Habitat influences on reproductive allocation and growth of the mummichog (<Emphasis Type="Italic">Fundulus heteroclitus</Emphasis>) in a coastal salt marsh

To determine whether life history differences can occur in salt marsh fishes that occupy different habitats within the same marsh, we compared reproductive allocation in female mummichog (Fundulus heteroclitus) inhabiting creeks and ponds of a coastal salt marsh in southern New Jersey, USA during the spring and summer of 2001 and 2002. Females were collected in phase with the lunar spawning cycle from four sites of each habitat type, and assessed for gonad-to-body-mass ratio and growth increment. Annual reproductive allocation, expressed as a percent of somatic mass, was estimated for each site and year from the gonadosomatic indices of individuals collected during each spawning period. Mummichogs from creeks showed little change in annual reproductive allocation from 2001 to 2002, whereas those inhabiting ponds showed a significant increase between these years. Seasonal reproductive patterns indicated that pond females cease spawning at least one lunar cycle earlier than creek fish. While ponds experienced considerably higher maximum summer temperatures than creeks as well as near-anoxic pre-dawn conditions, neither of these variables explained a significant amount of variation in annual reproductive allocation. In contrast, annual reproductive allocation of mummichogs in a pond correlated with its flood frequency in both years of study. Our results suggest that while the length of the spawning season differs in mummichogs inhabiting marsh creeks and ponds, annual reproductive allocation depends more upon the hydrodynamic conditions of the particular waterbody than its habitat type per se.     

Tidal and diurnal influence on food consumption of a salt marsh killifish Fundulus heteroclitus

Feeding patterns during four 24-h periods, sampled at 3-h intervals, were investigated for the mummichog Fundulus heteroclitus, in a Delaware, USA tidal marsh. Two factors potentially influencing feeding patterns, time of day and tide height, were examined. On 2 of the sampling periods a low tide occurred in the morning, while on the other 2 sampling periods a high tide occurred in the morning. Results are reported as g-dry wt. of food per g-dry wt. of fish. F. heteroclitus is primarily a daytime feeder that most actively feeds at high tide, regardless of whether or not the high tide inundates marsh surface areas. When tide height was sufficient to inundate the marsh surface, fish invaded these areas and consumed prey characteristic of the marsh surface. F. heteroclitus is an important link in energy transfers between the marsh surface and subtidal systems, enhancing its own energy supplies by consuming marsh surface prey whenever available.     

Effects of chronic oil pollution on a salt-marsh grass community

A small, naturally defined saltmarsh in Chesapeake Bay (USA) was repeatedly dosed with a No. 2 fuel oil. The production, decay rates, physical characteristics and hydrocarbon content of the dominant grass species present in the marsh were routinely monitored. Spartina alterniflora was the only grass species displaying effects of the oil dosing; this is attributed to the grass's location in the marsh. A substantial portion of the S. alterniflora in the marsh was killed by the oil dosing; the remaining portion evidenced sublethal effects including delayed development in the spring, increased density, and reduced mean weight per stem. The second annual cohort of shoots, usually produced in late summer and early fall, was suppressed almost entirely. Decay rates of S. alterniflora in the oiled marsh were higher and peaked later than decay rates in a control marsh. Oil which entered the roots and rhizomes of dead S. alterniflora was retained in a relatively undergraded state for at least 7 months after dosing stopped.Virginia Institute of Marine Science Contribution No. 945     

Growth,production and energy transformations in the salt-marsh killifish Fundulus heteroclitus

Populations of Fundulus heteroclitus (L.) at the beginning of a growth season in a New England salt marsh consist of 3 yearly age classes, with the 1-year-old class contributing most of the biomass. Calculation of production rates revealed that the young fish were the most active part of the population and that females were more productive than males. The entire population spent equal amounts of energy in growth and in metabolism. Including the young of the year, we obtained total production of 160 kg dry weight/ha, a value among the highest obtained for natural fish populations. About 5 to 15% of the production is available to predators. Rates of food consumption by F. heteroclitus are high enough to turn over the population of prey relatively often. Popululations of F. heteroclitus are thus capable of exerting an important influence on the abundance and distribution of their prey.Contribution No. 3721 from the Woods Hole Oceanographic Institution.     

