Recoveries of juvenile Giant Petrels in regions of ocean productivity: potential implications for population change
2011; Wiley; Volume: 2; Issue: 7 Linguagem: Inglês
10.1890/es11-00083.1
ISSN2150-8925
Autores Tópico(s)Marine and fisheries research
ResumoEcosphereVolume 2, Issue 7 art75 p. 1-13 ArticleOpen Access Recoveries of juvenile Giant Petrels in regions of ocean productivity: potential implications for population change John van den Hoff, Corresponding Author John van den Hoff Australian Antarctic Division, 203 Channel Highway, Kingston 7050 Tasmania, AustraliaE-mail:john_van@aad.gov.auSearch for more papers by this author John van den Hoff, Corresponding Author John van den Hoff Australian Antarctic Division, 203 Channel Highway, Kingston 7050 Tasmania, AustraliaE-mail:john_van@aad.gov.auSearch for more papers by this author First published: 08 July 2011 https://doi.org/10.1890/ES11-00083.1Citations: 5 Corresponding Editor: D. P. C. Peters. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Explaining long-term population change for migratory seabirds such as Giant Petrels (Macronectes spp.) has proven elusive because only aspects of adult life-histories have been studied. There is a paucity of demographic data for juveniles however; there is a considerable amount of leg-band recovery location data for younger age-classes. Nestlings were leg-banded at two sub-Antarctic islands, and at two higher latitude Antarctic breeding colonies. Bands were most often recovered in winter within six months of liberation. Banded fledglings were recovered in known productive upwelling biomes. The birds had travelled between 350 and 6150 km downwind from their natal colonies to reach these biomes. The distances fledglings travelled to reach the biomes were correlated with the observed population trends at their natal colonies; negative/positive population trends were correlated with great/lesser distances travelled. This study highlights the potential importance juvenile life-history attributes when considering population change for long-lived seabird species, and identifies rich upwelling regions such as the Humboldt and Benguelen Currents that might be important foraging areas for this and other Procellariiform species with high conservation values. Introduction Knowing where highly migratory marine species, such as seabirds, acquire sufficient (or insufficient) resources to maintain their population levels is of increasing importance because the synoptic situation for many upper trophic-level predators seems bleak. Predator offspring growth parameters, breeding success, survival and population status are known to respond to resource and climate variability (e.g., Boersma and Parrish 1998, Pauly et al. 1998, Spear and Ainley 2008, Anisimov et al. 2007, Surman and Nicholson 2009, Nevoux et al. 2010), and habitat alteration/destruction (Innes and Frisvold 2009). Of increasing concern is the prediction that extreme climate events are to increase in severity and occurrence (Anisimov et al. 2007, Young et al. 2011). For migratory seabirds extreme weather events can occur either locally (during the breeding season), while migrating over open oceans, or at their distant non-breeding foraging grounds. One group of seabirds of particular interest from a conservation viewpoint are the long-lived charismatic Procellariiformes (albatrosses, petrels and shearwaters) of which the Giant Petrels (Macronectes ssp.) are the largest petrel species. Recent population estimates report almost five times as many Southern Giant Petrels (SGP, M. giganteus, approx. 50,170 breeding pairs) compared with Northern Giant Petrels (NGP, M. halli, approx. 11,800 breeding pairs) (ACAP 2008a, b). Both species breed on sub-Antarctic islands but the southern species begins breeding later in the year and has an extended distribution to include coastal sites on Antarctic continent and South America (ACAP 2008a, b). Population trends based on breeding-age bird numbers are reasonably well documented at some colonies but not at others (Delord et al. 2008, Wienecke et al. 2009). The most recent published data show that negative and positive trends can be site specific (Patterson et al. 2008, ACAP 2008a, b), the causes of which appear complex and are proving difficult to pin-down from adult numbers alone. For example, analyses of long-term population trend data found no direct relationship between breeding adult numbers at Possession Island and both commercial fishing effort and a climate variable (the Southern Oscillation Index) (Delord et al. 2008). Some Procellariiform populations continue to decline despite negligible fisheries related mortality to adults during the breeding season (Phillips et al. 2008). Other studies have concluded the most likely source of mortality, incidental by-catch in commercial trawl and long-line fisheries (Gales 1998, BirdLife International 2004), was too low (ca. 10%) to account