OF CORMORANTS, CLARITY, AND A CONUNDRUM
by Jack Henke

 

Oneida is a troubled lake in this millennium year. Cormorants, zebra mussels, and a drop in dissolved nutrients have altered the lake's rich food web. Fish populations have suffered. Angling, once a mainstay of the lake area's economy, has been poor.

What follows are a series of commonly asked questions and their answers, related to cormorants, water clarity, and other changes in the lake's ecosystem.

While the Department of Environmental Conservation, the Oneida Hatchery, and the Cornell Field Station have provided the Oneida Lake Bulletin with statistics and analyses, the opinions and some interpretations expressed in this article are the OLA’s.

Q:  How much has Oneida Lake's walleye population dropped?
There's been a steady decline during the past decade. Cornell estimated that the lake's population of adult walleyes (fish that are age 4 and older) was slightly less than 600,000 during 1990 and 1991. Walleye numbers slipped to around 315,000 by April, 1995. The population was about 215,000 last spring. The latter figures are well below walleye population averages from past years.

Q:  How have cormorants affected  the lake's walleye population?
A lot. Cormorants feed heavily on walleyes. Biologists talk about our lake's walleyes in terms of "year classes". A year class simply means the walleyes born in a particular year. The birds have taken a heavy toll from some year classes in the 1990's. Cornell biologists originally estimated that the 1991 class would contribute about 407,000 adult walleyes to the lake. Instead, the class produced about 144,000. Cormorants ate a large portion of the difference--well over 100,000 fish. Other year classes in the 1990's showed significant differences. The 1993 year class should have contributed about 73,000 adult walleyes to the lake, but instead only 8,000 were there at counting time. Cornell predicted that about 112,000 walleyes would reach adult size from the class of 1995, but only 32,000 did. The birds have been eating well.

Q:  What size walleyes do cormorants eat?
Larger fish, 15" and over, are harder for the birds to swallow. They prefer smaller walleyes. Now, listen very carefully to this important point:  You'll sometimes read or hear things like, "Cormorants and anglers each take about 15,000 adult walleyes from Oneida Lake every year." That's true but, by itself, the statement's misleading. It minimizes the cormorants' effect by not representing the whole picture. The key word is "adult". Both anglers and cormorants harvest about the same number of adult walleyes. However, the birds also consume tens of thousands of juvenile fish, those that are ages 1, 2 and 3. Anglers don't take these.

Q:  How many walleyes do Oneida Lake's cormorants eat in an average year?
We don't know average year numbers, but we can give you statistics from 1996-1997. From the spring of 1996 to the spring of 1997, the lake's cormorants gobbled about 33,200 walleyes that were age 1. They also ate 24,300 that were age 2 and 43,100 that were age 3 or older. That's a total of 100,600 walleyes! These fish had excellent chances of reaching adult size ... but the birds got them first. As one Cornell biologist said, "When walleyes get to age 1, they have few natural enemies--except for cormorants."

Q:  What's been happening to our lake's yellow perch population?
There's been a decline throughout the 1990's. Cornell says that in 1991, 1992, and 1993 the lake had 1,260,000, 965,000, and 1,562,000 adult perch, respectively. In the years 1997, 1998, and 1999, however, perch censuses revealed only 808,000, 905,000, and 765,000 fish. The population has not exceeded one million since 1993 and, in 1996, it bottomed at 561,000.

Q:  How have cormorants affected yellow perch?
Perch are a big part of the birds' diet. Since cormorants arrived in the 1980's, they've eaten millions of perch. Again, let's look at the 1996-1997 year. During that period, cormorants consumed about 67,800 perch that were age 3 and older. Remember, though, that bigger fish are harder for these birds to swallow. In that same year, over 138,000 perch that were age 2 and over 900,000 that were age 1 were devoured by cormorants.  These fish were more to the birds' liking. Just for comparison, walleyes ate about 3,600,000 perch that were age one during the '96-'97 year. However, the lake's walleyes ate no perch that were age 2 or age 3 in that year while cormorants, as you can see, took over 200,000.

