A PRODUCTIVITY INDEX FOR MARBLED MURRELETS IN ALASKA BASED ON SURVEYSAT SEA

Because the reproductive success of marbled murrelets (Brachyramphus m armoratus) cannot be efficiently monitored directly, we investigated a productivity index for marbled murrelets in Alaska that uses at-sea c ounts of hatching-year (HY) birds and after-hatching-lear (AHY) birds. Our objectives were to define seasonal patterns of abundance by age c lass 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 s ites, 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 perce ntage of HY birds increased. Numbers of HY birds in July-August ranged among sites from 2-13% of total murrelets. Comparing HY densities amo ng sites and) ears, rather than HY:AHY ratios, might avoid problems as sociated with postbreeding migration of adults. The HY densities in Ju ly-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 vari ance in juvenile densities by selecting a ''core'' survey period of ap proximately 3 weeks, centered around peak juvenile abundance. One pote ntial method of identifying the core period is the numbers of adults h olding fish for deliver), to inland nests. Power to detect a 50% chang e in juvenile abundance generally was >80% when 5 surveys occur each l ear for greater than or equal to 10 > ears (when CV = 0.4 and alpha = 0.1). Power to detect a 50% change in abundance in 5,ears is >80% when 8 surveys are conducted each year. The slope formed from the regressi on of June AHY versus July-August HY counts may also provide a regiona l index of marbled murrelet reproductive success. Our results suggest that marbled murrelet productivity in PWS is high compared to regions farther south.