Recruitment in Macoma balthica after mild and cold winters and its possible control by egg production and shrimp predation

In several species of bivalve molluscs recruitment is more successful in a summer after a cold than after a mild winter. Results of a long-term monitoring program on Balgzand, a 50-km(2) tidal-flat area in the westernmost part of the Wadden Sea, included a significantly negative relationship between winter temperatures and subsequent densities of Macoma balthica recruits in August. We show that this correlation may have arisen from two winter-temperature related processes: 1. Numbers of eggs spawned in April (both per female and per m(2)) were higher after cold than after mild winters. Results of experiments showed that this is due to higher body weights in late winter/early spring after cold than after mild winters, resulting from lower rates of weight loss at low than at high winter temperatures. 2. Densities of juvenile shrimps Crangon crangon on the tidal flats were significantly lower in springs after cold than after mild winters. Shrimps are important predators of post-larval just-settled Macoma. Numbers of Macoma recruits per spawned egg were generally between 0.001 and 0.0001 and were significantly higher at low than at high shrimp densities in late-spring. We conclude that the latter process was the decisive one.