recruitment, we measured the abundance of recently settled lobsters in mid-September, at the end of the post-larval season (Incze and Wahle, 1991; Incze et al.,.
ICES Journal of Marine Science, 57: 430–437. 2000 doi:10.1006/jmsc.1999.0603, available online at http://www.idealibrary.com on
Advection and settlement rates in a benthic invertebrate: recruitment to first benthic stage in Homarus americanus Lewis S. Incze, Richard A. Wahle, and Alvaro T. Palma Incze, L. S., Wahle, R. A., and Palma, A. T. 2000. Advection and settlement rates in a benthic invertebrate: recruitment to first benthic stage in Homarus americanus. – ICES Journal of Marine Science, 57: 430–437. Neustonic post-larvae of the American lobster (Homarus americanus) undertake exploratory forays to search for and select bottom habitat for settlement. Assessments at the end of the settlement season at multiple sites have shown that there may be about a 10-fold increase in concentration from the dispersed pelagic phase to the aggregated early benthic phase in lobsters. Here we examine recruitment at a single site over short time intervals (5–14 d) to evaluate the role of advection, estimate the proportion of potential settlers that recruited to the bottom, and evaluate whether time- or density-dependent effects influenced recruitment. We found that (1) tidal advection easily accounts for the average increased concentration from pelagic to benthic stages; (2) recruitment ranged from 0.1% to 1.8% of the potential settlers advected over the site; (3) the proportion of post-larvae that recruited was similar in high and low recruitment years and had a tendency to increase late in the settlement season; and (4) recruitment was positively correlated with post-larval supply. Densitydependent effects may have existed at the highest levels of recruitment. 2000 International Council for the Exploration of the Sea
Key words: advection, benthic recruitment, density dependence, Homarus americanus, lobster, post-larvae, settlement, young of year. L. S. Incze and R. A. Wahle: Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME 04575, USA; A. T. Palma: University of Maine, Darling Center, Walpole, ME 04573, USA.
Introduction Most marine invertebrates and fishes have larval stages that may disperse over substantial distances (Mileikovsky, 1971). The local supply of recruits for such species is therefore usually the result of transport from non-local sources or from a mixture of sources (Roughgarden et al., 1988; McConnaughey et al., 1994; Pearce and Phillips, 1994). While numerous studies have focused on relatively large-scale mechanisms of transport and their role in recruitment (Shanks, 1995), smaller-scale advective processes within the local environment also are important (Butman, 1987; Jacobsen et al., 1990; Pawlik and Butman, 1993). For organisms requiring specific habitats, recruitment involves the combined dynamics of the moving fluid and organismal behaviours adapted to site selection within the moving and complex features of the medium (Butman et al., 1988; Ekman et al., 1994). In the Gulf of Maine, new benthic recruits of the American lobster (Homarus americanus) are found 1054–3139/00/010430+08 $30.00/0
primarily in shallow sublittoral cobble habitat, which is actively selected by the organism (Wahle and Steneck, 1991) and which confers post-settlement protection from predators (Barshaw et al., 1994). Incze et al. (1997) reported that over a 7-year period in two study areas in Maine and Rhode Island, area-wide averages of youngof-year (YOY) recruitment in cobble ranged from about 0.2 to 1.5 m 2, with slightly more than 4 m 2 occurring at the best sites in some years. Annual neustonic postlarval production values were more than an order of magnitude lower than benthic recruitment per unit area. Under normal conditions, YOY lobsters do not relocate after settlement at the depths studied (Lawton and Lavalli, 1995; Wahle and Incze, 1997), suggesting that the increased concentration from pelagic to benthic stage resulted from the coupled effects of advection and site selection. We examine rates of transport and settlement at meter scale at a single recruitment site for the American lobster, with the objective of determining whether advection could account for the observed increases in concentration. 2000 International Council for the Exploration of the Sea
Advection and settlement rates in a benthic invertebrate
