Abundance, diversity and seasonal patterns of estuarine fish populations

Abstract

Fish populations of the middle Patuxent Estuary in Maryland were sampled over a four-year period with quarterly frequency using three sampling gears: haulseine, beam trawl and otter trawl. Data from each quarterly collection at 50 stations were examined using a three-dimensional grid to check for possible effect due to a steam electrical generating station (s.e.s.). No marked s.e.s. effect was noted although occasional temporal shifts were noted. Trend analyses were performed for the number of species, number of individuals and several diversity indices. All analyses show strong seasonal cycles that are related to the nursery function of the estuary and to migratory activities. Regression lines fitted to the cyclic-trend patterns all show decreasing functions with time. These patterns suggest that a loss in diversity, as measured by these indices, has occurred. These changes are interpreted as being caused by a true loss of species in some cases and a shift in population structure in others. Possible explanations for the trends are examined and the implication of the trends are discussed. Should these trends continue, structural complexity of an already simple system will be reduced and increased amounts of energy may be diverted to dominant forms. The altered system, because of the loss of buffering capacity provided by alternate food web interconnections, may become subject to crashes and booms.