Research Document - 2013/126

Trends in the trophic ecology of St. Lawrence beluga (Delphinapterus leucas) over the period 1988-2012, based on stable isotope analysis

By Véronique Lesage

Abstract

The paucity of information related to the trophic ecology and diet of St. Lawrence Estuary (SLE) beluga limits our understanding of contaminant pathways, and precludes the identification of period, region and prey species likely to be the most important for their successful reproduction and survival. We used a combination of gastro-intestinal data and stable C and N isotope ratios in beluga muscle to infer their trophic position and diet over the period 1988 to 2012, and to identify potential changes in ecosystem structure or functioning over this period. When controlling for inter-annual variability, δ13C and δ15N values were higher in males than females in both juveniles and adults, which is consistent with sex dimorphism and differential access to prey assemblages. Trends in isotopic signatures over the study period could not be examined for calves and juveniles given small overall sample sizes. In adults, δ13C values varied significantly over the study period, and in a similar way in males and females, with a slow increase from 1988 to 2002 followed by a decline of approximately 1‰ from 2003 through 2012. The decline observed in δ13C values after 2002 was echoed by δ15N values, but was overall modest at approximately 0.6‰ over the 10 yr period. There are several potential explanations for the observed decline in isotopic signature, including a change in beluga diet, or in the diet of some of their prey. Isotopic mixing models using 11 potential prey of SLE beluga suggested sandlance and squid, as well as capelin, herring and tomcod as important dietary sources for adult beluga. The model also suggested that groundfish species such as redfish, Atlantic cod and white hake were more important in the diet of adult males than females.

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