The pike killifish, Belonesox belizanus Kner, is the largest member of the live-bearing family Poeciliidae. The species is native to Central America, and was introduced into Dade County, Florida in 1957. B. belizanus is primarily piscivorous, and has been identified as a potential ecological "problem" species for Florida due to its heavy predation on small native fishes, especially the mosquitofish (Gambusia affinis). The population structure and reproduction of the Florida population were investigated in this study from field samples and aquarium observations. The mean standard length (SL) of mature females was about 103 mm. Most females larger than 75 mm SL contained yolked developing ova, fertilized eggs, or embryos. Mature males averaged about 72 mm SL, and the majority of males larger than 55 mm SL were mature. The sex ratio of mature fish was significantly skewed in favor of females, while immature fish did not differ significantly from a 1:1 sex ratio. The average brood size of field-caught females was about 99, and there was a significant positive correlation of brood size with size of the parent female. The gross morphology of embryological development was described. Although mature unfertilized eggs, abnormal embryos, and degenerating embryos from a previous brood were found at a low frequency in broods of normal embryos, no evidence of superfetation was found. Sizes and weights of embryos and neonates were the largest reported for the family. There was a significant decrease in mean embryo weight as development proceeded, indicating that developing embryos received little or no nutrient input from the maternal system after fertilization. The average interbrood interval of aquarium-held females was 42 days, and females were capable of storing viable sperm. Field-caught females were capable of year-round reproduction. Laboratory-born neonates were immediately piscivorous, and exhibited distinctive behaviors associated with filling of the swim bladder and with avoiding detection by predators. Comparisons of the reproductive patterns of B. belizanus with other members of the family Peociliidae are discussed.

Epibenthic marine fish populations were sampled in order to establish quantitative base-line data for use in long term monitoring of the northern Indian River lagoon system, Florida. Fish samples were taken by trawling monthly at 8 fixed stations from June through September, 1979 and 1980. Fifty-seven fish species representing 29 families were collected. The fish community was numerically dominated by a very few species. Anchoa mitchilli alone accounted for 87% of the 105,853 individuals collected. Abundance, domin ance, and species diversity were compared among the three lagoon subdivisions, Mosquito Lagoon, Indian River, and Banana River. Greater numbers of fishes were collected in the Indian River, while the highest diversity levels were in the Banana River. Although most species were collected in all three subareas, gross inequities occurred in the numbers of individuals taken. For example, Mosquito Lagoon produced significantly more Micropogonias undulatus and Leiostomus xanthurus than either the Indian or Banana rivers. Significantly more Syngnathus scovelli and Anchoa mitchilli were collected in the Indian River than in the Banana River, and the Banana River produced significantly more Lagodon rhomboides than the Indian River. The only measured environmental parameters that appeared to play significant roles in determining fish populations were vegetative cover and salinity. An importance value (IV) index, combining relative abundance, relative frequency of capture, and relative size (biomass) per individual, was calculated for each species in the community. Dominance ranking by IV has advantages over dominance ranking by relative abundance alone and is calculable for making comparisons in a long-term monitoring study.