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Feeding ecology of coexisting Heermann’s gull Larus heermanni and elegant tern Thalasseus elegans chicks, based on stable isotope measurements
Abstract
The Heermann’s gull and the elegant tern are considered Near Threatened by the IUCN Red List of Threatened Species and under special protection under Mexican environmental law. Close to 95% of the world populations of both species breed on Isla Rasa (Gulf of California). As both species breed on the same island, competition for trophic resources can be expected to be high. Here, the feeding ecology of chicks of both species, as well as the contribution by prey species provided by adults, was studied using stable carbon (δ ¹³ C) and nitrogen (δ ¹⁵ N) isotope analysis in 3 consecutive breeding seasons (2017, 2018, and 2019). Results of the stable isotope analysis revealed that although some segregation in the δ ¹³ C and δ ¹⁵ N values exists between both species, isotopic mixing model outputs indicated that fishes were the most important prey group for both. However, at the prey species level, the most important prey consumed by Heermann’s gulls were northern anchovy, Pacific sardine, and Pacific mackerel, followed by pelagic crab, whereas elegant terns consumed mainly northern anchovy. The trophic width estimated with the stable isotope values during 2017 and 2019 was higher for the Heermann’s gull than for the elegant tern. In contrast, for 2018, the trophic width of the elegant tern was higher than that of the Heermann’s gull. In addition, both species showed a high degree of trophic width overlap during 2017 and 2019 (overlap probability 0.67 and 0.79, respectively). These results highlight the ability of the Heermann’s gull to redirect its food intake to what might be the most accessible prey groups, a feeding strategy that allows the coexistence.
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Elegant terns Thalasseus elegans breed in a very limited area of the northern Gulf of California and the Pacific coast of southern California, with up to 95% (mean 78%, 1991–2014, Perez et al., 2020) of the population nesting on Isla Rasa in the northern Gulf of California. On Isla Rasa, the primary nesting colony, elegant terns suffered predation by rodents which raised the possibility of population extinction, with a substantial proportion of the world population nesting on this single island. Because of this threat, rodents were successfully removed from Isla Rasa in 1995. The removal of rodents from Isla Rasa led to a near immediate increase in the population of elegant terns. That increase was associated with a changing pattern in dispersal by the terns, including extraordinary movements to the Gulf of Mexico, the Atlantic coast of the United States north to Massachusetts, and, remarkably, to western Europe. A few elegant terns successfully bred at these European localities during 2009 to the present. In this paper we use this exceptional example of long-distance dispersal to illustrate how rapid population growth during ∼ 1995 to present can lead to successful colonization of remote sites through repeated instances of vagrancy. We tested four Hypotheses that together support the idea that the growing population of elegant terns has produced increasing numbers of young, and these young have spread, through the mechanism of vagrancy, to the Pacific Northwest, the east coast of the United States, and western Europe. Our Hypotheses are: (1) The nesting population of elegant terns within their core nesting range has increased since removal of rodents from Isla Rasa; (2) Occurrence of vagrant elegant terns in the Pacific Northwest is driven by population growth within the core breeding range. (3) Occurrence of vagrant elegant terns at the east coast of the United States is driven by population growth within the core breeding range. (4) Occurrence and colonization of western Europe by elegant terns is driven by nesting population size within the core breeding range. Corollaries of these Hypotheses are, (i) that there is a time lag in occurrence of vagrants at each of these areas, based on increasing distance from the core breeding range and (ii) the number of vagrants in any given year is also related to sea surface temperature (SST), as expressed by Oceanic Niño Index, a proxy for food resource levels. Generally we found strong statistical support for each of these Hypotheses; an exception was for the occurrence of elegant terns in the Pacific Northwest, which initially occurred following El Niño events (low food supply) and profound breeding failure, but later corresponding to cold water years with high breeding success. We use elegant terns, exceptional for the highly restricted breeding range and sustained population growth over 25 years, to illustrate how growing populations may colonize very distant habitats through repeated instances of vagrancy.
Oxygen, hydrogen, carbon and nitrogen stable isotopes were measured on a comprehensive sampling of feathers from two spring Hooded Warblers (Setophaga citrina) in Texas to evaluate isotopic variability between feathers and during molt. Isotopic homogeneity within each bird was found across all four isotopic systems, supporting the hypothesis that molt in these neotropical migrants is fully completed on the breeding grounds. This homogeneity suggests that the isotopic composition of a single feather is may be representative of the whole songbird. However, each bird was found to have one or two outlier feathers, which could signify regrowth of lost feathers after prebasic molt.
Gulls (Larus spp.) are described as generalist, opportunistic feeders that show great flexibility in habitat use. Despite an apparent advantage in changing landscapes, many Larus populations have declined in eastern North America since the 1990s. The main hypothesis explaining gull declines at a broad scale is a decrease in total food availability, especially anthropogenically derived fisheries discards and human refuse as industries and cities have improved their management practices. However, it is difficult to quantify the total proportion of gull diet subsidized by humans to test this hypothesis because many common prey items can be traced to both anthropogenic and nonanthropogenic sources. Our aim was to estimate the proportion of diet derived from anthropogenic food sources for Herring Gull (L. argentatus) during the breeding season at the two largest colonies in the Bay of Fundy, Canada, which are located 36 km apart. GPS loggers were deployed to quantify spatiotemporal movement patterns, and whole blood and feather samples were collected for δ13C and δ15N stable-isotope analysis to estimate diet composition during the incubation and chick-rearing stages of the breeding season. Results indicate that there is spatial segregation in the foraging areas used by gulls from the two colonies. All gulls relied on a variety of anthropogenic food sources, with some individuals selecting heavily on fisheries (i.e., active town wharfs, fish packaging plants, aquaculture pens) and mink (Neovison vison) fur farms. Landfills were not a significant source of food during the breeding season. Our study provides valuable information about the relative reliance of gulls on anthropogenic food subsidies, providing insight into how changing industry practices may affect patterns in nesting and foraging by gulls in the region.
