Search USGS⌕ Search

Geology topics

Robert O’Gorman

Publications and source records attributed to Robert O’Gorman.

43 records · Page 3Linked to original sources
USGS1987

Prey fish dynamics and salmonine predator growth in Lake Ontario, 1978-84

The size of hatchery-reared brown trout ( Salmo trutta ) and coho salmon ( Oncorhynchus kisutch ), 1 yr after release in Lake Ontario, declined when the stocking of salmonines was increased between 1978 and 1984. The principal prey species, alewife ( Alosa pseudoharengus ) and rainbow smelt ( Osmerus mordax ), failed to show the expected, predator-induced downturn in abundance. Instead, rainbow smelt remained moderately abundant and alewives very abundant. During this period, alewife year-classes were small, survival of yearling alewives was poor, growth of young-of-the-year of both alewives and rainbow smelt was slow (growth of most older alewives ceased), and rainbow smelt numbers gradually increased (the much larger alewife population presumably buffered older rainbow smelt from predation by large piscivores). When adult alewife numbers were halved by a winter die-off, the subsequent year-class of alewives was large and growth of brown trout during their first year in the lake increased. This suggested a causal relation between abundance of young alewives and brown trout growth. In the first year coho salmon were at liberty, their growth was related to abundance of young-of-the-year alewives; in their second year it was related to the abundance of yearling alewives and the condition of adult alewives. We hypothesize that abundant adult alewives suppressed production of young-of-the-year fish (necessary prey for salmonines during their first year in the lake) through competition for limited zooplankton production, and thus impeded the transfer of energy from the lowest trophic level to young salmonine predators.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS1987

Meeting future information needs for Great Lakes fisheries management

Description of information needs for management of Great Lakes fisheries is complicated by recent changes in biology and management of the Great Lakes, development of new analytical methodologies, and a transition in management from a traditional unispecies approach to a multispecies/community approach. A number of general problems with the collection and management of data and information for fisheries management need to be addressed (i.e. spatial resolution, reliability, computerization and accessibility of data, design of sampling programs, standardization and coordination among agencies, and the need for periodic review of procedures). Problems with existing data collection programs include size selectivity and temporal trends in the efficiency of fishing gear, inadequate creel survey programs, bias in age estimation, lack of detailed sea lamprey ( Petromyzon marinus ) wounding data, and data requirements for analytical techniques that are underutilized by managers of Great Lakes fisheries. The transition to multispecies and community approaches to fisheries management will require policy decisions by the management agencies, adequate funding, and a commitment to develop programs for collection of appropriate data on a long-term basis.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS1987

Discrepancies between ages determined from scales and otoliths for alewives from the Great Lakes

Discrepancies between ages determined from otoliths and those determined from scales were common and, sometimes, quite large in alewives Alosa pseudoharengus collected in fall 1983 from Lakes Ontario, Huron, and Michigan. Among fish with 'otolith ages' of 4 or more, the percentages having identical 'scale ages' were 1% in Lake Ontario, 35% in Lake Huron, and 56% in Lake Michigan. Among alewives with different otolith and scale ages, the percentages with discrepancies of 3 years or more were 51% in Lake Ontario, 23% in Lake Huron, and 6% in Lake Michigan. Among the three populations, variation in the magnitude of age disagreements were perhaps due to the different mortality rates in each lake, whereas variation in the frequency of disagreements appeared to be due to different ratios of food to fish in each lake. Previously reported age compositions and rates of mortality and production for Great Lakes alewives derived from scales have probably been inaccurate to various degrees.

