Feeding habits of the yellowtail flounder and production of its invertebrate prey on Georges Bank

Abstract

As part of the Georges Bank Benthic Infauna Monitoring Program, size-frequency analysis was used to study the life history and production of three benthic amphipod species: Ampelisca agassizi, Unciola inermis and Ericthonius fasciatus. Abundant over large areas of Georges Bank, these amphipods are important prey of yellowtail flounder. From benthic grab samples collected quarterly during 1981 to 1983, over 200 individuals of each species were measured for each sampling date. The computer program NORMSEP separated the length-frequency distributions into cohorts. Growth and mortality rates were determined by following cohorts through time; production was calculated as the sum of growth increments. Differences in the population dynamics of the three species were exemplified by annual production-to-biomass ratios (P/B). E. fasciatus had the highest P/B (2.8 to 4.1), followed by U. inermis (1.3 to 2.7) and A. agassizi (1.3 to 1.4). These results, the first direct production estimates for benthic macrofaunal species on Georges Bank, are compared to production estimates for other marine amphipod species. Amphipod production rates on Georges Bank are as high as production rates of related near-shore species with similar life-spans. The Northeast Fisheries Center sampled yellowtail flounder stomachs at three sites on four quarterly dates from August 1982 to May 1983. As closely as possible, stomach sampling coincided with the dates and locations of benthic monitoring to enable direct comparison between ingested and available food. Stomach contents were sorted and weighed and important prey species were measured. Yellowtail flounder feed mainly on benthic macrofauna; amphipods and polychaetes are the most important prey groups. Although the diet composition varied among stations and dates, in each case a few prey species constituted the bulk of the diet. Selection indices were calculated to compare the species composition of the stomach contents to that of the benthic macrofauna. Food selection by yellowtail flounder is explained on the basis of prey life history and prey size. The species- and size-selection indices account for most of the variability in diet composition. A log-normal curve was fit, by least squares, to the size-selection indices; according to this fit, the optimum prey weight for yellowtail flounder is 21 mg. The rate of food consumption by yellowtail flounder was calculated by two methods. The bioenergetic method sums the food required to satisfy the energetic requirements of the fish. The second method is based on the weight of stomach contents and the rate of food evacuation from the stomach. The bioenergetic method gave consistently higher results (1.4 to 1.6 kg fish-l yr-1) than the stomach-content method (0.5 to 0.8 kg fish-l yr-1). Yellowtail flounder abundance was estimated by a method that uses the commercial catch data to standardize the survey, relative-abundance index. These abundance estimates were divided by the appropriate areas to obtain yellowtail flounder density estimates for fall 1982 of 20.0 and 41.3 fish per hectare for Georges Bank and Southern New England, respectively. Consumption rates per fish were multiplied by flounder density to obtain estimates of consumption per unit area, which range from 1.2 to 6.3 gww m-2 yr-l. Annual production of six important prey species was calculated. Yellowtail flounder consumption as a percentage of prey production was 6 to 12%, 2 to 5%, and 1 to 3%, for the amphipods Unciola inermis, Ericthonius fasciatus and Ampelisca agassizi, respectively; 11 to 34% and 7 to 21% for the polychaetes Chone infundibuliformis and Nephtys incisa, respectively; and less than 1% for the sand dollar, Echinarachnius parma.