Population genetic structure can aide in developing conservation and management strategies by characterizing populations on local and regional scales. The Elegant Tern (Thalasseus elegans) has a restricted breeding range, with a majority of its nesting population historically found on Isla Rasa, Gulf of California, Mexico. Since the late 1950s, increased frequency of low marine productivity due to warm oceanographic anomalies, such as El Niño events, have caused northward expansion of Elegant Tern breeding colonies into southern California, USA. To test the hypothesis that high gene flow occurs between Gulf of California and southern California Elegant Tern breeding colonies, restriction-site associated DNA sequencing was used to analyze 5,510 single nucleotide polymorphisms from 69 Elegant Terns sampled across four known breeding sites: Isla Rasa, Mexico (n = 30), San Diego National Wildlife Refuge, USA (n = 17), Bolsa Chica Ecological Reserve, USA (n = 11), and Port of Los Angeles, USA (n = 11). Analyses revealed little population subdivision, with non-significant genetic differentiation (FST) among sites and no geographic association of individuals, but there was subtle clustering of individuals by breeding site. These results suggest a strong degree of gene flow among the Gulf of California and southern California nesting colonies and indicate that Elegant Terns have a fluid breeding distribution and move readily among nesting sites.
Background:
Human activities have profoundly altered the spatio-temporal availability of food resources. Yet, there is a clear lack of knowledge on how opportunistic species adapt to these new circumstances by scheduling their daily rhythms and adjust their foraging decisions to predicable patterns of anthropic food subsidies. Here, we used nearly continuous GPS tracking data to investigate the adaptability of daily foraging activity in an opportunistic predator, the yellow-legged gull (Larus michahellis), in response to human schedules.
Methods:
By using waveform analysis, we compared timing and magnitude of peaks in daily activity of different GPS-tracked individuals in eleven different habitat types, in relation to type of day (i.e., weekday vs. weekend).
Results:
Daily activity rhythms varied greatly depending on whether it was a weekday or weekend, thus suggesting that gulls' activity peaks matched the routines of human activity in each habitat type. We observed for the first time two types of activity as modelled by waveforms analysis: marine habitats showed unimodal patterns with prolonged activity and terrestrial habitats showed bimodal patterns with two shorter and variable activity peaks.
Conclusions:
Our results suggest that gulls are able to fine-tune their daily activity rhythms to habitat-specific human schedules, since these likely provide feeding opportunities. Behavioral plasticity may thus be an important driver of expansive population dynamics. Information on predictable relationships between daily activity patterns of gulls and human activities is therefore relevant to their population management.
Research focused on evaluating how human food subsidies influence the foraging ecology of scavenger species is scarce but essential for elucidating their role in shaping behavioral patterns, population dynamics, and potential impacts on ecosystems. We evaluate the potential role of humans in shaping the year‐round distribution and habitat use of individuals from a typical scavenger species, the yellow‐legged gull (Larus michahellis), breeding at southwestern Spain. To do this, we combined long‐term, nearly continuous GPS‐tracking data with spatially explicit information on habitat types and distribution of human facilities, as proxied by satellite imagery of artificial night lights. Overall, individuals were mainly associated with freshwater habitats (mean proportion, 95% CI: 40.6%, 36.9%–44.4%) followed by the marine‐related systems (40.3, 37.7%–42.8%), human‐related habitats (13.5%, 13.2%–13.8%), and terrestrial systems (5.5%, 4.6%–6.5%). However, these relative contributions to the overall habitat usage largely changed throughout the annual cycle as a likely response to ecological/physiological constraints imposed by varying energy budgets and environmental constraints resulting from fluctuations in the availability of food resources. Moreover, the tight overlap between the year‐round spatial distribution of gulls and that of human facilities suggested that the different resources individuals relied on were likely of anthropogenic origin. We therefore provide evidence supporting the high dependence of this species on human‐related food resources throughout the annual cycle. Owing to the ability of individuals to disperse and reach transboundary areas of Spain, Portugal, or Morocco, international joint efforts aimed at restricting the availability of human food resources would be required to manage this overabundant species and the associated consequences for biodiversity conservation (e.g., competitive exclusion of co‐occurring species) and human interests (e.g., airports or disease transmission). We investigate the spatial distribution and habitat associations of a yellow‐legged gull population inhabiting a highly anthropogenic landscape, and throughout a complete annual cycle. The tight overlap between the spatial distribution of gulls and that of human facilities (as proxied by satellite imagery of artificial night lights) supports the high dependence of this species on human‐related resources. Gulls relied differently on different types of human food subsidies throughout the annual cycle as evidenced by observed changes in habitat use.
We measured the fractionation of stable nitrogen (δ15N) and carbon (δ13C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ15N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ13C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ15N and 0.1‰ to 2.5‰ for δ13C. Studies that did not remove lipids from prey samples showed higher δ15N and δ13C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ15N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ13C fractionation, (2) high interstudy and interspecific variation in δ13C makes species-specific studies essential, and (3) use of muscle tissue rather than whole bodies of fish will minimize unexplained variation in δ15N fractionation.
