Lake Hopatcong Trout Habitat and Tagging Study

One of the most significant recreational draws to Lake Hopatcong is its trout fishery, recognized regionally by anglers and established as an important component of the local economy. Data collected over the past 30 years at the lake was recently analyzed and showed increasing surface water temperatures. This trend may suggest that the trout carryover habitat is being negatively impacted. Additionally, trout stocking practices at the lake have been modified over the last decade and have shifted towards stocking smaller trout that are potentially more vulnerable to changes in water quality and habitat quality. Together, these factors are viewed as potentially detrimental to the trout fishery.

Trout require relatively high dissolved oxygen (DO) concentrations and cool water temperatures. In regard to those habitat requirements, the critical period is the high summer months near peak water temperatures. At that time of year, trout holdover habitat, or the portion of the lake that meets the temperature and DO requirements for trout, compresses as a result of increased surface water temperatures and oxygen depression in the deeper water column. Research indicates larger and older trout have a higher tolerance of marginal habitat conditions like high water temperatures and low DO concentrations than smaller fish.

In response to the concerns regarding the current stocking of trout in Lake Hopatcong, the Lake Hopatcong Commission (LHC) Trout Committee was formed in 2021. The LHC, in cooperation with the Lake Hopatcong Foundation and the Knee-Deep Club, initiated a three-year trout tagging study. The study is focused on the introduction of larger trout to assess the long-term population dynamics of those stocked fish and the general health of the fishery. In particular, the Trout Committee is interested in the intersection between the stocking of larger brown trout (Salmo trutta) and trout carryover habitat quality.

One thousand tagged brown trout, approximately 12-14 inches in length, were released in Lake Hopatcong on March 26, 2022 to initiate the first year of the study. In addition to stocking fish and managing the tag and creel survey, the Trout Committee and other stakeholders also seek to better define carryover habitat in the lake. This includes habitat in the limnetic area (open waters) of the lake, as well as potential trout refuge habitat near seeps, springs, tributaries, or other attractant features around the shoreline. Thus, temperature and DO were measured at a high frequency in the limnetic zone, around the entire shoreline, and in 10 tributaries over the course of the summer of 2022. All technical aspects of the project, including the data collection, analysis, and subsequent report were conducted by Princeton Hydro and funded by a New Jersey Highlands Council grant.

Overall, little can be stated after the first year of the study regarding carryover trout populations. More data on which to analyze those factors will be generated in the spring of 2023 and will continue to be developed over the course of this multi-year study. The in-situ data indicates that although the summer was historically hot and dry in 2022, and carryover trout habitat was extremely limited at times from mid-July through mid-August, available habitat was dynamic on a weekly and likely diel basis during the peak summer months.

The study also documented multiple near-shore locations, mostly around the northern end of Lake Hopatcong, that provided carryover trout habitat on the same day where limnetic habitat was limited during the extremely warm August. Additionally, all 10 stream sites were cooler than the Mid-Lake station, and eight sites were over 4.0 °C cooler than the Mid-Lake station on the same date in mid-August. As a final component of the study, nine locations throughout the watershed were identified as candidates to be enhanced to protect the near-shore and stream habitat identified as carryover trout habitat during the 2022 sampling.