Long-term dynamics in Atlantic surfclam (Spisula solidissima) populations: The role of bottom water temperature

The potential linkages between warming bottom temperatures and increased mortality and/or reduced growth of the Atlantic surfclam (Spisula solidissima) were investigated using a model that simulates the temperature-dependent growth of the post-settlement population at specific locations on the Middle Atlantic Bight (MAB) continental shelf. External forcing for the individual-based surfclam model is provided by a 50-year simulation (1958-2007) of bottom water temperature obtained from an implementation of the Regional Ocean Modeling System for the northwestern Atlantic. The simulations show that in years with above average bottom water temperature (> 2 °C above average), surfclam assimilation rate is significantly reduced as a result of thermal stress, which leads to starvation mortality and an overall decline in the surfclam population of 2-9%, mainly in the inner shelf regions. Years with warmer bottom water temperatures were preceded by warm winters, which produced an earlier and longer summer season. These results suggest that the long-term observed decline in Atlantic surfclam populations on the MAB is a response to episodic warm years rather than a gradual warming trend in bottom water temperature, as previously suggested. These temperature driven population declines can persist for several years and have the largest effect on older and larger animals, which are the target of the commercial fishery.