Nicolás Suárez

Foraging Patterns of Breeding Olrog’s Gull at Golfo San Jorge, Argentina

Abstract The spatial distribution and use of feeding areas by Olrog’s Gull (Larus atlanticus) breeding within a 34 km2 inlet at Golfo San Jorge, Argentina was investigated. Radio-transmitters were deployed on eight breeding adult birds during the late incubation period in 2001. Feeding areas were identified by means of radio-telemetry on the basis of 93 bird locations during a total of 89 foraging trips, and by direct observation of instrumented birds on 23 occasions. Both telemetry and visual data indicated that Olrog’s Gull fed almost exclusively within the inlet. In all, 85% of mapped locations (N = 93) were within 3 km of the colony, near the mouth of the inlet. The birds were consistent in the use of one particular area, and most of the tracked birds foraged mainly in one or two areas. Mean number of trips per day was 4.1 ± 0.99 (range = 1-9) and mean trip duration was 109 ± 31.2 min (range = 15-645 minutes). Both variables differed significantly among individuals. Gulls foraged at all states of the tide, although all eight individuals were more likely to forage during mid and low tides than during high tides. The small foraging range of Olrog’s Gull is defined by the spatial extension of intertidal habitats with crabs, being their main prey during the breeding season. Our results confirm the relatively high ecological specialization of Olrog’s Gulls shown in previous diet studies and their dependence on intertidal areas with grapsid crabs.

Temporal Changes in Diet and Prey Selection in the Threatened Olrog's Gull Larus atlanticus Breeding in Southern Buenos Aires, Argentina

Summary. Temporal changes in diet and prey selection in the threatened Olrogs gull Larus atlanticus breeding in southern Buenos Aires, Argentina. We examined temporal changes in diet composition and prey selection in the threatened Olrogs gull Larus atlanticus breeding at Bahía San Blas, Argentina. Diet was assessed through pellet analysis (N = 360) and chick stomach samples (N = 120) during 2006 and 2007. Prey availability was sampled in 2,084 1m2 quadrats distributed throughout Olrogs gull potential feeding areas. Gulls fed almost exclusively on three crabs: Neohelice granulata, Cyrtograpsus altimanus and Cyrtograpsus angulatus. Diet composition was similar between years. The relative importance (%IRI) of different crab species, assessed through pellet analysis, varied across the breeding cycle. During incubation, N. granulata predominated (> 90%) while C. altimanus comprised less than 2% of the crabs taken. This relative contribution was reversed during the young chick stage (< 10% v. > 60%, respectively) and was 40–50% for both species during the old chick stage. Stomach sample analysis provided a similar pattern of %IRI values for the chick stages. Sizes of available crabs differed significantly between species, C. angulatus and N. granulata both being similar-sized but larger than C. altimanus. Gulls selected N. granulata during incubation and largely C. altimanus during the chick stages. Our study suggests that Olrogs gull depends on three crab species, confirming its specialised feeding ecology during the breeding season. Their relative consumption, however, can change temporally, probably in response to restrictions imposed by the different requirements of adults and chicks.

Kelp Gulls (Larus dominicanus) killed and injured by discarded monofilament lines at a marine recreational fishery in northern Patagonia

Among marine debris, monofilament fishing lines often result in negative impacts on marine organisms. We characterized marine debris and incidence of lost and discarded monofilament lines along beaches used by recreational fishers, and report the impact of lines on Kelp Gulls (Larus dominicanus) at the Bahía San Blas protected area, site of one of the main shore-based recreational fisheries of the southwestern Atlantic. Over 55% of the marine debris recorded originated from recreational fishing activities. Balls of tangled monofilament lines were found at a rate of 40.5 items per km. A total of 27 adult Kelp Gulls were found entangled with monofilament. All individuals were tangled to vegetation within colony boundaries. Four of the gulls had a monofilament line protruding from the bill, showing that they may be also killed when trying to obtain bait. Our results indicate that lost or discarded monofilament lines in the Bahía San Blas recreational fishing area result in undesired impacts on coastal wildlife.