Book chapter↗
USGS1986

Dynamics of alewives in Lake Ontario following a mass mortality

The U.S. Fish and Wildlife Service and the New York Department of Environmental Conservation assessed the population of alewives Alosa pseudoharengus in U.S. waters of Lake Ontario during 1976–1982 with bottom trawls. Alewives were abundant in 1976 but a die-off greatly reduced their numbers during the winter of 1976–1977. The population quickly recovered, however, adult abundance increasing nearly sevenfold during 1978–1981. In spring 1981 the bottom population in southern Lake Ontario was estimated to be 5.25 × 10 9 fish weighing 128,500 t. Estimated average alewife biomass per hectare during 1978–1982 far exceeded the estimates for either Lake Michigan during 1967–1982 or western Lake Huron during 1973–1982. Recruitment of age-II fish to the population was affected by abundance of adults in two ways: (1) the number of yearlings produced was directly related to adult abundance at low population levels but inversely related at high population levels; and (2) survival of yearlings to age II was inversely related to adult abundance. Growth in 1977 was exceptional, leaving a wide, unmistakable band on scales of the previously slow-growing adults. This wide growth zone served as a marker to identify survivors of the 1976–1977 die-off and to show that each year after 1978 a successively larger proportion of survivors was failing to grow in length or to form an annulus (54% in 1979, 96% in 1980, and 100% in 1981). There was no marker on scales of alewives recruited after the die-off, but the apparent age composition of our catches strongly suggested that most of them also failed to grow in 1981.

Transactions of the American Fisheries Society↗
USGS1984

Catches of larval rainbow smelt ( Osmerus mordax ) and alewife ( Alosa pseudoharengus ) in plankton nets of different mesh sizes

Four 0.5-m plankton nets (one each of 0.355-, 0.450-, 0.560-, and 0.750-mm mesh) were used to collect larvae of the alewife ( Alosa pseudoharengus ) and rainbow smelt ( Osmerus mordax ) in northwestern Lake Huron and the St. Marys River. The number of larvae collected varied inversely with mesh size and the average length of larvae varied directly with mesh size. Numbers of each species caught in 0.355- and 0.450-mm mesh nets were significantly greater (P < 0.01) than the numbers caught in 0.560- and 0.750-mm mesh nets. Numbers of alewives caught were significantly greater (P < 0.01) in the 0.355- than in the 0.450-mm mesh, but numbers of rainbow smelt caught in the two meshes were about equal. Between the two larger-mesh nets, numbers of each species caught did not differ significantly. Nets with mesh larger than 0.355 mm were less effective at catching recently hatched larvae, and this reduced effectiveness accounted wholly for the smaller numbers caught in the 0.450-mm mesh net. Smaller numbers were caught in the 0.560- and 0.750-mm mesh nets not only because they caught fewer recently hatched larvae but also because they failed to retain older, larger larvae. Information on the effect of mesh size and towing speed on catches of larvae is important for making accurate estimates of larval densities.

Journal of Great Lakes Research↗
USGS1983

Distribution and abundance of larval fish in the nearshore waters of western Lake Huron

Ichthyoplankton was collected at 17 nearshore (bottom depth &ge;5 m but &le;10 m) sites in western Lake Huron during 1973&ndash;75 with a 0.5-m net of 351-micron mesh towed at 99 m/min. Larvae of rainbow smelt ( Osmerus mordax ) dominated late spring and early summer catches and larvae of alewives ( Alosa pseudoharengus ) the midsummer catches. Larval yellow perch ( Perca flavescens ) were caught in early summer but were rarely the dominant species. The time of spawning and hatching, and thus occurrence of larvae, differed between areas but was less variable for alewives than for yellow perch. The appearance of larvae in Saginaw Bay was followed successively by their appearance in southern, central, and northern Lake Huron. Rainbow smelt were most abundant in northern Lake Huron and yellow perch and alewives in inner Saginaw Bay. Densities of either rainbow smelt or alewives occasionally exceeded 1/m 3 , whereas those of yellow perch never exceeded 0.1/m 3 . Abundance of alewives was usually highest 1 to 3 m beneath the surface and that of rainbow smelt 2 to at least 6 m beneath the surface. Important nursery areas of rainbow smelt were in bays and off irregular coastlines and those of yellow perch were in bays. All nearshore waters seemed equally important as nursery areas of alewives.

Journal of Great Lakes Research↗