Allocation of nitrogen and carbon in an estuarine salt marsh in Portugal

Above and below-ground biomass and nitrogen and carbon composition ofSpartina maritima, Halimione portulacoides andArthrocnemum perenne, dominating species in plant communities of the lower, middle and higher salt marsh, respectively, were compared in an estuarine salt marsh in Portugal. Plant and soil nitrogen and carbon pools were estimated. For all three species root biomass was significantly higher (70–92% of total biomass) than above-ground biomass. The percentage of root biomass was related to the location of the plants in the marsh: higher values were found in plants growing in the lower salt marsh where the sediment was more unstable and subject to tidal action, which stresses the role of the roots as an anchor. For all three species nitrogen concentrations were highest in leaves, reflecting the photosynthetic role of the tissue. For carbon higher concentrations were found in the stems, with the exception ofS. maritima. In general, lower nitrogen concentrations were found in summer, which can be explained by dilution processes due to plant growth. For both nitrogen and carbon, higher concentrations were found in the soil surface layers. Higher soil nitrogen and carbon levels were associated with higher organic matter contents. Most of the nitrogen in the salt marsh occurred in the sediments (0–40 cm) and only ca. 5.7–13.3% of the total was found in the plants. The greater portion (76.5%–86%) of carbon was found in the sediment.     

Growth,food utilization and effect of social interaction in the European bass Dicentrarchus labrax

Fingerling and juvenile bass, Dicentrarchus labrax (L.) fed on filleted saithe, Gadus virens, exhibited much higher K ₁ energy and nitrogen utilization efficiencies (growth as % energy or nitrogen intake) than those fed on trout pellets or shore crabs, Carcinus maenas, although the efficiency declined during prolonged feeding. Mean organic absorption efficiency for saithe was 99.5% and was independent of food intake but the efficiency for pellets and crab declined with increasing food intake from 99 to 92%. For each diet the regression of relative growth rate on food intake was linear up to the regression of relative growth rate on food intake was linear up to the highest feeding level and independent of body weight, enabling construction of a simple model for food transformation. The regression coefficient (equivalent to K ₃, the ratio of growth to food consumed in excess of maintenance) for saithe was about 3 times that for pellets in terms of energy and twice that for pellets in terms of nitrogen. The more efficient utilization of saithe for growth and of pellets for maintenance were related to differences in their nutritional value. With each diet, K ₃ and maximum K ₁ for energy were about twice those for nitrogen. Social interaction had a significant effect on food utilization in juvenile bass. Isolated fish showed poor growth and mean K ₁ efficiency in comparison with bass in pairs or groups. K ₃ efficiency and mainternance requirement were similar in individuals and groups, but were significantly greater in pairs. These results are related to behavioural observeations and environmental stress, and the limitations of extrapolating the results of laboratory feeding experiments on fish to other situations is discussed.     

Growth of the salt marsh periwinkleLittoraria irrorata on fungal and cordgrass diets

The growth of the salt marsh periwinkleLittoraria irrorata (collected from Sapelo Island, Georgia in 1991, initial shell length 6.2 to 11.5 mm) on various diets was measured. Growth was highest on a diet of standing-dead leaves ofSpartina alterniflora. Periwinkles provided with marsh sediment, yellow-green, sterile, or bacteria-colonized leaves lost organic mass. Fungal-colonized leaves and pure mycelia of fungi common on standing-dead leaves allowed intermediate growth. Growth onS. alterniflora-based diets was negatively correlated with the phenolics content of the food, and positively correlated with its lipid content. No correlation was found between growth and protein content. The digestibility ofS. alterniflora leaves, estimated with the acid-insoluble ash technique, was highest when yellow-green leaves were used. Colonization by fungi or bacteria caused it to decline. ForS. alterniflora-based diets, growth rates were positively correlated with the amount of time spent on the food.     

