Great Lakes species and age-groups within species (e.g., young-of-year and yearling-and-older) have different thermal and depth preferences (Brandt 1980, Brandt et al. 1980, Argyle et al. 1998, Parker Stetter et al. 2006). Age groups of bloater and rainbow smelt are often separated spatially, but this is not the case for alewife in all lakes. There are differences in adult alewife distributions among Lakes Michigan, Huron and Ontario. At present our understanding of distributions of the major forage fish species in the different lakes (summer-fall) are:
Epilimnion: young-of-year rainbow smelt, young-of-year alewife, and young-of-year bloater; adult alewife in Lake Ontario
Metalimnion: adult alewife and adult rainbow smelt
Hypolimnion: adult bloater, adult rainbow smelt, and adult alewife in Lakes Michigan and Huron.
In addition, we expect lake herring to be metalimnetic in the summer, and stickleback and shiners to be epilimnetic. Mysids are expected to be below the thermocline. Although complete physical separation may not occur, an acoustic survey conducted during thermal stratification may improve our ability to assign observed scattering groups to different species or age-groups. Direct sampling is needed to confirm thermal distributions of target and non-target species.
Another consideration for survey timing is seasonal schooling or spawning migration. In alewife, adults move inshore during late spring and early summer into water too shallow to survey with large vessels. This movement may cause a large proportion of the adult population to be missed during a spring survey.
A final seasonal consideration is the timing of young-of-year recruitment to sampling gear. If nets, such as midwater trawls, are used for target identification or collection, sampling should occur when the majority of young-of-year are available to the gear used.
Diel vertical migration (DVM) has been documented in the primary Great Lakes forage species (bloater, kiyi, rainbow smelt, alewife, (Janssen and Brandt 1980; Tewinkel and Fleischer 1998; Rudstam et al. 2003, Yule et al. 2007)). Change in pressure (i.e. depth) during DVM can affect swim bladder size and therefore TS. Differences in swimming behavior between day and night may affect average tilt angle, which strongly influences the echo intensity returned by a fish. Day-night differences in TS have been observed in marine surveys (Hjellvik et al. 2006).
Schooling behavior can result in biased abundance estimates because of acoustic shadowing (Appenzeller and Leggett 1992). Schooling is more common during the day than during the night in the Great Lakes.
Another possible bias may be introduced to density estimates made during DVM by fish bubble release. In Lake Erie, migrating rainbow smelt produce more bubbles at dusk than during the night and these bubbles have a TS similar to small fish. Surveys that began after dusk minimized the inclusion of bubbles in acoustic data (Rudstam et al. 2003).
Moon light can affect abundance estimates (Luecke and Wurtsbaugh 1993). A survey timing issue related to DVM is moon phase. Alewives form schools during periods of sufficient light intensity and although this behavior is typically observed during the day, it has also been observed under a bright moonlight (D. Warner, personal observation). Moonlight may attract alewife to the surface and allow them to feed by sight. Due to increased visibility of gear, trawl catches may also be affected by moonlight.