Materials and methods Recruitment was studied in a cove on the west side of Damariscove Island, located 3 km from shore in the western Gulf of Maine (Fig. 1). The island is 3 km long and 0.5 km at its widest, with a narrow isthmus in the mid-section which creates a cove on each side. The west cove gets consistently good recruitment because prevailing summer winds favour convergence of surface water along the western side of the island (Wahle and Incze, 1997). The cove has a narrow section of cobble-boulder habitat (about 3000–5000 m2) running approximately 100 m along the shore and extending to a depth of 5–8 m below mean low water (MLW: Fig. 1). Below this, sand flats dominate. Recruitment was sampled in the natural cobble habitat and in a series of adjacent experimental plots positioned approximately 5–7 m below MLW. We sampled with a diver-operated airlift suction sampler. One diver removed rocks while the second vacuumed the interstitial spaces and excavated several centimetres into the underlying sediment. All samples were kept in separate mesh bags and brought to the surface for sorting and measurement. YOY lobsters were identified as those c10 mm CL (Incze et al., 1997). Data used in this analysis were drawn from two previous studies (Wahle and Incze, 1997; Palma et al., 1998). We had 10 experimental plots of 2 m2 each in 1993 and eight plots 1 m2 each in 1994. Plots consisted of a standardized size and density of cobble intended to mimic dense natural cobble areas: ca. 0.16 m3 of cobble measuring 15–25 cm diameter per m2 of plot. Details of
69° 35' W
Johns Bay
100 m 1 km Damariscove Island
5m
In previous studies of post-larval supply and benthic recruitment, we measured the abundance of recently settled lobsters in mid-September, at the end of the post-larval season (Incze and Wahle, 1991; Incze et al., 1997; Wahle and Incze, 1997). Lobster post-larvae were present for 7 weeks or more in Maine, and during that period most were in middle or late moult-cycle stages (Incze et al., 1997) and competent to settle (Botero and Atema, 1982; Cobb et al., 1989). This means that settlement may have been occurring over several weeks, which introduces two potential problems with a single, end-of-season, assessment of recruitment. First, YOY at the time of sampling were of different ages and, if there were post-settlement losses, they might not have operated equally on all settlers. Second, density-dependent effects, if they existed, might have been masked by a single survey. Knowledge of time-dependent or densitydependent effects might influence future interpretations of post-larva:YOY relationships (Eggleston and Armstrong, 1995; Pile et al., 1996). Our second objective, then, was to determine whether density-dependent interactions or other short-term losses (mortality or emigration) took place.
431
43° 45' N
Figure 1. Study area in the western Gulf of Maine. Left panel shows the experimental site (arrow) on the west side of Damariscove Island and the location of Johns Bay, where the post-larval time series was collected. Right panel shows an enlargement of the island’s midsection and a schematic of the experimental site. Stippled areas are natural cobble and the open circles are mooring sites where tidal velocities were measured; the polygon on the west side indicates the area where experimental plots were placed.
design, deployment and sampling are in Wahle and Incze (1997). We performed a complete enumeration of lobsters in half the plots at approximately weekly intervals from mid-July to mid-September and censused the other half only once, in mid-September. We sampled the natural habitat once in mid-September using twelve 0.46 m2 quadrats placed haphazardly over cobble areas. We have used this method for 7 years at 8 sites in the region (Incze et al., 1997). Sample variance stabilized at 9–10 quadrats and the coefficient of variation for density of new recruits was about 1.0 (Incze and Wahle, 1991; Wahle and Steneck, 1991). We tested the hypothesis that time- or density-dependent effects would cause the sum of settlement from the intervals to be statistically greater than the end-of-season samples. To estimate the background population of post-larvae in coastal waters we used post-larval data collected as part of a time-series in Johns Bay, located 6 km northeast of Damariscove Island. The sampling methods are described by Incze and Wahle (1991). Incze et al. (1997, in press a) showed that samples from Johns Bay provided a good indicator of post-larval abundance in open coastal waters outside the bay and around Damariscove Island: for Johns Bay vs. Damariscove Island, samples collected within 1 d of each other had a statistically significant correlation with a slope of 1.1 (r2 =0.65, df=13, p
