Modeling Black-footed Albatross Dispersion in Cordell Bank and Gulf of the Farallones National Marine Sanctuaries
- Pam Michael
Hawai'i Pacific University
- Jaime Jahncke
Point Blue Conservation Science
- David Hyrenbach
Hawai'i Pacific University
- Applied California Current Ecosystem Studies (ACCESS)
- Nancy Foster Scholarship Program
- PRBO Conservation Science
- Cordell Bank National Marine Sanctuary
- Gulf of the Farallones National Marine Sanctuary
- Resources Legacy Fund
- California Sea Grant
- National Fish and Wildlife Foundation
- Anonymous donor
End Date: October 31, 2008
Cordell Bank National Marine Sanctuary (NMS) is a well-known albatross hotspot, with two north Pacific species regularly occurring at this site and another one visiting occasionally, as well as occasional southern hemisphere visitors (Stallcup & Terrill, 1996). In particular, the Black-footed Albatross (Phoebastria nigripes: BFAL) is the most common albatross species in the region, with the highest densities and a year-round presence (Stallcup, 2004; Fig 1). Despite breeding in the central and western North Pacific, BFAL forage on the central California shelf during the chick-rearing (April–June; when breeding adults must return to the colony to feed their chick) and the post-breeding (July–October; when adults can range freely unconstrained by breeding duties) seasons (Kappes et al., 2010).
Effective conservation of wide-ranging species such as the Black-footed Albatross requires an understanding of the factors that influence their movements and habitat use patterns, both locally and within a large-scale oceanographic context. While previous vessel-based surveys and satellite tracking studies have characterized Black-footed Albatross distribution in this region (e.g., Ainley et al. 1995; Hyrenbach et al. 2006; NCCOS, 2007), their habitat use and ecology remain poorly understood. For instance, their use of small-scale bathymetric features (banks, shelf-breaks) remains poorly understood.
To increase our understanding of Black-footed Albatross use of Cordell Bank and Gulf of the Farallones NMS waters, we used five years (2004-2008) of vessel survey data from Applied California Current Ecosystem Studies (ACCESS) cruises to characterize the occurrence (habitat), and abundance (areas of aggregation) of Black-footed Albatross during the rearing and post-breeding season (See Fig. 2 for study area). Specifically, we quantified the relative influence of oceanographic conditions on Black-footed Albatross occurrence (presence / absence) and abundance (when present) by assessing their relationship with: (1) seasonal and inter-annual variability, (2) static habitat features defined by bathymetry, (3) local and regional dynamic habitats, and (4) basin-wide fluctuations in oceanic – atmospheric conditions (Table 1). Each hypothesis-based model was developed with the aim of relating the resulting Black-footed Albatross habitat use patterns to spatially explicit management for this species.
Summary to DateWhile the results for Black-footed Albatross occurrence were stronger than for abundance, both indicated that static and dynamic features play a role in Black-footed Albatross habitat use. Specifically, Black-footed Albatross occurrence was greatest near the shelf break (200 m depth contour), especially in months with strong northern and southern upwelling. Because the validation of the occurrence models using a distinct, yet temporally and spatially overlapping dataset indicated that the models accurately identified the majority of Black-footed Albatross occurrences (combined 94.7% of presences, 86.1% of absences), we developed a map of predicted occurrence contours, highlighting the high predictability of Black-footed Albatross occurrence along the edge of the continental shelf (Fig. 3).
Though quite variable, high Black-footed Albatross abundance was associated with productive large-scale conditions (positive North Pacific Gyre Oscillation index values), and strong regional upwelling (high northern monthly upwelling, high sea surface salinity (SSS), low sea surface temperature (SST)). Spatially, Black-footed Albatross aggregations occurred near Rittenburg Bank, especially during the chick-rearing season. Though Black-footed Albatross occurrence decreased with increasing distance from the shelf break, abundance (when Black-footed Albatrosses were present) increased, suggesting that solitary birds occurred offshore and aggregated in larger flocks onshore. Accordingly, the most intense aggregations occurred on the shoreward extent of the Black-footed Albatross habitat (Fig. 4), where most Black-footed Albatrosses were observed sitting / eating on the water (G-test =26.255, df =8, p > 0.001; Fig. 5)
- Our findings suggest that Black-footed Albatross distribution within the sanctuaries is influenced by multiple features, including the distance to the shelf break and the intensity of upwelling, and that their abundance varies during different breeding seasons and responds to oceanographic changes at regional and basin-wide scales.
- Future assessment of Black-footed Albatross numbers and trends within sanctuary waters should incorporate these broad-scale perspectives to interpret local changes in their abundance and distribution.
- Continued monitoring of Black-footed Albatross within this dynamic region is essential for managers to keep a “finger on the pulse” of the sanctuary resources and to assess the state of the system.
DiscussionBlack-footed Albatross dispersion is a balance between static and dynamic habitat characteristics, with distinct occurrence and abundance patterns. The shelf break, particularly during months with strong upwelling, may be an important habitat for Black-footed Albatrosses searching for prey or transiting between the breeding colonies and West Coast sanctuaries. The low occurrence, but occasional high abundances of Black-footed Albatrosses, mainly sitting onshore of the shelf break may be the result of aggregations of becalmed birds during low wind conditions, or the signature of an earlier feeding event, possibly attracted to fishing vessels. For example, having consumed all available prey, Black-footed Albatrosses may remain within the area digesting or waiting for favorable wind conditions.
