Addressing Concerns for Water Quality Impacts
from Large-Scale Great Lakes Aquaculture

Based on a Roundtable
Co-hosted by the Habitat Advisory Board of the Great Lakes Fishery Commission
the Great Lakes Water Quality Board of the International Joint Commission

August, 1999

Appendix 10

Minnesota's Experience with Net Pen Aquaculture in Mine Pit Lakes

by Marvin E. Hora
Minnesota Pollution Control Agency, 520 Lafayette Road, St. Paul, Minnesota 55155


Minnesota Aquafarms, Inc. established a large-scale net pen salmon and trout culture facility near Chisholm, Minnesota in 1988. The aquaculture facility was located in an area which previously had been extensively mined for iron ore. The strip mining activities resulted in the development of several large, deep pits. The pits had to be continuously dewatered during mining operations to prevent the inflow of groundwater from interfering with mining activities. In some respects these large pits could be viewed as very large open wells. The mine pit lakes are very different than natural lakes for many reasons. Mine pit lakes in the project area range from 30-120 meters in depth, have very small watersheds, have very small surface areas, have very short fetches, and are filling up with groundwater in some cases at rates of several meters per year. Typically these lakes have very low nutrient concentrations, have very deep water clarity, and are meromictic. Shorelines and lake slopes are very steep, lack vegetation, and are highly erodible. Since a number of the mine pits were very close together, in many cases the pits were linked by mine shafts or tunnels. Sometimes the tunnels or shafts were collapsed upon closure of the mine, but even so, the fractured nature of the rock allowed for interchange of water between pits. Some of the mine pit lakes provide the drinking water supply to neighboring communities.

Aquaculture Operations

Chinook salmon and rainbow trout were the two principal species raised at the Chisholm facility. The facility received a National Pollutant Discharge Elimination System (NPDES) permit which authorized the production of 1.9 million kg per year of fish with the corresponding level of manure waste generated. Although the number remains proprietary, it is estimated that from 1988 to 1994 a little over 1.4 million kg of fish were produced. At the end of the production facility's existence there were 750,000 fish (173,000 kg) being held in 22 net pens in the Sherman pit.

The fish reared in the Chisholm mine pit lakes were held in several large net pens. The design of net pen fish rearing facilities is quite different than conventional or traditional on-land systems. At a net pen facility, floating cages are used to contain the fish. At the height of production, fish were held in sixty 15 meters deep nets. The salmon and trout within the net pens were fed a dry fish food which was hand thrown or mechanically blown into the nets. The fish food would sink toward the bottom of the net, and the fish would consume it on the way down. Several thousand kilograms of fish food per day were fed to the fish when the facility was operating under a normal mode. The food which was not consumed was lost through the bottom of the net and continued down to the bottom of the mine pit lake. The company experimented with utilizing a floating fish food, but rejected it due to increased cost and poor conversion rates.

The consequences of feeding several million fish held captive in the nets caused a concurrent problem of waste (manure) production. Fish excreted both soluble and insoluble waste into the net pens. The insoluble fish waste acted the same as the waste food in that it sank and passed through the net bottom and ultimately ended up on the mine pit floor. The soluble portion excreted in the net was lost to the surrounding water body. Violations of the company's NPDES permit phosphorus limit (33 µg/L) resulted in the Minnesota Pollution Control Agency (MPCA) requiring Minnesota Aquafarms to utilize a waste collection system for the facility.

The system consisted of a large funnel net below four net pens. The funnel net collected the insoluble waste that fell through the nets and concentrated the waste in the collector cone. Waste in the collector cone was then pumped out to a holding pond prior to land application. A pilot test of the system showed the efficiency of removal at 31% for solids, 39% for phosphorus and 10% for oxygen demand. Although the pilot showed these efficiencies, once in operation many problems occurred which we believed lowered the efficiencies and made operation of the system extremely problematic. Mechanical problems with the pumps and clogging of the net panels with waste and attached algal growth, which required the net to be hauled up and cleaned regularly, were two of the main issues. Ferric chloride was added to the net pens to enhance precipitation of phosphorus and collection effectiveness.

Environmental Concerns


The mine pit lakes are oligotrophic, having phosphorus levels less than 10 µg/L, less than 3 µg/L of chlorophyll a, and a secchi disc reading greater than three meters. The first pit lake to experience net pen activities was Twin City-South. This is a 28.3 hectare lake with a maximum depth of 69 m. During most of this time the lake was artificially mixed and aerated. The lake continued to be mixed after aquaculture activities ceased in July of 1993. Total phosphorus values began at µ10 g/L in 1988, rose to a high of 94 µg/L in 1992 and returned to near 10 µg/L in 1994 (Table 1). Dissolved inorganic nitrogen rose from 363 µg/L in 1988 to a high of 2,043 in 1993 before dropping to 1,228 µg/L in 1994. Chlorophyll a rose from 2.9 µg/L in 1988 to a high of 22.5 µg/L in 1991 before dropping to 1.5 µg/L in 1994.

