Subsurface Predators Research Articles

Sailfish generate foraging opportunities for seabirds in multi-species predator aggregations.

While various marine predators form associations, the most commonly studied are those between subsurface predators and seabirds, with gulls, shearwaters or terns frequently co-occurring with dolphins, billfish or tuna. However, the mechanisms underlying these associations remain poorly understood. Three hypotheses have been proposed to explain the prevalence of these associations: (1) subsurface predators herd prey to the surface and make prey accessible to birds, (2) subsurface predators damage prey close to the surface and thereby provide food scraps to birds, and (3) attacks of underwater predators lower the cohesion of prey groups and thereby their collective defences making the prey easier to be captured by birds. Using drone footage, we investigated the interaction between Indo-Pacific sailfish (Istiophorus platypterus) and terns (Onychoprion sp.) preying on schooling fish off the eastern coast of the Malaysian peninsula. Through spatio-temporal analysis of the hunting behaviour of the two predatory species and direct measures of prey cohesion we showed that terns attacked when school cohesion was low, and that this decrease in cohesion was frequently caused by sailfish attacks. Therefore, we propose that sailfish created a by-product benefit for the bird species, lending support to the hypothesis that lowering cohesion can facilitate associations between subsurface predators and seabirds.

An overview of the impacts of fishing on seabirds, including identifying future research directions

Abstract Knowledge of fisheries impacts, past and present, is essential for understanding the ecology and conservation of seabirds, but in a rapidly changing world, knowledge and research directions require updating. In this Introduction and in the articles in this Themed Set “Impacts of fishing on seabirds”, we update our understanding of how fishing impacts seabird communities and identify areas for future research. Despite awareness of the problems and mitigation efforts for >20 years, fisheries still negatively impact seabirds via the effects of bycatch, competition, and discards. Bycatch continues to kill hundreds of thousands of seabirds annually, with negative population-level consequences. Fisheries for forage fish (e.g. anchovy, sandeel, and krill) negatively impact seabirds by competing for the same stocks. Historically, discards supplemented seabird diets, benefitting some species but also increasing bycatch rates and altering seabird community composition. However, declining discard production has led to potentially deleterious diet switches, but reduced bycatch rates. To improve research into these problems, we make the following recommendations: (1) improve data collection on seabird–vessel interaction and bycatch rates, on fishing effort and vessel movements (especially small-scale fleets), and on mitigation compliance, (2) counter the current bias towards temperate and high-latitude ecosystems, larger-bodied species and particular life stages or times of year (e.g. adults during breeding), and (3) advance our currently poor understanding of combined effects of fisheries and other threats (e.g. climate change, offshore renewables). In addition, research is required on under-studied aspects of fishing impacts: consequences for depleted sub-surface predators, impacts of illegal, unreported and unregulated fishing, artisanal and emerging fisheries, such as those targeting mesopelagic fish, have received insufficient research attention. Some of these shortfalls can be overcome with new tools (e.g. electronic monitoring, remote sensing, artificial intelligence, and big data) but quantifying and addressing fishing impacts on seabirds requires greater research investment at appropriate spatio-temporal scales, and more inclusive dialogue from grassroots to national and international levels to improve governance as fishing industries continue to evolve.

At-sea movements of wedge-tailed shearwaters during and outside the breeding season from four colonies in New Caledonia

The wedge-tailed shearwater (WTS) population of New Caledonia is one of the largest in the world, yet its biology and foraging ecology are poorly known. We studied WTS from 4 colonies in New Caledonia. We examined foraging behaviour and habitats using GPS receivers and light sensors during and outside the breeding season, respectively, and compared our findings with those from other WTS populations worldwide. During breeding, New Caledonian WTS alternated short foraging trips close to the colony over the lagoon, or off the reef edge, with longer trips over distant, deep waters. Whereas neighboring colonies overlapped at sea, especially during short trips, there was a clear separation of foraging zones between the pairs of colonies located in the southern versus northwestern parts of New Caledonia. Although WTS actively foraged and commuted to foraging zones during the day, they mainly returned to the colony or rested at night, indicating that they feed mainly during the day. Active foraging did not take place in more productive areas, suggesting that it may instead be related to the presence of sub-surface predators. Outside the breeding season, birds from 3 colonies had similar trans-equatorial migratory behaviour. All left New Caledonia at the same time of the year with a fast, northeasterly movement and wintered over deep waters in the same sector of the northwestern tropical Pacific Ocean. At overwintering sites, they spent most of their non-foraging time presumably sitting on the water, especially at night, making a slow westward movement before returning to New Caledonia. WTS from New Caledonia forage over warm, oligotrophic deep waters throughout their life cycle, and the species appears to have a flexible foraging strategy adapted to the various environmental conditions encountered across its wide tropical range.

