty terns reproduce in summer and rely mostly on fish larvae. At Glorieuses (non-seasonal breeding), the diet was intermediate with fish larvae and flying squid being important prey items. The stable-carbon and nitrogen isotope values in blood confirm the differences observed in dietary analysis, and demonstrate different feeding strategies between colonies. δ¹³C values of feathers showed spatial segregation between birds from the Mozambique Channel and the Seychelles region. Terns from the Seychelles had also higher δ¹⁵N values. Feather δ¹³C values also suggest a significant shift from summer to wintering habitat for birds from Juan de Nova. This study emphasizes the high phenotypic plasticity of the species, which may explain its numerical dominance in all tropical waters of the World’s Ocean.     

Diet and nest attendance of incubating and chick-rearing northern fulmars (Fulmarus glacialis) in Shetland

We examined diets and nest attendance patterns of northern fulmars (Fulmarus glacialis) on Foula and Unst, Shetland, UK, during the breeding seasons of 1998 and 1999. Mean foraging trip duration, derived from nest attendance patterns, differed between incubation (32.5 h) and chick-rearing (early stage: 11.2 h, mid-stage: 20.4 h). It was influenced by chick age and obviously also by prey availability. The numbers of fulmars attending the colonies were influenced by wind direction, wind speed, time of day and state of reproduction. Diet samples were collected by regurgitations of adult and young fulmars. Diet was analysed by identifying prey items on the basis of otoliths, vertebrae, premaxillae (fish), cephalopod beaks and fragments of arthropod exoskeletons. Adult fulmars and chicks fed upon a wide range of prey types. Gadoid fish, including Norway pout (Trisopterus esmarkii), were the most common prey in the diet. Fish offal was found in 32% of regurgitates, clupeids in 15%. In contrast, the proportions of sandeels were very low (1%). This is different from previous studies where sandeels formed a large part of fulmar diet at Shetland in summer. Differences in food composition of incubating and chick-rearing fulmars were found for fish, but not for offal, crustaceans or squid. The percentage of non-discard fish (Clupeidae, Ammodytidae, Isospondylae) was significantly different between regurgitates from incubating and chick-rearing fulmars. Samples from chick-rearing adults contained non-discard items more frequently than regurgitates from incubating birds. The results strongly indicate that fulmars select to feed their chicks on energy-rich clupeids.     

Rapid changes in growth,condition, size and age of small pelagic fish in the Mediterranean

Since 2007, the ecosystem of the Gulf of Lions has shifted to a different regime, characterised by a low anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) biomass and a remarkably high sprat (Sprattus sprattus) biomass. Surprisingly, the abundance and recruitment of anchovy and sardine remained high. To understand which processes (bottom-up or top-down control, etc.) could have caused this shift, we studied the changes in body condition, growth and size and age of anchovy, sardine and sprat over 1984–1985 and 1992–2012, using data from scientific surveys. The annual age structure of anchovy and sardine was estimated using Bayesian mixture models based on size frequency data with priors on the age–length relationship derived from independent otolith readings. The results indicated periods during which anchovy and sardine were in an average (1992–2004), good (2005–2007) or poor (2008–2012) overall state of condition. For sardine, the shift towards smaller fish observed during these past 4 years was explained by a combination of slower growth and the disappearance of older individuals (ages 2+). Despite the increase in biomass of sprat since 2008, indications were found that sprat was also smaller than in the past. As growth and condition decreased and overexploitation has not been documented or suspected for those three species in this area, we propose that the current decline in sardine and anchovy biomass could be due to qualitative and/or quantitative modifications in the planktonic production (i.e. a bottom-up control) or mass mortalities of adults due to an epidemic disease.     

Foraging behavior of juvenile Carcinus maenas (L.) and Cancer pagurus L.

Information concerning the way juvenile crabs choose their diet from a variety of prey types can be useful for a better understanding of community dynamics, as well as for the adequate management of natural resources. Prey size and species selection by juvenile Carcinus maenas (15-35 mm carapace width, CW) and Cancer pagurus (20-40 mm CW) feeding on four bivalves of contrasting shell morphology were investigated. When offered a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas, and Cerastoderma edule presented individually, crabs generally showed evidence of size-selective predation. Cancer pagurus selected larger mussels relative to the size of their chelae (relative prey size, RPS) than did Carcinus maenas of similar and even larger carapace width. However, the RPS of selected O. edulis and Cerastoderma edule were similar for all crabs, suggesting that certain prey features constitute effective barriers even to the powerful chelae of Cancer pagurus. When offered a wide size range of mussels and oysters simultaneously, all crabs consistently selected mussels. When offered O. edulis and Crassostrea gigas, crabs consumed both these oyster species in similar numbers. Carcinus maenas consumed similar numbers of mussels and cockles; Cancer pagurus, however, showed no preference for either prey in the smaller size classes but selected more mussels than cockles as prey increased in size. Although previous studies report that adult Carcinus maenas select prey species according to their profitability (amount of food ingested per unit of handling time, milligrams per second), consumption rates of the size classes of prey selected by juvenile shore crabs did not always parallel prey value. Although variations in crab strength can account for many of the differences between the foraging strategy of juvenile and adult C. maenas, our results suggest that juvenile crabs are less species selective than adults as a result of the restrictions imposed on small individuals that have limited access to larger prey.     

