Distribution of marine birds in relation to water masses and fronts in the Strait of Belle Isle, northwestern Atlantic Ocean

LeGrow, Keith Herbert (1999) Distribution of marine birds in relation to water masses and fronts in the Strait of Belle Isle, northwestern Atlantic Ocean. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Strait of Belle Isle, which separates the Northern Peninsula of Newfoundland and mainland Canada, is a region of two water masses, moving in opposite directions, separated by a seasonal frontal boundary. There are major seasonal migrations of marine mammals, birds and fishes through the Strait. Cold water from the Labrador Current enters the Strait along the Labrador coast and moves into the Gulf of St. Lawrence along the North Shore of Quebec. Warm water flows out of the Gulf of St. Lawrence through the Strait of Belle Isle along the western Newfoundland coast. These different water masses along the two sides of the Strait are associated with different seabird colonies, Le. pursuit-diving alcids and pelagic kittiwakes nesting along the Quebec South Shore and only a few colonies of surface-feeding gulls and terns on the Newfoundland coast. Furthermore, a previous study (Rees 1963) reported that Thick-billed Murres were associated with the cold Labardor Current water mass and pursuit-diving alcids were associated with the frontal regions in the Strait of Belle Isle. However, no subsequent studies have been conducted on seabird distributions in relation to fronts in the Strait of Belle Isle. -- The present study investigated seasonal patterns of seabird abundance in the Strait of Belle Isle. Ship and land-based surveys were conducted across and on both sides of the Strait in order to address three questions: -- 1) What are the abundance patterns of pursuit-diving alcids and pelagic surface-feeding gulls in the Strait of Belle Isle? -- 2) Do alcids occur more frequently in the cold water (western) region and gulls occur more frequently in the warm water (eastern) region? -- 3) Do seabirds occur more frequently near the frontal region of the Strait of Belle Isle? -- It was also expected that the frontal water mass boundary would move over the season and would influence the distributions and abundances of seabirds. An independent data set of land-based observations from Point Amour, Labrador during spring 1996 was used to answer a fourth question: -- 4) Do wind conditions influence marine bird occurrences in the Strait, as has been reported in previous coastal studies? -- Alcids and gulls were the dominant avian groups during the study. Some transects revealed statistically significant differences in seabird densities between eastern and western water masses but there was no consistently higher seabird density associated with either water mass. Seabird density was also not significantly higher at the frontal region. Differences in surface temperatures between eastern and western water masses and frontal strength varied monthly. Statistical simulations revealed that at the observed level of variance, it would not be possible to run sufficient surveys in a season to find significant differences in seabird densities between eastern and western water masses or between frontal and non-frontal regions. Gulls, loons and ducks showed decreases in abundance immediately following wind events. In comparison, alcids showed a delayed positive correlation with longshore wind events, with maximum correlation occurring at a lag of 4 days. The results do not support the hypothesis that seabird occurrences are influenced by the presence of different water masses or frontal regions in the Strait of Belle Isle. Frontal occurrence in the Strait of Belle Isle may not aggregate prey for seabirds, as has been previously hypothesized. Power analysis should be undertaken before surveying seabirds relative to oceanographic processes or anthropogenic effects.

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