Q:  Has the cormorant harassment program worked?
Somewhat. It was estimated that, in 1998, the program saved at least 30,000 walleyes and 90,000 perch. However, even with harassment, cormorants munched at least another 30,000 walleyes and 90,000 perch. (When this article was written, the numbers for 1999 were, as yet, unavailable.)

Q:  Do other factors contribute to the lower numbers of walleyes and perch?
Yes. There are two "life stages" of walleyes and perch that have experienced an increased mortality in recent years. We've talked about one stage--fish from ages 1 upward. The other stage involves fish from when they hatch until age 1. Young walleyes just aren't surviving like they used to. Their mortality is very high. If little fish don't live we sure as heck won't have the big fish. Cornell doesn't know for sure what's causing this. It might be increased water clarity. This would make small fish more vulnerable to predation. We could label this problem as an Oneida Lake conundrum--a puzzling situation. The cormorants get a big chunk of the fish that survive and they're a primary reason why populations aren't what they should be. As Lars Rudstam--fisheries biologist at the Field Station--puts it, "Cormorants are the main reason we don't get as many walleyes and perch in Oneida Lake as we predicted. We examined other causes, but they didn't measure up. For ages 1, 2, and 3 perch and walleyes, the differences between predictions and reality are, by and large, due to cormorants."

Q: What else has changed in Oneida Lake?
Chemically speaking, our lake is very different. It contains less phosphorus. We need this substance to support algae growth. Algae is important because it makes up the bottom of the Oneida Lake food web. Improved wastewater treatment facilities and wiser farming methods within the lake's drainage basin have reduced the amount of phosphorus flowing into that watershed. In addition, zebra mussels feed on algae. They filter water through their tiny bodies and remove the algae. Our lake still makes plenty of algae, but it makes it later in the summer. The water stays clear for a much longer period of time, even until August. Clarity like this has never been observed in Oneida's recorded history.

Q:  Does this mean that the walleye and perch populations will stay low?
No. The lake still produces enough tiny creatures to support good-sized fish populations. In Cornell's last yearly report it said that "Because zooplankton production has not declined and benthic invertebrates have increased, there is currently no reason to believe that previous walleye populations are not sustainable." The zooplankton and invertebrates are little aquatic animals near the food web's bottom. Small fish eat these little guys and big fish eat the small fish. Evidently, even with the zebra mussels, the tiny creatures have enough to eat. That's good news.

Q:  So, if we get rid of cormorants, will walleye and perch populations recover?
They'll bounce back, but we may never have as many fish as in the "good old days". Remember that Cornell said the lake can support a larger walleye population. No one knows for sure how much larger. One thing's definite, though. If we have effective cormorant control we'll have more fish.

Q:  Will the 18", 3-fish limit help restore the walleye population?
Yes, it will. In fact, coupled with cormorant control, adherence to the new limit provides the quickest way to rebuild Oneida Lake's walleye numbers.

Q:  What’s the OLA’s position on the cormorant issue?
We want all cormorants removed from Oneida Lake. Until the last decade, cormorants have never been a major factor in the lake's food web. Their feeding has caused tremendous damage--not only to the lake's ecology, but to our region's economy.

Q:  What's the OLA doing about the problem?
Working through channels. This is the best way to address the problem. We will be presenting our case to the Fish and Wildlife Division of the Department of the Interior in a couple months. Cormorants are a protected, migratory bird, controlled by the Federal Government. We'd like the authority for cormorant control to be given to the states so that they can deal with problems created by these birds.

Q:  What can OLA members do?
Nothing, yet. We will need your help in the future. It is absolutely imperative that we avoid a situation like the one that occurred on Lake Ontario, where people took the law into their own hands and killed cormorants. This can only set back our efforts and lessen chances for success. If you know anyone who's thinking about shooting cormorants, discourage them. As long as our laws ban this practice, it must not occur on Oneida Lake.


Reprinted (with minor modifications) from The Oneida Lake Bulletin:  Spring/Summer, 2000.