Advancing presence and changes in body size of brown shrimp Crangon crangon on intertidal flats in the western Dutch Wadden Sea, 1984-2018

Upon settlement after a pelagic larval phase, brown shrimp Crangon crangon depend on intertidal flats. During low as well as high tide the young brown shrimp play roles as predators of meiofauna and as prey for fish and birds. Unlike the biology of the commercially important adults, knowledge on these juveniles remains sketchy. Here we provide an analysis of 35 years (1984-2018) of brown shrimp monitoring in May-June on intertidal flats in the westernmost Dutch Wadden Sea. Intertidal shrimp densities were sampled bi-weekly at three stations during low tide, using sampling corers. We show that over this 35-year period the appearance of shrimp on mudflats advanced by 12 days (- 0.34 days yr(-1)). Simultaneously, densities on 7 May increased by more than 2.4 times, from 28 shrimp m(-2) in 1984 to 69 shrimp m(-2) in 2018. Across years, mean shrimp length decreased from 12.6 to 10.7 mm, but length in early May did not change. The advancement in settlement and the increasing shrimp densities correlated with increases in the seawater temperatures in April more than during earlier times of the year. We propose four interpretations of these changes: (1) shrimp settle on the mudflat when they reach a certain 'threshold' length, (2) settlement of shrimp is controlled by a critical period of 'threshold' temperature sensitivity, (3) timing of shrimp settlement is a response to food availability on mudflats or (4) a direct response to inferred predation pressure. The different interpretations will lead to different scenarios of change in a warming world.