What Causes Scombrotoxin and How Is It Controlled?

Fish species like tunas, mahi-mahi, bluefish, mackerel, marlin and amberjack are prone to the development of scombrotoxin (histamine). When these fish are exposed to elevated temperatures for an extended period of time, spoilage bacteria can grow and produce an enzyme that can change a harmless amino acid called histidine into histamine and other related compounds that are toxic at high levels. The process that produces scombrotoxin can start as soon as the fish dies, and can continue at any time thereafter if temperature abuse allows histamine producing bacteria to grow or the enzymes they produce to be come active. Lowering the temperature as quickly as possible will prevent these bacteria from growing and producing scombrotoxin. Fish species prone to histamine development must be handled properly, chilled rapidly, and kept cold to ensure their safety and quality. Scombrotoxin can develop if the temperature of these fish is allowed to remain above 40^oF for an extended period of time, and it can develop faster at higher temperatures. To ensure safety and maintain quality, you should always ice your fish as soon as possible.

FDA Regulations to Prevent Histamine

The Food and Drug Administration (FDA) Seafood Hazard Analysis Critical Control Point (HACCP) regulations require that processors, who receive any of these fish species from a fishing vessel, monitor how the fish were handled aboard the vessel or implement a fish sampling and histamine-testing program. It may be necessary for fishermen to provide harvesting records that show all of the fish caught were properly iced or cooled to safe temperatures within a specific period of time. Current FDA guidance provides specific time and temperature targets necessary to prevent the development of scombrotoxin in susceptible fish species.

Generally, fish should be placed in ice or in refrigerated seawater within 6 to 12 hours after death (harvest), and then rapidly chilled to an internal temperature of 40^oF or less. The amount of time needed for proper cooling depends on the chilling method, size of the fish, air and water temperature, and if the fish were gutted. Without proof that these fish reached the proper temperature in time to prevent scombrotoxin, buyers will not be able to comply with the FDA's HACCP regulations and cannot accept the fish without con ducting expensive tests for histamine. There are a number of effective alternative control strategies that can be used to prevent histamine. Before using these alternatives, scientific data must be gathered to prove that these controls are effective, and all elements of a HACCP based system of control including monitoring and record keeping must be included. For more information, please see FDA Seafood HACCP Regulations.

Developing a Scombrotoxin (Histamine) Control Plan

Fishing vessel operators need to consider a number of factors when developing and implementing procedures to prevent scombrotoxin including:

• How to chill the fish you catch.
• The equipment or supplies needed to properly handle and chill your catch.
• The air and water temperatures during the fishing trip.
• The species, size, and amount of fish you expect to catch.
• How you will monitor the catch to show all fish were properly chilled and handled.
• The records needed to document that the catch was properly chilled.

A complete scombrotoxin (histamine) prevention strategy will include all of these items. Decisions about chilling strategies and what equipment and supplies are needed should be part of the planning and prepa ration for each fishing trip. Discussing your plans for handling, cooling, and monitoring your catch with your packer or buyer may be necessary to make sure that your control strategy will meet both regulatory and com pany requirements. One strategy or procedure might work for all of the different types of fish you catch, or you may need variations for specific types or sizes of fish or for different times of the year (e.g. one control plan for the colder months of the year when water temperatures are low and another plan for the warmer months). For more details, please see FDA HACCP Seafood Guidelines, Monitoring Plan Examples and Coot It: On-Board Fish Handling.

Chilling Fish Properly to Prevent Histamine and Comply with FDA Guidelines

1. the amount of ice used, or the temperature of the ice slurry or RSW;
2. the temperature of the fish when brought on board the vessel;
3. the size of the fish and/or the amount of fish added to the slurry or RSW;
4. if the fish has been gutted, and
5. the air temperature on the deck and in the storage hold.

To achieve the most rapid cooling, as much surface area of each fish as possible should be in direct contact with the cooling medium (ice or sea water). Most of the heat will be removed from the fish during the initial cooling stages. The ice will melt faster, or the temperature of the slurry or RSW will increase faster, during this initial chilling period. More ice or refrigeration is necessary to cool fish after they are landed than during storage.

