Our analyses of aggregation intensity and behavior suggest that BFAL use shelf-break and onshore habitats differently. Specifically, the high overall occurrence and disproportionate amount of flying/ship-following BFAL near the shelf-break suggests that the birds search for prey (including fisheries discards) through this area as they transit between the breeding colonies and West Coast sanctuaries. The highest BFAL aggregation intensity in onshore waters is in agreement with previous observations of BFAL aggregations during the chick-rearing season, north of the Monterey Bay canyon, and south of our study area (Hyrenbach et al., 2006). The overall low occurrence, but occasionally high abundances, of sitting/feeding BFAL onshore of the shelf-break may represent aggregations of becalmed birds during low wind conditions, or the signature of an earlier feeding event caused by an ephemeral prey patch or attraction to a fishing vessel. Upon the dispersal or vertical emigration of the prey beyond the birds' reach, BFAL may remain in the area digesting or awaiting for favorable wind conditions to resume their search for food. Although vessel survey observations do not enable the tracking of individual bird behavior, previous satellite-tracking studies have documented that BFAL movement rates slow down (or altogether stop) during periods of low wind (Adams and Flora, 2009).
Following Burnham and Anderson (2002), we developed the scaled average weight to facilitate the comparison between individual variables used in a different number of models, when the model-based approach does not yield clear results. The resulting weights, though un-bounded (< 0 to infinity), are easy to interpret, as the scaled average weight is the proportional increase in likelihood for models containing the variable in question. We hope that other researchers will continue to develop, refine, and standardize techniques addressing the challenges of abundance data for individual species and communities (McGill et al., 2007).
Our findings suggest that BFAL dispersion within CBNMS and GFNMS is influenced by features at multiple spatial and temporal scales. Thus, the assessment of BFAL distribution and abundance within sanctuary waters should consider both local and larger-scale environmental drivers.
We developed separate model predictions for the BFAL range (occurrence data) and the areas of aggregation (abundance data, when present). The occurrence data can be used to implement certain diffuse management and monitoring actions, such as fishery observers and vessel-based survey, during the time periods and areas in which the species is expected to occur. While seabird abundance data are highly variable and problematic to model, they augment the inferences from the occurrence models. The abundance models identify areas of aggregation, which can be targeted with special management actions, such as time–area closures (Hyrenbach et al., 2006; Louzao et al., 2006).
Our results will enable NMS managers to refine their definition of the BFAL range and aggregation areas off central California. Additionally, our analysis of BFAL behavior highlights the unique importance of habitats encompassed within this region. Monitoring could be extended to include important features (e.g., the shelf-break) adjacent to sanctuary boundaries, helping managers to identify if BFAL habitat use and aggregation are focused within the sanctuaries or extend past the current boundaries.
Despite the broad range of processes influencing BFAL habitat use and the limited spatial scale of our surveys, the existing CBNMS and GFNMS management framework encompasses important features of BFAL aggregation. Furthermore, it appears that chick-rearing birds commute to these bathymetric features from distant breeding areas (Hyrenbach et al., 2006), further underscoring the ecological significance of these sanctuary waters. However, because dynamic oceanographic processes also play a role, sanctuary management could be enhanced by developing dynamic approaches, like those used to mitigate loggerhead turtle (Caretta caretta) bycatch in driftnet fisheries off California (NMFS, 2002) and in the Hawaii-based longline fishery (Howell et al., 2008).
In particular, the predicted BFAL habitat use patterns can provide a dynamic layer for inclusion in the NOAA coastal and marine spatial planning (CMSP) framework; the sectioning of the marine environment into spatially/temporally explicit areas of compatible and future uses (IOPTF, 2009; Young et al., 2007). To this end, this study has begun to address some of the specific CMSP needs, such as the identification of important static features (Rittenburg Bank) and dynamic processes (upwelling) influencing BFAL dispersion, and the description of the ‘relative ecological importance’ of different habitat features (shelf-break and continental shelf) within CBNMS and GFNMS. The next step of this process would entail overlaying information on the distribution of potential threats over the current understanding of BFAL spatial/temporal dispersion.
Fisheries represent an ongoing threat to BFAL, as bycatch is known to occur in demersal longlines in the Bering Sea, the Gulf of Alaska, and the West Coast of North America (Dietrich and Fitzgerald, 2010; Jannot et al., 2011). In particular, bycatch has been reported by limited-entry sablefish-endorsed vessels operating in the CCS, primarily north of our study area (Jannot et al., 2011). The precautionary management of BFAL interactions with fisheries can afford managers several bycatch reduction options. Managers could empower fishers to reduce their own bycatch by informing them of the location of bycatch-prone areas (Howell et al., 2008) or require the use of bycatch mitigation techniques, including the use of tori-lines or night-setting at times/areas of high BFAL abundance (see Bull, 2007 for more examples). Such condition-specific time–area closures have already been implemented in southern California, where drift gillnet fisheries are temporarily closed during El Niño conditions to reduce the bycatch of loggerhead turtles (first implementation; National Marine Fisheries Service (NMFS), 2002). Similarly, protective measures could be expanded to important shelf-break and shoreward habitats during those seasons (chick-rearing period) and years (elevated localized upwelling) of high BFAL abundance. Furthermore, the consideration of additional regional and basin-wide indices could provide a powerful tool for the spatial management of highly mobile mega-fauna and ocean productivity related to atmospheric/oceanographic forcing (e.g., Mantua et al., 1997; Benson et al., 2007).