|Other Abstract||Human activities have profoundly affected the environments and population structures of fishes. It is of great significance in the theoretic researches and practical application to assess the effects of alternation of habitats and exploitation on fish life histories. Honghu Lake and Dongting Lake, two important fisheries waterbodies have remarkably different biotic and abiotic environments, as well as different natural and human influences, so the fishes in these lakes may evolve the life-history strategies to adapt to their environments. It is important for management of the fish population resources to comparatively study the adaptive variations in life-history traits of fishes in the two lakes. Crucian carp, Carassius cturatus, a dominant fish species both in Honghu Lake and Dongting Lake, was selected to detect the differences in some of important life-history traits, such as population structures, growth, age and size at maturity, sex-ratio, and reproductive investments etc. and to analyze the causes of the variations in life-history traits and their adaptive significance. Field observations were carried out during January 1998 to September 1999, with a total 5620 samples collected. Moreover, in a attempt to make an insight into the underlying causes, the crucian carp sampled from the two lakes were reared in 'common garden' between December 1998 and May 1999, and Fl generations with triplicates from the two populations were reared by feed A(soybean cakes used as the main resources of protein, 32.22% crude protein ) and feed B (fishmeal used as the main resources of protein, 40.41% crude protein) during September 1999 to May 2000. The main results and conclusions summarized as: (1) Scales were reliable for determining the ages of crucian carp in Dongting Lake which showed a faster growth and a relatively simple age structure. The annuli on the scales were obvious and could be read easily. The agreement between two readers and between scales and otoliths reached 90.5% and 91.7%, respectively. However, it was difficult to read annuli when the ages of fish is above 4 and the percent agreement between two readers, and between scales and otoliths for age estimations of population in Honghu Lake are only 50.9% and 56.6%, respectively. It is suggested that scales is not well suitable for age estimation in populations which have slow growth and relatively complex age structure because it underestimate the ages of older individuals.(2) The whole or sectioned lapillus and asteriscus both could be used for age determination of crucian carp. Agreement between whole and sections of otoliths were above 80%, and the errors were minor, less than one year. It is demonstrated that the annuli are formed once in one year, by analyzing the opaque zone at the edge. The annuli on the otolith, especially in sectioned ones are very clear and can be read easily, and the agreement between two readers reached 97% for the population in Dongting Lake and 92.6% for the population in Honghu Lake. It is suggested that otolith is better than scale for age determination, particularly in populations with complex age-structure and slower growth.
(3) The otolith-weight among different age groups were significantly different and could be used for validating the accuracy of age determination. It is of great value in separating the ages of the individuals with similar body-length.(4) The population of crucian carp in Honghu Lake had relatively complex age-structure with eight age classes and the individuals older than three-year-old accounted for about 25%. The age-structure in Dongting Lake was simple, generally composed of three age classes, and the individuals of age 3 accounted for less than 5% It showed that the population of crucian carp in Dongting Lake was confronted with a higher adult mortality than in Honghu Lake, which might result from the differences in environments, fishing methods and fishing intensities.(5) The mean back-calculated body-length-at-age, based on otolith, showed that the growth of crucian carp in Honghu Lake was significantly slower than in Dongting Lake. The back-calculated body-length at age 3 and age 5 in Honghu Lake were similar to that at age 1 and age 2, respectively, in Dongting Lake. The differences in growth rate between the two populations might be attributed to the different fishing and predation pressures, forage resources and aquatic macrophytes abundance.(6) In Honghu Lake, about 70% females reached maturity at age 1, 94% at age 2 and 100% at age 3. while nearly all the females in Dongting Lake matured at age 1. It might be related to variation in growth and different environmental pressures.(7) The sex-ratio of crucian carp was 1.46:1 in Honghu Lake and 1.50:1 in Dongting Lake, with no significant difference. The sex-ratio was length-dependent in both populations and would increase with the body-length. The sex-ratio in Honghu Lake was age-dependent, and the average of the sex-ratio for age 0 to age 2 was 1.34:1 and 2.14:1 for age 3 to age 8, the sex-ratio in Dongting Lake showed little variation among age groups. The sex-ratio also changed seasonally in relation to reproductive
activities.(8) The reproductive investments, indicated by gonadosomatic index and fecundity, for crucian carp in Honghu Lake was significantly lower than in Dongting Lake. However, cmcian carp in Honghu Lake showed a tendency to produce relatively larger eggs than in Dongting Lake, based on measurements of the egg diameter in the ovary at stage IV . The higher reproductive investments in Dongting Lake was a adaptation to relatively higher adult mortality, which is in agreement with the life-history theory prediction.(9) Based on the comparative spawning power, the individuals of age 2 and age 3 made greatest contribution to the recruitment, age 4 the second, the other age groups relatively low in Honghu Lake. Meanwhile, age 2 played a most important role in the recruitment, age 1 and age 3 are nearly the same. The contribution to the recruitment can't be underestimated although the individuals of age 3 had the lowest percentage in the spawning stock in Dongting Lake,(10) The experimental results under indoor 'common garden' condition showed that there were no significant differences in growth, but significant differences in reproductive investments between the populations from Honghu Lake and Dongting Lake. It was suggested that the differences in growth between the two populations in the field might be caused by environmental factors, not by genetic factor, but the differences in reproductive investments might be genetically-controlled (11)The Fl crucian carp from the population in Honghu Lake displayed a significantly higher growth rate, but lower reproductive investments than that from the population in Dongting Lake, either fed by feed A or feed B. The results support the preliminary conclusions of the previous experiment. The population from Honghu Lake exhibited growth priority over reproduction, but the reverse in Dongting Lake.(12)The feed quality affect the growth of Fl crucian carp from the two populations. The groups fed by feed A displayed a lower growth rate and growth efficiency than that by feed B. However, the two populations showed a remarkably different response to the feed quality in the reproductive investments. Fl from Honghu Lake displayed no significant differences either fed by feed A or feed B, but Fl from Dongting Lake displayed significant differences between two feeds, the higher investments in the groups fed by feed B were observed.|