PRESETTLEMENT AND POSTSETTLEMENT PROCESSES IN RECRUITMENT OF THE AMERICAN LOBSTER

In censuses conducted between 1989 and 1993 the cause of consistent lo cal differences in benthic recruitment of the American lobster (Homaru s americanus Milne-Edwards) to coastal sites in Maine was unclear. Fie ld experiments were conducted to assess the role of pre-and post-settl ement processes in causing high and low extremes in recruitment on opp osite sides of an outer coastal island in our study area. The west sid e of the island has some of the highest population densities measured in New England. Our results indicate that postlarval supply determines these differences in recruitment between the two sites. Standardized replicate cobble plots deployed on each side of the island ruled out h abitat differences as an explanation for these differences, because th ey exhibited the same east-west difference in recruitment as the natur al habitat. We also ruled out differing rates of post-settlement loss because we recovered previously marked and released settlers in equal numbers in similar plots from both sites. The distribution of neustoni c postlarvae and hydrographic evidence indicate that wind-driven surfa ce transport produces an asymmetric postlarval supply to the two sides of the island during the settlement season. Differences in the degree of asymmetry from year to year correspond to differences in the magni tude of eastward transport. We also conducted experiments at the site receiving high recruitment to assess whether new recruits or older yea r classes were near the saturation of cobble habitat for these animals . The combination of saturation seeding trials, using hatchery-reared lobsters, and weekly counts of natural recruits and immigrants suggest s that lobsters may become increasingly subject to crowding as they gr ow. Movements away from the initial settlement site, probably in part caused by crowding, tend to smooth the inequality in population densit y which is set initially by postlarval supply. (C) 1997 Elsevier Scien ce B.V.