Effects of tidally mediated litter moisture content on decomposition of Spartina alterniflora and S. patens

Relationships between flooding frequency, flooding duration, litter moisture levels, and litter decay rates were investigated across the natural hydrologic gradient common to intertidal salt marshes. The effects on litter decay of natural and experimental alterations of litter moisture content were assessed in both field litterbag experiments (conducted in a southern New Jersey salt marsh from 1989 to 1990) and laboratory incubations (1990). Overall, tidally mediated litter moisture content was the dominant factor controlling litter decay throughout the vegetated marsh. Rates of carbon loss were most closely related to litter moisture levels (r=0.84), which were directly related to flooding frequency (r=0.66) and duration (r=0.63). Litter moisture levels were related to elevation within the tidal range due to increasing surface levation from creekbank to high marsh (ca. 54 cm) and height of litter above the sediment surface. Carbon losses from litter of short and tall form Spartina alterniflora Loisel. and S. patens (Aiton) Muhl. along the marsh elevation gradient indicate that while some of the variations in decay rates may be due to litter type, litter moisture accounted for most of the observed variation between marsh zones and within each litter type. Mousture levels are also affected by the water retention capacity of each litter type, which may also secondarily influence decay rates. Short-term incubations of litter indicated that CO₂ evolution was positively related to moisture content with negligible respiration at moisture levels below 15% (fresh mass), increasing to a maximum between 65 and 75% depending upon litter type. Since most Spartina spp. litter remains above the marsh surface where it maintains a lower moisture content than surface litter, the use of surface litterbags may overestimate rates of carbon loss in some systems. In addition, since changes in elevation of only a few centimeters had significant effects on both litter moisture levels and decay rates, slight changes in tidal regime may have important consequences for organic matter cycling in salt marshes by affecting litter decomposition processes.     

Feeding,conversion efficiencies,and growth of larval mummichogs,Fundulus heteroclitus

Feeding rates, conversion efficiencies and growth of larvae of the mummichog Fundulus heteroclitus, an extremely abundant estuarine fish, were measured at temperatures ranging from 18° to 30°C. The food used was Artemia salina nauplii. At the time of total yolk sac absorption (5 to 7 days after hatching), the feeding rate decreased for a short time, an indication of a shift in metabolism. Higher feeding rates and growth occurred at higher rearing temperatures. The highest conversion efficiency (gross growth efficiency) was 1.1%, at 22°C. Mummichog larvae may be energetically inefficient compared with other fish species, but efficiency might not be critical for this fish, which is an opportunistic omnivore in an energy-rich environment.Contribution No. 291 of the Belle W. Baruch Institute for Marine Biology and Coastal Research, supported by DOE contract No. EY-76-5-09-0869.     

Adaptations of wild populations of the estuarine fish Fundulus heteroclitus to persistent environmental contaminants

Many aquatic species, including the estuarine fish Fundulus heteroclitus (mummichogs), adapt to local environmental conditions. We conducted studies to evaluate whether highly exposed populations of mummichogs adapt to toxic environmental contaminants. These fish populations are indigenous to an urban estuary contaminated with persistent and bioaccumulative contaminants (dioxin-like compounds, or DLCs) that are particularly toxic to the early development of fish. We conducted laboratory challenge experiments to compare mummichog embryos and larvae from reference sites and this highly contaminated site [New Bedford Harbor (NBH), Massachusetts, USA] for their sensitivity to DLCs. While there was variation in DLC-responsiveness within each group, fish from NBH were profoundly less sensitive to DLCs than reference fish. Specifically, concentrations of DLCs similar to those measured in NBH-collected mummichog eggs were lethal to reference embryos. Further, DLC-responsiveness was inherited and independent of maternal contaminant contributions. These findings are consistent with the conclusion that DLC contamination in NBH has contributed to the selection of fish that are resistant to the short-term toxic effects of these environmental-contaminant exposures. This adaptation may be a critical mechanism by which fish populations persist in this highly contaminated site. Further evaluation of this ecosystem may provide important information concerning the direct and indirect consequences of this “unnatural” selection. Received: 12 July 1998 / Accepted: 16 January 1999     

