Bathythermal Habitat Use by Strains of Great Lakes- and Finger Lakes-Origin Lake Trout in Lake Huron after a Change in Prey Fish Abundance and Composition

A study conducted in Lake Huron during October 1998–June 2001 found that strains of Great Lakes-origin (GLO) lake trout Salvelinus namaycush occupied significantly higher temperatures than did Finger Lakes-origin (FLO; New York) lake trout based on data from archival (or data storage) telemetry tags that recorded only temperature. During 2002 and 2003, we implanted archival tags that recorded depth as well as temperature in GLO and FLO lake trout in Lake Huron. Data subsequently recorded by those tags spanned 2002–2005. Based on those data, we examined whether temperatures and depths occupied by GLO and FLO lake trout differed during 2002–2005. Temperatures occupied during those years were also compared with occupied temperatures reported for 1998–2001, before a substantial decline in prey fish biomass. Temperatures occupied by GLO lake trout were again significantly higher than those occupied by FLO lake trout. This result supports the conclusion of the previous study. The GLO lake trout also occupied significantly shallower depths than FLO lake trout. In 2002–2005, both GLO and FLO lake trout occupied significantly lower temperatures than they did in 1998–2001. Aside from the sharp decline in prey fish biomass between study periods, the formerly abundant pelagic alewife Alosa pseudoharengus virtually disappeared and the demersal round goby Neogobius melanostomus invaded the lake and became locally abundant. The lower temperatures occupied by lake trout in Lake Huron during 2002–2005 may be attributable to changes in the composition of the prey fish community, food scarcity (i.e., a retreat to cooler water could increase conversion efficiency), or both. *Corresponding author: [email protected] 1Retired. Received August 4, 2010; accepted June 20, 2011 Published online February 14, 2012 263 D ow nl oa de d by [ M ic hi ga n St at e U ni ve rs ity ] at 0 9: 34 0 9 M ar ch 2 01 5 264 BERGSTEDT ET AL. Temperature is an important environmental variable that governs food consumption and growth, and hence affects the distribution of fishes in lakes (Fry 1947). Temperature selection (Goddard and Tait 1976) and the ability to maintain position at different depths (Ihssen and Tait 1974) are known to be heritable characteristics in lake trout Salvelinus namaycush. The combined expression of these multigenic traits among fish species during summer periods of thermal stratification, along with other environmental variables (Fry 1947), yields the temperature and depth distributions of predators and their prey. Numerous laboratory studies have contributed to our understanding of the thermal behavior of lake trout, including preferred temperatures (e.g., McCauley and Tait 1970; Peterson et al. 1979; Edsall and Cleland 2000), optimum temperatures for growth (e.g., Elliott and Hurley 1999; Edsall and Cleland 2000), and temperature tolerance or lethal limits (e.g., Ihssen 1973; Grande and Andersen 1991). The temperature relationships described by these studies are consistent (Jobling 1981), which suggests that a component of temperature selection is genetic and defines the fundamental niche axis as defined by Hutchinson (1957). The description of a fundamental niche for temperature considers only one of a number of environmental variables affecting the distribution of fish; thus, a species is not necessarily found at optimal temperatures indicated in laboratory studies (Magnuson et al. 1979). Describing the realized thermal niche, as recorded by field observations and which includes the effects of all environmental variables, is of greater interest to fishery ecologists (Magnuson et al. 1979). Historically, little was known about the depths and temperatures of waters occupied by lake trout on a daily and seasonal basis in the Laurentian Great Lakes. Our limited knowledge was deduced from captures in different fishing gears. Capture data, however, only provide point estimates of location within the depths and temperatures sampled (e.g., Elrod and Schneider 1987; Elrod et al. 1996) and suffer from the sampling bias of each fishing gear. Observations of temperature at the time of capture are also limited to where gear is set and do not necessarily reflect the range of habitats occupied. Archival tags implanted in lake trout strains of two genetic origins, both of which were stocked in the Great Lakes, provided the first continuous seasonal measures of temperatures occupied by lake trout in Lake Huron from October 1998 through June 2001 (Bergstedt et al. 2003). During that time, lake trout of Great Lakes origin (GLO) occupied significantly higher temperatures than did lake trout of Finger Lakes (New York) origin (FLO). Although that information contributed substantially to our knowledge of lake trout behavior in the Great Lakes, the tags measured temperature but not depth. The lack of depth data was problematic in that it was not possible to directly conclude that the difference in occupied temperature between origins in summer was due to the use of different depths. Uncertainty existed because the average thermocline depth can be affected by prevailing winds and bathymetry and can vary geographically within Lake Huron. Therefore, temperature differences could have been due to similar depth distributions but with differences in geographic distributions between origins (Bergstedt et al. 2003). Since the temperature distribution data for lake trout reported for 1998–2001 were collected, substantial changes have taken place in the prey fish community of Lake Huron. The abundance of most prey fish species in Lake Huron declined sharply in the early 2000s (Riley et al. 2010), and alewives Alosa pseudoharengus virtually disappeared between 2003 and 2004. The round goby Neogobius melanostomus first appeared in prey fish surveys in U.S. waters of Lake Huron in 1997 and were widespread by 2001 (Schaeffer et al. 2005). By 2002, they had entered the lake trout diet (Michigan Department of Natural Resources [MDNR] Alpena Fisheries Station, unpublished data). The distribution of predators such as lake trout in Lake Huron would potentially shift after the loss of a key pelagic prey species, such as the alewife (Diana 1990), and the appearance of a novel demersal prey species, such as the round goby. By 2001, a tag became available that not only measured temperature and depth but also had substantially more memory than the tag used by Bergstedt et al. (2003). To examine whether GLO and FLO lake trout occupied different temperatures and different depths, we examined data recorded with the new tags during 2002–2005. In addition, we were able to compare the temperatures occupied by the lake trout tagged in this study with those reported for 1998–2001 (Bergstedt et al. 2003).