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Predator/Prey Balance and Stocking

prepared in 2000 by
The New York State Department of Environmental Conservation

The Lake Ontario ecosystem is very complex and dynamic. Phosphorus is the nutrient which limits the amount of primary productivity (phytoplankton or algae) which ultimately determines how many fish the lake can produce. Long-term downward trends in phosphorus loading resulting from the ban on phosphates in detergents in the early 1970s and improved levels of sewage treatment in the Great Lakes Basin have resulted dramatic reductions in phosphorus loading in Lake Ontario over the last 20 or so years. Sea lamprey control and stocking of trout and salmon led to the spectacular sport fisheries we now have on Lake Ontario. By the early 1990s, prey fish species once thought to be virtually unlimited, began showing signs of stress brought on by reductions in primary productivity and excessive pressure from predation by trout and salmon.

As if this were not complicated enough, add the exotic Dreissenid mussels (zebras and quaggas) to the picture in the early 1990s and it gets more complex. While the impact of these mussels is not fully understood, they are known to filter large quantities of phytoplankton from the water. Recent studies revealed that primary productivity in the nearshore areas was less than would be predicted based upon the amount of phosphorus available. This may be a direct result of mussels filtering out the algae in these areas.

Alewives and smelt (to a lesser degree) are the dominant prey fish species for trout and salmon in Lake Ontario. By the early 1990s, both species had experienced reductions in overall biomass and in the proportion of larger individuals in the populations compared with the early 1980s. These signs were believed to be a direct result of excessive predatory pressure from trout and salmon. In addition, alewives, which are particularly important to chinook salmon, were also in very poor condition (low fat content) suggesting that they were being constrained from both the "top down" by predators and the "bottom up" by primary productivity.

These alarming conditions led to the formation of a board of technical experts from academia and the resource agencies in Ontario and New York to attempt to determine what level of productivity Lake Ontario could sustain under these changing circumstances. The findings of the committee suggested that a reduction in predatory pressure from trout and salmon of 50% would result in the best chance to maintain a healthy alewife population that could sustain trout and salmon populations.

Managers from the Ontario Ministry of Natural Resources (OMNR) and the New York State Department of Environmental Conservation (NYSDEC) consulted their respective constituents with the problem seeking input on how to deal with it. Most agreed that it was important to maintain the alewife population as well as the trophy fishing opportunities and the diversity of the trout and salmon available from the fishery. The resulting changes in stocking targets were a 64% reduction in chinook salmon, a 50% reduction in lake trout, a 10% increase in steelhead and no change in brown trout or coho salmon. Although chinook salmon were maintained to provide a trophy fishery, they bore the greatest reduction because being the largest and fastest growing of all the trout and salmon species in the lake, they accounted for a large majority of the predator demand. To maintain chinooks at their previous level, it would have been necessary to eliminate the stocking of all other species to achieve the 50% reduction in predatory pressure. This would have destroyed the diversity of the fishery, which was one of the attributes most highly valued by anglers.

Another panel of experts was convened in 1996 (Panel '96) to review the earlier panel's findings and to review more recent information. Panel '96 concluded that a complete collapse of the alewife population was unlikely and that alewives were more "resilient" than previously thought. However, they also suggested that, even at lower stocking levels, the risk was high that the alewife population would become sufficiently depressed to cause significantly reduced growth and survival of chinook salmon for prolonged periods, and that increasing stocking of chinook salmon would amplify this risk.

Poor catch rates for chinook salmon by the mid-1990s led to a great deal of dissatisfaction among many of New York's Lake Ontario stakeholders. A Fisheries Congress was assembled by NYDEC late in 1996 consisting of fishermen from the lake and tributaries, charter fishermen, business people, and elected officials from lakeside communities to gather input for Lake Ontario management decisions. The overwhelming recommendation from the Congress was to accept the additional risk and return the stocking of chinook salmon to the 1992 level (2.7 million). A similar effort conducted by OMNR on the Canadian side resulted in a much more conservative recommendation for a very modest increase. Subsequent negotiations between NYDEC and OMNR resulted in New York increasing chinook stocking from 1.0 to 1.6 million. The effects of this increase on the fishery, if any, will be evaluated over the next five years.

Just when the adult alewife population was approaching its lowest point since the monitoring program began in 1978, alewives produced an excellent year-class in 1998. The very warm fall of 1998 and mild winter were probably very advantageous to these fish. These fish, which were sampled as yearlings in the spring of 1999 represented the highest catch rate ever for yearlings during the 22 years that the survey has been conducted. They, along with individuals from the 1995 year-class (the last good year-class), made up the majority of the diets of trout and salmon this year.

Many changes in the fish community occurred in the lake as a result of the lower alewife abundance in recent years. Many species which were believed to be suppressed by alewives became more abundant. There have been dramatic increases in emerald shiners, burbot, lake herring, whitefish and three-spined stickleback. Yellow perch populations have also rebounded. A SUNY ESF graduate student is also conducting a large scale study to assess the effects of the changing alewife population and the resurgence of other species on trout and salmon diets. The reduced alewife population in recent years is also at least partially responsible for the natural reproduction we have seen by lake trout because alewives have been shown to feed on lake trout fry. Some level of natural reproduction in lake trout has been documented every year since 1994.

The dynamic and unpredictable nature of the fish community has also been expressed by fluctuations in the fishery. Harvest rates for boats fishing Lake Ontario were lowest for chinook salmon in 1995 when the fishery was the result of an annual stocking of 2.7 million. Interestingly, the large and abundant chinooks caught in 1999 were from the 1995 (age-4 +) and 1996 (Age-3 +) year-classes which were from stockings of 1.15 and 1.3 million fish, respectively. Clearly, other (but poorly understood) factors besides the numbers of fish stocked are playing an important role in determining fishing quality.

We expect more surprises and unpredictability over the next several years as the Lake Ontario ecosystem adjusts to the effects of lower productivity and the influences of zebra and quagga mussels. Despite this uncertainty our fishery management goal for Lake Ontario will remain to provide trophy fishing opportunities for a diverse assemblage of trout and salmon species.

This document is public information prepared by the The New York State Department of Environmental Conservation

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