for the observed population trends at breeding colonies (Sullivan and Reid 2002, Sander et al. 2010). One study (Quintana et al. 2006) even suggested that commercial fisheries have a positive influence on population status because extra food (offal discharge) is available to the birds from trawlers. Increased carrion from escalating populations of seals and seabirds close to Giant Petrel breeding colonies has also been proposed as a possible factor regulating their populations (Reid and Huin 2005, also see references within Copello et al. 2008), but again no clear dependence could be found between those factors at Marion Island (de Bruyn et al. 2007). Human disturbance and pollutants were also considered (González-Solís et al. 2000, Copello et al. 2008, Delord et al. 2008, Patterson et al. 2008) but the findings were inconclusive, suggesting it is important to look elsewhere for factors that might regulate petrel populations world-wide. Adult survival is an important factor regulating population status for long-lived species such as seabirds (Davis et al. 2005). However, given the findings of the studies mentioned above, it seems prudent to examine aspects of juvenile life-histories that might influence population trends. The main problem with such an approach is that little or no demographic (e.g., survival, breeding success, recruitment) data exist for juvenile Giant Petrels at the majority of their breeding colonies (ACAP 2008a, b, BirdLife International 2004), even though mark-recapture studies commenced in the early 1950s. What is known is that band recovery rates are low (>2%; e.g., Hunter 1984, Woehler and Johnstone 1988, Patterson and Hunter 2000) and fledglings dispersed vast distance down-wind with the mid-latitude prevailing westerly winds on their maiden migrations. Banded fledglings are also thought to circum-navigate Southern Ocean latitudes perhaps up to three times before returning to their natal islands (Weimerskirch et al. 1985, Trivelpiece and Trivelpiece 1998). From banding studies it is now generally accepted that, unlike adult birds (Hunter 1984, González-Solís et al. 2000, González-Solís et al. 2008, Quintana et al. 2010), the ca. 25,000 fledglings (figure based on total population estimates and breeding success (Patterson and Hunter 2000)) produced each year disperse in a north-easterly direction from their breeding colonies (Sander et al. 2010). Recoveries and at-sea sightings for young birds banded at a broad range of breeding locations have been consistently reported from coastal margins of Southern Hemisphere continents especially during colder winter months (May–Sept), shortly after fledglings leave the nest (Stonehouse 1957, Hitchcock and Carrick 1958, Ingham 1959, Tickell and Scotland 1961, Conroy 1972, Jehl 1973, Hunter 1984, Woehler and Johnstone 1988, Voisin 1990, Trivelpiece and Trivelpiece 1998, Patterson and Hunter 2000, Sander et al. 2010). Although not always explicitly recognised in Giant Petrel studies the continental margins where banded birds were reported are also known productive upwelling biomes (Longhurst 1998). The productivity of these Eastern and Western Boundary Currents (EBC and WBC) sustains a high biomass of mid-trophic level schooling fishes upon which predatory fish, squid, seabirds and marine mammals feed (e.g., Shannon 1985, Thomas et al. 2004, Thiel et al. 2007). South of Africa are the south flowing Agulhas and the north flowing Benguela Currents to its east and west coasts, respectively; South America has the Humboldt (Peruvian) Current flowing northward along its west coast toward the Galapagos Islands and to the east the Malvinas and Brazilian Currents flow south; the Leeuwin Current flows along the West Australian coast and the East Australian Current along the east coast. It seemed reasonable then to expect that upwelling biomes might be important feeding grounds for juvenile Giant Petrels. The birds can take advantage of the spatially (though not temporally) predictable productivity and therefore perhaps improve their survival prospects. It also seemed reasonable to assume that the distance fledgling's transit to reach these upwellings might exert pressure the survival of the juvenile age-classes that could be measurable at the population level. A key limitation to fledgling survival is that they carry only a finite fat reserve (Reid et al. 2000) before they need to forage for themselves. Copello et al. (2008) also recognised that the proximity and productivity of coastal feeding grounds might influence population trends but did not investigate further. From this and other previously published band-recovery studies I sought to identify the broad-scale areas that may be critical foraging areas for Giant Petrels during their first winter foraging journeys. I modelled the relationship between the distance fledglings travelled down-wind to the nearest upwelling biome and the long-term (>10 years) population