Many closely related seabirds nest in mixed colonies, and this association may result in interspecific interactions such as competition for common resources and kleptoparasitism. Trophic interactions were evaluated between the Kelp Gull (Larus dominicanus Lichtenstein, 1823) and Royal and Cayenne terns (Thalasseus maximus maximus (Boddaert, 1783) and Thalasseus sandvicensis eurygnathus (Saunders, 1876), respectively) nesting at a mixed-species colony in an area with high availability of recreational fishery waste for the opportunistic Kelp Gull. Diet analyses were based on gull chick stomach content samples and direct observations of food delivered to tern chicks in 2013 and 2014, complemented in 2014 with carbon and nitrogen stable isotope analysis of chick whole-blood samples. Main prey species of Kelp Gull chicks were Cynoscion guatucupa (Cuvier, 1830), a demersal species obtained from recreational fishery waste, Argentine anchovies (Engraulis anchoita Hubbs and Marini, 1935), and insects. Engraulis anchoita and Odontesthes spp. were the main prey of both tern species. Trophic niche and isotopic niche overlap between the Kelp Gull and Royal and Cayenne terns was low. Kelp Gull kleptoparasitism on Royal and Cayenne terns was ≤2.5% and <0.6%, respectively. The use of anthropogenic food subsidies by Kelp Gulls may be mediating the trophic relationships among species, favouring their use of predictable and abundant fishery waste over a more unpredictable pelagic schooling fish such as E. anchoita.
A frequently used method to estimate diet composition is based on the identification of fish otoliths present in pellets and feces. However, whether pellets and feces provide similar unbiased estimates of the diet remains poorly understood. The diet of African Royal Terns (Thalasseus maximus albididorsalis) breeding in the Parc National du Delta du Saloum, Senegal, was studied. Prey species composition based on otoliths in freshly regurgitated pellets and a mixture of pellets and feces (excrement) accumulated near nests during the incubation period were compared. Altogether, 59 fish species were identified. Pellets contained far less prey species than excrement. Maximum diet overlap between excrement and pellets varied between 0.34 and 0.43 (mean = 0.36). Differences between minimum and maximum overlap between both sample types were small in all years. Pellets contained almost exclusively large otoliths (widths 3.0-8.5 mm), whereas excrement contained two fractions: large sized ones, identical to those present in the pellets and smaller-sized ones (0.5-3.0 mm) originating from feces. It is hypothesized that large otoliths cannot pass the intestinal tracts of the birds and are therefore regurgitated. Differences in prey species composition in pellets and excrement could potentially be explained by a combination of seasonal changes in availability of prey species and size of otoliths. Neither pellets nor feces alone give an unbiased picture of the diet of African Royal Terns.
We analyze recently-collected feather tissues from two species of seabirds, the sooty tern (Onychoprion fuscatus) and brown noddy (Anous stolidus), in three ocean regions (North Atlantic, North Pacific, South Pacific) with different human impacts. The species are similar morphologically and are similar in the trophic levels from which they feed within each location. In contrast, we detect reliable differences in trophic position amongst the regions. Trophic position appears to decline as the intensity of commercial fishing increases, and is at its lowest in the Caribbean. The spatial gradient in trophic position we document in these regions exceeds those detected over specimens from the last 130 years in the Hawaiian Islands. Modeling suggests that climate velocity and human impacts on fish populations strongly align with these differences.
Partitioning of resources by competing species of seabirds may increase during periods of food shortages and elevated energy demands. Here, we examined whether food resource partitioning (differential use of foraging habitat or the consumption of different prey species) between common murres (COMU, Uria aalge) and thick-billed murres (TBMU, U. lomvia) breeding on the same colony in the Bering Sea increases with a predictable increase in energy demands between the incubation and chick-rearing stages of reproduction. We assessed the seasonal dynamics of food availability via corticosterone (CORT) levels and examined adult diet (via stable isotope analysis of nitrogen and carbon, SI) and chick diets (based on nest observations). We compared chick provisioning patterns and examined the characteristics of parental foraging habitat via deployment of bird-borne temperature-depth recorders. We found that CORT levels remained low and similar between the species and reproductive stages, reflecting relatively stable and favorable foraging conditions for both murre species during the study period. Comparisons of SI between murres and their potential prey indicated that diets were similar between the species during incubation and diverged during chick-rearing. Chick-rearing common and thick-billed murres also used different foraging habitats, as reflected in travel distances to foraging areas and sea surface temperature distributions of their foraging dives. TBMUs performed shorter foraging trips, deeper dives and delivered squid to their chicks, while COMUs foraged farther from the colony, performed shallower dives, and delivered fish species to their chicks. These results suggest that food resource partitioning between murre species increased during chick-rearing under favorable foraging conditions. Whether the dietary segregation reflected species-specific differences in adults' foraging efficiency, differences in chicks' dietary requirements, or was a way of reducing competition remains unknown. Regardless of the causal mechanism(s), food resource partitioning might ameliorate interspecific competition between sympatrically breeding birds during periods of increased energy demands.