Long-term dietary shift and population decline of a pelagic seabird-A health check on the tropical Atlantic?

In the face of accelerating ecological change to the world's oceans, seabirds are some of the best bio-indicators of marine ecosystem function. However, unravelling ecological changes that pre-date modern monitoring programmes remains challenging. Using stable isotope analysis of feathers and regurgitants collected from sooty terns (Onychoprion fuscatus) nesting at a major Atlantic colony, we reconstructed a long-term dietary time series from 1890 to the present day and show that a significant dietary shift occurred during the second half of the twentieth century coinciding with an apparent population collapse of approximately 84%. After correcting for the "Suess Effect," δ13 C in feathers declined by ~1.5‰ and δ15 N by ~2‰ between the 1890s and the present day, indicating that birds changed their diets markedly over the period of population decline. Isotopic niches were equally wide before and after the population collapse but isotopic mixing models suggest that birds have grown ever more reliant on nutrient-poor squid and invertebrates as teleost fish have declined in availability. Given that sooty terns rely heavily on associations with sub-surface predators such as tuna to catch fish prey, the rapid expansion of industrialized fisheries for these species over the same period seems a plausible mechanism. Our results suggest that changes to marine ecosystems over the past 60years have had a dramatic impact on the ecology of the most abundant seabird of tropical oceans, and highlight the potentially pervasive consequences of large predatory fish depletion on marine ecosystem function.

Evaluating habitat, prey, and mesopredator associations in a community of marine birds

Abstract Offshore wind energy development on the US Atlantic Continental Shelf has brought attention to the need for marine spatial planning efforts to reduce potential conflict between wind turbines and marine animals, including seabirds. We evaluated the effects of marine mammals, fishes, and habitat characteristics on the distribution and relative abundance of marine birds off the coast of Delaware, Maryland, and Virginia. From May 2012 to 2014, we collected line transect data from 14 shipboard surveys, and novel high-resolution digital videography data from 14 aerial surveys. We compiled five habitat covariates: three static (distance to shore, sea floor slope, and sediment grain size), and two dynamic (sea surface temperature, salinity). We additionally analysed two seabird community covariates: the density of observed marine mammals and detected fish. Using zero-altered models, we tested our hypothesis that plunge-diving seabird species would show positive associations with marine mammals. Our results provide statistical evidence that, alongside competition, facilitative interactions occur among pelagic communities, where subsurface predators improve the detectability and accessibility of prey to surface-feeding seabirds. This study highlights the importance of quantifying community and ecological influences on avian abundance, particularly in predicting the potential exposure of marine birds and mammals to offshore development.

Seabird distribution patterns observed with fishing vessel\u2019s radar reveal previously undescribed sub-meso-scale clusters

Seabirds are known to concentrate on prey patches or at predators aggregations standing for potential feeding opportunities. They may search for prey using olfaction or by detecting visually feeding con-specifics and sub-surface predators, or even boats. Thus, they might form a foraging network. We hypothesized that conditionally to the existence of a foraging network, the visual detection ability of seabirds should have a bearing on their medium-scale distribution at sea. Using a fishing-boat radar to catch the instantaneous distribution of seabirds groups within 30 km around the vessel, we conducted a spatial clustering of the seabird-echoes. We found 7,657 clusters (i.e. aggregations of echoes), lasting less than 15 minutes and measuring 9.2 km in maximum length (median). Distances between seabirds groups within clusters showed little variation (median: 2.1 km; CV: 0.5), while area varied largely (median: 21.9 km2; CV: 0.8). Given existing data on seabirds’ reaction distances to boats or other marine predators, we suggest that these structures may represent active foraging sequences of seabirds spreading themselves in space such as to possibly cue on each others. These seabird clusters were not previously described and are size compatible with the existence of a foraging network.

