Abstract
As global ocean-bound commerce increases, managing human activities has become important in reducing conflict with threatened wildlife. This study investigates environmental factors determining abundance and distribution of blue whales (Balaenoptera musculus), humpback whales (Megaptera novaeangliae) and their prey (Euphausia pacifica and Thysanoessa spinifera) in central California. We provide insights into environmental drivers of the ecology and distribution of these species, model whale distributions and determine coincident hotspots of whales and their prey that will help decrease human threats to whales and protect critical feeding habitat. We developed separate predictive models of whale abundances (using negative binomial regression on count data) and krill abundance (using a two-part hurdlemodel combining logistic and negative binomial regressions) over a 14 year period (2004–2017). Variables included in situ surface and midwater oceanographic measures (temperature, salinity, and fluorescence), basin-scale climate indices, and bathymetric- and distance-related data. Predictions were applied to 1 km2 cells spanning the study area for May, June, July, and September during each of the 14 years of surveys to identify persistent distribution patterns. Both whales and krill were found to consistently use the northeast region of Cordell Bank, the Farallon Escarpment, and the shelf-break waters. The main identified blue whale hotspots were also krill hotspots, while co-occurrence was more limited and varied seasonally for humpback whales and krill. These results are valuable in identifying patterns in important areas of ecological interaction to assist management of whales. Areas north of Cordell Bank are of particular management concern since they overlap with the end of the San Francisco Bay northern shipping lane. Our findings can help decrease threats to whales, particularly in important foraging areas, by supporting implementation of vessel management and informing potential conflicts with other human uses.
Highlights
Our oceans face increasing pressures from human activities, including commercial fishing and shipping [1,2,3,4]
We used data collected by the Applied California Current Ecosystem Studies (ACCESS) program, an ongoing collaboration between Point Blue Conservation Science, CBNMS, and GFNMS (Fig 1)
Significant variables in the logistic portion of the krill model included all midwater oceanographic variables, distance to bathymetric features, contour index, and 4 climate indices (SOI, Pacific Decadal Oscillation (PDO), NPGO, UI); all the above were significant at p
Summary
Our oceans face increasing pressures from human activities, including commercial fishing and shipping [1,2,3,4]. An increase in traffic in conjunction with other human activities (e.g. fisheries, energy production, climate change) places additional stress upon marine ecosystems, potentially resulting in loss of biodiversity and declines in endangered species [2,3]. Previous research has highlighted areas in central California within two National Marine Sanctuaries as important to foraging humpback (Megaptera novaengliae) and blue whales (Balaenoptera musculus) [6,7,8]. This high-use whale area near San Francisco Bay has seen numerous documented instances of ship strikes and 24 reported whale mortalities from 2008 to 2018. Modeling spatial patterns of habitat use by species within the Sanctuaries can help managers make more informed decisions to preserve a healthy, functioning ecosystem and recover depleted animal populations
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