COMPARATIVE LIFE HISTORIES OF FOUR POPULATIONS OF ORCONECTES PROPINQUUS (GIRARD, 1852) IN SOUTHWESTERN ONTARIO, CANADA (DECAPODA, ASTACIDEA) BY SUSAN COREY Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1 RÉSUMÉ Étude de certains aspects de la biologie de l'écrevisse Orconectespropinquus provenant de quatre localités différentes, sur trois cours d'eau appartenant à deux systèmes fluviaux séparés, dans le sud-ouest de l'Ontario. Cette étude avait pour but de déterminer si des différences existent entre des populations de la même espèce à l'intérieur d'une aire relativement petite (30 km de rayon). Il n'y avait pas de différences statistiques pour beaucoup d'aspects de la biologie entre le site d'Eramosa River et les deux sites de Speed River. La population de Nottawasaga différait de la population Eramosa/Speed en ce que la ponte et l'éclosion des œufs avaient lieu deux semaines plus tard. La fécondité des femelles de Nottawasaga était approximativement deux fois plus élevée que celle des femelles Eramosa/Speed. INTRODUCTION Crayfish are one of the commonest and the largest of benthic invertebrates inhabiting freshwaters and are among the most energetically important benthic invertebrates feeding on plant, animal and detritial material and in turn are prey for many birds, mammals and fish. In North America there are 300 or more species and subspecies of crayfish of which life history aspects are known for only 14 species. Aspects of life histories of most of these species have been based on one location and one population and are summarized and compared by Momot (1984). Over the past several years, Orconectes rusticus (Girard, 1852) has been intro- duced into many freshwater areas in Ontario and the northern United States where it has replaced or is replacing 0. propinquus (Girard, 1852) or O. virilis (Hagen, 1870) (Crocker & Barr, 1968; Capelli, 1982). In several rivers and lakes in southwestern Ontario such as the Eramosa River, 0. propinquus is becoming extinct. Thus a population of 0. propinquus in the Eramosa River was compared to three other 0. propinquus populations to determine the basic life history of the threatened population and to determine the extent of possible intrapopulation differences occurring within a relatively small portion of the species total range. The abbreviation CL is used for carapace length. 130 MATERIALS AND METHODS Specimens of Orconectes propinquus were collected within an area of 30 km radius at the following localities in southwestern Ontario, Canada, during 1983 and 1984: Eramosa River near Guelph, 43 ° 32' S2 "N 80° 10' 59 "W; Speed River in Guelph, 43 ° 31 ' 30 "N 80 ° 15' 10 "W ; Speed River 10 km north of Guelph, i.e., about 5 km above Guelph Lake, 43°38' 50 "N 80° 16' 45 "W; Nottawasaga River, 43 ° 58' 30 "N 80 °00' 30 "W in the Hockley Valley. Collec- tions commenced in mid to late April and continued at 2 to 5 week intervals throughout the summer and into the fall, ceasing in October or November depending on the locality (figs. 1, 2, 3, 4). Most collections consisted of a minimum of 100 animals. Each animal was sexed and placed into one of the following categories: immature, no secondary sexual characteristics; mature non-ovigerous female; ovigerous female; male II (non-breeding stage), and male I (breeding stage). The carapace length ( ± 0.5 mm) was measured from the base of the eyestalk to the mid-dorsal point of the carapace. Travelling length frequency histograms were drawn and the mean of each cohort was determined. The cohorts were successfully separated using the method des- cribed by Harding (1949). The number of eggs, stage of embryological development, and presence or absence of internal parasites, were recorded for each ovigerous female. Other observations taken at time of collecting were: presence and number of mating pairs, habitat and water temperature. Water temperature data were supplemented with information from the Inland Waters Directorate, Guelph, Ontario. Data on discharge rates of water and temperatures were supplied by Environment Canada. RESULTS In all four populations, a total of 46 mating pairs were collected and measured: in 56. 5 9lo of the pairs, the male was larger than the female, 8. 7 9lo both sexes were of equal size and in 34.8% of the pairs, the male was smaller than the female; 9 1 . 2 9lowere year class II animals and year class I and III each accounted for 4.4%. Two periods of mating were noted for all four popula- tions : a major peak just prior to the laying of eggs and a second minor peak in late July and August. Egg release by females began about mid-April in the Eramosa and late April in the two Speed River populations in 1983 and 1984. In the Nottawasaga population the first appearance of eggs occurred two to three weeks later in both years. In all four sites, ovigerous females were not found until the water temperature was >_5°C. In 1983, the greatest number of ovigerous females expressed as a percentage of the total population was: Nottawasaga 15.3%, Eramosa 10.3%, Upper Speed 34.6% and Lower Speed 16.8 % . During the spawning season of 1984 these percentages were higher in the Nottawasaga .