Evan A. Howell

The transition zone chlorophyll front, a dynamic global feature defining migration and forage habitat for marine resources

Abstract Pelagic ecosystem dynamics on all temporal scales may be driven by the dynamics of very specialized oceanic habitats. One such habitat is the basin-wide chlorophyll front located at the boundary between the low chlorophyll subtropical gyres and the high chlorophyll subarctic gyres. Global satellite maps of surface chlorophyll clearly show this feature in all oceans. In the North Pacific, the front is over 8000 km long and seasonally migrates north and south about 1000 km. In the winter this front is located at about 30–35°N latitude and in the summer at about 40–45°N. It is a zone of surface convergence where cool, vertically mixed, high chlorophyll, surface water on the north side sinks beneath warm, stratified, low chlorophyll water on the south side. Satellite telemetry data on movements of loggerhead turtles and detailed fisheries data for albacore tuna show that both apex predators travel along this front as they migrate across the North Pacific. The front is easily monitored with ocean color satellite remote sensing. A change in the position of the TZCF between 1997 and 1998 appears to have altered the spatial distribution of loggerhead turtles. The position and dynamics of the front varied substantially between the 1998 El Nino and the 1999 La Nina. For example, from May to July 1999 the transition zone chlorophyll front (TZCF) remained between about 35°N and 40°N latitude showing very little meandering, whereas in 1998, during the same period, the TZCF exhibited considerable meandering and greater monthly latitudinal movement. Catch rates for albacore were considerably higher in 1998 than in 1999, and we hypothesize that a meandering TZCF creates regions of convergence, which enhances the foraging habitat for apex predators along the front.

WhaleWatch: a dynamic management tool for predicting blue whale density in the California Current

Summary Management of highly migratory species is reliant on spatially and temporally explicit information on their distribution and abundance. Satellite telemetry provides time-series data on individual movements. However, these data are underutilized in management applications in part because they provide presence-only information rather than abundance information such as density. Eastern North Pacific blue whales are listed as threatened, and ship strikes have been suggested as a key factor limiting their recovery. Here, we developed a satellite-telemetry-based habitat model in a case–control design for Eastern North Pacific blue whales Balaenoptera musculus that was combined with previously published abundance estimates to predict habitat preference and densities. Further, we operationalize an automated, near-real-time whale density prediction tool based on up-to-date environmental data for use by managers and other stakeholders. A switching state-space movement model was applied to 104 blue whale satellite tracks from 1994 to 2008 to account for errors in the location estimates and provide daily positions (case points). We simulated positions using a correlated random walk model (control points) and sampled the environment at each case and control point. Generalized additive mixed models and boosted regression trees were applied to determine the probability of occurrence based on environmental covariates. Models were used to predict 8-day and monthly resolution, year-round density estimates scaled by population abundance estimates that provide a critical tool for understanding seasonal and interannual changes in habitat use. The telemetry-based habitat model predicted known blue whale hot spots and had seasonal agreement with sightings data, highlighting the skill of the model for predicting blue whale habitat preference and density. We identified high interannual variability in occurrence emphasizing the benefit of dynamic models compared to multiyear averages. Synthesis and applications. This near-real-time tool allows a more accurate examination of the year-round spatio-temporal overlap of blue whales with potentially harmful human activities, such as shipping. This approach should also be applicable to other species for which sufficient telemetry data are available. The dynamic predictive product developed here is an important tool that allows managers to consider finer-scale management areas that are more economically feasible and socially acceptable.

Sensors detect biological change in mid‐latitude North Pacific

High temporal and spatial resolution ocean color data for the global ocean were collected for January–June 1997 by the Ocean Color and Temperature Scanner (OCTS) from the Japanese ADEOS satellite and for September 1997 to the present by the Sea-viewing Wide Field-of-view sensor (SeaWiFS). These sensors show the North Pacific Subtropical Gyre characterized by surface chlorophyll less than 0.15 mg/m3, while to the north, the Transition Zone and Subarctic Gyre exhibit surface chlorophyll in excess of 0.25 mg/m3 (Figure 1). The boundary between the low and high chlorophyll domains can be characterized by the 0.2 mg/m3 chlorophyll contour line (Figure l). This boundary is termed the Transition Zone Chlorophyll Front (TZCF) because it moves seasonally between the southern and northern limits of the Transition Zone, coinciding with the convergence of cool, vertically mixed, high chlorophyll water found to the north with warmer, stratified, low chlorophyll water on the south. In addition to simply marking the separation between high and low chlorophyll regions, the TZCF is used as a migratory and forage habitat by apex predators including sea turtles and tunas [Polovina et al., 2000].