Herring Gulls and Lobster Bait
Wing Goodale '01, College of the Atlantic, Bar Harbor, Maine.
Herring Gull (Larus argentatus) populations increased dramatically after 1900 (Drury 1973, 1974) due to an increase in food supplied by humans (Harris 1970, Hunt 1972) and laws limiting hunting and egg collecting (Kadlec and Drury 1968, Spaans 1971). Gull populations in Maine expanded at the expense of terns, eiders, alcids, and other colonial nesting seabirds because gulls feed on the eggs and chicks of these species (Drury 1965, Nisbet 1973, 1975). Consequently, seabird restoration has become closely linked to lethal and non-lethal techniques of gull control (Kress 1998). Today the non-lethal techniques are effective on a small scale, but they do not completely rid restoration sites of gulls. These methods may be enhanced if the gulls' primary food source was removed.
Studies in other areas suggest that fishery wastes subsidize gull and other scavenging seabird populations (Garthe et al. 1996). In the late 1960's George Hunt (1972) documented that 40 - 60 % of Penobscot Bay herring gull diet came from human sources: lobsterbait, fish factories, chicken farms, and open dumps. Today, 30 years since Hunt's study, the dumps, farms, and plants have closed, leaving bait as the remaining food subsidy for gulls. Gulls may now depend more heavily on lobsterbait than they did in the past.
During the summers of 1999 and 2000 for my masters at College of the Atlantic, I tested if bait is a significant food subsidy to herring gulls during the breeding season in Penobscot Bay, Maine. I collected diet samples to determine the percentage of bait consumed by herring hull chicks, I collected data from lobsterboats to determine how much bait was available to the birds, and I marked breeding adult gulls with a rhodamine red b dye to document how far the birds flew to consume lobsterbait.
The frequency of lobster bait in herring gull chick diet on 5 study islands was 57% in 1999 (n = 251) and 41% in 2000 (n = 608). Bait was consumed by the population as a whole, at all times of day, at all tides, at all weeks during my study, by all aged chicks, and by both juveniles and adults. In addition, the frequency of bait increased in areas of high fishing effort.
I found that 73% (n = 26) of lobstermen studied in Penobscot Bay discarded approximately 14% full (n = 911) bait bags overboard, of which gulls recovered approximately 45% (n = 49). Data from marked gulls indicated that 81% of the gulls flew 0-9km to forage at lobsterboats, although some flew as far as 18km. The birds appeared to seek bait by flying to areas with a greater density of fishermen.
Attempting to find a long-term solution to the increased gull populations in Maine is difficult. There are many density-independent and density-dependent factors driving the gull populations. But one major factor appears to support the gulls: food supplied by humans. Around the world gull populations have increased as the gulls were provided with greater amounts of food. The resulting population is a nuisance to humans and a detriment to small seabirds. My research suggests that like many other places, Maine herring gull populations are supported by waste from human activities. In Penobscot Bay, Maine, this waste is discarded lobsterbait. Therefore, gulls in Maine may be controlled by reducing lobsterbait discards through a collaborative effort with lobstermen.
Drury, W.H. 1965. Gulls vs. terns: Clash of coastal nesters. Massachusetts Audubon 1965: 207-211.
Drury, W.H. 1973. Population changes in New England Seabirds. Bird Banding 44: 267-313.
Drury, W.H. 1974. Population changes in New E