Kj. Kuletz et Sj. Kendall, A PRODUCTIVITY INDEX FOR MARBLED MURRELETS IN ALASKA BASED ON SURVEYSAT SEA, The Journal of wildlife management, 62(2), 1998, pp. 446-460
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.