Spatial and temporal variations in the abundance, distribution, and aggregation of krill (Thysanoessa raschii and Meganyctiphanes norvegica) in the lower estuary and Gulf of St. Lawrence Ian H. McQuinn a,⇑ , Stéphane Plourde a , Jean-François St. Pierre a , Maxime Dion b a Institut Maurice-Lamontagne, Fisheries and Oceans Canada, Mont-Joli, QC G5H 3Z4, Canada b Sherbrooke University, Sherbrooke, QC J1K 2R1, Canada article info Article history: Received 16 April 2014 Received in revised form 17 December 2014 Accepted 18 December 2014 Available online 30 December 2014 abstract The opportunistic collection of hydroacoustic data during multipurpose, large-scale surveys since 2000 has led to the detection of several, newly-documented krill concentrations throughout the Gulf of St. Lawrence. Based on both historical and these recent results, a series of regional-scale acoustic surveys was initiated in the northwestern Gulf and estuary to quantify the spatial and temporal variations in the regional abundance, distribution and aggregation of northern and Arctic krill. By using a recently- developed multi-frequency method to classify these two krill species, our study re-evaluated the spe- cies-specific krill distribution and abundance, previously monitored by plankton-net sampling alone. We show that Thysanoessa raschii is the more coastal species, with its center-of-mass in the shelf and slope habitats. Meganyctiphanes norvegica forms generally less dense aggregations in the slope and chan- nel habitats. Biomass in the northwestern Gulf and estuary was split 60:40 between T. raschii and M. nor- vegica, and totaled 1.0 million tonnes in August 2009; substantially more than has been reported previously. This re-evaluation should have considerable impact on ecosystem models which have assumed much lower values for these important forage populations. The pattern of krill aggregations identified from large- and regional-scale surveys will have important implications for the interpretation of critical habitat for endangered marine mammals which depend on krill. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved. Introduction Krill in the north Atlantic has long been recognized as an impor- tant forage species group in arctic and boreal ecosystems for a wide variety of marine predators from invertebrates and fishes (Hanson and Chouinard, 2002), to birds and whales (Mauchline, 1980). In eastern north Atlantic waters, euphausiid abundance and distribution has received wide attention, from the Norwegian Sea (Skjoldal et al., 2004) to the Mediterranean Sea (Warren et al., 2004). A review of the biology of Meganyctiphanes norvegica, the largest species in the north Atlantic Ocean, was recently under- taken (Tarling et al., 2010) in acknowledgment of its standing as a fundamental component of marine food webs. However, despite the recognition of the key ecological role played by this very suc- cessful group (Mangel and Nicol, 2000; Peterson et al., 2009), mon- itoring of krill in the western north Atlantic has been sporadic and surprisingly little is known of their abundance, distribution and behavioral ecology in eastern North American waters. Some funda- mental work was undertaken in the 1970s in the Gulf of St. Lawrence (GSL) which covered large areas on scales meaningful to the understanding of krill population dynamics (Berkes, 1976; Sameoto, 1976, 1980). Since then, mostly spatially and/or tempo- rally limited studies have been carried out, particularly in the Bay of Fundy (Kulka et al., 1982; Nicol, 1984), the eastern Scotian Shelf (Cochrane et al., 1994, 2000; Sameoto and Lewis, 1980) and in the St. Lawrence Estuary (e.g. Harvey et al., 2008; Simard et al., 1986; Simard and Lavoie, 1999). Accurate estimates of krill density on small, medium and large spatial scales are essential for understanding the specific ecological role that they play, on the scale of a patch or swarm to whole eco- systems. However, it has been recognized throughout the world that there can be large discrepancies in the estimation of krill abundance and density depending on, among other things, the sampling method (Mangel and Nicol, 2000). Several studies con- ducted over a wide range of habitats and involving several species have shown that plankton-net sampling of krill can be strongly biased (Brinton, 1967; Hovekamp, 1989; Sameoto et al., 1993; Wiebe et al., 1982), with the under-sampling of some species reaching 20-fold. The use of a stroboscope or other artificial light mounted on the sampler has been shown to increase catches http://dx.doi.org/10.1016/j.pocean.2014.12.014 0079-6611/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Progress in Oceanography 131 (2015) 159–176 Contents lists available at ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean