Fringing floodplains and assemblage structure of fishes in the Desoto National Forest, Mississippi
This project addressed floodplain use by stream fishes of a low-order blackwater stream (Pascagoula River drainage) in southern Mississippi. The initial phase of the study lasted 32 months (January 1992-September 1994) and explored aspects of flooding and floodplain access by the stream fish assemblage. Forty-nine floods were recorded during the period, and 35 were sampled for fishes. Randomly chosen stream sections were sampled quarterly during non-flooding periods. Thirty-six species were documented from the study area; 32 on the floodplain and 26 in the stream. The eight numerically abundant stream species represented five families and comprised 95% of the relative abundance. One species, Lythrurus roseipinnis, accounted for 69% of the total specimens collected. A non-metric multi-dimensional scaling analysis showed no change in the stream assemblage during the study (stress = 0.008). Ten of the 32 floodplain species accounted for 81% of the relative abundance. Floods were short (x = 43 h, 95% CI $\pm$ 8 h) with 92% of floods $<$80 h. The frequency of flooding was greatest from November-April (5-9 floods/month) while floods for May-October ranged from 1-3 per month. Thirteen species occurred infrequently ($<$20% of the 35 floods sampled) while five species were common, occurring in $>$60% of the floods. Canonical correspondence analyses indicated no definable pattern in species occurrence on the floodplain based on environmental variables which included temperature, season and hydrological conditions. The potential use of floodplains as a nursery habitat for larval fishes was investigated during the second phase of the study. Floodplain and stream habitats were sampled concurrently using multiple gears deployed on 42 sampling dates (May 1992-May 1993) with 12.94 mean fishing hours per sampling date (95% CI $\pm$ 0.7 hrs). Combined efforts yielded 3,492 larval and juvenile fishes representing 10 families. Larval occurrence on the floodplain was sparse (336 larvae/juveniles). Larvae were typically more abundant in the stream and their occurrence coincided with the general spawning period reported for most southeastern fishes (mid-February to mid-September). The greatest percentage of total larval abundance (92%) was from April-August with most larval taxa occurring from May-August when the frequency of flooding was at its lowest (1-3 floods/month). Comparison of pre- and post-flood samples (n = 12) indicates little change in the number of larval taxa present in the stream. In contrast, the number of larval taxa present on the floodplain was low before flooding but generally increased immediately following inundation. Overall, the role of floodplains as a nursery habitat is minor, and the occurrence of most larvae on the floodplain is likely due to transport of larvae from the stream during periods of inundation.