Science Advisory Report 2013/076

Status Of Beluga (Delphinapterus Leucas) in the St. Lawrence River Estuary

Summary

• The beluga is an Arctic species, and the St. Lawrence Estuary (SLE) population is at the southernmost limit of the species distribution. It occurs primarily in the SLE and seasonally in the Gulf of St. Lawrence. Its current range is about 65% of the extent used historically, whereas the size of its annual core distribution is at the lower limit of areas of occupancy described for any population of this species.
• The SLE population of beluga is listed as threatened under the Canadian Species at Risk Act (SARA); Critical Habitat has been identified, and corresponds to the summer area occupied by females accompanied by calves and juveniles (FCJ).
• Data from a carcass monitoring program indicate year-to-year variation, but no trend, in the number of adult beluga carcasses (male and female) reported over the period 1983-2012, with a median of 10.5 whales annually. The number of newborn death reports varied annually from 0 to 3 until 2007, but was unusually high in 2008, 2010, and 2012, with 8, 8 and 16 carcasses, respectively, and close to or within the previous range in 2009 (n=1) and 2011 (n=4). The age composition of the carcasses indicates an increase in the mortality of young adult females during the 2000's compared to the 1990's.
• Necropsies conducted on 222 carcasses indicate that the primary cause of death for juvenile beluga was parasitic pneumonia (52%). Cancer (17%) and bacterial diseases (13%) were the most common causes of death in adult beluga. Cancer was not observed in beluga estimated to be born after 1971, which coincides with the regulation of several chemical substances, including polycyclic aromatic hydrocarbons (PAHs) and polychlorinated byphenyls (PCBs).
• Necropsies also show the occurrence of problematic births (dystocias) and post-partum complications. These reproductive issues were associated with the death of 19% of adult females, and increased in frequency over the past decade, with several cases observed since 2010 when numbers of dead calf reports were also high in some years.
• With the exception of one calf caught accidently in a net, none of the calves examined over the years presented any pathologic changes that could have accounted for their death.
• The abundance of the SLE beluga population has been estimated from 8 aerial photographic strip surveys and 28 aerial visual line-transect surveys flown between 1988 and 2009. The 2009 estimates are the lowest in the two times series. However, there was no significant long-term trend in abundance in the visual or photographic time series.
• The proportion of 0-1 year-old calves counted during photographic surveys has declined at the end of the 1990s, from 15.1-17.8% in the 1990s to 3.2-8.4% in the 2000s.
• Abundance estimates and percentage of 0-1 year-old calves, as well as the number of beluga carcasses reported annually during 1983-2012, were incorporated into an age-structured population model. The model estimates that the population was stable or increasing at a slow rate until the early 2000s (reaching approximately 1000 individuals in 2002), and has declined since then to a total population of 889 individuals (95%CI 672-1167) in 2012.
• The model also suggests that the SLE beluga population has experienced changes in vital rates and age-structure, moving from a relatively stable to an unstable period characterized by an apparent shift from a 3-year calving cycle to a 2-year cycle, increasing variability in newborn mortality and pregnancy rates, and a decline in the proportion of immature individuals and newborns in the population.
• A long-term photo-identification program of live SLE beluga (1989-2012) indicated changes in age structure and calf production similar to those suggested by the model. In particular, years of high pregnancy rates predicted by the model for the period 2004-2012 were corroborated by observations of high calf production in the field the following year. The photo-identification program also suggests a slight increase in the proportion of grey individuals (juveniles and young adults) from 1989 to the mid-2000s, with a recent transition to a negative trend echoed in the model as a reduction in the proportion of immature individuals.
• SLE beluga are one of the world's most contaminated marine mammals. While some chemical substances (e.g., PCBs, DDTs) have decreased in beluga over the past decade, others such as the polybrominated diethyl ethers (PBDEs) compounds, increased exponentially during the 1990s, and are at their maxima since then. Their role in the recent elevated frequency of complications at parturition and mortalities of newborns in SLE beluga could not be determined. However, these different classes of chemical substances are known to have various endocrine disrupting effects in mammals, with possible impacts on reproduction, immune system and behaviour, and on offspring development.