Table 1. Twin City-south Pit Lake surface summer (May-Sept.) values (Axler et al. 1995)

Total Phosphorus (µg/L) Dissolved Inorganic Nitrogen (µg/L) Chlorophyll a (µg/L)
1988 10.2 ± 7.8
363 ± 105
2.9 ± 3.6
1989 34.0 ± 7.0
420 ± 108
11.4 ± 5.5
1990 65.5 ± 16.4
881 ± 58
8.9 ± 3.3
1991 77.5 ± 6.2
1,249 ± 146
22.5 ± 9.4
1992 94.6 ± 7.5
1,969 ± 146
4.0 ± 2.0
1993 51.6 ± 13.4
2,043 ± 233
5.5 ± 2.3
1994 14.6 ± 3.4
1,228 ± 180
1.5 ± 0.4

The second pit to experience net pen aquaculture was Sherman Lake. This lake has not undergone the scrutiny that the Twin City-South Mine has undergone. The one key parameter measured in the Sherman pit until the operation ceased in 1995 was phosphorus. The phosphorus concentration rose from below 10 µg/L before aquaculture began to levels above 250 µg/L towards the end of the operation.


The aquaculture company assured the MPCA at the beginning of the operation that no therapeutics would be necessary to treat the fish for diseases. This was soon proved to be untrue. Within a short time period, the company submitted a request for the use of therapeutics to control fish diseases. The use of therapeutics in the mine pits which were connected to other mine pits and ground water used for human drinking water became a volatile emotional issue with the people who used the drinking water. Eventually the company was given approval to use Terramycin antibiotic for disease control at levels that were not hazardous to the drinking water supply. A concern raised by the Minnesota Department of Natural Resources was that the widespread use of therapeutics would potentially select out disease organisms resistant to the antibiotic and that these antibiotic-resistant organisms could infect natural populations.

Persistent toxics

The presence of low levels of persistent bioaccumulative chemicals such as PCBs in the food fed to the fish was identified as a concern. The chemicals were present in the food due to the use of fish oils and other parts in making the food. No problems were found in the fish flesh offered for sale.

Political Concerns

The political ramifications of this aquaculture issue were very strong. The loss of the mining industry in this area reduced the jobs available to local citizens and was devastating to the local economy. The aquaculture venture promised a new industry and jobs. The mine pits were viewed as sterile bodies of water with little or no use by many. Those who took their drinking water from the pits felt otherwise, so a natural confrontation arose. The threat to drinking water and the pollution of the mine pits versus jobs for the local economy split the local community and the state legislature. The MPCA found itself in a common position of being in the middle and being attacked by both sides.

Lessons Learned

Restoration efforts

The restoration effort of the mine pit lake, Twin City-South, which included intensive aeration, alum addition, and fallowing, has led to a rapid return to near baseline conditions, which is oligo-mesotrophic status. This restoration was facilitated greatly by the fact that the steep shoreline conditions caused high ambient erosion which buried the nutrient-rich sediments below a layer of inorganic sediment. This isolated the nutrient-rich sediments from the overlying water column. Also, continual inflow of nutrient-poor groundwater diluted the nutrients.

Closure efforts

Minnesota Aquafarms filed for bankruptcy in February 1995 and the operation was taken over by Inter-Tribal Business Network. This appeared at a critical juncture due to problems which were being experienced by Minnesota Aquafarms. Minnesota Aquafarms had no resources with which to continue paying staff, paying for power for lake circulation, or maintaining the net cleaning activities. The net pens had to be regularly cleaned or else waste and algae would clog the nets, reducing the amount of water flow-through and oxygen available to the fish. The fish in the net pens were in danger of dying and the state was set to declare an emergency. No one wanted 750,000 fish to die needlessly in the nets, but the basis for the state to declare a health emergency, which is required by state law for the MPCA to act, was difficult to determine. Two solutions were available: (1) let the fish go, or (2) provide the resources to clean the nets. The state did not own the fish, so they couldn't just be released because recapture would have been extremely difficult and providing state resources to a private entity was not viewed with favor. No decision was required on the state's part since Inter-Tribal took over the entire operation. They maintained the nets, sold the fish that were at the appropriate size, and eventually released the rest of the fish to the lake.

Over the years of net pen aquaculture, several hundred thousand fish had escaped the net and had formed a free-roaming population which fed beneath the operating nets. All natural food had been consumed. When feeding stopped and the smaller fish were released, they became food for the larger fish. The released fish and the free-roaming fish were utilized as a sport fishing operation for approximately two years after the aquaculture operation ceased. Inter-Tribal would rent boats and charge a fee for fishing in the mine pit lake.


Minnesota's experience with net pen aquaculture was a long and tortuous affair. Environmental and political issues caused a great deal of controversy for the local community, Minnesota state government, the state Legislature and the project owners. At the completion of the effort an estimate of over 20,000 MPCA staff hours went into managing and dealing with this one facility.

Major References Utilized

Axler, R.P., S. Yokom, C. Tikkanen, J. Henneck and M. McDonald. 1995. Effects of Aquaculture on Mine Pit Lakes near Chisholm, MN: Restoration of Twin City-South pit lake by fallowing and status of Fraser pit lake.

Barr Engineering Company. 1993. Aquaculture Waste Collection and Treatment System for the Sherman Mine Pit Lake.

Barr Engineering Company. 1993. Twin City-South Mine Pit Lake Restoration Plan.