ABSTRACT We monitored the diet of the Heermann’s gull (Lavus heermannz] and the elegant tern (Sterna elegans) between 1983 and 1992 and compared,the proportion,of each fish species in the diet, through , low, marginally positive correlation was found between,the proportion of mackerel in the diet and the proportion of mackerel landed. Dietary studies of these seabirds pro-
Assessments of species vulnerability to climate change should increase the effectiveness of interventions in the current decline in biodiversity. Species vulnerability to climate change is a consequence of their sensitivity and adaptive capacity, in combination with their exposure to climate change. We apply a vulnerability assessment framework to 243 bird species inhabiting the tropical savannas of northern Australia. We build on previous vulnerability studies by including detailed data for variables relating to species sensitivity to change (relative abundance, clutch size, sensitivity to fire and distribution area), species adaptive capacity (movement behaviour and dietary breadth) and proportional changes predicted for their geographic range (i.e. exposure to climate change). These are integrated to provide a ranking of vulnerability. Our analysis found that birds of Australian tropical savannas cluster together with high sensitivity, with a few wide-ranging increasing species with very low sensitivity. Australian tropical savanna birds have a range of adaptive capacities, and the impact of climate change on these species is predicted to be substantial. Two already endangered species are among the most vulnerable. Species largely restricted to Cape York Peninsula (a geographically distinct region) had the greatest overall vulnerability; these species were, in general, sensitive due to small distributions, sensitivity to fire frequency and had a lower capacity for dispersal. It will be important for the future of Australian tropical savanna birds to mitigate ecological threats and maintain extensive areas of suitable habitat to facilitate species dispersal.
Parallel studies of nesting colonies in Mexico and the United States show that Elegant Terns (Thalasseus elegans) have expanded from the Gulf of California Midriff Island Region into Southern California, but the expansion fluctuates from year to year. A strong inverse relationship between nesting pairs in three Southern California nesting areas [San Diego saltworks, Bolsa Chica Ecological Reserve, and Los Angeles Harbor (1991 to 2014)] and Isla Rasa in the Midriff (1980 to 2014) shows that terns migrate northward when confronting warm oceanographic anomalies (>1.0°C), which may decrease fish availability and hamper nesting success. Migration pulses are triggered by sea surface temperature anomalies localized in the Midriff and, secondarily, by reductions in the sardine population as a result of intensive fishing. This behavior is new; before year 2000, the terns stayed in the Midriff even when oceanographic conditions were adverse. Our results show that terns are responding dynamically to rapidly changing oceanographic conditions and fish availability by migrating 600 km northwest in search of more productive waters.
Considerable progress has been made in the development of statistical tools to quantify trophic relationships using stable isotope ratios, including tools that address size and overlap of isotopic niches. We build upon recent progress and propose a new probabilistic method for determining niche region and pairwise niche overlap that can be extended beyond two dimensions, provides directional estimates of niche overlap, accounts for species-specific distributions in niche space, and, unlike geometric methods, produces consistent and unique bivariate projections of multivariate data. We define the niche region (N R) as a given 95% (or user-defined a) probability region in multivariate space. Overlap is calculated as the probability that an individual from species A is found in the N R of species B. Uncertainty is accounted for in a Bayesian framework, and is the only aspect of the methodology that depends on sample size. Application is illustrated with three-dimensional stable isotope data, but practitioners could use any continuous indicator of ecological niche in any number of dimensions. We suggest that this represents an advance in our ability to quantify and compare ecological niches in a way that is more consistent with Hutchinson's concept of an ''n-dimensional hypervolume.''
Seabirds occupy a range of trophic levels in marine foodwebs, and as top predators, they potentially may compete with large fish for prey. Here, we examine trophic segregation among seabird species breeding on Sa˜o Pedro and Sa˜o Paulo Archipelago (SPSPA), in the tropical Atlantic Ocean, as well as the trophic relationships between the seabirds and pelagic fish. Weused stable isotopes (d15N,d13C) combined with a conventional dietary nalysis of brownbooby (Sula leucogaster), brownnoddy (Anous stolidus), and black noddy (Anous minutus) and also analysed stable isotopes in the muscles of tropical two-wing flying fish (Exocoetus volitans), blackfin tuna (Thunnus atlanticus), and common dolphinfish (Coryphaena hippurus).
Brownboobies shared a similar trophic level with large predatory fish probably due to their intense consumption of flying fish, but seabirds are most likely not competing with blackfin tuna and common dolphinfish, despite relying on the same prey. Food resources seem to be abundant around SPSPA, minimizing competition. Despite relying mostly on two-wing flying fish, the three seabirds studied had non-overlapping trophic niches during the breeding period, with segregation apparently occurring by prey size. Sex-related trophic differences were not found, but age-related trophic segregation was observed in brown boobies and brown noddies. Overall, although seabirds and marine pelagic fish rely on the same
prey species, the overlap in their trophic niches was limited, most likely due to an overabundant food resource, i.e. flying fish, a common prey of large predators in most pelagic tropical oceans.
Isla Rasa, a small (0.68 km2) but globally important seabird island in the Gulf of California, has a fl ora of only 14 vascular plant species found in three vegetation zones. Signifi cant physical alteration of the island's surface and the introduction of non-native rodents, eradicated in 1995, add restoration ecology to the remarkable biology of the island. Over a century of botanical collections and observations record a consistently depauperate fl ora, best understood in the context of extreme aridity, isolation, and elevated levels of nitrogen and phosphorus from bird guano. The shaping factor of guano on the dearth of floristic diversity illustrates close connection between marine and terrestrial ecosystems in the Gulf of California. El Niño events that trigger collapses in marine productivity and crashes in seabird reproduction bring above-average winter rainfall pulses and rapidplantgrowth.Two new island records are reported (Rhizophora mangle and Viscainoa geniculata). Matched photographs show signifi cant increase in the cholla cactus (Cylindropuntia fulgida) since 1971. Baseline knowledge of seabird breeding ecology and theflora of the island position Isla Rasa as a preeminent locality to study and monitor the ecological dynamics of a desert island in the context of restoration ecology and global environmental change.