trend at their respective breeding colonies. The objective being to identify an alternative explanation to those already forwarded regarding the observed population trends for Giant Petrels at their breeding colonies. Methods Northern Giant Petrels (NGP) and Southern Giant Petrels (SGP) fledge their chicks during March and March to late May, respectively (ACAP 2008a, b). An undetermined total number of Giant Petrel nestlings were banded at four breeding locations (Fig. 1). The distribution of these colonies represents a cross-section of their geographical range. Two sites (Macquarie and Heard Islands) were mid-ocean Sub-Antarctic island sites and two were coastal east-Antarctic continental sites (Hawker Island and the Frazier Islands). The majority of band deployments analysed for this study (1951–2006) occurred before the two species were separated by Bourne and Warham (1966). Separation of the two species was considered unnecessary for the spatial analyses undertaken herein because SGP's outnumber NGP's at a ratio of 5:1, satellite tracked fledglings from both species followed broadly similar migration routes (Trebilco et al. 2008), band recoveries were more different between two populations of SGP than between species (Weimerskirch et al. 1985), the foraging ecology for adults is very similar with inter-sexual differences in trophic niche being greater than inter-specific differences (González-Solís et al. 2000) and Giant Petrels of mixed ages feed at-sea during winter on squids and smaller planktonic organisms (Harper 1987). Figure 1Open in figure viewerPowerPoint Southern Hemisphere ocean basins and land masses, showing the giant petrel breeding colony locations (squares) and band recovery sites for fledgling and one-year-old chicks (circles). The band recovery (circle) colour corresponds to the colour of the breeding colony (square) from where the banded bird originated. Biome (blue-shaded areas) labels from Longhurst (1998) are expanded in the materials and methods. Band recovery data presented herein include previously published summaries from Downes et al. 1954 (n = 16), Ingham 1959 (n = 28), Murray 1972 (n = 2), Orton 1963 (n = 2), Woehler and Johnstone 1988 (n = 73) Patterson and Hunter 2000 (n = 3) and Trebilco et al. 2008 (n = 27). Fifty (50) unpublished records are included (this study). Where available, band recovery observations included date of recovery, sighting location (latitude and longitude), some measure of the bird's condition at recapture, and a comment on the circumstance surrounding the recovery (e.g., alive, dead, shot, and injured). Leg-bands were often removed from birds captured alive, effectively removing that individual from the marked population and future study. Age was accurately determined only for birds recaptured alive. Each band recovery location was categorised into an ecological biome/province as defined by Longhurst (1998:81–82 downloaded from 〈http://www.vliz.be/vmdcdata/vlimar/downloads.php〉 on 04/05/2009). Longhurst's (1998) provinces are based on features, such as the regional surface chlorophyll field relative to surface circulation, mixed layer topography, wind stresses, heat flux and distribution of ocean frontal systems. Since weather systems and ocean circulations are not stable, the provincial boundaries are not fixed in time and space (Longhurst 1998), however, they do have some predictive value suitable for broad-scale investigations such at this. Biome abbreviations and their associated ocean currents follow Longhurst (1998): AUSE, East Australian Current; AUSW, Leeuwin Current; BENG, Benguela Current; BRAZ, Brazil Current; EAFR, Agulhas Current; FKLD, Falkland (Malvinas) Current; HUMB, Humboldt/Peruvian Current; NEWZ, Circumpolar Frontal Systems; and SPSG, South Pacific Subtropical Gyre. Available population trend data, excluding Bird Island, for both species were taken from the ACAP web site (〈http://www.acap.aq/acap-species〉 accessed 31/March/2010). Population trends for NGP's and SGP's at Bird Island were calculated from data presented in González-Solís et al. (2000). I selected only trend data for colonies with greater than 10 years of observations (Table 1). I chose the ACAP population data because these are the most recent summaries of the longest data available, including unpublished data. ACAP used TRIM software with a stepwise selection of change point (ACAP 2008a, b) to provide population trend data. From that analysis I selected only the longest continuous trend results for my analyses. Positive rates of population change have been reported for SGP's at Gough Island (Patterson et al. 2008) and the Falkland Islands (Reid and Huin 2005) but no actual rates of change could be found in the published literature. Table 1. Annual percentage population change at Northern and Southern Giant Petrel breeding colonies. Distances fledglings travelled in a straight line between their natal colony and the