Long-term declines in two apex predators, bull sharks (Carcharhinus leucas) and alligator gar (Atractosteus spatula), in lake pontchartrain, an oligohaline estuary in southeastern Louisiana

We analyzed historic and current fishery independent data to determine if the abundance of two apex predators, bull sharks (Carcharhinus leucas) and alligator gar (Atractosteus spatula), in Lake Pontchartrain had changed significantly over the last half century. Lake Pontchartrain is an environmentally degraded oligohaline estuary in southeastern Louisiana that has experienced considerable changes in fish assemblage composition over this interval. Using gillnet, beach seine, and trawl data collected during three time periods (1953-1955, 1977-1978, and 1996-2005), we analyzed trends in abundance for C leucas and A. spatula using generalized linear models with a negative binomial error structure and a log link. Lake Pontchartrain data were divided into four spatial locations (northwest, northeast, southwest, southeast) since each region represents a unique combination of anthropogenic and natural influences that could affect catches. For each species and gear type, we produced log-likelihood profiles for the instantaneous rate of change in relative abundance through time. Raw catches were generally lower for both species in the later surveys. C leucas were not captured in beach seines since the 1950s and A. spatula were rarely captured in trawls or seines since the 1970s. Likelihood profiles of changes in abundance for C. leuvas and A. spatula showed very large dectines in both species since 1953. C. leucas declined by 98.6% (95% Cl: 73.499.9%) in gillnets and became functionally extirpated in beach seines with a decline of 99.9% (95% Cl: 23-99.9%). Among all gears, C. leucas declined by the same rate as in gillnets. The decline in A. spatula was also large with a decrease of 98.6% (95% Cl: 73.4-99.9%) in beach seines and a decline of 99.2% (95% Cl: 54.8-99.9%) in trawls since 1953. Catches of A. spatula in gillnets did not show a significant change over the study period. The continued decline of these two apex predators could seriously affect efforts to restore this degraded estuarine ecosystem.