We wish to use stable-isotope analysis of flight feathers to understand the feeding behavior of pelagic seabirds, such as the Hawaiian Petrel (Pterodroma sandwichensis) and Newell's Shearwater (Puffinus auricularis newelli). Analysis of remiges is particularly informative because the sequence and timing of remex molt are often known. The initial step, reported here, is to obtain accurate isotope values from whole remiges by means of a minimally invasive protocol appropriate for live birds or museum specimens. The high variability observed in δ13C and δ15N values within a feather precludes the use of a small section of vane. We found the average range within 42 Hawaiian Petrel remiges to be 1.3‰ for both δ13C and δ15N and that within 10 Newell's Shearwater remiges to be 1.3‰ and 0.7‰ for δ13C and δ15N, respectively. The δ13C of all 52 feathers increased from tip to base, and the majority of Hawaiian Petrel feathers showed an analogous trend in δ15N. Although the average range of δD in 21 Hawaiian Petrel remiges was 11‰, we found no longitudinal trend. We discuss influences of trophic level, foraging location, metabolism, and pigmentation on isotope values and compare three methods of obtaining isotope averages of whole feathers. Our novel barb-sampling protocol requires only 1.0 mg of feather and minimal preparation time. Because it leaves the feather nearly intact, this protocol will likely facilitate obtaining isotope values from remiges of live birds and museum specimens. As a consequence, it will help expand the understanding of historical trends in foraging behavior.
En el futuro, deseamos usar análisis de isótopos estables de plumas de vuelo para entender el comportamiento alimenticio de aves marinas pelágicas como Pterodroma sandwichensis y Puffinus auricularis newelli. El análisis de las rémiges es particularmente informativo porque generalmente se conoce la secuencia y el momento en que ocurre la muda. Nuestro primer paso, que reportamos aquí, es obtener valores de isótopos precisos a partir de plumas rémiges completas utilizando un protocolo poco invasivo apropiado para aves vivas o especímenes de museo. La alta variabilidad de los valores de δ13C y δ15N observada en las plumas no permite el uso de una pequeña sección de la pluma. Encontramos que el rango promedio observado en 42 plumas rémiges de P. sandwichensis es de 1.3‰ tanto para δ13C como para δ15N, mientras que en 10 plumas rémiges de P. auricularis newelli es de 1.3‰ y 0.7‰ para δ13C y δ15N, respectivamente. El δ13C para 52 plumas estudiadas aumentó desde la punta hacia la base, y la mayoría de las plumas de P. sandwichensis mostraron una tendencia similar para δ15N. Aunque el rango promedio en δD para 21 rémiges de P. sandwichensis fue de 11‰, no encontramos una tendencia longitudinal. Discutimos la influencia del nivel trófico, de la localidad de forrajeo, del metabolismo y de la pigmentación sobre los valores de isótopos y comparamos tres métodos para obtener promedios de valores de isótopos para la pluma entera. Nuestro nuevo protocolo de muestreo de las barbas sólo requiere de 1.0 mg de pluma y un tiempo de preparación mínimo. Debido a que este protocolo deja las plumas casi intactas, éste podrá facilitar la obtención de valores de isótopos a partir de plumas rémiges de aves vivas y de especímenes de museo. Consecuentemente, ayudará a entender las tendencias históricas en el comportamiento de forrajeo.
The big (Raja binoculata), California (R. inornata), longnose (R. rhina), and sandpaper (Bathyraja kincaidii), skates are commonly found on soft-bottom regions of the central California continental shelf and upper slope. The feeding ecology of this assemblage was compared to evaluate the degree of trophic separation among species, based on the results of previous species-specific diet studies. Specimens were collected from fishery independent trawl surveys conducted during September 2002–March 2003 at depths of 9–536 m. Using single and compound measures, diet composition of small (£60 cm TL) and large (>60 cm TL) individuals were compared within continental shelf (£200 m) and slope (>200 m) regions using traditional, multivariate, and novel techniques. Diet compositions within size classes were similar in both regions. Diet compositions between size classes generally differed, however, with fishes more important and crustaceans (especially shrimps and euphausiids) less important in the diets of large individuals. Crabs contributed substantially to skate diet compositions on the shelf, but were uncommon prey items at deeper depths, probably because of their relative scarcity in slope waters. Conversely, cephalopods were common prey items at slope depths, but were rarely ingested at shelf depths. The studied skate assemblage appears to consist primarily of gener-alist crustacean and fish predators that exhibit high dietary overlap at similar sizes. It is possible that resource competition among skates and groundfish species has been reduced because of considerable recent declines in the biomass of upper trophic level groundfishes. Skates may therefore play important roles in contemporary benthic food web dynamics off central California.
The measurement of stable isotopes of nitrogen and carbon in tissues of marine consumers can provide trophic-level information that is a time-integrated approximation of assimilated diet. By measuring deltaN-15 and delta-C-13 concentrations in liver, muscle and bone collagen of 7 species of seabirds inhabiting the Barrow Strait - Lancaster Sound region, Northwest Territories, Canada, estimates of short-, intermediate-, and long-term trophic level (TL), respectively, were established. Trophic estimates were based on a simple model incorporating diet-tissue fractionation established through the captive rearing of piscivorous birds on known isotopic diets. DeltaC-13 was not a useful indicator of trophic position in this marine food web but may indicate inshore or benthic vs offshore or pelagic feeding preferences in seabirds. DeltaN-15 was a useful indicator and ranged from 12.7 +/- 0.3 parts per thousand (TL 3.3) in liver of dovekies Alle alle to 19.1 +/- 0.8 parts per thousand in the bone collagen of glaucous gulls Larus hyperboreus. Consistent with their known diets, dovekies and common eiders Somateria mollissima showed lowest trophic positions for all tissues; black-legged kittiwakes Rissa tridactyla, thick-billed murres Uria lomvia, northern fulmars Fulmarus glacialis and black guillemots Cepphus grylle showed intermediate trophic positions (TL 3.9 to 4.3) and glaucous gulls showed highest trophic position (TL 4.5 to 4.8). In general, seabirds increased their consumption of lower-trophic-level invertebrates during the breeding season over their lifetime averages but fed their chicks diets consisting of high proportions of fish. Isotopic estimates suggest greater dependence on lower-trophic-level prey by murres, fulmars, kittiwakes and guillemots than previously established through conventional dietary techniques.