closest edge of the nearest of Longhurst's biomes were calculated using the measuring tool in Google Earth (〈http://earth.google.com〉). A downwind direction was chosen because it is well established from band and satellite studies that SGP and NGP fledglings move from west to east across southern oceans (see references in the Introduction). Linear regression was used to model the relationship between distance travelled to Longhurst's biomes and the population trends for Giant Petrels at their breeding locations. In order to show if any differences or similarities were present between the two species I analysed the population trend data either separately for each species and both species pooled. Results Two-hundred and one (201) uniquely identifiable individual Giant Petrels were resighted over the study period (Table 2). Seventy percent of bands (n = 141) were recovered within the first year of life; of those recoveries 53% (n = 75) of the birds were alive, 38% (n = 54) were dead, and 9% (n = 12) were of unknown status. Twenty-two birds resighted were aged one year; 10 birds were recaptured alive, 11 birds were found dead, and the status of one bird was not recorded. The remaining 27 resights occurred 2 to 36 years after band deployment; 11 birds were alive, 14 dead and the status of 2 birds was undetermined. The time elapsed between banding and recovery could not be determined for 11 birds (3 live, 4 dead and 4 unknown) because the exact date of recovery was not noted. Table 2. Numbers of band recoveries for Giant Petrels (Macronectes spp.) banded at four breeding colonies in Australian territories. Of the birds recaptured alive, 73 were released alive, 18 were shot or killed and eight birds died in captivity. Twenty-nine live birds were "exhausted"; five subsequently died during attempts to rehabilitate them and the remainder released. Six birds were entangled in fishing gear (line/net), the band details noted and the bird released alive. Leg-band recovery biomes The first year of life (Fledglings) One hundred and forty-one (141) bands were resighted on fledgling Giant Petrels. Ten birds were recaptured at the original banding location and are excluded from further analyses. The remaining 131 birds were resighted at distant locations (Fig. 1). Most (62%) of the fledglings were seen in the HUMB and NEWZ provinces while the remaining 7 provinces individually accounted for 8% or less of the total resights (Fig. 2). Most (56%) birds were seen alive, 34% were dead and for the remaining 9% their status went unrecorded. Live fledglings were common in the HUMB (n = 27 live vs. 7 dead). Dead birds (n = 23) outnumbered live birds (n = 13) in NEWZ. Fledglings were most frequently seen in winter, between May and October. Figure 2Open in figure viewerPowerPoint Number of band recoveries for fledgling giant petrels in each biome categorised according to the natal breeding colony. Figures the top of each histogram represent the percent contribution to total bands recovered. Second year of life Only two of the 22 one-year-old birds were seen at their breeding colonies, one from Macquarie Island and one from the Frazier Islands. Overall the HUMB biome (n = 7) was the most important province to one-year-old birds, followed by NEWZ (n = 5), AUSW (n = 5), BENG (n = 2) and EAFR (n = 1). Half (50% of the birds seen were alive and most of these were in the HUMB (n = 5). Recaptures per month were few but again occurred most commonly in the winter; August for both the HUMB and NEWZ provinces. Two birds from the Antarctic Frazier Islands were recovered in the HUMB province, one was the most northern recovery recorded in this study. Population trends and the distance fledglings fly to the nearest upwelling The data used for this analysis are shown in Table 1. There was a modest (r2 = 0.46) but not significant (F(1,5) = 4.26, P = 0.094) linear relationship between the distance newly fledged SGP chicks dispersed to reach the nearest downwind upwelling and the rate of population change for their natal colonies (Fig. 3). The same relationship for the NGP was statistically significant (F(1,2) = 19.75, P = 0.047) and had a strong correlation (r2 = 0.91) (Fig. 3). Regression analysis for both species combined (not shown on Fig. 3 for clarity) showed the relationship was statistically significant (F(1,9) = 10.97, P < 0.009) and the correlation coefficient modest (r2 = 0.55). Figure 3Open in figure viewerPowerPoint Linear relationships between the long-term annual rates of population change (%) for Northern (solid line, open circle) and Southern (broken line, filled triangles) Giant Petrel breeding colonies, and the distance (km) fledglings travelled down-wind to reach the nearest continental boundary current biome. The regression line for both species pooled is not shown for reasons of clarity. Discussion Like other Procellariiformes (see BirdLife International 2004: Figs. 3.2 and 3.3) Giant