Larval Fish As Indicator of Changes in the Ecosystem State in the Upper St. Lawrence Estuary

The Upper St. Lawrence River Estuary (USLE) is used as a nursery by several key aquatic species. This study investigates the use of fish larvae as indicators of the ecosystem state in the USLE, testing the hypothesis that extreme temperature conditions occurring in 2012 have affected larval abundance and condition. Since 2002, fish larval abundance is monitored in two coastal nursery areas (Anse Ste Anne, ASA and Baie de Rivière du Loup, BRL), in July. The relationships between the abundance and condition of larval rainbow smelt (Osmerus mordax) and Atlantic herring (Clupea harengus), and oceanographic and climatic variables were examined from 2002 to 2012. Smelt was the most abundant species at the upstream, less saline site (ASA), and herring at the downstream site (BRL). The abundance of both species was strongly correlated to spring freshwater flow. In 2012, a combination of extremely low freshwater flow and high temperature in the spring was associated with extremely low abundance at ASA and high body length at BRL for both species. Morphometric measurements of herring larvae sampled in BRL indicate extremely low body condition in 2012. These results support the use of fish larvae as cost-effective indicators of ecological conditions.