Eric P. Bjorkstedt

Population Characteristics of the Mangrove Crab Scylla serrata (Decapoda: Portunidae) in Kosrae, Federated States of Micronesia: Effects of Harvest and Implications for Management

ABSTRACT Apparent declines in abundance of mangrove crabs Scylla serrata (Forsskål, 1755) in Kosrae, Federated States of Micronesia, have prompted concern regarding long-term persistence of this important cultural and economic resource. To support development of effective management strategies, we gathered basic biological information about mangrove crabs on this island, where S. serrata is the only mangrove crab species present. In particular, we were interested in understanding movement patterns and evaluating spatial variation in population structure. Many population characteristics, including estimated life span, ontogenetic shifts in habitat use, sex-specific allometric relationships, male-biased sex ratios, and evidence for limited (<2 km) alongshore movement, are similar to those reported elsewhere in the range of the species. Therefore, insights from S. serrata populations elsewhere might usefully inform management of the species on Kosrae. Moreover, information reported in this study, for which there is no ambiguity about species identification, has broader relevance. Spatial variation in size structure of the population appears to be driven by variable harvest pressure that reflects distribution of the human population and location of emerging commercial harvest operations. Effective management of mangrove crabs is therefore likely to benefit from application of size-based or sex-based restrictions on harvest and might usefully incorporate spatially explicit strategies, such as partial or complete reserves. Development and implementation of effective management will necessarily depend on cultural as well as scientific information.

Massive Mortality of a Planktivorous Seabird in Response to a Marine Heatwave

Climate change has exacerbated the occurrence of large-scale sea-surface temperature anomalies, or marine heatwaves (MHW) - extreme phenomena often associated with mass mortality events of marine organisms. Using a combination of citizen science and federal datasets, we investigated the causal mechanisms of the 2014/15 die-off of Cassins Auklets (Ptychoramphus aleuticus), a small zooplanktivorous seabird, during the NE Pacific MHW of 2013-2015. Carcass deposition followed an effective reduction in the energy content of mesozooplankton, coincident with the loss of cold-water foraging habitat caused by the intrusion of the NE Pacific MHW into the nearshore environment. Models examining interannual variability in effort-controlled carcass abundance (2001-2014) identified the biomass of lipid-poor zooplankton as the dominant predictor of increased carcass abundance. In 2014, Cassins Auklets dispersing from colonies in British Columbia likely congregated into a nearshore band of cooler upwelled water, and ultimately died from starvation following the shift in zooplankton composition associated with onshore transport of the NE Pacific MHW. For Cassins Auklets, already in decline due to ocean warming, large-scale and persistent MHWs might represent a global population precipice.

The origins of the anomalous warming in the California coastal ocean and San Francisco Bay during 2014–2016

During 2014 exceptionally warm water temperatures developed across a wide area off the California coast and within San Francisco Bay (SFB) and persisted into 2016. Observations and numerical model output are used to document this warming and determine its origins. The coastal warming was mostly confined to the upper 100 meters of the ocean and was manifested strongly in the two leading modes of upper ocean (0-100 m) temperature variability in the extra-tropical eastern Pacific. Observations suggest that the coastal warming in 2014 propagated into nearshore regions from the west while later indicating a warming influence that propagated from south to north into the region associated with the 2015-16 El Nino event. An analysis of the upper ocean (0-100 m) heat budget in a Regional Ocean Modeling System simulation confirmed this scenario. The results from a set of sensitivity runs with the model in which the lateral boundary conditions varied supported the conclusions drawn from the heat budget analysis. Concerning the warming in the SFB, an examination of the observations and the heat budget in an unstructured-grid numerical model simulation suggested that the warming during the second half of 2014 and early 2016 originated in the adjacent California coastal ocean and propagated through the Golden Gate into the Bay. The finding that the coastal and Bay warming are due to the relatively slow propagation of signals from remote sources raises the possibility that such warming events may be predictable many months or even several seasons in advance.

Ichthyological Assemblage and Variation in a Northern California Zostera marina Eelgrass Bed

Abstract Fish assemblages on an Eelgrass (Zostera marina) bed located in Humboldt Bay, California were studied using an epibenthic otter trawl. Sampling was conducted on a monthly basis from 1994 to 1995, and 2003 through 2006. A total of 204 tows over 51 mo yielded 16,760 individual fish representing 20 families and 43 species. The catch was dominated by a few species, with 4 species comprising 74.3% of the total number of fish collected. Black Rockfish (Sebastes melanops) were the most abundant species taken, accounting for 22.5% of the total collected. Annual peaks in abundance and species richness were observed during the late spring and summer months, with smaller peaks noted in winter. Fish assemblages differed seasonally, with late spring and summer assemblages significantly different from other times of the year, due to recruitment of young-of-the-year life stages. Temporal patterns of community structure were also analyzed by decade and season, with no interaction among main effects. Seasonal and temporal patterns were analyzed using permutational multivariate analysis of variance and plotted using nonmetric multidimensional scaling. These results demonstrate the importance of Eelgrass habitats to juvenile fishes, and the value of data sets collected over various temporal scales.

Predation on Juvenile Pacific Salmon Oncorhynchus spp. in Downstream Migrant Traps in Prairie Creek, California

Abstract Downstream migrant traps are a widely applied fishery management tool for sampling anadromous Pacific salmon Oncorhynchus spp. and steelhead O. mykiss smolts along the West Coast of North America and elsewhere, yet predation on juvenile salmonids in traps has not been studied quantitatively. We assessed the frequency of occurrence and abundance of juvenile salmonids in the stomachs of coastal cutthroat trout O. clarkii clarkii, coho salmon O. kisutch, steelhead, and prickly sculpin Cottus asper (>70 mm fork length) captured in traps and in nearby stream habitats. All four predator species took juvenile salmonids with much greater frequency in traps than in stream habitats. Among free-swimming predators, only coastal cutthroat trout were observed with salmonid fry in their stomachs, but they took fewer salmonid prey and appeared to rely more heavily on insect prey than did coastal cutthroat trout captured in traps. Predators consumed up to 25% of the available prey over a broad range of prey abund...