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58 rocks. At night they would come out to forage for food, returning to the burrow, especially after a molt. During several nights of movement, they may move 50 to 200 meters. Females tended to remain in one area longer than males but, when they moved, they moved further. 96 In one neat study, video monitoring was used to observe the movements of marked Northern crayfish individuals in a natural stream. These crayfish were solitary animals and encounters with other crayfishes resulted in a "fight or flight" interaction. Small crayfish used the shallow waters at the stream edge while larger ones used deep water and were more nocturnal. Burrows were used and defended by individual crayfish and, occasionally, a fight for a burrow occurred where the larger crayfish usually won. 43 One study observed their response to a non- crayfish threat which was usually several quick tail-flips for a quick backwards retreat. Larger crayfish might respond to a fish with the "claws-up" meral spread. 43 In interactions between the Northern crayfish and the Calico crayfish, the Northern crayfish was the more aggressive species and would displace the Calico crayfish from crevices. 15 In Ontario, Canada, it was noted that the Northern crayfish was rarely found in swift streams. When tested in the lab, it was found that when the current got over 28 cm/sec (0.92 ft/sec) they started to slip downstream. 153 This is quite low and seems to contradict its frequent presence in Nebraska streams that flow much faster than that. It may be that crayfish may be using the thin boundary layer next to the substrate where velocities are much lower. REPRODUCTION In the Northern crayfish, ovarian maturation depends on an extended period of four to five months of darkness and low temperatures. Increased water temperatures in the spring induce egg laying. 5 Females lay their eggs in the spring and the number of eggs produced by a female depends on body size. As females deposit their eggs while in open water, quite a few are lost but they have been found to have an average of 94 though not all will hatch. 163 When hatched, a baby crayfish looks like a tiny adult. They remain attached to the female for their first two molts then leave to make their own lives. While a female is brooding, she moves little and remains in hiding. 181 In Kings Creek, Kansas, it was noted that the abundance of Water Nymph crayfish began to increase in June and July with the appearance of newly hatched individuals. 55 PRODUCTION AND GROWTH In Michigan lakes, the mean standing crop varied from 9.4 to 30.3 kg/ha while annual production ranged from 71.9 to 169.7 kg/ha. 167 In small lakes in Ontario, annual production was 18.9 to 70.4 kg/ha/year. 168