Foraging profitability can be strongly affected by the size structure of different prey, so that predator distributions are not a simple function of total prey biomass. For a bottom-feeding avian predator, the surf scoter Melanitta perspicillata, we assessed effects of prey size and other prey attributes on seasonal shifts in scoter use of 2 major foraging habitats in Puget Sound, Washington, USA. During early winter, many thousands of scoters fed at an unvegetated site where profitable prey appeared limited to mussels Mytilus trossulus of smaller sizes (2 to 30 mm) despite their much lower biomass relative to larger mussels and several other prey types. Accordingly, scoter numbers decreased at that site as small mussels declined over winter. During pre-migratory fattening in spring and feather molt in summer, >8000 surf scoters aggregated at a seagrass site where they fed mainly on epifaunal crustaceans (50 to 73%) and gastropods (12 to 27%). Body sizes of most crustacean prey had increased substantially since winter. Thus, prey size had opposite effects on the profitability of unvegetated habitats that provide mainly mussels (smaller items likely reduce shell processing costs) versus seagrass crustaceans (larger items are likely more visible and yield greater energy per prey item, although relative mobility of prey can alter their value). Total prey biomass, and prey distributions relative to water and sediment depths, appeared less important than prey size to shifts in scoter diets and numbers. Our synthesis of past studies indicates that biomass and production of mussel beds are typically an order of magnitude greater than for entire assemblages of seagrass macroinvertebrates. However, because of seasonal shifts in prey size structure, seagrass sites can be an important complement to mussel beds when the narrow size fraction of mussels that are profitable to scoters declines.
Many productive ocean ecosystems are also highly variable, resulting in complex trophic interactions. We analyzed interannual patterns in the diet of a seabird, the common murre Uria aalge, in a region of high oceanographic productivity, the northern California Current, to investigate how these top predators adjust their chick provisioning to cope with environmental variability. Murres relied chiefly on Pacific herring Clupea harengus pallasi and surf smelt Hypomesus pretiosus to provision chicks, although they regularly returned 8 other fish taxa. Provisioning success was measured by the energy return rate to chicks, which in turn was disarticulated into energy per meal (quality) and meal delivery rate (quantity). Parents exhibited ‘compensation’ during 2 years in which smaller, low quality prey were returned more quickly than in years with normal (i.e. ‘good’) provisioning. Despite the increased delivery rate, energy return rates were still lower in ‘compensation’ vs. ‘good’ years. The lowest energy return rates occurred in 3 ‘poor’ years, during which ocean productivity was also depressed. Our results suggest that murres in this system have the ability to shift provisioning strategies to deal with some variability in prey resources, but not when limited by exceptionally poor environmental conditions.
Two pilot trials and one study in a closely related grebe species suggest that Western grebes (Aechmophorus occidentalis) will not tolerate intracoelomic transmitter implantation with percutaneous antennae and often die within days of surgery. Wild Western grebes (n = 21) were captured to evaluate a modified surgical technique. Seven birds were surgically implanted with intracoelomic transmitters with percutaneous antennae by using the modified technique (transmitter group), 7 received the same surgery without transmitter implantation (celiotomy group), and 7 served as controls (only undergoing anesthesia). Modifications included laterally offsetting the body wall incision from the skin incision, application of absorbable cyanoacrylate tissue glue to the subcutaneous space between the body wall and skin incisions, application of a waterproof sealant to the skin incision after suture closure, and application of a piece of porcine small intestine submucosa to the antenna egress. Survival did not differ among the 3 groups with 7 of 7 control, 6 of 7 celiotomy, and 6 of 7 transmitter birds surviving the 9-day study. Experimental birds were euthanized at the end of the study, and postmortem findings indicated normal healing. Significant differences in plasma chemistry or immune function were not detected among the 3 groups, and only minor differences were detected in red blood cell indices and plasma proteins. After surgery, the birds in the transmitter group spent more time preening tail feathers than those in the control and celiotomy groups. These results demonstrate that, in a captive situation, celiotomy and intracoelomic transmitter implantation caused minimal detectable homeostatic disturbance in this species and that Western grebes can survive implantation of intracoelomic transmitters with percutaneous antennae. It remains to be determined what potential this modified surgical procedure has to improve postoperative survival of Western grebes that are intracelomically implanted with transmitters with percutaneous antennae and released into the wild.