Petrel fledglings dispersed widely over the Earth's oceans but sightings and band recoveries were concentrated close to continental margins within boundary current biomes (Fig. 1), especially during colder winter months. Eastern and Western Boundary Currents are amongst the most productive upwelling systems in the world (Shannon 1985, Longhurst 1998, Carr and Kearns 2003, Thomas et al. 2004). The Humboldt (HUMB, or Peruvian), Benguelan (BENG) and Falkland (FKLD, also known as the Malvinas) biomes provide local and visiting predator aggregations with spatially abundant zooplankton, fish and squid resources (Shannon 1985, Croxall and Wood 2002, Thomas et al. 2004, Thiel et al. 2007). Some upwelling biomes are important winter fishing grounds for humans (e.g., BRAZ, Bugoni et al. 2008) and cephalopod feeding Procellariiform species, such as black-browed albatrosses and white-chinned petrels (e.g., BENG, Abrams 1983; BRAZ, Olmas 2002). While no stomach contents or stable isotope analyses are available to gain insights into the diet of these birds recovered from coastal biomes during winter it is known that mixed age-groups and sexes of both Giant Petrels are known feed on live fish and squid during winter (Hunter 1985, Harper 1987, Hunter and Brooke 1992). Thus their presence within coastal upwelling biomes during the winter is not entirely unexpected. Additional observations report young Giant Petrels scavenging for fish, cephalopods, fur seal and seabird corpses near islands within the HUMB and FKLD biomes (Jehl 1973, Duffy 1989, Ayala 2007, Zavalaga et al. 2009). Individuals of unspecified age were seen off the Patagonian (Yorio and Caille 1999) and west South Africa coasts (Abrams 1983, Duffy 1989) during winter, and Holgersen (1957) and Sander et al. (2010) reported the young Giant Petrels they observed were largely absent from the open sea but frequent in coastal waters. The locations of these observations overlap tightly with the resight locations presented in this study (Fig. 1), and also with earlier banding accounts from breeding colonies on the Antarctic Peninsula (Trivelpiece and Trivelpiece 1998), at Signy Island (Roberts and Sladen 1952, Tickell and Scotland 1961), and at Isle Crozet (Voisin 1990) among others. The quantity of sightings and feeding observations suggest boundary current upwelling ecosystems are important foraging grounds for young Giant Petrels. There is little doubt that upwelling ecosystems can provide migrating seabirds with abundant food resources but newly fledged Giant Petrels must travel between 350 and 6 150 kilometres from the breeding colony location to reach these productive waters. The distances fledglings travelled were correlated with the observed long-term (>10 years) population trends for both species at their natal colonies (Fig. 3); breeding locations for populations with negative trajectories tended to be further from upwelling biomes than populations with positive trajectories. Fledgling survivorship may play an important role in the direction a population may take because their survival is initially dependent upon finite body reserves the chicks have stored before leaving the nest (Reid et al. 2000) and acquiring sufficient foraging skills for self-maintenance. Fig. 3 shows distances that exceed 4000 km between a breeding colony's location and the nearest downwind coastal upwelling biome may be critical to the population status. Perhaps only those fledglings carrying sufficient body reserves to reach an upwelling biome actually survive. Weighing banded nestlings just prior to fledging would provide a measure of potential first-year survival and contribute to understanding how variation in chick mass might influence survival and population change. Such a study would be strengthened if banded fledglings were recaptured and re-weighed during the first years of their lives. Simply reaching a productive upwelling ecosystem doesn't ensure a fledgling's survival prospects because these ecosystems experience extreme temporal cycles in productivity. Ecosystem regime shifts have been correlated with variability in atmospheric and oceanographic properties (Shannon et al. 1986, Chavez et al. 2003, Alheit and Niquen 2004, Surman and Nicholson 2009), the results of which have catastrophic local and regional consequences (Rodhouse 2001, Sydeman et al. 2001, Thompson and Grosbois 2002, Alheit and Niquen 2004, Monticelli et al. 2007, Rolland et al. 2008). The effects of the El Nino Southern Oscillation (ENSO), for example, can be seen locally along the western coastline of South America or can propagate through global atmospheric teleconnections (Alexander et al. 2002, Garcia et al. 2003, Pshennikov et al. 2005) to influence distant ecosystems (Behrenfeld et al. 2006, Trathan et al. 2007). Depending on the ENSO phase, the HUMB can alternate between sardine (Sardinopsis sp.) and anchovy (Engraulis sp.) regimes; the dominance of one fish