Settlement, first-year growth, and mortality of surfclams, Spisula solidissima

Abstract

As part of a long-term study of factors affecting inshore recruitment of surfclams, Spisula solidissima (Dillwyn) at 12-m deep sites off New Jersey, USA, we followed patterns of settlement and growth of surfclams and their invertebrate predators in benthic samples taken from July 1993 to July 1996. A large pulse of surfclam settlement occurred in late June or early July in all four years. There was also evidence of low-level settlement as late as December in some years. The highest settling density, which occurred in 1993, was 2.4×10 5 m −2. This was the only cohort to survive to the following year in substantial numbers. Settlement appeared to be correlated with downwelling following near-shore upwelling events. The naticid gastropod predator Neverita duplicata Say settled at approximately the same time as surfclams while the naticid Euspira heros Say appears to have settled later. Crab predators ( Carcinus maenas (Linnaeus), Cancer irroratus Say, and Pagurus longicarpus Say) settled later in the summer. Timing of settlement by sea stars Asterias forbesi (Desor) was variable relative to surfclam settlement. Predator abundances were highly variable among samples, but in general were three orders of magnitude lower than surfclam abundances. Naticid boreholes in the juvenile surfclam death assemblage were used to estimate the impact of naticid predation, to determine whether or not naticids were size-selective predators, and to estimate naticid growth rates. Naticid predation never accounted for >21% of surfclam mortality, and was usually <10%. From June to October, the mean size of surfclams consumed by naticids was close to the mean size available. From October to December, coincident with a sharp decline in surfclam growth, the mean size of clams consumed by naticids was greater than the mean size available. Based on maximum abundances of sea stars and previously determined laboratory feeding rates, recently settled sea stars may have consumed 4.2×10 3 clams m −2 d −1, and naticids may have consumed 90 clams m −2 d −1. Surfclam growth rates varied during the summer, and between years (range = 14.6 to 63.5 μm shell length d −1), in part reflecting low and high bottom temperatures during near-shore upwelling and downwelling events, respectively. Surfclams ceased growing over the winter. Maximum length of surfclams at 6 months post-settlement was 16 mm, smaller than the length reported by other researchers. This suggests site specific differences in first-year growth, or the possibility that others may have overestimated first-year growth. We conclude that inshore surfclam growth is temperature dependent, and that survival to one year is dependent on high-density surfclam settlement in the preceding summer.