Length-structured analysis of the reproductive season of anchovy and common sardine off central southern Chile

The anchovy (Engraulis ringens) and common sardine (Strangomera bentincki) are two small pelagic fish which have a similar reproductive strategy off central southern Chile. The seasonal reproductive dynamics of both the species was investigated by taking into account the gonadosomatic index (GSI) and visual maturity data as a function of female size for the period 1993–1999, and also the seasonal pattern in condition factor per size classes. Larger females of common sardine (repeat spawners) have a reproductive peak earlier in the season than younger females at first maturity, while larger anchovy have a reproductive peak delayed as compared with first-time spawners. The condition factor of females exhibited an inverse cycle with the seasonal pattern of GSI, and delayed by approximately 6 months. Although larger females tend to present better and wider condition than smaller females, the condition of females seems to be delayed in larger females probably mediated by the seasonal pattern in food availability. This “energy storage strategy” in spring and summer time seems to be size-dependent and past energetic reserves could also affect the egg production and timing of reproduction. We concluded that the reproductive season of anchovy and common sardine is different and mediated by the length structure in the seasonal upwelling system off central southern Chile.     

Use of Fatty Acid Analysis to Determine Dispersal of Caspian Terns in the Columbia River Basin,U.S.A.

Abstract: Lethal control, which has been used to reduce local abundances of animals in conflict with humans or with endangered species, may not achieve management goals if animal movement is not considered. In populations with emigration and immigration, lethal control may induce compensatory immigration, if the source of attraction remains unchanged. Within the Columbia River Basin (Washington, U.S.A.), avian predators forage at dams because dams tend to reduce rates of emigration of juvenile salmonids (Oncorhynchus spp.), artificially concentrating these prey. We used differences in fatty acid profiles between Caspian Terns (Hydroprogne caspia) at coastal and inland breeding colonies and terns culled by a lethal control program at a mid‐Columbia River dam to infer dispersal patterns. We modeled the rate of loss of fatty acid biomarkers, which are fatty acids that can be traced to a single prey species or groups of species, to infer whether and when terns foraging at dams had emigrated from the coast. Nonmetric multidimensional scaling showed that coastal terns had high levels of C₂₀ and C₂₂ monounsaturated fatty acids, whereas fatty acids of inland breeders were high in C18:3n3, C20:4n6, and C22:5n3. Models of the rate of loss of fatty acid showed that approximately 60% of the terns collected at Rock Island Dam were unlikely to have bred successfully at local (inland) sites, suggesting that terns foraging at dams come from an extensive area. Fatty acid biomarkers may provide accurate information about patterns of dispersal in animal populations and may be extremely valuable in cases where populations differ demonstrably in prey base.     

In-depth studies of Magellanic penguin (<Emphasis Type="Italic">Spheniscus magellanicus</Emphasis>) foraging: can we estimate prey consumption by perturbations in the dive profile?

A new concept based on analysis of dive depth data was developed to help estimate prey consumption in ten free-ranging Magellanic penguins (Spheniscus magellanicus) that were brooding chicks. By simultaneously analysing the undulations in the dive depth profile (measured by time-depth recorders, TDRs) and beak opening (obtained from the recently developed intra-mandibular angle sensors, IMASEN), it was possible to determine the proportions of the undulations in the dive profile that resulted (or not) in prey capture. This methodology allowed the number of prey consumed to be estimated with a mean error of 10±6% using TDR data alone. If the mean mass of prey is known, then the overall mass of prey consumed per unit time can be determined. Additionally, the method allows estimation of the depth at which prey is taken and thus indicates how penguins exploit the water column. Due to its simplicity, the proposed methodology has applications for other Spheniscus penguin species and should be considered for other marine endotherm divers that show undulations in the dive depth profile.Communicated by O. Kinne, Oldendorf/Luhe     