• SLE beluga are chronically exposed to noise and disturbance due to commercial shipping, recreational activities, and an extensive marine mammal observation industry, especially in the North Channel of the SLE and the lower Saguenay Fjord where most of this activity is taking place. Vessel traffic related to tourism and recreational activities peak in July-August when SLE beluga give birth, and has increased between 2003 and 2012 in some sectors of their Critical Habitat as a result of newly established whale-watching companies operating in the Upper SLE and targeting beluga.
• Currently, there are proposals to reroute some of the shipping traffic from the North to the South Channel in the SLE. While the North Channel is highly ensonified by current marine traffic, islands in the middle of the SLE create a sound shadow for FCJ habitat located along the south shore. Increased shipping in the South Channel would considerably reduce the amount of noise-shelter areas available to FCJ beluga.
• Chemical tracers (stable isotope ratios) in beluga carcasses collected over the period 1988 to 2012 indicate a steady change in sources of carbon (and presumably diet) exploited by adult males and females since 2003. Prey species and/or ecosystem factors responsible for this change are unknown.
• The analysis of 28 ecosystem indices describing ecosystem variability of the Gulf of St. Lawrence from 1990 to 2012 indicates that environmental conditions shifted from periods of above, to below long-term averages in the late 1990s, i.e., approximately at the same period as when SLE beluga population shifted from a stable to an unstable age structure, to lower proportion of calves in the population, and increased number of dead calf reports. The period of below long-term average environmental conditions was when large demersal fish and 4T spring herring were at their lowest biomass, ice conditions were below-average, and water temperature above-average. The evolution of these variables since 1971 also identified the period starting in the late 1990s as a sustained 14-y long period when conditions were below long-term average, with unprecedented extreme conditions in 2010 to 2012.
• Many dead marine organisms, including an unusually high number of beluga and their prey, tested positive for saxitoxins during a period coincident with a bloom of the dinoflagellate Alexandrium tamarense, indicating the toxic algal bloom was responsible for these deaths, on its own or in conjunction with other unfavourable environmental conditions.
• The 8 and 16 times above-median number of carcasses of newborn calves documented in 2010 and 2012 cannot be explained by high calf production alone, suggesting that the observed mortalities resulted from a combination of increased calf production, and reduced calf survival. This is supported by the high pregnancy rates predicted in 2009 and 2011 by the model, and the high index of calf production observed in the field the following year (2010 and 2012). The above-median numbers of dead calves in the SLE in 2010 and 2012 occurred during an unprecedented period of well-below long-term average ice conditions in the Gulf of St. Lawrence and for 2012, high water temperatures, conditions which were also favourable to increased boating activities, which may have resulted in potentially higher disturbance of FCJ during a sensitive period.
• Sources of uncertainty include 1) the rate of decline of the population, which varies depending on the choice of input data to the model, 2) the summer distribution range which may extend further downstream than currently recognized, 3) biases from using data on beluga carcasses as a proxy for age-and sex-specific mortality rates, 4) mortality rates outside of the summer period, 5) prey resources important seasonally for SLE beluga, and abundance and availability of these prey in the SLE, 6) importance of sea ice to beluga, 7) sources of anthropogenic disturbance in the Critical Habitat, for which the portrait is incomplete, 8) trends in emerging contaminants and new toxins in beluga and their prey, and effects on beluga.
• The decline of the SLE beluga population since the beginning of the 2000s suggested by the model, occurred during a period of changing environmental conditions in the Gulf of St. Lawrence, concomitant with high levels of some contaminants (PBDEs) in SLE beluga, chronic and increasing exposure to noise and marine traffic, and occasional toxic algal bloom s in the SLE.
• Climate variability resulting, among other things, in increased water temperature and associated decline in ice-cover may further affect this beluga population through, for example, changes in food resources and increases in inter-specific competition as other species expand their ranges due to loss of ice cover. In the short term, efforts can be directed to reducing anthropogenic stressors such as disturbance in sensitive areas, chemical contamination, nutrient enrichment, habitat loss for beluga and their prey, and competition for food resources from fisheries.

This Science Advisory Report (SAR) is from the October 7-11 2013 Annual Meeting of the National Marine Mammal Peer Review Committee (NMMPRC). Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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