Although conceptually robust, it has proven difficult to find practical measures of niche width that are simple to obtain, yet provide an adequate descriptor of the ecological position of the population examined.
Trophic niche has proven more tractable than other niche dimensions. However, indices used as a proxy for trophic niche width often suffer from the following difficulties. Such indices rarely lie along a single scale making comparisons between populations or species difficult; have difficulty in combining dietary prey diversity and evenness in an ecologically meaningful way; and fail to integrate diet over ecological time‐scales thus usually only comprise single snapshots of niche width.
We propose an alternative novel method for the comparison of trophic niche width: the use of variance of tissue stable isotope ratios, especially those of nitrogen and carbon.
This approach is a potentially powerful method of measuring trophic niche width, particularly if combined with conventional approaches, because: it provides a single measure on a continuous axis that is common to all species; it integrates information on only assimilated prey over time; the integration period changes with choice of tissue sampled; and data production is theoretically fast and testing among populations simple.
Empirical studies are now required to test the benefits of using isotopic variance as a measure of niche width, and in doing so help refine this approach.
During 1980 and 1981, I studied nest-site selection and reproductive success in Heermann's Gull (Larus heermanni) on Isla Rasa in the Gulf of California, Mexico. About 95% of the world population of this species nests on Isla Rasa. In 1980, about 130,240 nests were present within an area of 0.8 km 2. In the Midriff Island Region (which includes Isla Rasa), oceanic productivity is high even during E1 Nifio events because of tidal upwelling. My results indicate that the Heermann's Gull is characterized by high nesting density (up to 110 nests per 100 m 2) and high nesting synchrony. Massive synchronized communal flights to nesting areas occurred every evening during several weeks prior to egg laying. Nesting density in valleys (71 per 100 m ) was higher than on rocky hills (9.5 per 100 m2). Clutch size was higher in valleys in 1981 than in 1980 and was lowest on rocky substrates on hills. Hatch- ing success was independent of clutch size but varied with substrate and year. Fledging suc- cess among broods was independent of hatching order. Fledging success and reproductive success (i.e. the number of chicks fledged/number of eggs laid) were higher in valleys in 1981 and were lowest on rocky hills in 1980, and reproductive success was significantly high- er for three-egg clutches than for one-egg clutches. The observed patterns of reproductive performance may be correlated with annual variation in food abundance and with exposure to predators in different nesting habitats and among different nesting densities. Received 2 December 1996, accepted 30 October 1998.
Small pelagic fisheries show wide fluctuations, generally attributed to oceanographic anomalies. Most data on these fisheries come from landings, often reporting sustained catches-per-unit-effort (CPUEs) until a decline occurs. Fishery-independent data are important as management tools. In this study we show that the proportions of Pacific Sardine and Northern Anchovy in the diet of three seabird species (California Brown Pelicans, Heermann's Gulls, and Elegant Terns) nesting in spring in the Gulf of California show significant relationships with CPUEs during the following season in gulls and terns, or during the same season in pelicans. As sardine availability for seabirds declines, CPUEs remain high until the fishery falls, one or two seasons later. A declining proportion of sardines in the seabirds' diet, combined with the status of the Pacific warm-phase anomaly (El Niño), give a reliable forecast of diminishing CPUEs and signals the need to reduce fishing efforts in the ensuing season.
Analysis of adult anchovy samples collected in the Catalan Sea during the spawning pen-ods in 1994 and 1995 showed that feeding activity by this engraulid was mainly diurnal, taking place particularly after noon. Copepods and other small prey were primary dietary items, the former pre-sumably being concentrated at the level of the deep chlorophyll maximum during daytime. At night only sporadic consumption of large prey items, mostly decapod larvae and mysids, was recorded. The feeding pattern descnbed results in spatio-temporal segregation of feeding and spawning activity and may account for the absence of cannibalism on anchovy eggs. An assessment of the impact of con-sumption by the anchovy population in the Catalan Sea on zooplankton production underscored the major role played by this specles in channelling energy of the pelaglc food webs to higher trophic levels KEY WORDS: Engraulis encrasicolus . Trophic ecology Pelagic ecosystem Energy flows . N W Mediterranean INTRODUCTION
Determining the importance of prey taxa in the diets of predacious species is a frequent objective in fisheries research. Various indices of prey importance are in common use, and all give different results because of their emphasis on different aspects of fish diets. We explored these differences by empirically comparing four well-known indices—percent weight (%W), percent occurrence (%O), percent number (%N), and percent index of relative importance (%IRI)—as well as a modified %IRI (%MIRI), as applied to an extensive data set on the diets of six fish species in Spirit Lake, Iowa. Correlations among all indices were positive but were weaker among component indices (%W, %O, and %N) than between the two compound indices (%IRI and %MIRI); correlations among component indices were also weaker than correlations of compound with component indices. Correlation strength of %MIRI with the three component indices varied greatly (%N < %O < %W), whereas the correlation strength of %IRI with component indices was similar. Importance values based on %W, %MIRI, and %N depend more on prey size than those based on %IRI and %O. The %W and %MIRI emphasized the importance of large prey taxa, whereas %N emphasized small prey in diets; %IRI and %O were similarly unbiased with respect to prey size. The %O yielded substantially higher importance values than all other indices. Thus, for use as a general index of dietary importance, we believe %IRI provides the optimal balancing of frequency of occurrence, numerical abundance, and abundance by weight of taxa in fish diets.