Biogeography of top predators – seabirds and cetaceans – along four latitudinal transects in the Atlantic Ocean

The distribution, abundance, and species assemblage of top predators - seabirds and cetaceans - can be correlated to water masses as defined by hydrological parameters. In comparison to other oceans, information about the structuring effects of water masses on top predators in the Atlantic Ocean is limited. The present study aims 1) to provide baseline distributional data of top predators for future comparisons, for instance in the course of climate change, and 2) to test how water masses and seasons affect distributional patterns of seabirds and cetaceans in the temperate and tropical Atlantic. During four trans-equatorial expeditions of the RV Polarstern between 2011 and 2014, at-sea observation data of seabirds, cetaceans and other megafauna were collected. Counts of top predators were generally low in the surveyed regions. Statistical analyses for the eight most abundant seabird species and the pooled number of cetaceans revealed water masses and seasons to account for differences in counts and thus also distribution. In most cases, borders between water masses were not very distinct due to gradual changes in surface water properties. Thus, top predator counts were correlated to water masses but, in contrast to polar waters, not strongly linked to borders between water masses. Additional factors, e.g. distance to locally productive areas (upwelling), competition effects, and seabird associations to prey-accumulating subsurface predators may be similarly important in shaping distributional patterns of top predators in the tropical and temperate Atlantic, but could not be specifically tested for here.

Trans-equatorial migration and non-breeding habitat of tropical shearwaters: implications for modelling pelagic Important Bird Areas

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 550:219-234 (2016) - DOI: https://doi.org/10.3354/meps11713 Trans-equatorial migration and non-breeding habitat of tropical shearwaters: implications for modelling pelagic Important Bird Areas Fiona McDuie*, Bradley C. Congdon Centre for Tropical Environmental and Sustainability Science & College of Marine & Environmental Science, James Cook University, MacGregor Rd, Smithfield, Cairns, QLD 4870, Australia *Corresponding author: fionamcduie@gmail.com ABSTRACT: Declining prey availability drives many seabirds to migrate following breeding. While long-distance, latitudinal migrations are common in temperate-breeding species (including temperate-breeding Procellariiformes), regional dispersal or longitudinal migration is more common in tropical-breeding species. We used geolocators to track adult, tropical-breeding wedge-tailed shearwaters Ardenna pacifica from the Great Barrier Reef, Australia, through a ~6000 km migration to non-breeding grounds in Micronesia. This lengthy, trans-equatorial migration was similar to that undertaken by temperate-breeding Procellariiformes, but contrasted with patterns previously observed for tropical-breeding species. However, the oceanographic characteristics of tropical non-breeding habitats differed significantly from those of temperate sites. Core-use habitat had high sea-surface temperatures, very low wind speeds and low primary productivity, features normally associated with poor foraging habitat. However, activity was strongly linked to positive sea-level anomalies, indicating the presence of anti-cyclonic eddies at foraging sites. Such eddies are often associated with oceanic fronts and are known to aggregate micronekton and facilitate sub-surface predator feeding. Consequently, our results suggest that eddies, frontal activity and feeding associations with sub-surface predators enhance prey availability to non-breeding shearwaters beyond levels expected based on standard indices of primary production. This is the first tropical study to simultaneously assess the full set of oceanographic features considered important for modelling pelagic Important Bird Areas (IBA). Our findings demonstrate the need for IBA modelling in the tropics to go beyond standard indices of productivity by including measures of frontal activity and assessments of biological interactions. Consequently, this study provides a framework for better predicting candidate Marine IBAs throughout tropical regions. KEY WORDS: Overwinter · Tropical Procellariiform · Sea-level anomalies · Sub-surface predator · Electronic tracking · Marine Important Bird Areas (MIBAs) · Habitat modelling Full text in pdf format PreviousNextCite this article as: McDuie F, Congdon BC (2016) Trans-equatorial migration and non-breeding habitat of tropical shearwaters: implications for modelling pelagic Important Bird Areas. Mar Ecol Prog Ser 550:219-234. https://doi.org/10.3354/meps11713 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 550. Online publication date: May 25, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Combining Methods to Describe Important Marine Habitats for Top Predators: Application to Identify Biological Hotspots in Tropical Waters.