Large fish will cool much more slowly than small fish. For large fish, more time is required to transfer the heat from inside the fish to the surface. For this reason, gutting large fish and then packing the gut cavity with ice, or immersing it in slush ice or RSW, will cool the fish faster. The following graphs show how cooling can be affected by the size of the fish, the type of cooling medium (ice or slurry ice) and whether the fish is gutted or whole before cooling.

Cooling Curve for 25 Pound & 13 Pound Whole Albacore Tuna

Cooling Curve for 22 Pound Whole Albacore Tuna in Ice and in SuperChill Solution

Using Ice to Chill Fish

A general rule of thumb for rapid cooling and storage is to use 2 pounds of ice for every pound of fish. Flaked or crushed ice is best because it will cool fish more rapidly. Ice pieces or particles should be as small as possi-ble to ensure that the greatest surface area of each fish will be in direct contact with ice. Fish should be completely covered in flaked or crushed ice during cooling and storage. Large fish should be carefully gutted when possible, and the gut cavity packed with ice to speed cooling. Careless gutting may spread bacteria found in the entrails that cause histamine formation.

When using ice to cool histamine-producing fish, there are several critical conditions that are currently recommended by FDA. All fish, except tuna larger than 20 pounds, should be packed in ice as soon as possible, never later than 12 hours after death (harvest), and then cooled quickly to 40^oF or less. For many fishing methods, time of death will coincide with when fish are landed on the vessel. The length of the tow or set may need to be considered for other fishing methods. When received by the processor, the fish must be at 40^oF or less if delivered 24 hours after death (harvest), 50^oF or below if delivered between 12-24 hours after death (harvest), or there must be evidence that chilling began on board the vessel when necessary if delivered less than 12 hours after death (harvest). For tuna larger than 20 pounds see the requirements described below. Talk to your buyer to be sure that your plans for icing your catch are adequate to meet these requirements.

Using Slurry Ice or Refrigerated Seawater (RSW) Ice slurries or RSW can cool fish even faster because each fish is completely surrounded by cold water, which maximizes the rate that heat is removed from the fish. A general rule for rapid cooling of fish with slurry ice is to use 2 parts ice to I part seawater or brine. The ice slurry mixture should be cooled to a temperature of 40oF or below before adding fish. To maintain the temperature below 40oF, add more ice as the fish cools and the ice melts. The quantity of ice needed to maintain this temperature will vary depending on the amount of fish added, their temperature, and the temperature of the deck or hold where the fish are cooled. Refrigerated seawater should be at 40oF or below before fish are added, and sufficient refrigeration capacity should be available to maintain the seawater below 40oF during the cooling process and storage. When using slurry ice or RSW to cool histamine producing fish several critical conditions must be met to meet current FDA recommendations. All fish, except tuna larger than 20 pounds, must be placed in an ice slurry or RSW at 40oF or less within 12 hours after death (harvest). If the slurry or RSW is only at 50oF or less, fish must be packed in the slurry or RSW within 9 hours after death (harvest). Cooling must continue until the fish reach 40oF or less. The target internal fish temperatures at delivery are the same as for cooling with ice. Talk to your buyer or RSW specialist to determine if your system is adequate to meet these requirements.

Chilling Fish When Water Temperature is 83^oF or Higher

When the water temperature is above 83^oF, it can take significantly longer to cool your catch to a safe temperature, and the risk of histamine formation in the fish is much greater. Under these conditions, the same time and temperature limits described above for tuna larger than 20 pounds must be met for all species that are prone to the development of scombrotoxin.