Movements and food habits of striped bass (Morone saxatilis) in Delaware Bay (USA) salt marshes: comparison of a restored and a reference marsh

 There has been much recent interest in restoration of salt-marsh habitats to their natural structure and function. However, the criteria for success of such restorations are not well-defined. As part of a larger program to evaluate the restoration of a former salt-hay farm bordering Delaware Bay, New Jersey, USA, we monitored the response of a large predator, the striped bass Morone saxatilis, to the restoration. During June to October 1998 we compared tidal and diel movements and food habits of juvenile and adult striped bass (n = 82, 212 to 670 mm fork length) between a restored marsh and an adjacent reference marsh with similar physical characteristics (depth, salinity, temperature). Striped bass movements at both sites were characterized by ultrasonic tracking with small, surgically implanted tags (21 d rated battery-life). Striped bass (n = 23, 421 to 610 mm fork length) were tagged and released near the main creek mouths at both the restored (n = 14) and reference (n = 9) marshes. At both sites, striped bass tended to move up the main creek during ebb tide. At the restored site, ebb tide upstream-movements ranged from 0.1 to 3.5 km from the main creek mouth (mean = 1.2 km). During the upstream movement, the fish typically stopped every 200 to 300 m (presumably to feed) for 1 to 2 h. At the reference site, few of the tagged fish moved farther than 100 to 200 m upstream from the main channel mouth at ebb tide, perhaps in response to somewhat lower dissolved oxygen at this site. During flood tide, tagged fish at both sites moved out into Delaware Bay, where they remained within 200 to 500 m of the creek mouth. Striped bass were sampled with gill nets to determine additional aspects of habitat use and food habits. Striped bass in both marshes were much more abundant at creek mouths (catch per unit effort, CPUE = 1.17) than in the upper reaches of the creeks (CPUE = 0.13). In the creek mouths, CPUE was greater at the restored site (CPUE = 1.8) than at the reference site (CPUE = 0.5). At both sites, most fish (approx. 80%) were collected on the late ebb or early flood tides, i.e. around low tide, when prey were presumably concentrated at the creek mouths. Stomach contents of bass from both restored and reference marshes (n = 59, 212 to 670 mm fork length) revealed that striped bass were eating mostly blue crab (Callinectes sapidus), grass shrimp (Palaemonetes vulgaris), sand shrimp (Crangon septemspinosa), mummichog (Fundulus heteroclitus), and various unidentifiable fishes (probably anchovies, Anchoa mitchilli, and Atlantic silverside Menidia menidia). In conclusion, the restored marsh supported larger numbers of striped bass than the reference marsh, but there was little difference in the pattern of creek utilization or food habits at either site. Thus, the restored marsh appears to be functioning in a similar manner to the reference marsh for these large predators. Received: 28 June 1999 / Accepted: 1 August 2000     

Feeding energetics and metabolism in demersal fish species from Antarctic,temperate and tropical environments