In tropical waters resources are usually scarce and patchy, and predatory species generally show specific adaptations for foraging. Tropical seabirds often forage in association with sub-surface predators that create feeding opportunities by bringing prey close to the surface, and the birds often aggregate in large multispecific flocks. Here we hypothesize that frigatebirds, a tropical seabird adapted to foraging with low energetic costs, could be a good predictor of the distribution of their associated predatory species, including other seabirds (e.g. boobies, terns) and subsurface predators (e.g., dolphins, tunas). To test this hypothesis, we compared distribution patterns of marine predators in the Mozambique Channel based on a long-term dataset of both vessel- and aerial surveys, as well as tracking data of frigatebirds. By developing species distribution models (SDMs), we identified key marine areas for tropical predators in relation to contemporaneous oceanographic features to investigate multi-species spatial overlap areas and identify predator hotspots in the Mozambique Channel. SDMs reasonably matched observed patterns and both static (e.g. bathymetry) and dynamic (e.g. Chlorophyll a concentration and sea surface temperature) factors were important explaining predator distribution patterns. We found that the distribution of frigatebirds included the distributions of the associated species. The central part of the channel appeared to be the best habitat for the four groups of species considered in this study (frigatebirds, brown terns, boobies and sub-surface predators).

Biomass consumption by breeding seabirds in the western Indian Ocean: indirect interactions with fisheries and implications for management

AbstractFisheries potentially affect seabirds both directly and indirectly. Well-documented direct effects have resulted in significant losses to seabird populations, but indirect effects are less well known. One way in which tropical seabirds may be indirectly affected is through overexploitation of large subsurface predators. Tropical seabirds must forage over wide areas to attain sufficient prey and have evolved various methods of increasing foraging efficiency. One strategy is their association with surface-feeding tunas. When feeding, these predators drive prey to the surface, making them available to seabirds feeding from above. Losses in predator biomass will reduce prey accessibility (but not necessarily prey abundance) for seabirds, contributing to declines in bird populations. To explore indirect fisheries effects, we compared estimates of the magnitude and spatial distributions of consumption by breeding seabirds with fisheries offtake in the western Indian Ocean (WIO). Data from the literature were compared with Indian Ocean Tuna Commission longline and purse seine landings of selected tuna and billfish species from between 2000 and 2009. Breeding seabird populations (adults and immature birds) were estimated at ∼19 million individuals, assuming 50% breeding success. Based on the literature, these birds will consume between 150 000 and 500 000 metric tonnes (t) of prey; values that are of the same magnitude as mean annual longline (904±632 t) and purse seine (349 861±61 820 t) landings for the region. Spatial overlap between fisheries and seabirds is high, especially around the Seychelles, suggesting that the indirect impacts of fisheries on seabird populations may be great. Sooty Tern (Onychoprion fuscatus) is by far the dominant seabird in the study area, accounting for over 80% of numbers and consumption estimates. Our results highlight the importance of seabirds within WIO marine trophic webs and emphasize the potential indirect effects of industrial tuna fisheries on their populations.

Oceanography and seabird foraging: within-season impacts of increasing sea-surface temperature on the Great Barrier Reef

Previously we have demonstrated that prey availability to seabirds of the Great Barrier Reef (GBR) decreases in direct association with within-season increases in sea-surface temperature, independent of prevailing El Nino conditions. These negative impacts occur throughout the GBR and affect multiple seabird species. Currently, the oceanic processes driving these impacts or the potential for them to occur in other marine systems are unknown. Here, we use satellite and in situ data obtained during a thermal stress event to identify the within-season links between ocean dynamics and seabird foraging success on the southern GBR. In February 2006, a major mesoscale eddy formed coastward of the East Australian Current flow, adjacent to our study site. In mid-February, strengthening of this eddy caused an intrusion of cool, dense waters at depth across the GBR shelf. This intrusion intensified vertical stratification and caused a pronounced warming of sea-surface layers. Prey availability to seabirds significantly decreased during this period and remained low until eddy intensity decreased and surface waters cooled. Prey availability increased following this episode, clearly indicating that loss of prey was associated with a short-term vertical and/or horizontal redistribution of forage-fish, or subsurface predators, rather than an overall decrease in productivity linked to seasonal-scale El Nino processes.