species having the potential to restructure the entire food-web from the phytoplankton to top-predators (Shannon et al. 1988, Chavez et al. 2003, Alheit and Niquen 2004, Jaksic 2004, Sato et al. 2004). Such alterations in food web structure could affect the foraging success and future survival of Giant Petrels with limited foraging experiences. As might be expected temporal fluctuations in population trends might respond to cyclic climatic events, but as yet no link has been detected for Giant Petrels (Delord et al. 2008). Climatic conditions experienced by fledglings during their long flights over open oceans seemingly present an additional challenge in reaching upwelling biomes. An assessment of the fledglings recovered within the HUMB, NEWZ and BENG biomes showed some birds were exhausted, close to death or dead. Many starving and emaciated Procellariiformes including young (six-month-old) Giant Petrels were blown ashore onto New Zealand's coastline following heavy storms (Sibson 1969). Extreme climate events are predicted to increase (Anisimov et al. 2007); global trends in wind speed and, to a lesser extent, wave height are already increasing (Young et al. 2011). For migratory seabirds storm events can occur either locally at the breeding colonies, during their oceanic migrations, or at the distant foraging grounds. There are clear negative repercussions for the survivorship of inexperienced foragers with limited body reserves who encounter adverse weather conditions (Sibson 1969, Hunter 1984). However the positive effect of an increase in extreme weather might be a reduction in the time fledglings take to traverse open oceans to reach upwelling biomes. The impacts of climate variability and change are difficult to predict with certainty and thus requires further study with fresh insights. Immigration and emigration are life-history features that may also contribute to population change. To what degree these factors influence Giant Petrel population status remains unquantified even after about 50 years of banding studies, perhaps because band recovery rates are so low (<2%; e.g., Patterson and Hunter 2000). I have seen two SGP's that were leg-banded as nestlings on the Frasier Islands (66°14′ S, 110°10′ E) present within breeding colony on Hawker Island (68°38′ S, 77°51′ E), but in the limited time I had I could not determine if the birds were attending chicks. Although small in number, the addition of just two to three individuals to an already small breeding population such as that at Hawker Island (ca. 20 breeding pairs; Wienecke et al. 2009) could make a considerable (≥10%) difference to the population status. There are inherent biases associated with band recovery data such as that presented herein. For example, the likelihood of band recovery is clearly greater in populated coastal regions (Sander et al. 2010) compared with the open ocean or less populated coasts. Never-the-less the long-term consistency in the band recovery locations and at-sea sightings of non-banded birds in coastal areas, combined with the well established link between regional productivity and seabird foraging migrations (e.g., BirdLife International 2004) suggests long-term usage of these biomes even before commercial fisheries operations escalated during the 1950s (Duffy 1983, Tuck and Polacheck 1997). It is possible Procellariiform species such as Giant Petrels may have been foraging in these productive waters long before fisheries operations knew of them, a situation similar to that reported for the guano-birds of Southern America and Africa (Crawford and Jahncke 1999). Observing juvenile Giant Petrels in upwelling biomes today may not simply be the result of their attraction to fisheries operations, it may be a historical foraging strategy. Single-point band resights alone shed little insight into the migration paths of marked individuals. However, the connectivity between the band recovery locations and the migration paths can be more fully understood after Trebilco et al. (2008) satellite tracked fledgling Giant Petrels from Macquarie Island. Their successfully migrating birds flew from Macquarie Island across the Southern Pacific Ocean, via New Zealand, toward the west coast of South America (HUMB) and beyond into the Atlantic Ocean after spending variable amounts of time, perhaps feeding, in commercial fisheries regions. The spatial pattern of leg-band recoveries for fledglings from this study is similar to Trebilco's findings, supporting the idea that survival (of fledglings) may be dependent upon the bird's ultimately reaching spatially predictable upwelling regions, while avoiding mortality due to fisheries interactions. For seabird populations, juvenile and sub-adult demographics are a major source of population variability (Nur and Sydeman 1999). Unfortunately demographic studies focussing on juvenile survivorsh
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