The energetics of feeding has been investigated in demersal fish with similar sedentary lifestyles from the Antarctic (Notothenia neglecta Nybelin), North Sea (Myoxocephalus scorpius L.) and Indian Ocean (Cirrhitichys bleekeri Bleeker). In general, the metabolic rates of fasting individuals were positively correlated with adaptation temperature: values for a standard 100 g fish (mg O₂/h) were 3.3 for N. neglecta at around 0 °C, 2.7 for winter-acclimatized M. scorpius at 5 °C, 4.3 for summer-acclimatized M. scorpius at 15 °C, and 7.0 for C. bleekeri at 25 °C. In all species, following a single satiating meal, oxygen consumption increased to a peak of 2 to 3.5 times the fasting values. Maximum rates of oxygen consumption after feeding were several-fold higher in the warm-than in the cold-water species. After controlling for the effects of body mass and energy intake by analysis of covariance, the duration of the increase in metabolic rate, referred to as specific dynamic action (SDA), was found to be 3 to 4 times shorter in the warm- than in the cold-water fish, ranging from 57 h in C. bleekeri to 208 h in N. neglecta. In contrast, the SDA was not significantly different in the various species, corresponding to 15 to 23% of the energy ingested. Seasonal influences on metabolism and feeding were also studied in N. neglecta acclimated to simulated winter (-1.0 to-0.5 °C; 3 h light:21 h dark) or summer (0 to 0.9 °C; 21 h light:3 h dark) conditions. The metabolic rates of fasting and fed individuals, and the characteristics of the SDA were found to be independent of acclimation conditions. This suggests that N. neglecta is capable of processing food at similar rates throughout the year. Energy stores and enzyme activities were measured in the swimming muscles and liver of fish fed ad libitum. Summer-acclimated fish had higher concentrations of liver triglyceride stores and elevated activities of some enzymes of intermediary metabolism relative to winter-acclimated fish. The observed changes in intermdiary metabolism are probably related to annual cycles of growth and reproduction. It is suggested that the low aerobic scope for physiological performance in Antarctic fish may necessitate the seasonal switching of energy allocation between growth and reproduction.     

Nitrogen accumulation and plant species replacement in three salt marsh systems in the Wadden Sea

Salt marsh development on the coastal barrier island of Schiermonnikoog (The Netherlands) was compared with two other salt marsh systems in the Wadden Sea. Accretion rate, nitrogen accumulation and changes in plant species composition were investigated using chronosequences. The age of the marsh was estimated from aerial photographs and old maps. In 7230 plots, the elevation of the marsh surface, the thickness of the sediment layer (clay) and the presence of plant species was recorded. In addition, the nitrogen pool was measured at each successional stage. Accretion rates were similar in the three salt marshes. Higher accretion rates were found at younger marshes. A strong linear relationship between nitrogen pool size and thickness of the clay layer was found for the three marshes. The accumulation rate of nitrogen is therefore strongly related to the accretion rate. Thus, more nitrogen is present in the sediment of later successional stages where more clay has accumulated. On the high salt marsh (55 cm+MHT),Ameria maritima disappeared andArtemisia maritima, Juncus gerardi andElymus athericus established at sites with a thicker clay layer. On the low salt marsh (25 cm+MHT),Plantago maritima, Puccinellia maritima andLimonium vulgare disappeared andAtriplex (Halimione) portulacoides established. Apparently, with the accumulation of clay and therefore of nitrogen, tall growing species take over in salt marshes not grazed by livestock.     

Transformation of food in the fish Megalops cyprinoides

Comparative studies on feeding, digestion, absorption and conversion have been made in the fish Megalops cyprinoides fed with mosquito fish, Gambusia affinis, and prawn, Metapenaeus monoceros, (Pandian, 1967). Feeding as well as digestion rates are faster in individuals fed with Gambusia than in those fed with prawn. The extent of the differences in food intake is more pronounced in smaller individuals than in larger ones. The prawn exoskeleton prolongs the duration of digestion and thus further reduces rate and amount of food intake. Changes in food quality do not affect absorption efficiency. Both, rate and efficiency of food conversion, are higher in M. cyprinoides fed with Gambusia than with prawn.     