Foraging of seabirds on pelagic fishes: implications for management of pelagic marine protected areas

Scientists and managers have become increasingly interested in how pelagic marine protected areas (PMPAs), or protected areas away from the coast, can be used to protect pelagic species. Subsurface-predator facilitated foraging ('facilitated foraging') between seabirds and subsurface predators, such as tunas, is a key ecological interaction in the tropical oceans where a number of large PMPAs have been created. In facilitated foraging, subsurface predators drive for- age fish to the surface, where they are made available to seabirds. Because this is a critical inter- action for seabirds, a reduction or cessation of fishing effort within tropical PMPAs may increase subsurface predator density, resulting in more foraging opportunities for seabirds closer to colonies. This interaction is well documented in the Eastern Tropical Pacific, and to a lesser extent in the Indian Ocean, but is poorly studied in the Central Tropical Pacific. Here, we review our cur- rent state of knowledge of facilitated foraging, particularly in relation to questions critical to PMPA management. We specifically consider 2 components of the foraging ecology of tropical seabird species: (1) diet and foraging methods, including flock feeding behavior, i.e. how species forage in association with other species or conspecifics, and (2) association with subsurface pred- ators. We consider the spatial scale of this interaction as it pertains to the distribution of seabirds and tunas and to PMPA boundaries. We conclude with discussion of data gaps and the implica- tions for managers. The present review of facilitated foraging will help focus future efforts and multidisciplinary collaborations on this important but understudied interaction.

Overnight foraging trips by chick‐rearing Nazca Boobies Sula granti and the risk of attack by predatory fish

Most tropical booby species complete breeding foraging trips within daylight hours, thus avoiding nights at sea. Nazca Boobies Sula granti are unusual in this respect, frequently spending one or more nights away from the nest. We used GPS dataloggers, time‐depth recorders, and changes in body weight to characterize foraging trips and to evaluate potential influences on the decisions of 64 adult Nazca Boobies to spend a night at sea, or to return to their chicks on Isla Española, Galápagos, in daylight hours. The tagged birds foraged east of Isla Española, undertaking both single‐day (2–15 h, 67% of trips) and overnight trips (28 h–7.2 days, 33%), and executing 1–19 foraging plunge‐dives per single‐day trip. Birds might forage longer if they are in nutritional stress when they depart, but body weight at departure was not correlated with trip length. Birds might be expected to return from longer trips with more prey for young, but they returned from single‐day and overnight trips with similar body weights, consistent with previous indications that Nazca Boobies forage until accumulating a target value of prey weight. Birds with a lower dive frequency during the first 5 h of a trip were more likely to spend the night at sea, suggesting that they might choose to spend the night at sea if prey capture success was low. At night, birds almost never dived and spent most of their time resting on the water’s surface (11.8–12.1 h, > 99% of the time between civil sunset and civil dawn). Thus, the night is an unproductive time spent among subsurface predators under low illumination. The birds’ webbed feet provided evidence of this risk: 24% of birds were missing > 25% of their foot tissue, probably due to attacks by predatory fish, and the amount of foot tissue lost increased with age, consistent with a cumulative risk across the lifespan. In contrast, other tropical boobies (Blue‐footed Sula nebouxii and Brown Boobies Sula leucogaster), which do not spend the night on the water, showed no such damage. These results suggest that chick‐rearing Nazca Boobies accept nocturnal predation risk on occasions of low prey encounter during a foraging trip’s first day.