Distribution and diet of juvenile Patagonian toothfish on the South Georgia and Shag Rocks shelves (Southern Ocean)

The distribution and diet of juvenile (<750 mm) Patagonian toothfish are described from four annual trawl surveys (2003–2006) around the island of South Georgia in the Atlantic sector of the Southern Ocean. Recruitment of toothfish varies inter-annually, and a single large cohort dominated during the four years surveyed. Most juveniles were caught on the Shag Rocks shelf to the NW of South Georgia, with fish subsequently dispersing to deeper water around both the South Georgia and Shag Rocks shelves. Mean size of juvenile toothfish increased with depth of capture. Stomach contents analysis was conducted on 795 fish that contained food remains and revealed that juvenile toothfish are essentially piscivorous, with the diet dominated by notothenid fish. The yellow-finned notothen, Patagonotothen guntheri, was the dominant prey at Shag Rocks whilst at South Georgia, where P. guntheri is absent, the dominant prey were Antarctic krill and notothenid fish. The diet changed with size, with an increase in myctophid fish and krill as toothfish grow and disperse. The size of prey also increased with fish size, with a greater range of prey sizes consumed by larger fish.     

Food resource partitioning by the pufferfishes Sphoeroides spengleri and S. testudineus from Biscayne Bay,Florida

Partitioning of the food resources by two coexisting pufferfishes (Sphoeroides spengleri and S. testudineus) from Biscayne Bay, Florida, USA, was investigated. Gut contents from 453 bandtail and 339 checkered puffers were analyzed. The diets of both species consisted of a variety of benthic prey, but only crustaceans and molluscs were important prey groups. While differences were found in the proportions of general prey categories eaten by these fishes, both species consumed substantial quantities of brachyuran crabs, bivalves, and gastropods. Specific identification of the prey items within these three food categories revealed additional differences in prey between the two puffer species. This partitioning of the food resources by bandtail and checkered puffers was found between both species overall, between overlapping size ranges, and between both species' most abundant size group. Differences in food habits between these two fishes illustrate that congeners with virtually identical mouth structure and complete spatial overlap can significantly partition the food resources.Contribution No. 78-58M from the U.S. Department of Commerce, NOAA, National Marine Fisheries Service, Southeast Fisheries Center, Miami Laboratory, Miami, Florida, USA.     

Species-specific antipredator capacities and prey refuges: interactions between piscivorous perch (Perca fluviatilis) and juvenile perch and roach (Rutilus rutilus)

The outcome of predator-prey interactions depends on the characteristics of predators and prey as well as the structure of the environment. In a replicated field enclosure experiment, we tested the effects of quantity and quality of different prey refuges (no structure, structure forming a partial refuge, and structure forming a complete refuge) on the interaction between piscivorous perch (Perca fluviatilis) and juvenile perch and roach (Rutilus rutilus). We quantified the behaviour of the predators and the prey and predator-induced prey mortality. The piscivores stayed in or close to the prey refuge and were more dispersed in the presence than in the absence of prey refuges. Survival of juvenile perch and roach decreased in the presence of predators and was higher for juvenile roach than for juvenile perch. In addition, juvenile perch survival increased with refuge efficiency Roach formed schools which were denser in the presence of predators, had a higher swimming speed (both in the open water and in the refuge) and used a larger area than juvenile perch. Both prey species decreased their distance to the prey refuge and increased the proportion of their time spent in the refuge in the presence of predators. The number of switches between the open-water habitat and the prey refuge was higher for juvenile roach than for juvenile perch. Juvenile perch used different parts of the prey refuge in a flexible way depending both on presence of predators and refuge type whereas juvenile roach used the different parts of the prey refuge in fixed proportions over all refuge treatments. Our results suggest that juvenile roach had a overall higher capacity to avoid predation than juvenile perch. However, in the presence of qualitatively different prey refuges juvenile perch responded to predators with more flexible refuge use than juvenile roach. The differences in antipredator capacities of juvenile perch and roach when subjected to piscivorous perch predation may depend on differences in life history patterns of the two species.     