References Ainley DG, Veit RL, Allen SG, Spear LB, Pyle P (1995) Variations in marine bird communities of the California Current, 1986-1994. California Cooperative Oceanic Fisheries Investigations Reports 36:72-77 Andrew NL, Mapstone BD (1987) Sampling and the description of spatial pattern in marine ecology. Oceanography and Marine Biology 25:39–90. Green RH (1966) Measurement of non-randomness in spatial distributions. Researches in Population Ecology 8:1-7 Hyrenbach KD, Keiper C, Allen SG, Ainley DG, Anderson DJ (2006) Use of marine sanctuaries by far-ranging predators: commuting flights to the California Current System by breeding Hawaiian albatrosses. Fisheries Oceanography 15:95-103 Kappes MA, Shaffer SA, Tremblay Y, Foley DG, Palacios DM, Robinson PW, Bograd SJ, Costa DP (2010) Hawaiian albatrosses track interannual variability of marine habitats in the North Pacific. Progress in Oceanography-Climate Impacts on Oceanic Top Predators CLIOTOP International Symposium 86:246-260 NOAA National Centers for Coastal Ocean Science (NCCOS) (2007) A biogeographic assessment off north/central California: in support of the National Marine Sanctuaries of Cordell Bank, Gulf of the Farallones ad Monterey Bay. Phase II - environmental setting and update to marine birds and mammals. Prepared by NCCOS’s Biogeographic Branch, R. G. Ford Consulting Co. and Oikonos Ecosystem Knowledge, in cooperation with the National Marine Sanctuary Program. NOAA Technical Memorandum NOS NCCOS 40, Silver Spring, MD. 302 pp Stallcup R (2004) The amazing seabirds of Cordell Bank National Marine Sanctuary. National Marine Sanctuaries Program Olema, CA Stallcup R, Terrill S (1996) Albatrosses and Cordell Bank. Birding 28:106-110
- Habitat association
- Temporal variation
- Proximity to feature
- Bathymetric domain
- Bathymetric gradient
- Hydrographic fronts
- Movement of weather systems
Study MethodsWe modeled the seasonal (chick-rearing, post-breeding) and inter-annual (2004 – 2008) variation in BFAL distribution and abundance within the productive continental shelf / slope system of central California using vessel-based survey observations. Vessel survey data were collected as a part of the Applied California Current Ecosystem Studies (ACCESS, www.accessoceans.org) cruises and sampled seven east – west parallel survey lines, from slightly south of the Farallon Islands to slightly north of Cordell Bank, extending from coastal waters (50 m depth) within 15 km of the coast to the upper continental slope (Fig. 2). To rank the influence of temporal variation, static, local and regional dynamic features, and basin-wide fluctuation on BFAL occurrence and abundance, we used a broad range of vessel-based and integrated habitat variables developed by Pacific Fisheries Environmental Group (http://las.pfeg.noaa.gov/las6_5 /servlets/dataset; Table 1). We applied an information-theoretic approach, creating a suite of hypothesis-based models which we ranked to identify the model(s) / feature(s) most related to BFAL occurrence and abundance. The predictive power of our best occurrence models were assessed using ‘validation data’, which were also collected by ACCESS during the same study period as the survey lines used in our analyses (‘training data’) (Fig. 2). We also evaluated the intensity of BFAL aggregations, when present, relative to their onshore/offshore location from the shelf break. We quantified aggregation intensity using Green’s Index of Dispersion (Gx): Gx = (S2 * X-1) * (Σx-1)-1, where Gx = 1 indicates intense aggregation, Gx = 0, random distribution and Gx = small negative number indicates a uniform distribution (Green, 1966; Andrew & Mapstone, 1987). Lastly, we evaluated the proportion of the BFAL sighted during surveys that were engaged in three different behaviors (following the boat, flying, and sitting / eating on the water) using a G-test contingency table.
Figures and Images
Figure 1: Photograph of a flock of Black-footed Albatross near Cordell Bank.
Figure 2: Study area including Black-footed Albatross (BFAL) abundance, training (solid lines) and validation (dashed lines) data from the rearing and post-breeding season, 2004-2008. CB = Cordell Bank, RB= Rittenburg Bank. Note the high abundance (large circles) northeast and southeast of Rittenburg Bank.
Figure 3: Predicted occurrence contours for Black-footed Albatross (BFAL) in Cordell Bank and Gulf of the Farallones National Marine Sanctuaries (CBNMS, GFNMS, respectively) during the rearing and post-breeding season, 2004 - 2008. CB = Cordell Bank, RB= Rittenburg Bank.
Figure 4: Black-footed Albatross aggregation intensity (Green’s Index of Dispersion: Gx) relative to the distance (negative) offshore / (positive) onshore from the shelf break (dashed line). Gx was incalculable more than 20 km onshore of the shelf break because there were no sightings.
Figure 5: Percent of Black-footed Albatross observed relative to the distance onshore (positive) and offshore (negative) of the shelf break (dashed line). Three behaviors are considered: (black) following the ship, (yellow) flying, or (blue) on the water relative to the distance onshore (positive) and offshore (negative) of the shelf break (dashed line). Total number of BFAL observed shown in parentheses above figure.
Table 1: Summary of the ecological interpretation of habitat variables evaluated. SB = shelf break (200 m isobath), CB = Cordell Bank (100 m isobath), RB = Rittenburg Bank (100 m isobath), CV = coefficient of variation, SST (SSS) = sea-surface temperature (salinity), SSH = sea-surface height. Wind modulus was the vector (hypotenuse) of zonal (east-west) and meridional (north-south) wind speeds. Change (∆) in atmospheric pressure calculated as pressure at time of survey – pressure 24hr before survey. Monthly and 6hr upwelling indices were from 39o N 125o W and 36o N 122o W. PDO = Pacific Decadal Oscillation, NPGO = North Pacific Gyre Oscillation.