Foraging movements of great frigatebirds from Aldabra Island: Relationship with environmental variables and interactions with fisheries

Great Frigatebirds ( Fregata minor) are large tropical seabirds that rely primarily on sub-surface predators such as tunas or cetaceans to capture their prey. We studied the foraging movements of 14 Great Frigatebirds breeding on Aldabra Island (9.4°S, 46.4°E), the largest colony in the Indian Ocean. This colony is located at more than 500 km from the main fishing grounds of a very important industrial purse-seine fishery targeting surface-dwelling tunas. Despite their slow flight speeds (16 km h −1), frigatebirds are able to forage at more than 1000 km from the colonies when breeding, using 2500–4750 km long foraging loops over oceanic waters. All trips were directed to the north of the island up to the equator. Foraging bouts, indicated by reduced flight speeds, were rare and located throughout the trips. Foraging spots tended to be more frequent on higher surface chlorophyll concentration and in association with some cyclonic vortices. However, mesoscale activity is relatively weak between Aldabra and the equator and the chlorophyll variability is mostly the result of wind-mixing processes during the southwest monsoon. These results suggest that frigatebirds forage for widely distributed resources to the north of Aldabra. The northernmost foraging bouts were located in the vicinity of the purse-seine fishing grounds, but without a significant overlap between frigatebirds and tuna fleets. The results of the study are compared with those from another population at Europa Island (22.3°S, 40.3°E) where birds were foraging on predictable features, the edge of cyclonic eddies that are marked in the Mozambique Channel. We discuss the consequences of the reliance of populations on contrasted oceanographic conditions on foraging strategies and on the evolution of life histories in these long-lived animals in a changing climate, as well as the possible effects of overfishing on frigatebird populations.

Movements, at-sea distribution and behaviour of a tropical pelagic seabird: the wedge-tailed shearwater in the western Indian Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 391:231-242 (2009) - DOI: https://doi.org/10.3354/meps07717 Movements, at-sea distribution and behaviour of a tropical pelagic seabird: the wedge-tailed shearwater in the western Indian Ocean Teresa Catry1,*, Jaime A. Ramos1, Matthieu Le Corre2, Richard A. Phillips3 1Instituto do Mar (IMAR), Departamento de Zoologia, Universidade de Coimbra, 3004-517 Coimbra, Portugal 2Laboratoire ECOMAR, Université de la Réunion, 97715 Saint Denis message cedex 9, France 3British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK *Email: teresa_catry@yahoo.com ABSTRACT: This is the first study using geolocators (global location sensing, GLS) to track the movements of a pelagic tropical seabird. We used GLS to describe at-sea distribution and activity patterns of wedge-tailed shearwaters Puffinus pacificus breeding on Aride Island, Seychelles, in the late chick-rearing, non-breeding and pre-breeding periods. During late chick-rearing and pre-breeding periods, shearwaters foraged relatively close to the colony. In the non-breeding period, shearwaters were found on a west–east gradient along the equator, between 5°N and 10°S. Some of the tracked individuals showed little dispersion, staying as close as 1000 km to Aride Island, while others travelled 3500 km to the Central Indian Ocean Basin. Individual core areas of activity showed little overlap. Overall, wedge-tailed shearwaters showed short-distance movements and exploited relatively unproductive oceanic waters. At-sea distribution largely matched that of yellowfin and skipjack tunas, emphasising the importance of the association with subsurface predators rather than associations with physical oceanographic features that enhance primary productivity. During the non-breeding period, the feeding activity of shearwaters was mainly concentrated in the daylight period, when tunas also forage. A different behaviour, characterised by a much lower proportion of the night sitting on the sea surface, was recorded in the pre-laying exodus of 1 female to more productive waters, suggesting a different feeding strategy and/or targeting of different prey. Knowledge of the at-sea distribution of wedge-tailed shearwaters allows quantification of the overlap with industrial fisheries, which will be crucial to devise fisheries policies for the Indian Ocean with important implications for the conservation of this species. KEY WORDS: Wedge-tailed shearwater · Activity patterns · At-sea distribution · Foraging behaviour · Pelagic · Habitat preferences · Indian Ocean · Tuna distribution Full text in pdf format PreviousNextCite this article as: Catry T, Ramos JA, Le Corre M, Phillips RA (2009) Movements, at-sea distribution and behaviour of a tropical pelagic seabird: the wedge-tailed shearwater in the western Indian Ocean. Mar Ecol Prog Ser 391:231-242. https://doi.org/10.3354/meps07717 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 391. Online publication date: September 28, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Associations between seabirds and subsurface predators around Oahu, Hawaii