Understanding and quantifying upwelling is of great importance for marine resource management. Direct measurement of this process is extremely difficult and observed time-series do not exist. However, proxies are commonly derived from different data; most commonly wind-derived. A local wind-derived coastal upwelling index (CUI) is reported for the period 1970-1996 and is considered representative for the eastern central Gulf of California, an important fishing area where no proxies exist. The index is well related to pigment concentration distribution, surface water temperature, and population dynamics of important fish resources over the seasonal time-scale. There is a biological response to ENSO activity not reflected by this index, indicating that improvement of biological enrichment forecasting also requires water column structure input. A clearly increasing seasonal amplitude signal is detected in the coastal upwelling index and sea surface temperature since the mid 1970s. Understanding the nature of these long-term trends, the incorporation of remote tropical ocean signals into the enrichment proxy, the dynamics of atmosphere and ocean, and the biological responses are major challenges to the proper management of fish resources in the Gulf of California.
The Gulf of California (fig. 23.1; described in detail in chapter 9) is an elongated, semi-enclosed sea separating the Baja California peninsula from north western mainland Mexico. Its southern boundary is somewhat subjective, but based on geological and bathymetric parameters, it should be extended south to a line joining Cabo San Lucas at the tip of the Baja California peninsula to Cabo Corrientes, Jalisco. Following this view, the south-ern Gulf corresponds here to a triangular area ly-ing between the coast of the Mexican mainland from Mazatlán, Sinaloa south to Cabo Corrientes; the Gulf’s southern boundary, as defined above; and another imaginary line joining Cabo San Lucas to Mazatlán (fig. 23.1).
This updated and expanded A New Island Biogeography of the Sea of Cortés, first published nearly 20 years ago, integrates new and broader studies encompassing more taxa and more complete island coverage. The present synthesis provides a basis for further research and exploration in upcoming years of the biologically fascinating Sea of Cortés region. The Gulf region is increasingly being exploited, for its natural resources by way of marine fisheries, and for its stunning natural beauty by way of a burgeoning tourism industry. Further, the region's human population is increasing apace. It is appropriate, therefore, that this volume discusses these evolving circumstances, and the efforts of the Mexican government to regulate and manage them. The new Biogeography includes a section on the conservation issues in the Sea of Cortés, past accomplishments and conservation needs as yet outstanding. This book should be of strong interest to conservation biologists, ecologists, and evolutionary biologists more generally.
Effects of variable energy intake on chick growth, organ development, and gut-passage time in seabirds were investigated by raising Elegant Tern (Sterna elegans) chicks in the laboratory on three diets involving a major prey species, northern anchovy (Engraulis mordax), and a minor prey species, topsmelt (Atherinops affinis). Anchovies are ∼25% higher in energy content than topsmelt and represent a more abundant but highly fluctuating prey resource. Chicks were raised from age 15 to age 35 days posthatch on one of three diets (n = 8): (1) high-energy anchovy at the average daily intake rate, (2) low-energy topsmelt at the same intake rate, and (3) high-energy topsmelt (i.e. at a biomass equal in energy content to the anchovy diet). Chicks raised on the high-energy anchovy diet had a significantly faster gain of body mass and wing length. All three groups, however, had attained a statistically similar wing length at 35 days posthatch and did not differ in lean dry mass or pectoral muscle mass, indicating that energy was allocated preferentially toward wing development and flight capacity. Most digestive organs were heavier and gut-passage rates faster for chicks fed topsmelt. Bill dimensions and tarsus length showed little difference among chicks on the three diets. Lipid content of chicks that were fed anchovy was twice that of those fed low-energy topsmelt and one-third higher than that of chicks on the high-energy topsmelt diet. Our results indicate that Elegant Terns have evolved certain capacities for growth and digestion in response to variable prey quality that may increase the probability of chick survival. Nevertheless, if Elegant Tern chicks were switched to a lower quality, primarily topsmelt diet, they would be underweight at fledging with heavier digestive organs, lower lipid reserves, and faster gut-passage times, which would place added provisioning demands on the parents. Under that scenario, the prospects for survival and long-term reproductive success might diminish for such an Elegant Tern colony.
Small and medium pelagic fishes play a central role in marine food webs by transferring energy from plankton to top predators. In this study, direct calorimetry was used to analyze the energy density of seven pelagic species collected over four seasons from the western Mediterranean Sea: anchovy Engraulis encrasicolus, sardine Sardina pilchardus, round sardinella Sardinella aurita, horse mackerels Trachurus trachurus and T. mediterraneus, and mackerels Scomber scombrus and S. colias. Inter-specific differences in energy density were linked to spawning period, energy allocation strategies for reproduction and growth, and feeding ecologies. Energy density of each species varied over time, with the exception of S. colias, likely due to its high energetic requirements related to migration throughout the year. In general, higher energy density was observed in spring for all species, regardless of their breeding strategy, probably as a consequence of the late-winter phytoplankton bloom. These results provide new insights into the temporal availability of energy in the pelagic ecosystem of the Mediterranean Sea, which are pivotal for understanding how the population dynamics of small and medium pelagic fishes and their predators may respond to environmental changes and fishing impacts. In addition, the differences found in energy density between species highlighted the importance of using species specific energy-values in ecosystem assessment tools such as bioenergetic and food web models.
