Observations on Some Factors Affecting the Hatching of Eggs and the Survival of Young Shallow‐Water Cisco, Leucichthys artedi LeSueur, in Lake Mendota, Wisconsin

Rather suddenly, in the early 1940's, the cisco in Lake Mendota became scarce. The evidence suggested that the decline was caused by heavy summer mortalities in 1940 and 1941, but further study indicated that the summer mortalities had involved mainly older fish and that the primary cause for the decline was a scarcity of young fish in the population at the time of mortalities. Accordingly, our efforts were turned toward a search for the cause of weak year-classes, or year-class fluctuations in general. It was postulated that year-class failures might arise if the eggs hatched under the ice because then the illumination would be insufficient to enable the larval cisco to feed. Experiments were conducted in the laboratory to determine the effects of illumination and of various patterns of incubation temperatures on the date of hatching of the eggs. Eggs exposed continuously to room illumination for 135 to 140 days hatched 7 to 8 days earlier than eggs incubated continuously in darkness. Different values of incubation temperatures acting on the eggs for about one month from the time of fertilization induced the expected proportional rates of development of eggs, but when the eggs were refrigerated at a common temperature after this first month, the rate of development of advanced embryos was retarded while the less developed embryos gained. Under such treatment, eggs incubated at temperatures differing by 1°C in the range 1°–6°C, and then incubated for the remainder of the period at a common temperature below 4°C, hatched about one day apart. Eggs incubated from the time of fertilization at a common temperature below 3°C, and then incubated for the last month at higher temperatures differing by 1°C in the range 1°-6°C, hatched 7 days apart. Eggs which had developed rapidly toward the hatching stage lay dormant for about 3 months at temperatures below 3°C. They hatched about 4 days earlier than eggs incubated continuously at temperatures below 3°C, having accumulated about 150 degree-days greater exposure. The evidence clearly indicated that the eggs of the cisco in Lake Mendota never hatch under the ice regardless of natural seasonal variations in temperatures and illumination incurred during the incubation period. Moreover, the dates of hatching are probably not affected significantly by any factor except possibly different temperatures incurred after the departure of the ice from year to year. Experiments conducted on larval cisco indicated that they require illumination for feeding. They starve if held in darkness with natural densities of food. They begin to swallow water on the first day after hatching as indicated by keeping them in a solution of 2,3,5-triphenyltetrazolium chloride. They also begin to swallow particulate materials within the first day. By the third day they began to consume adult Cyclops sp. and by the fifth day better than 90% of the larval fish fed on adult Cyclops sp. During the first week, the swimming ability of the food organisms was a decisive factor in the degree of feeding success shown by the larval fish. The test consisted of offering living Cyclops sp. to one group of fish and freshly killed Cyclops sp. tothe other group. After the first week, the fish fed equally well on both types of food. The larval fish tolerated about 108–120 degree-days of starvation without harm. Yet, when provided with food they began to feed before the yolk sac was absorbed. The initial postulated cause for year-class failures has been disproved. It is estimated that the eggs never hatch prior to April 25, and since 1880 the ice has not remained on Lake Mendota later than April 21.