Aggresive competition and individual food consumption in wild brown capuchin monkeys (Cebus apella)

Summary The impact of aggresive competition on food intake at all the resources used is analysed for every member of a group of brown capuchin monkeys (Cebus apella) in the Manu National Park, Peru, where they live in groups of 8–14 animals. An individual's food intake at a given tree was affected independently both by its domirance rank (Fig. 1) and by how much aggression it received (Table 5). Food intake was not strongly affected by body size when dominance rank was held constant by partial correlation. At food sources where high rates of fighting occurred, an individual's food intake depended more on its domirance status than on the rate of aggression it received (Fig. 2). However, food intake at resources where rates of fighting were low depended mostly on the rate of aggression received. When aggression over food was absent, the food intakes of dominants and subordinates were equal. Dominants had significantly greater total energy intake (20.5% more) than did subordinates, even though more than one third of their diet came from food sources where little or no fighting occurred (Fig. 3). Energy intake was also significantly greater for individuals that received little aggression. The only adult that emigrated from the main study group was the individua with the lowest energy intake. Competition for food within groups was more than ten times as intense as competition between brown capuchin groups.     

Effects of Invasive Cordgrass on Presence of Marsh Grassbird in an Area where It Is Not Native

The threatened Marsh Grassbird (Locustella pryeri) first appeared in the salt marsh in east China after the salt marsh was invaded by cordgrass (Spartina alterniflora), a non‐native invasive species. To understand the dependence of non‐native Marsh Grassbird on the non‐native cordgrass, we quantified habitat use, food source, and reproductive success of the Marsh Grassbird at the Chongming Dongtan (CMDT) salt marsh. In the breeding season, we used point counts and radio‐tracking to determine habitat use by Marsh Grassbirds. We analyzed basal food sources of the Marsh Grassbirds by comparing the δ¹³C isotope signatures of feather and fecal samples of birds with those of local plants. We monitored the nests through the breeding season and determined the breeding success of the Marsh Grassbirds at CMDT. Density of Marsh Grassbirds was higher where cordgrass occurred than in areas of native reed (Phragmites australis) monoculture. The breeding territory of the Marsh Grassbird was composed mainly of cordgrass stands, and nests were built exclusively against cordgrass stems. Cordgrass was the major primary producer at the base of the Marsh Grassbird food chain. Breeding success of the Marsh Grassbird at CMDT was similar to breeding success within its native range. Our results suggest non‐native cordgrass provides essential habitat and food for breeding Marsh Grassbirds at CMDT and that the increase in Marsh Grassbird abundance may reflect the rapid spread of cordgrass in the coastal regions of east China. Our study provides an example of how a primary invader (i.e., cordgrass) can alter an ecosystem and thus facilitate colonization by a second non‐native species. Efectos de Spartina alterniflora Invasora Sobre Locustella pryeri en un Área Donde No Es Nativa     

Adaptive peaks and alternative foraging tactics in brook charr: evidence of short-term divergent selection for sitting-and-waiting and actively searching

Some recently emerged brook charr (Salvelinus fontinalis) inhabiting still-water pools along the sides of streams are sedentary and eat crustaceans from the lower portion of the water column. Others are more active and eat insects from the upper portion of the water column. We provide evidence that this divergent foraging behavior reflects short-term divergent selection brought about by intraspecific competition in the presence of alternative food sources. Rates of encounters and interactions between individuals were density dependent, and encounter and interaction events were closely timed with prey capture attempts. In addition, aggressive fish made more foraging attempts per minute than nonaggressive fish. Aggressive fish were also either inactive or very active, while nonaggressive fish exhibited intermediate levels of activity. Growth rate potential, an important component of fitness during the early life stages of brook charr, was assessed using tissue concentrations of RNA and found to be highest for sedentary fish and for active fish making frequent foraging attempts, and lower for fish exhibiting intermediate levels of activity. Our findings support contentions that individual behavior plays an important role during initial steps in the evolution of resource polymorphisms. Received: 27 July 1998 / Accepted after revision: 16 November 1998     

The use of RNA:DNA ratios to predict growth limitation of juvenile summer flounder (Paralichthys dentatus) from Delaware and North Carolina estuaries