Chilling Tuna Larger Than 20 Pounds

For tuna larger than 20 pounds, the fish need to be iced or put into slurry ice or RSW sooner after they die than smaller fish to ensure that the entire fish cools rapidly enough to prevent scombrotoxin formation. According to current FDA guidelines, if the fish are gutted, they need to be iced (including the gut cavity) or put into slurry ice or RSW at 40^oF or less within 6 hours of death (harvest). If large tuna are not gutted they must be chilled to an internal temperature of 50^oF or less within 6 hours of death (harvest). Getting the temperature of large fish below 50^oF in 6 hours can be a significant challenge using just ice. More rapid cooling can be accomplished using ice slurries or refrigerated seawater. Care should also be taken when gutting these fish. The bacteria that produce histamine can be found in high numbers in the fish's digestive tract. The digestive tract should not be cut or spilled into the gut cavity or other parts of the fish, and care should be taken to avoid cutting or puncturing the edible portions of the fish.

Type, Size and Amount of Fish

Before leaving the dock, fishermen need to consider the type and size of fish that they expect to catch, how much fish will be landed per tow or set, and how long the fishing trip will last. These factors determine what supplies (e.g. amount of ice, refrigeration capacity etc.) and crew are needed to handle, sort, chill and store the fish properly to prevent histamine formation. For example, longline fishermen targeting large tuna will need to be prepared to kill and dress individual fish as they come on board. Getting these large fish chilled fast enough and keeping them cold throughout the fishing trip will be the greatest challenge.

Other fisheries such as a trawler targeting bluefish or mackerel might expect to land a large number of fish at one time. In this situation the major challenge may be to ensure that all of the fish get sorted and packed in ice quickly enough to prevent temperature abuse on the deck or in the hold of the vessel.

Monitoring Your Catch

You need to monitor the critical cooling and holding conditions described above to show that the fish were landed, handled and chilled properly for each fishing trip. Monitoring may require making several observations that could include: water temperature, the temperature of the ice slurry or RSW, and the temperature of a representative sample of the fish as they are cooled. Alternative strategies that have been proven to be effective to prevent histamine may require monitoring these and/ or other variables that demonstrate that the catch was cooled properly. You may also need to estimate when the fish died to decide how much time you have to get the catch under temperature control. For many fishing methods, time of death will coincide with when fish are landed on the vessel. The length of the tow or set may need to be considered for other fishing methods.

The tools needed to do this monitoring are a clock and a thermometer. There are many types of thermometers or temperature monitoring devices available. Common dial type thermometers are the least expensive, but are also the least reliable. They frequently need to be re-calibrated, take longer to provide a stable temperature reading, and must be inserted a considerable distance into the fish or water to work properly. Digital thermometers (thermocouple or resistor type) can cost from $20 to $150, but are much more durable and reliable and can provide almost instantaneous temperature readings. Other products such as temperature loggers or maximum temperature sensors are also available that will automatically record temperatures over time.

It is important to check that your catch is cooled properly. This may involve manually checking variables such as whether the temperature of your ice slurry or RSW is below 40^oF or 50^oF for cooling, and that the fish reach an internal temperature below 40^oF within the required amount of time. You may also need to check the ocean water temperature if it is likely to be 80^oF or above. When checking the internal temperature of fish it may only be necessary to take the temperature of one or two fish at the top of the iced container or one or two fish in a container of slurry ice or RSW to show that they have cooled to the proper temperature. This may need to be repeated for the catch from each tow or set during the fishing trip. If you have a system that has been validated and scientifically shown to meet the required time and temperature targets, other predetermined measurements may be monitored to show that these time and temperature limits have been met. For example, the time of landing and the time that fish are placed in a standardized slush ice mixture may be sufficient without routinely recording fish temperatures. Monitoring your catch will not only ensure safety but will also help you to ensure that quality (and high market value) is also maintained.

Keeping Records

You should keep records of how the fish was chilled and stored on the vessel. Receivers of histamine producing fish species are required by FDA HACCP regulations to obtain records that show the fish were chilled and stored properly on board the fishing vessel. These records should show that the fish were iced or put into slurry ice or RSW in time to prevent histamine formation, and that they were chilled to the proper internal temperature. A simple logbook or form is one method to record the time and temperature observations needed to assess the relative risk of scombrotoxin formation. Electronic time/temperature records may provide a second option. Please see Monitoring Plan Examples for examples of histamine control strategies and record forms for various fisheries are available.