Fisheries Biologists in the Sebago Region have been spending the bulk of our 2016 and 2017 summer field seasons working on Sebago Lake to assess the lake trout (aka “togue”) population.
Learning even more about this important fishery in the State’s deepest and second largest lake (30,000 surface acres, with an average and maximum depth of 101 and 316 feet respectively), is a priority for regional fisheries staff.
In 2015, we reviewed fisheries literature on lake trout sampling procedures and discovered SPIN (Summer Profundal Index Netting). This lake trout assessment tool was developed by the Aquatic Science Unit of the Ontario Ministry of Natural Resources in Canada and published in 2009. After reviewing the protocol, and discussing it with the researchers from Canada and fisheries field staff in Colorado we decided to implement the program in 2016.
The primary objectives of the Sebago SPIN project are to: (1) estimate the density of harvestable size lake trout in the lake; (2) determine the distribution of lake trout in the lake and water column; (3) examine the population structure, and (4) attain a representative sample of lake trout across the range of lake depths and areas.
From each fish sample we collect age and growth data, size/age of maturity, diet information, and survival/mortality estimates. All of this information is useful for evaluating current and future regulations and guiding the management of this important resource.
Before starting the program, we had to map the lake and divide it into 1,000 meter grids and different depth zones. Net sites were then randomly or semi-randomly chosen by grid and the number of net sets per depth zone were sampled proportionally to the surface area of each zone.
Our goal was to get a good distribution of sets across the entire lake and across all depths to get a representative sample of the lake trout population (Map 1 – right). We began the project in the 2nd week of July 2016, and overcame several obstacles along the way including thunderstorms and high winds, improperly constructed nets, a lack of rope (Sebago is deep!), and boat engine failure. Even with the challenges, we managed to set 140 sample sites over a seven week period. 
Our sampling yielded 135 lake trout with minimal bycatch of non-target fish species. Preliminary analyses indicate 1.3 lake trout/acre with a lake-wide estimate of 39,200 harvestable size lake trout (12 inches or larger).
The lake trout appear to be well distributed around the lake, but there were several interesting patterns regarding their vertical distribution in the water column including no lake trout sampled shallower than 32 feet (not surprising) or greater than 262 feet in depth and that the density of lake trout progressively increases with depth.
Interestingly, 75% of the lake trout sampled were located in water over 98 feet in depth; and in general, the size of lake trout decreases with increasing depth. Since most anglers do not or cannot fish very effectively below 100’ in depth, angler observations and perceptions of the lake trout fishery can be very accurate, but are not reflective of the entire population. These differences between our biological data and angler’s observations are important to consider as we continue to manage Sebago Lake’s lake trout population.
The lake trout sampled ranged in size from 10.3 to 33.9 inches in length with an average length and weight of 19.7 inches and 2.8 pounds respectively. While most of the lake trout sampled were juvenile to average sized fish, we did catch several exceptional lake trout (Photo 1 – Below). We are in the process of determining ages of the fish sampled via their ear bones (called otoliths), which will provide us with additional and valuable population statistics including growth rates, population age structure, and mortality/survival data.
We will continue the SPIN sampling on Sebago Lake for 2017, and as of this writing we have sampled 25 of the previous sites with a target of 128 sample sites. This additional data will give us an idea of the year to year sampling variability and provide Fisheries Biologists with a solid baseline of lake trout data for future evaluations on Sebago Lake. We plan to complete our otolith aging over the winter of 2017/2018 and to then provide a report on our findings and recommendations. In addition, we are considering collecting SPIN data on other important lake trout waters in the region over the next 4-5 years, so stay tuned!