Lake Mahopac Triploid Grass Carp Stocking Assessment and Proposal for Restocking in 2015

The aquatic vegetation in Lake Mahopac has been well documented over the last 20 years.  Since 1993, annual studies have been conducted to monitor the densities of the various vegetation types in the lake.  In a 1997 report written by John Grim of Northeastern Biologists, the history of when “aquatic weeds” started to become a problem was described.  John Grim wrote: “Anecdotal evidence of aquatic weed abundance indicates that before 1982, aquatic weeds of any species were not noticeably abundant.  In 1984 or 85, Eurasian Water Milfoil was identified in some abundance and became very abundant by 1987.  From that year on, action was taken to control it, culminating in the purchase of an aquatic weed harvester.  The harvester saw continual use until the triploid grass carp were introduced in October of 1994.”

The stocking of Triploid Grass Carp (TGC) was initiated due to a very high abundance of Eurasian Watermilfoil.  This species of vegetation is extremely invasive and will often outcompete native vegetation to the point that it becomes the dominant, or only, vegetation present in a lake.  This was certainly the case in Lake Mahopac by the early 1990’s.  Although many native vegetation types provide good habitat for aquatic organisms and are essential to the healthy ecology of a lake, over abundant vegetation can substantially change the ecology, resulting in problems for aquatic life and the human uses.  After a full environmental review, New York State Department of Environmental Conservation (DEC) issued a TGC stocking permit to the Lake Mahopac Park District.  This permit allowed the Park District to stock up to 15 Triploid Grass Carp per vegetated acre.  DEC anticipated that at this stocking rate, the grazing TGC would reduce the vegetation biomass throughout the lake by 70%.  DEC felt that this would be both protective of the lake’s ecology and provide relief to recreational users of the lake who found it hard to fish, swim, boat, and otherwise enjoy the near shore areas of the lake.

Within two years of the 1994 TGC stocking, drastic changes in the vegetation were seen.  This was documented in the annual monitoring reports that the Lake Mahopac Park District contracted.  Within several years of stocking, TGC eliminated virtually all the Eurasian Watermilfoil, as well as nearly all the other species of submerged vegetation.  All that remained in the lake were some filamentous algae and small pockets of lily pads until 2013.  During this period of scarce vegetation, many lake residents were very satisfied with the conditions of the lake.  Beneath the water’s surface, however, other changes had occurred that were maybe only noticed by people who fished the lake before TGC were stocked.  DEC conducted several fisheries surveys and showed that the lack of vegetation resulted in a decline of certain fish species.  Largemouth Bass (the primary gamefish) numbers declined by 66% between 1994 and 2004.  This species, as well as others, thrive in environments where they can find submerged vegetation for its protective cover (especially when they are small) and to feed in (during all life stages).

Since 2013, several species of vegetation have come back, including Eurasian Watermilfoil.  The predominant submerged vegetation in 2014 was found to be Najas flexilis (Waternymph), making up 92% of the biomass in the lake.  This Waternymph species is a native vegetation that has important qualities for the lake ecology that should benefit fish and wildlife.  Waternymph also rarely becomes a nuisance for people since it grows to a maximum height of only around 2 feet and should not form tangled mats on the surface of the water like Eurasian Watermilfoil.  The concern of many around the lake however, is the presence of Eurasian Watermilfoil and the potential it may have to expand quickly throughout the lake.  The Eurasian Watermilfoil is currently at a comparatively low density, nonetheless, it has already become a nuisance to some in a few locations where it exists around the lake.

The best way to control this limited amount of Eurasian Watermilfoil is not entirely clear.  Several options exist, but most of the people familiar with the history of vegetation control at Lake Mahopac know that TGC were very effective at controlling Eurasian Watermilfoil in the past.  Herbicides are one possible way to treat Eurasian Watermilfoil, and they could potentially be used in a precise way to only treat the areas where the milfoil is present.  Hand pulling or mechanical removal is another option, but this would need to be done carefully since it can often result in fragmenting the milfoil, enabling it to drift and take root in other locations.  Biological methods other than TGC, such as a weevil, are only sometimes effective and are expensive.

The Park District has formally requested a new permit to stock additional TGC to control the Eurasian Watermilfoil.  They are concerned that history will repeat itself and that the Eurasian Watermilfoil will expand and take over all the habitat from a depth of 15 feet shoreward, as it did in the early 1990’s.  The concern with this, however, is that if too many TGC are stocked again, it will result in too much vegetation being removed, which may have negative impacts to the ecology of the lake.  The DEC Regional Fisheries Unit feels that if TGC are stocked, the number stocked should be in proportion to the area currently covered by the problematic Eurasian Watermilfoil.  In 1993, 100% of the near shore area, to a depth of 15 feet, was covered with Eurasian Watermilfoil.  The milfoil was also quite dense at 1,785 grams per square meter.  As of the 2014, Eurasian Watermilfoil was present in all the five areas checked, but the estimated biomass was 118 grams per square meter.  If these five sites were representative of the rest of the lake, the Eurasian Watermilfoil is only 6.6% of what it was in 1993.  In 1994, a total of 2,565 TGC were stocked.  Given the estimate that the Eurasian Watermilfoil is 6.6% of the 1993 levels, the proportional number to stock for the current level of milfoil is 170 TGC.

The current coverage of Najas flexilis (Waternymph) may be a very good sign for the ecology of the lake.  This native submerged vegetation will provide an important component of the lake’s ecology that has been absent for nearly 20 years.  For fish species in the lake, this may allow them to rebuild their numbers and provide a better fishery.  We do not know, however, if the Triploid Grass Carp may choose to eat the Waternymph as opposed to the Eurasian Watermilfoil.  This is a risk, because if the carp do start to eat the Waternymph, it may hasten the expansion of Eurasian Watermilfoil.  Eurasian Watermilfoil has the ability to do this without the presence of TGC, but recent monitoring has shown that the opposite may be occurring.  In 2014 the biomass of Waternymph expanded by 395%, while the Eurasian Watermilfoil had declined by 6.2% since the previous year.

Continued monitoring will be needed to assess changes in the vegetation with time.  Regardless of whether TGC are stocked this year or not, it will be important to monitor the vegetation to determine the appropriate control methods in the future.  Ideally, it would be nice to have a map of the lake delineating the areas where each vegetation type is growing so that their expansion and contraction can be measured.

 DEC would like to hear from people who have an interest in the submerged aquatic vegetation control at Lake Mahopac and find out what they think about the Lake Mahopac Park District’s proposal to stock additional Triploid Grass Carp to control Eurasian Watermilfoil.  The DEC at this point feels comfortable about issuing a stocking permit for up to 170 TGC to be stocked in 2015.  DEC plans to issue this permit, unless substantive evidence is brought to our attention that would suggest a more appropriate option should be further explored.
 
Michael J. Flaherty
Fisheries Manager
NYS DEC Region 3
21 S. Putt Corners Road
New Paltz, NY 12561
Phone: 845-256-3066
Michael.Flaherty@dec.ny.gov