Many species of tropical seabirds rely on subsurface predators such as tuna and dol- phins to drive prey close to the ocean's surface. We observed seabird foraging events from fishing vessels around the island of Oahu, Hawaii, to determine the prevalence and relative importance of different subsurface predators to seabird foraging. Sixty-nine seabird foraging events were observed, in 62 of which the presence and/or identity of subsurface predators could be determined. Of those 62, all but two involved subsurface predators and accounted for 99.8% of all foraging birds. Skipjack tuna Katsuwonus pelamis were the predominant (81%) subsurface predator involved in seabird feed- ing events, with odontocetes Stenella attenuata and Pseudorca crassidens (8%), mahimahi Coryphaena hippurus (6%), and yellowfin tuna Thunnus albacares (2%) being less common. Wedge- tailed shearwaters Puffinus pacificus and brown noddies Anous stolidus foraged in association with tuna schools more often than would be expected by chance alone. Red-footed boobies Sula sula, in contrast, did not associate with any subsurface predator in greater proportion than what would be expected by chance. Nine additional seabird species were observed foraging in association with sub- surface predators, but not in numbers large enough for statistical analysis. This study highlights the value of regional investigations able to uncover the variable nature of seabird-subsurface-predator associations. The finding that, in Hawaii, at least 2 species of seabirds showed tight dependence on tuna underlines the need for tuna fishery management to address the potential indirect ecological effects of fishing.

Foraging strategy of a tropical seabird, the red-footed booby, in a dynamic marine environment

The foraging behaviour of animals depends on the distribution, abundance and predictability of their food resources. In the marine environment, top predators such as seabirds are known to concentrate their foraging effort in specific oceanic features where productivity is elevated or prey concentrated. When marine productivity is low and prey distribution unpredictable, such as in tropical waters, selection should favour the evolution of flexible foraging strategies. By using GPS, Argos transmitters and activity recorders, we studied the foraging strategy of red-footed boobies (RFBs) Sula sula breeding on Europa Island in the Mozambique Channel, to examine the way a central place forager searches for prey in tropical waters. RFBs only foraged during the day-time and never sat on the water at night; thus, they consequently had a limited maximum foraging range of 148 km. Primary production within this range was low overall (average 0.165 mg chlorophyll m–3), with a high spatial and temporal heterogeneity due to the presence of strong geostrophic currents and surface mixing by storms. RFBs appear to adjust their foraging strategy to cope with this situation. Individual birds do not return to the same oceanic sites from one trip to the next, but generally head in the direction of zones where productivity is enhanced. Active foraging, i.e. periods when birds spent time on the sea surface or dived, occurred at the most distant part of the foraging trip. Despite their limited foraging range and the low productivity of surrounding waters, red-footed boobies are able to track zones of prey availability located in areas of enhanced primary production. Searching is probably favoured by the presence of sub-surface predators that chase prey to the surface. The foraging strategy of this tropical sulid appears to differ extensively from that of temperate or polar seabirds whose prey distribution and availability are more predictable.

Seabird community structure in a coastal tropical environment: importance of natural factors and fish aggregating devices (FADs)

In tropical waters, productivity is low and seabirds only feed at the surface, often in association with sub-surface predators. We studied the distribution and abundance of seabirds in the coastal marine environment around Reunion Island, SW Indian Ocean, between February 2001 and October 2002. We investigated the distribution and abundance of birds at sea with respect to the effects of natural factors, such as bathymetry, distance to the shore, season and associations with subsurface predatory schools, and the presence of anchored fish aggregating devices (FADs). A total of 13572 birds of 15 species were observed, 8 of which were rare. Among the 7 abundant species, lesser noddies appeared to favour the shelf area. Barau's petrels and sooty terns were more abundant offshore, and wedge-tailed shearwaters, Audubon's shearwaters, white-tailed tropicbirds and brown noddies were present over all habitats. When foraging, most species searched for prey solitarily or in very small groups. When feeding, they were generally in large multi-species flocks, especially when feeding events occurred in association with sub-surface predatory schools. Associations with surfacefeeding schools of tunas, mainly skipjack, increased the abundance of most seabird species that fed on small fish or squids. The notable exception was for the white-tailed tropicbird, which was rarely found in association with sub-surface predators and, when associated, preferentially selected dolphin pods rather than tuna schools. Although they were often associated with tuna schools, lesser noddies were also strongly concentrated on the shelf, feeding on small pelagic schools. While the aggregating power of FADs on pelagic fish has previously been demonstrated, the associations between schools of sub-surface predators and seabirds were not more frequent close to FADs. However, 2 seabird species were more abundant close to FADs when feeding, and 1 when foraging, suggesting that they may occasionally take advantage of the attractive power of FADs. Seasonal variations in the occurrence of associations indicated a turn-over of fish species that induced variations in the nature of the associations, with the greatest aggregating power of dolphin fish in spring during their peak of abundance in the vicinity of the island

