GROWTH AND ABUNDANCE OF THE CRAYFISH ORCONECTES-PROPINOUUS IN A HARD WATER AND A SOFT-WATER STREAM

Two populations of O. propinquus were studied from the hard water Ford River and soft water Baraga Creek in northern Michigan. The streams were physically and chemically similar, except for water hardness, alkalinity, conductivity and pH. Each population was sampled regularly by kick-seining into block nets. Mean abundance of young-of-the-year and year one size classes was higher in the hard water stream than in the soft water stream in 1989 (10.7 young-of-the-year/m2 vs. 1.2/m2, 9.2 year one individuals/m2 vs. 3.0/m2). Total annual net production of O. propinquus was higher in the hard water stream (5.0 g/m2 vs. 3.0 g/m2). Individual growth of both young-of-the-year and year one size classes was greater in the soft water stream. Thus, there was an inverse relationship between individual growth and crayfish density in the two streams. There was considerable year-to-year variation in mean abundance and growth (inferred from mean size) in the Ford River (10.1 year one individuals/m2 and 0.729 g dry weight/individual in 1989 vs 5.6/m2 and 0.472 g dry weight/individual in 1990). A rapid, steady decline in abundance of the Ford River year one population occurred during the summer of 1989. Water hardness may be affecting the abundance and growth of O. propinquus in the two streams, both directly and indirectly, but our results do not allow us to establish causality.