Science Advisory Report 2012/052
Ecosystem Research Initiative Advisory Report: Forage Species Responsible for the Presence of Blue Whales (Balaenoptera musculus) in the St. Lawrence Estuary
This document provides information on: (1) the effect of natural processes and factors on the abundance and distribution of forage species (euphausiids, or krill) that are likely to impact the distribution of blue whales, and (2) the impact of human activity on the dynamic of some of the ecosystem’s key species and the consequences of this on the whales’ use of resources.
Issue: Forage species – Krill distribution
- In 2009, krill biomasses were estimated for the Estuary, the north-western Gulf, and off the coast of Gaspé based on acoustic surveys with high spatial resolution. The results suggest that Thysanoessa raschii, and not Meganyctiphanes norvegica, is the dominant species (by biomass). In June 2008, a survey of the Pentecôte region with more limited spatial resolution estimated the largest one-time level of krill ever observed in the Gulf.
- In 2009, the spatial distribution of T. raschii showed significant differences between June and August, although total biomass remained similar. In June, there were several accumulations located along the northern and southern coasts of the Estuary and at the head of the Laurentian Channel. There was also a large aggregation extending from Mont-Louis to Petite Vallée, and another off the coast of Gaspé. In August, there were fewer high-accumulation areas, but density was higher.
- In June and August 2009, the highest densities of T. raschii were often located above the slope (100–180 m) of the Laurentian Channel. However, a higher density of T. raschii was located above the deep-water channel at Les Escoumins, and the highest densities along the southern shore of the Estuary were concentrated above the shelf. The seasonal distributions of M. norvegica were more uniform between bathymetric areas. T. raschii was consistently located higher in the water column (20 m on average) than M. norvegica.
Issue: Forage species – Estuary supply and connectivity between regions
- The primary exchange between the Gulf and the Estuary by hydrodynamic circulation occurs off the coast of Pointe-des-Monts, where the current can be directed westward (entering into the Estuary) or southward (bypassing the Estuary), depending on the mode of regional circulation—hence the concept of a “valve”. Depending on the direction of the current, the krill is directed either toward the Estuary (open valve) or south to the Gaspé Peninsula (closed valve), where it can be re-circulated into the Anticosti Gyre or carried downstream toward Gaspé and the southern Gulf.
- The average relative densities of krill were higher at the oceanographic mooring stations in the Estuary than those in the north-western Gulf, when averaged over the year. However, monthly densities varied significantly, which is consistent with the movement of the aggregations. Some of these aggregations appeared to extend over several dozen kilometres. The cumulative transport of the krill biomass indicates net transport in an upstream direction at all stations, except the Gaspé Current station, where it is downstream. The results show the persistence of nycthemeral vertical migration throughout the year, even under the ice in winter, and illustrate how it is controlled by photoperiod.
- The circulation patterns corresponding with the open and closed valve conditions were found both in current-meter readings (mooring stations) and in the results of a numerical circulation simulation. The analysis revealed two distinct advection modes for the krill in the Estuary: a seasonal mode, and short-time major events (i.e. storms).
- Irrespective of the open/closed valve concept, the transport of krill upstream in deeper waters (80–180 m) is more prevalent in winter and spring and does not necessarily occur along the north shore of the Estuary. At the surface (0–40 m), upstream krill transport along the north shore (open valve) occurs primarily in summer and fall. The passage of low-pressure systems can generate significant inflow from the north-western Gulf toward the Estuary, adding another level of variability to the seasonal pattern.
- A species’ potential for transport depends on its daily migration pattern, diurnal depth, and influencing factors (e.g. seasonal and inter-annual variations in the light attenuation coefficient). The probability of transport is therefore potentially higher for T. raschii than M. norvegica, as the latter is found at a lower depth. However, it is difficult to predict how the estuarine circulation will be affected by future changes in the river flow of the St. Lawrence.
Issue: Forage species – Blue whale habitats in the St. Lawrence
- A new analysis of the marine mammal photo-identification data from the Mingan Island Cetacean Study (MICS) research station revealed that between 1987 and 2007, 333 blue whales were individually photo-identified in the Estuary and Gulf. Of this number, between 23 and 96 different blue whales (average ± SD = 60 ± 21) were recorded annually in the Estuary and Gulf. No time trends were observed.
- Baleen whales (blue, fin and Minke) in the north-western Gulf have been more strongly associated with concentrations of T. raschii than M. norvegica, making T. raschii the preferred prey for large whales. Blue whales were associated with aggregations where T. raschii were distributed in the first 100 m of the water column (surveys conducted during the day).
- In a study of the diving behaviour of blue whales in the Estuary, repeated observations of feeding at least 30 m from the surface, even in the daytime, confirmed a preference for shallow dives when food is available. This supports the idea that, to a certain extent, the quality of a habitat is not defined only by the density and abundance of prey, but also by its accessibility.
- The quantity of krill does not appear to be a limiting factor in the Estuary, even though the blue whale’s competition for this resource ranges from secondary producers (other microzooplankton) to top predators (fish, baleen whales).
Issue: Impact of marine traffic noise
- Marine traffic in the St. Lawrence Seaway (approximately 20 ships per day) produces sound levels that are above the Wenz reference level for high traffic, for at least 75% of the time within the first 200 metres of the water column. The noise levels were relatively stable throughout the 12-month ERI study, with slightly lower levels in winter (late January to early February) due to reduced traffic. Blue whales produce low-frequency sounds (below 200 Hz). Anthropogenic noise, such as marine traffic, can interfere with the detection of these sounds, and as a result, with a number of the whales’ vital activities.
- Over the course of the period between 1994 and 2009, concentrations of persistent organic pollutants (POP) in M. norvegica were either decreasing (e.g. DDT and PBDE) or showed no significant trends (e.g. PCB and HCB). Contamination of M. norvegica by POPs appears to be a good indicator of recent changes in the environmental quality of the Estuary.
- The ERI helped to determine potential physical, chemical and biological indicators of the ecosystem’s status. A large number of these indicators are influenced by the Estuary’s circulation. It is currently difficult to predict how this will be affected by future changes in the flow of the St. Lawrence river.
This science advisory report is from the February 14–16, 2012 meeting on the St. Lawrence Estuary Ecosystem Research Initiative: formulation of scientific advice in support of ecosystem management. Additional publications from this process will be posted as they become available on the Fisheries and Oceans Canada Science Advisory Schedule.
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