Foraging strategy of a top predator in tropical waters: great frigatebirds in the Mozambique Channel

Although oceanic tropical waters occupy almost 50% of the total area of pelagic oceans, knowledge of the foraging ecology of top predators in these low productivity waters is limitied. This is particularly the case for tropical seabirds that are believed to rely on scarce and unpredictable resources and have developed specific foraging strategies to exploit these resources. Frigatebirds are tropical seabirds that rely on subsurface predators such as tuna or cetaceans to feed. We studied the foraging strategy at sea of great frigatebirds breeding on Europa Island in the Mozambique Channel using satel- lite transmitters and altimeters. When foraging, birds moved at slow speeds (average 16.4 km h -1 ) and stayed at an average altitude of 180 m, continuously climbing and descending. During climbs, they reached high altitudes (maximum 2867 m) and during descent rarely came close to the surface to feed. Birds came to the surface on average 6.2 times d -1 . Feeding opportunities occurred only during the day, peaking early in the morning and late in the afternoon. Frigatebirds foraged over extensive distances, up to 612 km from the island, usually during the incubation or post-breeding periods, concentrating their effort in the western oceanic waters of the channel where overall productivity, although low, was still higher than in the eastern part of the channel. The higher productivity in the western waters is due to the presence of a persistent field of mesoscale anticyclonic gyres. Birds tended to avoid the centre of cold eddies and remained at the edge of eddies. When brooding chicks, birds foraged closer to the island, at an average distance of 94 km, mainly in the northwest of the island, in the vicinity of shallow waters of the Bassas da India Bank. During both long and short trips, birds did not return to the same area. Diet was composed essentially of flying-fish and Ommastrephid squids. The results of the study indicate that the strategy of frigatebirds is based on wide ranging foraging directed toward scarce prey patches that are encountered irregularly, and are not predictable in exact location at a coarse scale. At a mesoscale level, birds appear to favour large areas with slightly enhanced productivity such as a zone of strong eddies. Since they feed in close association with subsurface predators, mainly tuna in our area, it can be predicted that large predators such as seabirds or tuna are spread over extensive areas and have an unpredictable localisation at a coarse scale, but have some specific preferences on a regional scale.

Optimal diving under the risk of predation

Many air-breathing aquatic foragers may be killed by aerial or subsurface predators while recovering oxygen at the surface; yet the influence of predation risk on time allocation during dive cycles is little known in spite of numerous studies on optimal diving. We modeled diving behavior under the risk of predation at the surface. The relationship between time spent at the surface and the risk of death is predicted to influence the optimal surface interval, regardless of whether foragers accumulate energy at a constant rate while at the food patch, deplete food resources over the course of the dive, or must search for food during the dive. When instantaneous predation risk during a single surface interval decreases with time spent at the surface, a diver should increase its surface interval relative to that which maximizes energy intake, thereby increasing dive durations and reducing the number of surfacings per foraging bout. When instantaneous risk over a single surface interval does not change or increases with increasing time at the surface, divers should decrease their surface interval (and consequently their dive duration) relative to that which maximizes energy intake resulting in more dives per foraging bout. The fitness consequences of selecting a suboptimal surface interval vary with the risk function and the way divers harvest energy when at depth. Finally, predation risk during surface intervals should have important consequences for habitat selection and other aspects of the behavioral ecology of air-breathing aquatic organisms.