Trophic ecology of Grey-headed albatrosses from Marion Island, Southern Ocean: insights from stomach contents and diet tracers

During chick-rearing, albatrosses can alternate between long foraging trips that provide the main source of food for the adults and short foraging trips that they use to feed their young. This flexibility in foraging behaviour can lead to differences in diet composition between adults and chicks and implies that they may be vulnerable in different ways to food shortages. The trophic ecology of the Grey-headed albatross Thalassarche chrysostoma was investigated at the sub-Antarctic Prince Edward Islands during the chick-rearing period in April 2006 using a combination of approaches. Diets of adults and chicks were assessed using stable isotope ratios and fatty acid (FA) profiles of blood and/or stomach oils, in addition to stomach contents analysis. Fish from the family Macrouridae and cephalopods (particularly the onychoteuthid Kondakovia longimana) were the primary prey, whereas crustaceans (krill Euphausia superba) represented a smaller proportion of the stomach contents. Stomach oil FA profiles contained more monounsaturated FA than the profiles of plasma, which were richer in saturated FA and arachidonic acid (20:4n-6). There was also a distinct separation of adults from chicks, with higher levels of monounsaturates in chick plasma, and higher saturated FA levels (particularly 16:0) in the adult plasma. Stable carbon isotope ratios of whole blood were similar in adults and chicks, whereas stable nitrogen isotope ratios showed significant enrichment by >1‰ in chicks. The combined FA, stable isotopes and stomach contents analyses suggest clear differences in diet quality between adults and chicks, with chicks feeding at a higher trophic position through feeding more on highly nutritious fish and adults keeping much of the less nutritious zooplankton for themselves.     

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.     

Experimental evaluation of the energy balance in Octopus vulgaris, fed ad libitum on a high-lipid diet

A complete energy balance equation was estimated for the common octopus Octopus vulgaris at a constant temperature of 20°C, fed ad libitum on anchovy fillet (Engraulis encrasicolus). Energy used for growth and respiration or lost with faeces and excreted ammonia was estimated, along with total energy consumption through food, for six specimens of O. vulgaris (with masses between 114 and 662 g). The energy balance equation was estimated for the specimens at 10-day intervals. During each 10-day interval, food consumed, body mass increase and quantity of faeces voided were measured. The calorific values of octopus flesh, anchovy flesh and faeces were measured by bomb calorimetry. Oxygen consumption and ammonia excretion rates were monitored for each specimen during three 24-h experiments and daily oxygen consumption and ammonia excretion were estimated. It was found that 58% of the energy consumed was used for respiration. The amount of energy invested in somatic and gonadal growth represented 26% of the total energy budget. The energy discarded through faeces was 13% of consumed energy. The estimated assimilation efficiency (AE) values of O. vulgaris feeding on anchovy (80.9–90.7%) were lower than the AE values estimated for other cephalopod species with different diets of lower lipid content such as crabs or mussels. Specific growth rates (SGR) ranged 0.43–0.95 and were similar to those reported for other high-lipid diets (bogue, sardine) and lower than SGR values found for low-lipid, high-protein diets (squid, crab, natural diet). Ammonia excretion peak (6 h after feeding) followed the one of oxygen consumption (1 h after feeding). The values of atomic oxygen-to-nitrogen (O:N) ratio indicated a protein-dominated metabolism for O. vulgaris.     

Age and sexual differences in the exploitation of two anthropogenic food resources for an opportunistic seabird

The availability of food resources has been suggested as a major factor in the substantial increase in reproductive output, survival, recruitment and, ultimately, population growth rates in most organisms. In fact, the artificial increase in food availability resulting from human activities has been suggested as a factor in the substantial increase in population size of several seabirds in recent decades. In the present study, our primary aim was to estimate the importance of the main natural prey and two alternative feeding resources, fishery discards and the invasive American crayfish Procambarus clarkii, for an opportunistic seabird, the Audouin’s gull Larus audouinii. We also assessed the influence of age and sex in the use of those three types of food. For this purpose, we compared the analyses of δ¹⁵N and δ¹³C in blood of male and female adults of known age and chicks with those in their potential prey. Our results reveal sex-related and age-related differences in the consumption of fish discards, small pelagic fish and American crayfish. Differences in the diet of males and females and also between adults and chicks could be related to different nutritional requirements. Age differences were probably related to their different foraging proficiency and the tendency of young breeders to opportunistically exploit anthropogenically derived food. This study illustrates the importance of considering the age and sex of individuals to obtain feasible dietary information and to understand how the exploitation of food of human origin could affect population growth.     

Growth rates of the anchovyEngraulis encrasicolus and the sardineSardina pilchardus in the Northwestern Mediterranean Sea

The growth rates of the anchovyEngraulis encrasicolus Linnaeus, 1758 and the sardineSardina pilchardus Walbaum, 1792 in the Northwestern Mediterranean Sea between January 1987 and June 1989 was determined by means of otolith interpretation validated by length-frequency analysis. The growth-performance index was higher for the anchovy than for the sardine, in contrast to other areas where these species coexist, where the sardine generally displays a higher growth rate than the anchovy. The possible causes of this difference between geographical areas are discussed.