Seasonal effects of intraspecific density and predator exclusion along a shore-level gradient on survival and growth of juveniles of the soft-shell clam, Mya arenaria L., in Maine, USA

The relative roles that predation and competition play in regulating populations of infaunal marine bivalves in soft-bottom systems are strikingly different. Exploitative competition for food typically occurs at elevated densities, but crowding rarely results in mortality and competitive exclusion. Predation by decapods, gastropods, and, sometimes asteroids, is more important in controlling patterns of distribution and abundance. Most field tests leading to this synthesis have been conducted between 35 degreesN and 35 degreesS and/or with bivalves in the families Veneridae and Tellinidae. To test the robustness of these ecological processes at another geographic setting (45 degreesN) using a species from another family within the suspension-feeding guild (Myidae), we per-formed a short-term field manipulation at an intertidal mud flat in eastern Maine, USA. We followed survival and growth of 10,080 juveniles (12.4-mm shell length (SL)) of the soft-shell clam, Mya arenaria L., in field enclosures with and without predator-deterrent netting at three densities (330, 660 and 1320 m(-2)) along a tidal gradient over four sampling intervals from April to December 1996. We used a generalized completely randomized block design to assess variation in these dependent variables within a given tidal height (high, mid, and low) on a particular date. Mortality varied seasonally, peaking (13.6%) between August and September when seawater temperatures were warmest. No significant mortality occurred after September, when mean (+/- 95% CI) percent survival pooled over all treatments was 72.9 +/- 8.5%. Netting (6.4-mm aperture) effectively excluded predators along the tidal gradient as overall mean clam survival, independent of tidal position, was 88.7 +/- 4.1% in protected units (plastic plant pots with area = 181 cm(2)), but decreased from upper and mid tide levels (82.9 +/- 6.1%) to lower on the shore (66.3 +/- 9.7%) in unprotected units. Density-dependent mortality resulted in reduced survival (-4.6%) in clams stocked at the two highest levels (P < 0.001); however. numbers of dead clams with undamaged valves provided little evidence that this effect was due to starvation. Incremental growth also varied seasonally with greatest a-mounts of shell added during June-August at all tidal levels. Shell growth stopped or slowed significantly after September at all tidal positions. Mean SL increased with decreasing tidal height (December sizes: high = 20.6 2.9 mm, mid = 24.1 +/- 1.0 mm; low = 28.2 +/- 1.2 mm), however. submergence time alone failed to explain completely these differences. Density-dependent growth was detected once (August-September). Animals at the two highest densities experienced a growth depression of ca. 7%. We conclude predation, rather than competition, is more important in regulating populations of soft-shell clams in this intertidal location. (C) 2001 Elsevier Science B.V. All rights reserved.