Evidence of Changes in the Prey Assemblages of Florida Bay, Everglades National Park Using a Fisheries-Independent Method

Beginning in the late 1980s, Florida Bay underwent dramatic ecological changes due to altered freshwater inflows from the Everglades that caused a cascade of effects, including changes in prey-fish assemblages.  At nearly the same time, the local Bald Eagle (Haliaeetus leucocephalus) population began to decline and has continued ever since.  We documented food habits of the struggling Bald Eagle population to employ a fisheries-independent method of monitoring the prey assemblages of Florida Bay.  We collected prey remains from Bald Eagle nest sites at the end of the breeding season and compared these to similar historical data from 1973-1974.  571 prey remains (30 species) from 34 collections at 21 nest sites were collected in 1973-74, and 419 remains (33 species) from 13 collections at 11 nest sites were collected in 2010-11.  We found remains consisted of 81% fish and 16% birds in the 1973-74 and 77% fish and 21% birds in 2010-11.  Hardhead Catfish (Arius felis) made up the majority of prey remains in both time periods, and increased from 68% to 81% of fish and 55% to 63% of all remains from 1973-74 to 2010-11.  The overall compositions of prey remains from all nest sites are significantly different between time periods (ANOSIM, r-value=0.171, p=0.024).  When comparing only nest sites where collections were made during both time periods, there still remains a significant difference (p=0.007).  The species that contribute most to these differences are mullet, jack, and the fish-eating Double-crested Cormorant (Phalacrocorax auritus) and Red-breasted Merganser (Mergus serrator).  Hardhead catfish skulls were the most common prey part found at nest sites and total length measurements were taken.  The mean length was 120.06 mm (n=145) in 1973-74 and significantly dropped to 106.92 mm (n=153) in 2010-11 (p<0.0001).  These results help show that prey assemblages of Florida Bay have recently been altered, and that this has possibly affected the food habits and success of a top-level predator.