Differential Use of Food Resources Between the Kelp Gull Larus dominicanus and the Threatened Olrog's Gull L. atlanticus

Summary. We assessed the potential overlap in diet composition of the kelp gull Larus dominicanus and the threatened Olrogs gull L. atlanticus breeding syntopically at Bahía San Blas, Argentina, during two breeding seasons (2006 and 2007). Diet was studied using regurgitated pellets (180 per species and year) and chick stomach samples obtained through the water offloading technique (60 per species only in 2007). Kelp gulls fed on at least 18 prey types. Fish was the most important diet component (73.3–85%, depending on breeding stage and year), mainly stripped weakfish Cynoscion guatucupa (63.3–75%). Crustaceans were the main prey encountered in Olrogs gull diet, particularly the crabs Neohelice granulata and Cyrtograpsus altimanus, with frequencies of occurrence of over 98% at all breeding stages. Fish in Olrogs gulls diet were only recorded in the old chick stage and in less than 3.3% of samples, while the occurrence of crustaceans in kelp gull diet throughout the study period was never greater than 13%. The specialised crab-based diet of Olrogs gulls contrasts sharply with the generalist and opportunistic diet of the kelp gull. The characteristics of prey remains and the large size of stripped weakfish found in kelp gull diet samples suggest they were obtained from coastal sport and artisanal fisheries. Future studies should monitor the consumption of fish waste by kelp gulls as a function of changes in fishing effort, and assess the dietary overlap between both gull species in coastal sectors where breeding birds have no access to this human-derived food source.

Energy Density Empirical Predictor Models for Three Coastal Crab Species in the Southwestern Atlantic Ocean

Energy density was measured for key intertidal and subtidal crab species in the Southwestern Atlantic Ocean. Empirical models were developed to link energy density to water content (expressed as dry weight) and the fit to data was compared between different taxonomic and gender aggregations. Predictive power of models with different levels of aggregation and the effect of using a model of a surrogate species or group were also evaluated. Neohelice granulata (Dana, 1851) (3728 J/g wet weight) contained the highest energy density followed by Cyrtograpsus angulatus Rathbun, 1914 (3334 J/g wet weight), and C. altimanus Dana, 1851 (3042 J/g wet weight). Females (3645 J/g wet weight) showed higher energy density compared to males (3074 J/g wet weight). Dry weight of whole individuals provided good energy density predictions, with low predictive error when using species or same genus models (6.4-9.6% for the median error). Predictive power improved when more specific levels of taxonomic aggregation were employed. Interestingly, the two congeneric Cyrtograpsus did not differ in their energy density predictor model, suggesting a unique model could be used for this genus. This paper provides valuable inputs for bioenergetic modeling in coastal ecosystems in the Southwestern Atlantic.

Nest site selection of the Kelp gull (Larus dominicanus) in the Beagle Channel, Tierra del Fuego, Argentina

Nest site selection of Kelp gulls Larus dominicanus breeding in Conejo and Bridges islands, Tierra del Fuego, Argentina, was assessed between 14 and 30 December 2005 to describe variability in nest site features and determine variables affecting nest choice. Fourteen microhabitat variables were quantified at 40 nest sites and at 40 random points in each island during the late incubation stage. Nests at the two colonies were placed on different kinds of substrates across areas with varying degrees of cover provided either by rocks or vegetation. Despite the variability observed in nest site features, rock cover and rocky substrates were the main factors determining nest site selection. At Conejo Island, nest sites presented more rock cover, less percentage of vegetation cover and of vegetation debris substrate, and were placed further from the nearest vegetation in comparison to random points. Similarly, gulls at Bridges Island selected nest sites with more rock cover which were placed mainly on rocky and vegetation substrates. This contrasts with results previously obtained in Argentina, which indicated that vegetation is a key factor influencing Kelp gull nest site selection. Over 80% of the nests at both study colonies were placed on the northern slopes of the islands, relatively more protected from the strong prevailing southwest winds, and nests at both colonies tended to be more protected on their southern side either by rocks or vegetation. Results obtained at Tierra del Fuego confirm the plasticity of microhabitat use by Kelp gulls, and their ability to take advantage of nesting sites according to availability and local environmental factors.

Effect of Feeding Technique and Prey Characteristics on the Feeding Rate of Olrog's Gulls (Larus atlanticus)

Abstract The Olrogs Gull (Larus atlanticus) is a species that feeds almost exclusively on intertidal crabs. The influence of feeding technique, food source, and habitat characteristics on the feeding rate of breeding individuals was examined at Bahía San Blas, Argentina, from September to December 2007. Olrogs Gulls used four feeding techniques, with more prey captured by pecking than by peck-digging, head submergence, or up-ending. Olrogs Gulls fed throughout the tidal cycle using different intertidal zones when they were exposed and covered by water. However, feeding techniques used in exposed intertidal areas (pecking and peck-digging) resulted in significantly higher feeding rates. Feeding rate increased with average prey density and was higher when feeding in structured environments, characterized by the presence of rocks or oysters and high densities of the intertidal crab Cyrtograpsus altimanus. The burrowing crab Neohelice granulata is the main component of the Olrogs Gull diet during most of the breeding cycle. Although average density of N. granulata in feeding patches was lower than that of C. altimanus, body size of the former was significantly larger and energy density of individuals higher, resulting in a higher energy profitability of unstructured habitats. Prey switching toward C. altimanus during the young chick stage observed in previous studies may result from the need to feed chicks with smaller prey.

