Foraging ecology, bioenergetics and predatory impact of breeding double-crested cormorants (Phalacrocorax auritus) in the Beaver Archipelago, Northern Lake Michigan

As Double-crested Cormorant (Phalacrocorax auritus) populations have increased throughout the Great Lakes, many sport and commercial fish populations have declined. A high density of birds combined with their fish eating habits has led to their implication in these declines. From 1999 through 2002, a Smallmouth Bass (Micropterus dolomieui) population study was rekindled after a twenty plus year hiatus in the Beaver Archipelago. This work documents an intensive study on the population dynamics and foraging ecology of breeding cormorants of the same area between 2000 and 2004. The population size of breeding birds, well as their reproductive output, was estimated and the diet of birds was determined using regurgitates and the stomachs of harvested birds. Pellets were not used in analysis because they did not provide reliable quantitative data regarding bird diet. Breeding population size, as well as reproductive output, appears to vary substantially from year to year, and may be linked to the availability of alewife (Alosa pseudoharengus ). Breeding bird diet consists primarily of species of little commercial or sport value; the importance of individual prey species in bird diet varies temporally and spatially.; To determine important foraging areas, VHF radio telemetry was used to track the foraging activities of ten cormorants. Using triangulation, birds were monitored from both land and water daily, weather permitting, throughout the breeding season. In addition, rafting locations of cormorants were documented by boat survey throughout the breeding season. Radiotelemetery indicated that cormorants typically foraged 2.5 km for away from the colony, at the northeastern end of Beaver Island. This area overlaps with the area determined by rafting locations, however the latter were centered further south. Neither area overlaps with known bass habitat.; Colonial waterbirds are an important component of Great Lakes ecosystems. One important aspect is the role of these birds as top predators in aquatic food webs. In order to investigate this role, bioenergetics models, using allometric equations, were applied to breeding cormorants and their offspring in the study area. The models estimated the total prey biomass consumed as 1444.11 tonnes of prey in 2000, and 1586.17 tonnes of prey in 2001. Each year the majority of the prey biomass was aewife, with these fish comprising a greater percentage of prey biomass in 2001.; In addition, two types of simulation models, Rebuild and Forecast, to investigate a possible connection between the declining bass populations and the avian predators. Rebuild models indicate that is was unlikely that cormorants alone caused the observed bass decline; birds may have contributed to the decline in synergism with low recruitment, angling mortality or other factors. Forecast models suggest that direct cormorant predation on bass is not currently the leading factor limiting the bass population size in the region, even if bass are experiencing relatively low recruitment. Combined with the data collected on bird diet, foraging locations and bioenergetics, these models enhance our understanding of the relationships between cormorants and fish populations in the study area and may benefit fisheries managers in other systems.