July 18, 2019 at 9:01 am
By Fisheries Biologist Jim Pellerin
In a 2017 blog, the Region A fisheries staff reported on our SPIN sampling efforts for Sebago Lake and ended by saying we would be using the same protocol on other important togue lakes in the future. Last week we will began our second year of togue sampling on Thompson Lake, and we expect to finish up by the end of this week. As a brief refresher, SPIN is a lake trout assessment tool developed by the Aquatic Science Unit of the Ontario Ministry of Natural Resources in Canada (2009). The primary objectives of the Thompson SPIN project are to: (1) estimate the density of togue; (2) determine the distribution of togue in the lake and water column; (3) examine the population structure, and (4) attain a representative sample of togue across the full range of lake depths and areas. From the fish sample, we can collect age and growth data, size/age of maturity, diet information, and survival/mortality estimates. All this information will be useful for evaluating current and future regulations and guide the management of this important fishery resource.
The first part of the SPIN process was to create a new depth map of the lake and divide it into 1,000 meter grids and different depth zones or strata. Net sites where then semi-randomly chosen by grid and the number of sets per depth strata were sampled proportionally to the surface area of each strata. The goal was to get a good distribution of net sets across the entire lake and across all depths to get a representative sample of the lake trout population. Next, field sampling begins and for Thompson Lake that equates to about 6 full days on the water.
Sampling in 2018 yielded 37 lake trout with minimal bycatch of non-target fish species. Preliminary analyses of the 2018 data suggest 5.2 togue/acre with a lake-wide estimate of 16,181. The togue ranged in size from 13.9 to 27.0 inches in length with an average length, weight, and condition factor of 17.9 inches, 2.1 pounds, and 0.88, respectively. While most of the togue sampled were moderate in size, we did catch one unusually large laker for Thompson Lake that tipped the scales at almost 7 pounds (Photo 1). All of the captured lake trout were aged via their otoliths (ear bones), and they ranged from 4-17 years of age. The majority (57%) of the togue were 5-7 years old with only 1-3 fish representing the other age classes. The current sample size is too small to determine mortality and survival, but we should be able to calculate these statistics after this field season by pooling the two years of data.
Interestingly, the lake trout population at Thompson Lake appears to be quite different from Sebago Lake. First, the density of lake trout was almost 4x higher at Thompson Lake and preliminary results suggest they are likely a small-bodied morph. Small bodied lake trout populations are generally smaller in size with few large individuals, mature at an earlier age, tend to exhibit more pelagic behavior, and their diet is less reliant on forage fish like smelt. In addition, although the togue were well distributed around the lake the largest density of fish was found in the shallowest strata immediately below the thermocline whereas on Sebago density generally increased by depth.
Other work currently being conducted on Thompson Lake includes: annual fall sampling of the landlocked salmon population; open water and ice creel surveys; and Department staff are scheduled to meet with Town representatives to explore if dam operational changes may be related to more recent observations of salmon out-migration from the lake. Information from these efforts are intended to help us manage the fisheries of Thompson Lake and to maintain its positon as one of the region’s better cold-water fisheries.