Kelp Gull (Larus dominicanus) Use of Alternative Feeding Habitats at the Bahía San Blas Protected Area, Argentina

Abstract. Kelp Gull (Larus dominicanus) foraging patterns were studied by deploying GPS loggers on 20 incubating individuals at their colony in the Bahía San Blas protected area, Argentina. Mean number of trips per day was 1.5 ± 0.5, and mean trip duration was 272.6 ± 165.2 min. Mean maximum distance from the colony was 19.6 ± 24.4 km. Incubating Kelp Gulls visited natural and anthropogenic environments. Individuals switched between two or three different habitat types 47% of the time during a given foraging trip. Kelp Gulls showed a differential use of feeding areas, with a significantly higher use of refuse dumps (75%; n = 151 trips) than coastal (47%), terrestrial (10%) and offshore (10%) habitats. In 72% of the recorded trips, Kelp Gulls targeted the dump located in the small town of Bahía San Blas, where waste generated by recreational fishing is regularly disposed. Moreover, most visited shoreline locations were those regularly used by recreational fishers. Despite showing plasticity in foraging habitat use, the local refuse dump and nearby shoreline sites where fish waste is regularly disposed were the main feeding habitats for incubating Kelp Gulls.

Differences between colonies and chick-rearing stages in Imperial Cormorant (Phalacrocorax atriceps) diet composition: implications for trophic studies and monitoring

ABSTRACT We assessed differences in diet composition between Imperial Cormorants (Phalacrocorax atriceps) breeding at Isla Arce and Isla Vernaci Este, Golfo San Jorge, Argentina. Both colonies are located within the Patagonia Austral marine park, and commercial trawl fisheries operate in adjacent waters, thus knowledge of their food requirement is fundamental to assess potential conflicts and monitor interactions. We obtained stomach samples from 63 adult Imperial Cormorants during the chick-rearing period in 2014: 32 at Isla Arce and 31 at Isla Vernaci Este. We recorded 27 and 30 prey taxa in stomach samples at Isla Arce and Isla Vernaci Este, respectively, and at least 21 were common to both islands. Significant differences in diet composition in terms of importance by mass were found between young and old chick stages at Isla Arce but not at Isla Vernaci Este. Rock cod (Patagonotothen spp.) were dominant at Isla Vernaci Este during both chick stages (75.7% and 86.5%, respectively). At Isla Arce, rock cod, Argentine anchovy (Engraulis anchoita), and Argentine hake (Merluccius hubbsi) contributed similarly to their diet (26–33%) during the young chick stage, whereas Argentine anchovy dominated during the old chick stage (76.6%). Diet composition in terms of importance by mass was significantly different between colonies. Imperial Cormorant consumed benthic, demersal, and pelagic prey, confirming plasticity in feeding habits, but their contribution differed depending on the site and chick stage. Our results suggest that diet assessments and monitoring the potential interaction between Imperial Cormorant and fisheries should not be based on information from a single breeding stage or location, but requires the analysis of diet composition throughout the breeding cycle with a representative sampling of the 17 colonies within the marine park.

Persistent organic pollutants in sediments, intertidal crabs, and the threatened Olrog's gull in a northern Patagonia salt marsh, Argentina

Persistent organic pollutants (POPs) are of great concern for the environment. In this study we (a) determine levels and distribution of OCPs, PCBs, and PBDEs in sediments and two crab species (Neohelice granulata and Cyrtograpsus altimanus), (b) assess bioaccumulation in crabs, and (c) explore the occurrence of POPs in the Near Threatened Olrogs gull (Larus atlanticus) chicks and eggs in one of the most important salt marsh environments in the South West Atlantic. Sediments, crabs, and gull chicks and eggs showed POPs presence at low levels; being α-endosulfan, PCB-153, and BDE-47 the most represented compounds. In sediments, pollutant concentrations were lower than those reported in Canadian guidelines for the protection of the aquatic life. POP bioaccumulation was recorded in crabs, suggesting a risk to upper trophic level predators. Further studies are needed to understand the trophic effects of POPs in San Blas bay, particularly on the threatened Olrogs gull.