The influence of hydrography on pelagic fish distribution and communities in a sub-arctic estuary (Lake Melville, Labrador)

Small, Tiffany (2023) The influence of hydrography on pelagic fish distribution and communities in a sub-arctic estuary (Lake Melville, Labrador). Masters thesis, Memorial University of Newfoundland.

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Abstract

Estuaries are semi-enclosed bodies of water that are influenced by freshwater runoff and saltwater from neighbouring oceans. Water masses in estuaries can be strongly stratified and are often influenced by geological, hydrological and hydrographical features. The stratification of the water column in estuaries creates important nursery habitats for larvae and juvenile fish populations. The Lake Melville system, a highly stratified sub-arctic estuary in Labrador, spans 2100 km2 and is considered the longest single body of water in Labrador (250 km). By pairing seasonal hydroacoustic surveys conducted in summers 2018-2019 and winters 2019-2020 with ichthyoplankton and gill nets sampling as well as environmental DNA, we test the hypothesis that the strong water stratification prevailing in upper Lake Melville provides a refuge for early life stages of fish. We found that adult pelagic fish occupied the Atlantic waters below the steep pycnocline, while ichthyoplankton aggregated just above and within the pycnocline (as deep as 25 m). Nine species of fish were captured in gill and fyke nets and 53 fish species were detected with environmental DNA. Larvae of Rainbow smelt (Osmerus mordax) were ubiquitous in July and August and represented 100 % of the ichthyoplankton assemblage during these months. No fish larvae were caught in winter (February). As seen in other estuaries, we conclude that the layer of freshwater provides a refuge for Rainbow smelt larvae, a key forage species in the estuary. The recent completion of a large hydroelectric project on the main affluent of Lake Melville could alter the seasonal flow of freshwater and, ultimately, fish distribution. This study contributes to the growing library of resources from which to assess future changes in biodiversity and distribution of fish in the estuary.

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