A Productivity Index for Marbled Murrelets in Alaska Based on Surveys at Sea

Abstract

Because the reproductive success of marbled murrelets (Brachyramphus marmoratus) cannot be efficiently monitored directly, we investigated a productivity index for marbled murrelets in Alaska that uses at-sea counts of hatching-year (HY) birds and after-hatching-year (AHY) birds. Our objectives were to define seasonal patterns of abundance by age class and to develop a protocol for estimating productivity. In Prince William Sound (PWS), we conducted replicate boat surveys at 2 sites in 1994 and 6 sites in 1995 (45-60 km of shoreline per survey). At all sites, numbers of AHY birds peaked in late July and declined throughout August, whereas HY birds appeared after 18 July and peaked between 28 July and 28 August. Because AHY numbers decreased in August, the percentage of HY birds increased. Numbers of HY birds in July-August ranged among sites from 2-13% of total murrelets. Comparing HY densities among sites and years, rather than HY:AHY ratios, might avoid problems associated with postbreeding migration of adults. The HY densities in July-August were highly correlated with June AHY densities; thus, HY:AHY ratios derived from AHY densities during incubation phase may be more reliable than concurrent July-August HY:AHY ratios. We decreased variance in juvenile densities by selecting a core survey period of approximately 3 weeks, centered around peak juvenile abundance. One potential method of identifying the core period is the numbers of adults holding fish for delivery to inland nests. Power to detect a 50% change in juvenile abundance generally was >80% when 5 surveys occur each year for ≥10 years (when CV = 0.4 and α = 0.1). Power to detect a 50% change in abundance in 5 years is >80% when 8 surveys are conducted each year. The slope formed from the regression of June AHY versus July-August HY counts may also provide a regional index of marbled murrelet reproductive success. Our results suggest that marbled murrelet productivity in PWS is high compared to regions farther south.