Lipid decomposition studies in frozen fish have led to the development of a simple and rapid method for the extraction and purification of lipids from biological materials. The entire procedure can be carried out in approximately 10 minutes; it is efficient, reproducible, and free from deleterious manipulations. The wet tissue is homogenized with a mixture of chloroform and methanol in such proportions that a miscible system is formed with the water in the tissue. Dilution with chloroform and water separates the homogenate into two layers, the chloroform layer containing all the lipids and the methanolic layer containing all the non-lipids. A purified lipid extract is obtained merely by isolating the chloroform layer. The method has been applied to fish muscle and may easily be adapted to use with other tissues.
The influence of diet on the distribution of carbon and nitrogen isotopes in feathers of 11 species of birds on a constant diet was studied. The standard deviation in δ 13C and δ 15N in naturally obtained foods like fish during 6 yr was c1‰. Feathers showed an enrichment relative to food in both 13C and 15N, although its extent varied among different bird species. Isotope ratios for 13C and 15N from diet to feathers converged. Neither enrichment showed an age dependency among adult birds, and neither depended on the nitrogen content in the bird's diet. These results show that the stable isotope approach can be applied not only to these 11 species but to birds whose isotope enrichment factors are so far unknown. -from Authors
The highly productive Gulf of California exhibits high biodiversity and an abundance of small pelagic, schooling fishes, important both to the ecosystem and to fisheries. These fishes show wide fluctuations in abundance due to oceanographic–atmospheric phenomena such as El Niño-Southern Oscillation, constituting a problem for effective fisheries management. In this work we propose several parameters in the diet of three seabird species as useful predictors of the eventual commercial catch per unit effort (CPUE) of Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax) in the Midriff Island Region of the Gulf of California. We found that seabirds are sensitive to fluctuations in the abundances of these fishes, their proportions in the diet becoming suitable predictors of future commercial CPUE. The precision of our prediction is high because the birds feed on pre-recruit fish, and the time between the sampling of the diets and the initiation of the fishing season is short, minimizing stochastic effects on recruitment and subsequent abundance due to environmental fluctuations.
It is suggested that Pielou's method (1966a) of estimating the diversity of a plant community from a series of randomly placed quadrats, can be applied to measuring dietary diversity of predatory populations. Treating each stomach as a sampling unit, an equivalent of a quadrat, it is possible to calculate for the whole predatory population the trophic diversity and the average of the trophic diversity with its standard error, and obtain a satisfactory representation of the prey species. The method proposed here may serve to produce important descriptive parameters in comparative ecological studies among sympatric species.
Approximately 32 species of marine mammals of the Gulf of California (GC) share habitat and resources in this ecosystem. Unusually high δ15N values at the base of the food web in the GC permeate up all trophic levels, distinguishing the isotopic signature of the GC from other ecosystems in the Eastern North Pacific. Values of δ13C and δ15N were acquired from particulate organic matter (POM) of surface sediment, zooplankton, mollusks, crustaceans, fish, and marine mammals in the GC to construct a general trophic structure and were complemented with additional isotope data from the literature. Aims were to: estimate marine mammal trophic levels and habitat preferences; distinguish between GC residents and visitors; and assess potential trophic overlap among the most common and abundant cetacean species. Trophic level 1 (TL1), represented by POM, showed average (± SD) δ13C and δ15N values of -21.4 ± 0.5% and 9.6 ± 0.7%, bulk zooplankton (TL2) showed -18.8 ± 0.7% and 11.8 ± 1.4%, while TL3, represented by baleen whales, some fish, squid, and seabirds, showed δ13C values between -13 and -16% and δ15N values between 16.5 and 20%. Marine mammals occupied the entire coastal and pelagic isotope gradient (δ13C values from -12 to -18.1%), with most of the species at TL4 (δ 15N values from 17 to 23%), whereas Orcinus orca occupied TL5 (25.8% maximum value). The odontocetes Mesoplodon peruvianus, Globicephala macrorhynchus, Grampus griseus, Kogia sima, Delphinus delphis, D. capensis, and Tursiops truncatus had both GC resident and visitor representatives, and the latter 3 species showed a significant degree of trophic and habitat overlap.
Stable isotopes are increasingly used in animal ecology, but little attention has been paid to the underlying physiological processes accounting for changes in 15 N/ 14 N and 13 C/ 12 C ratios, for example, the influence of protein balance on 15 N values. We in-vestigated a ''professional'' faster, the King Penguin (Aptenodytes patagonicus), to test the effect of long-term food deprivation on the isotopic signature of tissues that can be non-destructively sampled, i.e., blood and feathers. Fasting for 25 days induced a tissue 15 N enrichment, thus leading to a moderate increase in the apparent trophic levels of penguins. As expected, 15 N enrichment was higher in tissues with high protein turnover rates (e.g., plasma, 0.70‰) than in those with low turnover rates (e.g., blood cells, 0.24‰). Fasting decreased the 13 C value of plasma, which was due to an increase in its lipid content, as indicated by a concomitant rise in plasma C/N ratio. Finally, food deprivation induced a 15 N enrichment in keratin (1.68‰), as indicated by the lower nitrogen signature for portions of new feathers that were synthesized at sea than for those parts grown on land, thus illustrating the different pathways for resource allocation (dietary vs. endogenous reserves) in molting birds. The study also emphasized the usefulness of collecting whole blood (or blood cells) in the field to overcome both the fasting and lipid effects observed in plasma.