We present a prototype monitoring strategy for estimating the density and number of occupied burrows of burrow-nesting seabirds. We use data and management questions from Washington State as an example that can be applied to burrow-nesting seabirds at single- or multi-island scales. We also demonstrate how habitat assessments can be conducted concurrently. Specifically, we compared the density and occupancy of burrows of the Rhinoceros Auklet (Cerorhinca monocerata) at nesting colonies in the California Current and the Salish Sea and in the 1970s, 1980s, and today. We estimated 36 152, 1546, and 6494 occupied burrows on Protection and Smith islands (Salish Sea), and Destruction Island (California Current), respectively. Our estimates for the Salish Sea are 52% greater than those from the 1970s and 1980s, while that for the California Current is 60% less than that of 1975. This suggests that the Salish Sea population has increased, despite greater human effects on that ecosystem. However, some of the estimated changes between the periods could be the result of methodological and analytical differences. To address these issues we recommend an unbiased and representative sampling approach (stratified random) and an approach for optimally allocating the samples among strata within and among islands, depending on the scale of the question being addressed. Optimally allocating the sample would save a great deal of field effort; using this approach, we achieve relatively high power (>0.80) to detect moderate changes (20%) sampling hundreds of fewer plots than in a sample not optimally allocated.
Presentamos un estrategia de monitoreo prototipo para estimar la densidad y el número de madrigueras ocupadas en aves marinas que anidan en madrigueras. Empleamos datos y preguntas de manejo del estado de Washington como un ejemplo que puede ser aplicado a diversas aves marinas que anidan en madrigueras a la escala de una isla única o de múltiples islas. También demostramos como las evaluaciones de hábitat pueden ser conducidas conjuntamente. Específicamente, comparamos la densidad y la ocupación de madrigueras de Cerorhinca monocerata en las colonias de anidación de la Corriente de California y del Mar de Salish en los años 70s, 80s y hoy. Estimamos 36 152, 1546 y 6494 madrigueras ocupadas en las islas Protección y Smith (Mar de Salish) y la isla Destrucción (Corriente de California), respectivamente. Nuestras estimaciones para el Mar de Salish son 52% mayores que aquellas de los años 70s y 80s, mientras que para la Corriente de California son 60% menos que la de 1975. Esto sugiere que la población del Mar de Salish ha incrementado, a pesar de los mayores efectos antrópicos en este ecosistema. Sin embargo, algunos de los cambios estimados entre los períodos podrán ser el resultado de diferencias metodológicas y analíticas. Para abordar estos temas, recomendamos un enfoque de muestreo no sesgado y representativo (estratificado al azar) y un enfoque para asignar óptimamente las muestras entre los estratos dentro y entre islas, dependiendo de la escala de la pregunta abordada. La asignación óptima de la muestra ahorraría un gran parte del esfuerzo de muestreo; usando este enfoque, alcanzamos un poder relativamente alto (>0.80) para detectar cambios moderados (20%), muestreando cientos de parcelas menos que en un muestreo no asignado de modo óptimo.
Identifying important foraging sites for highly mobile marine predators has relied mainly on relating their distributions to broadly defined habitat data. However, understanding functional dependencies on foraging sites also requires knowledge of the relative contributions of foods to predator condition. We coupled predator distributions with measures of their diet and condition to assess the importance of Pacific herring Clupea pallasii spawning events to 2 closely related and declining sea duck species. In Puget Sound, Washington, the numerical response of scoters to spawn increased with increasing biomass of spawning herring; this response was 4-fold greater for surf scoters Melanitta perspicillata than for white-winged scoters M. fusca after accounting for local differences in their abundances. In the Strait of Georgia, British Columbia, diets estimated from fatty acids and stable isotopes indicated that both scoter species gained mass by consuming spawn during late March to early April. At a site without spawn during this period, only male white-winged scoters gained mass. In contrast, body mass of male surf scoters declined appreciably before spawn became available in one study year, suggesting greater dependence on spawn for restoring depleted reserves. From winter to spring, surf scoters attained greatest body mass during late April to mid-May while migrating through southeast Alaska; during this period, plasma triglycerides suggested that fattening was not related solely to spawn consumption, yet surf scoters aggregated to consume spawn whenever it was available. Although it is not clear whether herring are essential to their population processes, surf scoters and a range of other predators for which spawning areas are clearly preferred foraging sites would likely benefit from efforts that preserve declining herring stocks.