Nutritional indices were used to develop biochemical correlates of feeding and growth rates for juvenile summer flounder, Paralichthys dentatus (Linnaeus), from North Carolina (NC) and Delaware (DE). Six parameters (Fulton's condition K=10⁴xweight/(length³), wet weight/dry weight, [protein], [RNA], [DNA], and RNA:DNA) were related to feeding and growth rates of fish from previously reported 10 to 14-d experiments at temperatures ranging from 2 to 20 °C with varying feeding levels (0 to 100% and libitum). RNA:DNA ratios were the best predictors of growth rates, but inclusion of a temperature term improved the relationship between RNA:DNA ratios and growth rate for Delaware fish. Feeding rates were poorly correlated with all parameters. RNA:DNA ratios of fish in the laboratory changed significantly within 1 d of starvation and refeeding at 16 °C. RNA:DNA of juvenile summer flounder collected from one site in Indian River Bay, DE and two sites in the Newport River Estuary, NC, between January and June 1992 were used to estimate in situ growth rates following settlement. Predicted growth rates in both estuaries were close to maximum (suggesting ad libitum feeding) until early May. Growth rates of juveniles from Delaware were <0% d^-1 from December through early March, and were higher (0.6 to 3% d^-1) from April through early June. However, growth rates of DE juveniles during May were <50% of maxinum. North Carolina juveniles had growth rates of 2 to 5% d^-1 from February through early April. Juveniles from one of the Newport River sites (a marsh habitat) were also severely growth limited (<20% of maximum) after April. Prolonged periods of sub-optimal growth may be important to survival and recruitment of juvenile summer flounder in northern mid-Atlantic estuaries. A model is presented which illustrates the potential impact that small changes in temperature and growth limitation can have on recruitment success in both delaware and North Carolina estuaries.     

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.     

Nitrogen balance in marine fish larvae: influence of developmental stage and prey density

The utilization and fate of nitrogen in larvae of plaice (Pleuronectes platessa), blenny (Blennius pavo) and herring (Clupea harengus), from the stage of first-feeding to metamorphosis, was examined under laboratory conditions. Rates of ammonia excretion, primary amine defaecation, and growth in terms of protein-nitrogen were monitored throughout larval life. Data were used to calculate daily ration, the coefficient of nitrogen utilization (absorption efficiency), and gross and net growth efficiencies. The developmental pattern of nitrogen balance was similar for plaice and blenny larvae. These species showed increasing growth efficiency (k₁: 55 to 80%) with decreasing weight-specific waste nitrogen losses with age. Absorption efficiencies. were high (83 to 98%) in plaice and blenny larvae, and tended to increase with development in the former species. Ration relative to body weight decreased with growth in both species. Herring larval development, although at a slower rate than blenny and plaice, appeared normal up to 33 d, after which high mortality occurred. Absorption efficiency in this species tended to decline (83 to 43%) with age, until metabolic costs exceeded the absorbed ration and growth ceased. Artemia sp. nauplii proved a suitable food source for the rearing of plaice and blenny larvae, but this diet may have long-term toxicity or deficiency effects on herring. Availability and density of food affected nitrogen balance in the larvae of all three species. Feeding stimulated the output of wastes in excretion and defaecation by a factor of up to ten times the 12-h non-feeding basal rates. Waste nitrogen output reached a peak some 2 to 3 h after commencement of feeding and returned slowly to the baseline in 5 to 10 h after cessation of feeding. There was an asymptotic increase in ration, ammonia output and growth of larvae as prey density increased. Ration saturated at a higher prey density (>4 prey ml^-1) than either growth or excretion rate (1 prey ml^-1). Thus the efficiency with which food is absorbed and utilized for growth must eventually decline in response to high prey density. The idea that larval fish are adapted to maximize ingestion and growth rate, rather than optimize growth efficiency and thus to respond to prey occurring in either low density or in occasional patches, is supported by these results.